Alcohols

Alcohols are a family of organic compounds that contain the hydroxyl functional group, –OH.

The –OH group is the characteristic functional group of an alcohol and strongly affects its chemical and physical properties.

Some simple examples are:

Methanol: CH₃OH
Ethanol: CH₃CH₂OH
Propan-1-ol: CH₃CH₂CH₂OH
Butan-1-ol: CH₃CH₂CH₂CH₂OH

Alcohols contain carbon and hydrogen like hydrocarbons, but they also contain oxygen. This makes their properties different from those of alkanes and alkenes with similar carbon chains.

The Hydroxyl Functional Group

The functional group of an alcohol is:

–OH

This is called the hydroxyl group.

For example:

CH₃CH₂OH

can be thought of as:

CH₃CH₂–OH

The carbon-containing part is attached to the hydroxyl group.

The presence of –OH identifies the compound as an alcohol.

For example:

CH₃CH₂CH₃

is propane, a hydrocarbon.

But:

CH₃CH₂CH₂OH

is propan-1-ol, an alcohol.

The difference is the presence of:

–OH

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What Is a Functional Group?

A functional group is an atom or group of atoms that gives an organic compound many of its characteristic chemical properties.

Different families of organic compounds contain different functional groups.

For example:

The functional group often determines how an organic molecule reacts.

Therefore, recognizing functional groups is one of the most important skills in organic chemistry.

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Alcohols Are Not Hydrocarbons

A hydrocarbon contains only:

carbon + hydrogen

Alcohols contain:

carbon + hydrogen + oxygen

Therefore, alcohols are organic compounds, but they are not hydrocarbons.

Compare:

Ethane: C₂H₆

contains only carbon and hydrogen.

Ethanol: C₂H₆O

contains carbon, hydrogen and oxygen.

Ethanol can also be written:

C₂H₅OH

Writing the formula this way makes the hydroxyl group easier to recognize.

The Alcohol Homologous Series

Alcohols form a homologous series.

Members of a homologous series:

• have the same functional group
• have similar chemical properties
• follow a general formula
• show gradual changes in physical properties
• differ from neighbouring members by CH₂

For simple saturated alcohols containing one –OH group, the general formula can be written:

CₙH₂ₙ₊₁OH

For example:

Notice that each successive member differs by:

CH₂

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Naming Simple Alcohols

The names of simple alcohols are based on the corresponding alkane names.

The ending:

-ane

is changed to:

-anol

For example:

methane → methanol

ethane → ethanol

propane → propanol

butane → butanol

The ending:

-ol

indicates that the molecule contains an alcohol functional group.

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Counting the Carbon Atoms

The beginning of the name tells us how many carbon atoms are present.

For example:

CH₃OH

contains one carbon.

Therefore:

methanol

And:

CH₃CH₂OH

contains two carbon atoms.

Therefore:

ethanol

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Position of the –OH Group

When a molecule contains three or more carbon atoms, the position of the hydroxyl group can matter.

Consider:

CH₃CH₂CH₂OH

The –OH group is attached to carbon 1.

Name:

propan-1-ol

Now consider:

CH₃CH(OH)CH₃

The –OH group is attached to carbon 2.

Name:

propan-2-ol

These molecules have the same molecular formula:

C₃H₈O

but different structures.

They are therefore structural isomers.

Naming Alcohols Step by Step

A simple procedure can be used.

Find the Longest Carbon Chain Containing the –OH Group

This determines the basic carbon name.

Count the Carbon Atoms

For example:

3 carbons → prop-

4 carbons → but-

Number the Chain

Number from the end nearest the –OH group.

Identify the Position of –OH

Give the hydroxyl group the lowest possible number.

Add the Ending –ol

For example:

CH₃CH₂CH₂CH₂OH

Four carbons.

OH on carbon 1.

Name:

butan-1-ol

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Worked Example: Name CH₃OH

There is one carbon.

Prefix:

meth-

It contains –OH.

Ending:

-anol

Therefore:

methanol

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Worked Example: Name CH₃CH₂OH

There are two carbon atoms.

Prefix:

eth-

It contains an alcohol group.

Therefore:

ethanol

For a two-carbon alcohol, there is no need to specify the OH position because placing it on either end gives the same structure.

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Worked Example: Name CH₃CH₂CH₂OH

There are three carbon atoms.

Parent chain:

propane

The –OH group is attached to carbon 1.

Therefore:

propan-1-ol

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Worked Example: Name CH₃CH(OH)CH₃

There are three carbon atoms.

The –OH group is attached to the middle carbon.

Therefore:

propan-2-ol

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Worked Example: Name CH₃CH₂CH(OH)CH₃

There are four carbon atoms.

Number from the end closest to –OH.

The hydroxyl group is on carbon 2.

Therefore:

butan-2-ol

Not:

butan-3-ol

The lowest possible position number is used.

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Molecular Shape and Polarity

The oxygen atom in an alcohol attracts bonding electrons more strongly than carbon or hydrogen.

As a result, the:

O–H

and:

C–O

bonds are polar.

This gives alcohol molecules a region of partial positive and partial negative charge.

Alcohols are therefore more polar than comparable hydrocarbons.

This polarity helps explain several important properties of alcohols.

Hydrogen Bonding

Alcohol molecules can form relatively strong intermolecular attractions called hydrogen bonds.

These occur because of the –OH group.

The hydrogen attached to oxygen in one alcohol molecule can be attracted to the oxygen atom of another molecule.

Hydrogen bonding influences:

• boiling point
• viscosity
• solubility
• evaporation

This is why alcohols can have quite different physical properties from hydrocarbons of similar molecular size.

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Boiling Points of Alcohols

Alcohols generally have higher boiling points than similar-sized alkanes.

For example, ethanol has a much higher boiling point than ethane.

Why?

Ethane molecules experience relatively weak intermolecular forces.

Ethanol molecules can form hydrogen bonds.

More energy is required to separate ethanol molecules.

Therefore, ethanol has a higher boiling point.

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Boiling Point Trends

Within the alcohol homologous series, boiling points generally increase as the carbon chain becomes longer.

As molecular size increases:

• the number of electrons increases
• intermolecular attractions become stronger overall
• more energy is required to separate the molecules

Therefore:

methanol < ethanol < propanol < butanol

in general boiling-point trend.

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Solubility in Water

Small alcohols are quite soluble in water.

Examples include:

• methanol
• ethanol
• propanol

The –OH group can interact strongly with water molecules through hydrogen bonding.

This allows alcohol molecules to mix with water.

Why Solubility Decreases with Chain Length

An alcohol molecule has two important regions:

hydroxyl group → polar

hydrocarbon chain → mostly non-polar

In a small alcohol, the –OH group has a strong influence on the molecule's behaviour.

As the carbon chain becomes longer, the non-polar hydrocarbon portion becomes increasingly important.

Therefore, water solubility generally decreases as carbon-chain length increases.

For example, ethanol mixes very well with water, while longer-chain alcohols are much less soluble.

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Alcohols as Solvents

A solvent is a substance capable of dissolving another substance.

Alcohols are useful solvents because their molecules contain:

• a polar –OH region
• a less-polar hydrocarbon region

This allows some alcohols to interact with a wider range of substances than water alone.

Ethanol and propan-2-ol are widely used as solvents.

Applications can include:

• perfumes
• cosmetics
• cleaning products
• laboratory solutions
• pharmaceutical products
• inks
• coatings

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Evaporation

Many small alcohols are volatile liquids.

Volatile means they evaporate relatively easily.

This property is useful in applications such as:

• cleaning
• perfumes
• laboratory solvents

A solvent can dissolve a substance and then evaporate, leaving the dissolved material behind.

However, alcohol vapours may also be flammable, so they must be handled appropriately.

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Alcohols as Fuels

Many alcohols burn in oxygen and release energy.

Ethanol is an important example.

Complete combustion:

ethanol + oxygen → carbon dioxide + water

Balanced symbol equation:

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Energy is released during the reaction.

This makes ethanol useful as a fuel.

Ethanol as a Vehicle Fuel

Ethanol can be blended with gasoline.

Fuel blends containing ethanol are used in many vehicles.

Potential advantages include:

• ethanol can be produced from biomass
• it can reduce dependence on petroleum
• it has useful fuel properties
• it can be blended with conventional fuels

However, evaluating ethanol as a fuel requires considering the entire production process.

Factors include:

• land use
• crops
• fertilizers
• transportation
• processing energy
• greenhouse gas emissions

Therefore, "renewable" does not automatically mean "zero environmental impact."

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Bioethanol

Bioethanol is ethanol produced from biological materials.

Sources can include crops containing:

• sugars
• starch

Examples include:

• sugar cane
• corn
• other plant materials

Sugars can be converted into ethanol through fermentation.

A simplified equation is:

glucose → ethanol + carbon dioxide

Balanced equation:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Yeast provides enzymes that allow the fermentation process to occur.

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Fermentation

Fermentation is an important biological method for producing ethanol.

Typical requirements include:

• sugar solution
• yeast
• warm conditions
• limited oxygen

If the temperature is too low, fermentation proceeds slowly.

If it is too high, enzymes can be damaged and yeast cells may die.

The ethanol produced can later be separated and concentrated using processes such as distillation.

Ethanol by Hydration of Ethene

Ethanol can also be manufactured from ethene.

Ethene reacts with steam:

ethene + steam ⇌ ethanol

Symbol equation:

C₂H₄ + H₂O ⇌ C₂H₅OH

This is an addition reaction because water is added across the C=C double bond.

Ethene:

CH₂=CH₂

becomes ethanol:

CH₃CH₂OH

This connects alcohol chemistry directly to the chemistry of alkenes.

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Comparing Fermentation and Hydration

Ethanol can therefore be produced in different ways.

The best process depends on factors such as resources, cost, energy use and environmental impact.

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Methanol

Methanol is the simplest alcohol.

Formula:

CH₃OH

It is a colourless liquid and is used industrially as:

• a solvent
• a fuel
• a chemical feedstock
• a starting material for manufacturing other chemicals

Methanol is toxic and must not be consumed.

Even relatively small exposures can cause severe poisoning.

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Ethanol

Ethanol has the formula:

C₂H₅OH

It has many uses, including:

• fuels
• solvents
• disinfecting products
• perfumes
• cosmetics
• chemical manufacture
• alcoholic beverages

The same ethanol molecule is present whether the ethanol is produced industrially or biologically.

Its use and purity determine the context in which it is encountered.

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Propan-2-ol

Propan-2-ol has the structure:

CH₃CH(OH)CH₃

It is also commonly called isopropyl alcohol or isopropanol.

It is widely used in:

• cleaning
• electronics cleaning
• laboratory work
• some disinfecting products
• solvents

It evaporates readily and can dissolve many substances.

It is also flammable.

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Alcohols in Beverages

The alcohol present in alcoholic beverages is primarily:

ethanol

Ethanol can be produced when yeast ferments sugars.

Examples of fermented products include beverages produced from:

• grapes
• grains
• fruits
• other sugar-containing materials

Fermentation produces ethanol and carbon dioxide.

Distillation can be used to increase the concentration of ethanol in some products.

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Ethanol and the Human Body

Ethanol affects the nervous system.

Its effects depend on factors such as:

• amount consumed
• concentration
• rate of consumption
• body size
• food intake
• individual metabolism

High ethanol intake can impair:

• judgment
• coordination
• reaction time

Very high concentrations can be dangerous.

Long-term excessive consumption can also cause serious health effects.

From a chemistry perspective, it is important to recognize that the ethanol used as a solvent, fuel and beverage alcohol is the same chemical compound, although products differ greatly in concentration, purity and intended use.

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Not All Alcohols Are Safe to Drink

In chemistry, the word alcohol refers to a family of compounds, not just alcoholic beverages.

For example:

Methanol — CH₃OH

is highly toxic.

Ethanol — C₂H₅OH

is the alcohol found in alcoholic beverages.

Propan-2-ol — C₃H₇OH

is used as a solvent and disinfectant and should not be consumed.

Therefore:

alcohol ≠ automatically drinkable

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Combustion of Methanol

Methanol can also undergo complete combustion.

Word equation:

methanol + oxygen → carbon dioxide + water

Balanced equation:

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

Energy is released.

This is why methanol can also be used as a fuel in some applications.

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Complete and Incomplete Combustion

When sufficient oxygen is available, alcohols can undergo complete combustion.

Products:

carbon dioxide + water

If oxygen is limited, incomplete combustion may occur.

Possible products can include:

• carbon monoxide
• carbon particles
• water

Carbon monoxide is particularly dangerous because it is toxic.

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Alcohols and Flammability

Many low-molecular-mass alcohols are flammable.

This means they can ignite and burn readily under suitable conditions.

Examples include:

• methanol
• ethanol
• propan-2-ol

Therefore, alcohols used as solvents should generally be kept away from:

• flames
• sparks
• high temperatures
• other ignition sources

Flammability is useful when the alcohol is deliberately being used as a fuel, but it creates a safety hazard during storage and handling.

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Physical Properties of Simple Alcohols

Many small alcohols share several physical properties.

They are often:

• colourless
• liquids at room temperature
• volatile
• flammable
• soluble or partly soluble in water
• useful solvents

However, these properties change as molecular size increases.

In particular:

carbon chain length increases → boiling point generally increases

and:

carbon chain length increases → water solubility generally decreases

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Comparing an Alkane and an Alcohol

Compare ethane and ethanol.

The –OH group produces major differences in physical behaviour.

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Comparing Ethanol and Hexanol

Both ethanol and hexanol contain:

–OH

so both belong to the alcohol homologous series.

However, hexanol has a much longer non-polar carbon chain.

As a result:

• hexanol has a higher boiling point
• hexanol is less soluble in water
• the hydrocarbon portion has a greater influence on its properties

This demonstrates how both the functional group and carbon-chain length influence a molecule.

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Alcohols in Perfumes and Cosmetics

Alcohols are useful in perfumes and cosmetic products because some are effective solvents.

Ethanol can dissolve many fragrance compounds.

It also evaporates relatively quickly.

In a perfume:

1. ethanol helps dissolve fragrance molecules
2. the mixture can be sprayed
3. ethanol evaporates
4. fragrance molecules remain and disperse

This combines two useful properties:

solvent ability + volatility

Alcohols in Cleaning Products

Alcohols such as ethanol and propan-2-ol are used in many cleaning applications.

They can:

• dissolve certain oils and organic residues
• evaporate relatively quickly
• mix with water to varying degrees

Propan-2-ol is commonly used for cleaning electronic components because it can remove some residues and evaporates relatively quickly.

However, electrical equipment should be handled safely and according to manufacturer guidance.

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Alcohols as Chemical Feedstocks

Alcohols are also important starting materials for manufacturing other chemicals.

They can be converted into compounds such as:

• alkenes
• carboxylic acids
• esters

For example, ethanol can be oxidized to produce ethanoic acid under suitable conditions.

Alcohols are therefore important not only as final products but also as chemical feedstocks.

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Recognizing an Alcohol from a Structure

Consider:

CH₃CH₂CH₃

Does it contain –OH?

No.

Therefore, it is not an alcohol.

It is propane.

Now consider:

CH₃CH₂CH₂OH

Does it contain –OH?

Yes.

Therefore, it is an alcohol.

It is propan-1-ol.

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Worked Example: Hydrocarbon or Alcohol?

Compound A:

C₄H₁₀

It contains only carbon and hydrogen.

Therefore:

hydrocarbon

Compound B:

C₄H₉OH

It contains an –OH group.

Therefore:

alcohol

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Worked Example: Identify the Functional Group

Consider:

CH₃CH₂CH₂CH₂OH

The functional group is:

–OH

Name:

hydroxyl group

Organic family:

alcohol

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Worked Example: Name an Alcohol

Structure:

CH₃CH₂CH(OH)CH₃

Longest chain:

4 carbons → but-

Number from the nearest end.

OH is on carbon 2.

Name:

butan-2-ol

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Worked Example: Predict Solubility

Which is likely to be more soluble in water?

ethanol or hexanol

Both contain –OH.

However, hexanol has a much larger non-polar hydrocarbon chain.

Therefore:

ethanol is more soluble in water.

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Worked Example: Compare Boiling Points

Which would generally have the higher boiling point?

ethanol or ethane

Ethanol molecules can form hydrogen bonds.

Ethane molecules cannot.

Therefore:

ethanol has the higher boiling point.

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Common Mistakes

Thinking Every Compound Containing Oxygen Is an Alcohol

Alcohols must contain the appropriate:

–OH

functional group.

Other organic compounds can also contain oxygen.

Calling –OH "Hydroxide"

In an alcohol, –OH is the hydroxyl group.

A hydroxide ion is:

OH⁻

These are not the same thing.

Thinking Alcohols Are Hydrocarbons

Hydrocarbons contain only carbon and hydrogen.

Alcohols also contain oxygen.

Forgetting the –ol Ending

Alcohol names normally use:

-ol

Numbering from the Wrong End

Number the carbon chain so the –OH group receives the lowest possible number.

Calling CH₃CH(OH)CH₃ Propan-1-ol

The OH is attached to carbon 2.

Correct name:

propan-2-ol

Assuming All Alcohols Are Safe to Drink

Methanol and propan-2-ol are toxic.

The alcohol associated with alcoholic beverages is ethanol.

Thinking All Alcohols Have the Same Solubility

Water solubility generally decreases as the non-polar carbon chain becomes longer.

Forgetting Hydrogen Bonding

The –OH group allows alcohol molecules to form hydrogen bonds.

Thinking Hydrogen Bonding Is a Covalent Bond Within the Molecule

Hydrogen bonding is primarily an intermolecular attraction between molecules in this context.

Assuming Renewable Means Environmentally Harmless

Bioethanol can be renewable, but its overall environmental impact depends on how it is produced.

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Key Terms

Alcohol — An organic compound containing a hydroxyl functional group attached to a carbon framework.

Hydroxyl group — The –OH functional group characteristic of alcohols.

Functional group — An atom or group of atoms responsible for characteristic reactions and properties of an organic compound.

Organic compound — A carbon-based compound belonging to the broad field of organic chemistry.

Hydrocarbon — A compound containing only carbon and hydrogen.

Homologous series — A family of organic compounds with the same functional group and similar chemical properties.

General formula — A formula representing the common composition pattern of members of a homologous series.

Methanol — CH₃OH, the simplest alcohol.

Ethanol — C₂H₅OH, an alcohol widely used as a fuel, solvent and in alcoholic beverages.

Propan-1-ol — A three-carbon alcohol with –OH on carbon 1.

Propan-2-ol — A three-carbon alcohol with –OH on carbon 2; also known as isopropyl alcohol.

IUPAC nomenclature — A systematic method for naming chemical compounds.

Structural isomer — One of two or more compounds with the same molecular formula but different structural arrangements.

Polar molecule — A molecule with an uneven distribution of electrical charge.

Hydrogen bond — A relatively strong intermolecular attraction involving hydrogen bonded to a highly electronegative atom such as oxygen.

Intermolecular force — An attraction between separate molecules.

Solubility — The ability of a substance to dissolve in a solvent.

Solvent — A substance capable of dissolving another substance.

Volatile — Able to evaporate relatively readily.

Flammable — Able to ignite and burn.

Combustion — Reaction with oxygen that releases energy.

Complete combustion — Combustion with sufficient oxygen, producing carbon dioxide and water for an alcohol.

Incomplete combustion — Combustion with insufficient oxygen, potentially producing carbon monoxide or carbon particles.

Fermentation — A biological process in which microorganisms such as yeast convert sugars into products including ethanol.

Bioethanol — Ethanol produced from biological resources.

Hydration — Addition of water across a carbon-carbon multiple bond.

Feedstock — A starting material used in an industrial chemical process.

Distillation — Separation based on differences in boiling points.

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Key Takeaways

• Alcohols contain the hydroxyl functional group, –OH.
• The hydroxyl group determines many characteristic properties of alcohols.
• Alcohols contain carbon, hydrogen and oxygen.
• Alcohols are organic compounds but are not hydrocarbons.
• Hydrocarbons contain only carbon and hydrogen.
• Simple alcohols include methanol, ethanol, propanol and butanol.
• Simple saturated monohydric alcohols can be represented by CₙH₂ₙ₊₁OH.
• Alcohol names use the ending -ol.
• The carbon-chain prefix identifies the number of carbon atoms.
• The position of –OH must sometimes be included in the name.
• Propan-1-ol and propan-2-ol are structural isomers.
• Carbon chains are numbered to give –OH the lowest possible position number.
• The O–H and C–O bonds make alcohols more polar than comparable hydrocarbons.
• Alcohol molecules can form hydrogen bonds.
• Hydrogen bonding helps explain their relatively high boiling points.
• Small alcohols are often soluble in water.
• Water solubility generally decreases as carbon-chain length increases.
• Boiling point generally increases as molecular size increases.
• Many small alcohols are volatile and flammable.
• Alcohols are widely used as solvents.
• Ethanol and methanol can be used as fuels.
• Complete combustion of an alcohol produces carbon dioxide and water.
• Ethanol can be manufactured by fermentation.
• Ethanol can also be produced by hydration of ethene.
• Bioethanol can be produced from plant-derived sugars or starches.
• Ethanol is used in fuels, solvents, chemical manufacturing and alcoholic beverages.
• Methanol is toxic and must not be consumed.
• Propan-2-ol is widely used as a solvent and cleaning agent and must not be consumed.
• The term "alcohol" describes a chemical family and does not mean a substance is suitable for drinking.
• The –OH group can dramatically change the properties of a carbon-containing molecule.

The most important structural clue is:

Look for –OH attached to the carbon framework.

And for simple naming:

carbon-chain prefix + position of OH when needed + ol

Examples:

CH₃OH → methanol

CH₃CH₂OH → ethanol

CH₃CH₂CH₂OH → propan-1-ol

CH₃CH(OH)CH₃ → propan-2-ol

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Check Your Understanding

1. What functional group is found in alcohols?

2. What is the name of the –OH functional group?

3. Explain why alcohols are not hydrocarbons.

4. Identify the alcohol:

a. CH₃CH₃
b. CH₂=CH₂
c. CH₃CH₂OH
d. CH₃CH₂CH₃

5. Give the formula of methanol.

6. Give the formula of ethanol.

7. What does the ending -ol tell you about a compound?

8. Name:

CH₃CH₂CH₂OH

9. Name:

CH₃CH(OH)CH₃

10. Name:

CH₃CH₂CH(OH)CH₃

11. Explain why propan-1-ol and propan-2-ol have different names.

12. What is meant by a homologous series?

13. State the general formula for simple saturated alcohols containing one –OH group.

14. What structural difference separates ethanol from ethane?

15. Why are alcohols more polar than comparable hydrocarbons?

16. What intermolecular attraction occurs between alcohol molecules?

17. Explain why ethanol has a higher boiling point than ethane.

18. Describe the general boiling-point trend as alcohol carbon-chain length increases.

19. Why are small alcohols soluble in water?

20. Why does alcohol solubility generally decrease as the carbon chain becomes longer?

21. Give three uses of alcohols as solvents.

22. Why is ethanol useful as a fuel?

23. Write the word equation for complete combustion of ethanol.

24. Write the balanced symbol equation for complete combustion of ethanol.

25. What is bioethanol?

26. Write the balanced equation for fermentation of glucose.

27. Name another industrial method for producing ethanol.

28. Write the equation for hydration of ethene.

29. Why is methanol particularly hazardous?

30. Challenge: Four compounds are shown below:

A: CH₃CH₂CH₃

B: CH₃CH₂CH₂OH

C: CH₃CH(OH)CH₃

D: CH₃CH₂CH₂CH₂OH

a. Which compound is a hydrocarbon?
b. Which compounds are alcohols?
c. Identify the functional group in B, C and D.
d. Name compound A.
e. Name compound B.
f. Name compound C.
g. Name compound D.
h. Which two compounds have the same molecular formula?
i. What relationship exists between these two compounds?
j. Which alcohol would you expect to be most soluble in water? Explain.
k. Which alcohol would you expect to have the highest boiling point? Explain.
l. Explain why these alcohols generally have higher boiling points than similar-sized alkanes.
m. Predict the products when B undergoes complete combustion.
n. Explain one reason alcohols are useful as solvents.
o. Explain how the hydroxyl group affects the physical properties of an alcohol.

Amines and Amino Acids

Amines are nitrogen-containing organic compounds. In simple primary amines, the characteristic functional group can be represented as the amino group, –NH₂.

Examples include:

Methylamine: CH₃NH₂
Ethylamine: CH₃CH₂NH₂
Propylamine: CH₃CH₂CH₂NH₂

Amino acids are especially important biological molecules because they contain both an amino group and a carboxyl group. Amino acids can join together to form proteins, which are essential components of living organisms.

A simplified amino acid structure is:

H₂N–CH(R)–COOH

where R represents a side chain that differs among amino acids.

The Amino Functional Group

The functional group emphasized in simple primary amines is:

–NH₂

This is called the amino group.

It contains:

• one nitrogen atom
• two hydrogen atoms

For example:

CH₃CH₂NH₂

contains an amino group attached to a two-carbon chain.

This compound is:

ethylamine

The amino group gives amines characteristic chemical properties that are very different from those of hydrocarbons, alcohols, and carboxylic acids.

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What Are Amines?

Amines are organic compounds related to ammonia, NH₃.

Ammonia:

NH₃

If one hydrogen is replaced by a carbon-containing group, a primary amine is produced.

For example:

NH₃ → CH₃NH₂

The product is methylamine.

More broadly, amines can contain one, two, or three carbon groups attached to nitrogen, but at this level the simplest primary amines containing –NH₂ are the main focus.

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Recognizing a Simple Amine

Consider:

CH₃CH₂CH₂NH₂

Look for:

–NH₂

It is present.

Therefore, the molecule is a:

primary amine

Compare this with:

CH₃CH₂CH₃

This contains only carbon and hydrogen.

Therefore, it is a:

hydrocarbon

Adding an amino group changes the chemical behaviour of the molecule significantly.

Naming Simple Amines

One common introductory naming method uses the carbon group followed by:

amine

For example:

CH₃NH₂ → methylamine

CH₃CH₂NH₂ → ethylamine

CH₃CH₂CH₂NH₂ → propylamine

Systematic IUPAC names such as methanamine, ethanamine, and propan-1-amine may also be encountered.

The important recognition skill is identifying the nitrogen-containing amino group.

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Amines Are Basic

Many amines behave as weak bases.

A base can accept:

H⁺

Amines have a lone pair of electrons on the nitrogen atom. This allows the nitrogen to accept a hydrogen ion.

A simplified reaction is:

RNH₂ + H⁺ → RNH₃⁺

For example:

CH₃NH₂ + H⁺ → CH₃NH₃⁺

This ability to accept H⁺ explains the basic behaviour of amines.

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Amines in Water

Amines can react with water to a limited extent.

A simplified representation is:

RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻

The formation of OH⁻ makes the solution:

basic

Therefore, an aqueous solution of a simple amine typically has:

pH > 7

Because the reaction is incomplete, many common amines are described as weak bases.

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Weak Base Does Not Mean Dilute

Just as with weak acids:

weak and dilute do not mean the same thing.

A weak base only partially reacts with water or accepts H⁺ to a limited extent.

A dilute solution contains a relatively small amount of dissolved substance per unit volume.

Therefore, a concentrated solution of a weak base can still be hazardous.

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Amines React with Acids

Because amines are bases, they can react with acids.

For example:

methylamine + hydrochloric acid → methylammonium chloride

A simplified equation is:

CH₃NH₂ + HCl → CH₃NH₃Cl

The amine accepts H⁺ and forms a positively charged ammonium-type ion.

This demonstrates the acid-base behaviour of amines.

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Physical Properties of Amines

The properties of amines depend on:

• molecular size
• structure
• number of amino groups
• intermolecular forces

Many small amines are:

• soluble in water
• volatile
• basic
• strong-smelling

Some low-molecular-mass amines have odours often described as:

• fishy
• ammonia-like
• unpleasant

As molecular size increases, water solubility generally decreases because the non-polar carbon portion becomes more significant.

---

Hydrogen Bonding in Primary Amines

Primary amines contain:

N–H bonds

They can therefore participate in hydrogen bonding.

Nitrogen is less electronegative than oxygen, so the hydrogen bonding in amines differs in strength from that in alcohols.

Nevertheless, hydrogen bonding affects properties such as:

• boiling point
• solubility
• interactions with water

This helps explain why small amines can dissolve reasonably well in water.

---

Comparing Amines and Alcohols

Consider:

ethylamine: CH₃CH₂NH₂

and:

ethanol: CH₃CH₂OH

Both contain a two-carbon chain.

However:

Ethylamine contains:

–NH₂

Ethanol contains:

–OH

Their different functional groups give them different chemical behaviour.

Most importantly:

ethylamine is basic

while:

ethanol is not normally considered basic in the same way.

---

Comparing Functional Groups

You have now encountered several important organic functional groups:

Recognizing these structures allows you to predict many properties and reactions.

---

What Are Amino Acids?

Amino acids are organic compounds containing both:

an amino group, –NH₂

and:

a carboxyl group, –COOH

These two functional groups give amino acids some unusual and extremely important properties.

A general amino acid structure can be represented as:

H₂N–CH(R)–COOH

The central carbon is attached to:

• an amino group
• a carboxyl group
• a hydrogen atom
• an R group

The R group differs among amino acids.

The Central Carbon

Most amino acids used to build proteins have a central carbon atom called the:

alpha carbon

This carbon is bonded to four groups:

–NH₂

–COOH

–H

and:

–R

The R group is particularly important because it determines the identity and many properties of the amino acid.

---

The R Group

The:

R group

is also called the:

side chain

Different amino acids have different side chains.

These side chains can differ in:

• size
• shape
• charge
• polarity
• ability to form bonds
• chemical reactivity

As a result, different amino acids behave differently inside proteins.

This diversity allows proteins to form an enormous range of three-dimensional structures.

---

Glycine

Glycine is the simplest amino acid.

Its R group is:

H

A simplified structure is:

H₂N–CH₂–COOH

Glycine contains:

• an amino group
• a carboxyl group

It is one of the amino acids that can be incorporated into proteins.

---

Alanine

Alanine has the structure:

H₂N–CH(CH₃)–COOH

Its R group is:

–CH₃

Compare:

Glycine: R = H

Alanine: R = CH₃

The difference appears small, but changing the side chain changes the amino acid's properties.

Different Amino Acids

Proteins are commonly constructed from 20 standard amino acids encoded by the genetic code.

Examples include:

• glycine
• alanine
• valine
• leucine
• serine
• cysteine
• lysine
• glutamic acid
• phenylalanine
• tryptophan

They share the basic amino-acid framework but contain different R groups.

These different side chains give amino acids different chemical characteristics.

---

Amino Acids Can Behave as Acids and Bases

Amino acids contain both:

–NH₂

and:

–COOH

The amino group can behave as a base.

The carboxyl group can behave as an acid.

Therefore, amino acids can show both:

acidic and basic behaviour

A substance capable of behaving as both an acid and a base is described as:

amphoteric

This is an important property of amino acids.

---

Zwitterions

In many conditions, especially in solid form and near neutral aqueous conditions, amino acids can exist largely as zwitterions.

A proton can transfer from the carboxyl group to the amino group.

Instead of writing:

H₂N–CH(R)–COOH

we can represent the zwitterionic form as:

⁺H₃N–CH(R)–COO⁻

The molecule contains:

• a positive charge
• a negative charge

but its overall charge can still be:

zero

This is called a zwitterion.

---

Why Zwitterions Matter

The presence of charged groups helps explain several physical properties of amino acids.

Many amino acids:

• are crystalline solids
• have relatively high melting points
• are soluble in water to varying degrees
• have much lower volatility than many small organic molecules

These properties differ greatly from compounds such as small esters or hydrocarbons.

The ionic character of zwitterions creates strong attractions between particles.

---

Amino Acids Are the Building Blocks of Proteins

One of the most important roles of amino acids is their ability to join together.

Amino acids can form long chains called:

polypeptides

These chains can fold into:

proteins

Therefore:

amino acids → polypeptides → proteins

Proteins are essential to almost every living organism.

Forming a Peptide Bond

Two amino acids can react together.

The carboxyl group of one amino acid reacts with the amino group of another.

A molecule of water is removed.

A simplified reaction is:

amino acid + amino acid → dipeptide + water

The new bond formed between the amino acids is called a:

peptide bond

The characteristic linkage can be represented as:

–CO–NH–

---

Peptide Bond Formation

Suppose we represent two amino acids as:

H₂N–CH(R₁)–COOH

and:

H₂N–CH(R₂)–COOH

They can join to form:

H₂N–CH(R₁)–CO–NH–CH(R₂)–COOH + H₂O

The:

–CO–NH–

linkage is the peptide bond.

Because water is produced, this can be described as a type of condensation reaction.

---

Dipeptides

When two amino acids join, the product is called a:

dipeptide

A dipeptide contains:

two amino-acid residues

joined by:

one peptide bond

If another amino acid joins:

tripeptide

can form.

When many amino acids join:

polypeptide

forms.

---

From Amino Acids to Proteins

The basic sequence is:

amino acids → peptide bonds → polypeptide chain → folding → functional protein

A protein may contain:

• dozens
• hundreds
• or even thousands

of amino-acid residues.

The exact sequence of amino acids is extremely important.

Changing the sequence can change how the protein folds and therefore how it functions.

---

Protein Structure

A polypeptide chain does not usually remain as a simple straight chain.

Interactions between different parts of the molecule cause it to fold.

The final shape of a protein depends partly on interactions between amino-acid side chains.

These interactions can include:

• hydrogen bonding
• ionic attractions
• hydrophobic interactions
• disulfide bonds

The resulting three-dimensional shape is closely connected to the protein's function.

Why Protein Shape Matters

Proteins perform highly specific jobs.

Their shape allows them to interact with particular molecules.

For example, an enzyme may have an active site whose shape allows particular substrate molecules to bind.

If protein shape changes significantly, its function can also change.

This is why:

amino-acid sequence → protein structure → protein function

is such an important relationship in biology.

---

Functions of Proteins

Proteins perform an enormous range of functions.

Examples include:

Enzymes

Enzymes catalyse biochemical reactions.

Examples include digestive enzymes such as amylase and proteases.

Structural Proteins

Examples include:

• collagen
• keratin

They provide strength and support.

Transport Proteins

Hemoglobin helps transport oxygen in blood.

Antibodies

Antibodies are proteins involved in immune defence.

Hormones

Some hormones, such as insulin, are proteins or peptides.

Movement

Proteins such as actin and myosin contribute to muscle contraction.

Receptors

Many cell receptors are proteins that allow cells to detect chemical signals.

---

Proteins in Food

Dietary protein provides amino acids that the body can use.

Protein-rich foods include:

• meat
• fish
• eggs
• dairy products
• beans
• lentils
• soy products
• nuts
• seeds

During digestion, proteins are broken down into smaller peptides and amino acids.

The amino acids can then be absorbed and used by cells.

Essential Amino Acids

Humans can manufacture some amino acids from other molecules.

However, some amino acids cannot be produced by the body in sufficient amounts.

These must be obtained through the diet.

They are called:

essential amino acids

For adults, nine amino acids are generally classified as essential.

A varied diet can provide the amino acids needed for protein synthesis.

---

Protein Digestion

Proteins in food are too large to be absorbed intact in the same way as individual amino acids.

During digestion, enzymes called:

proteases

break peptide bonds.

A simplified sequence is:

protein → shorter peptides → amino acids

The amino acids can then be absorbed through the small intestine and transported around the body.

---

Protein Synthesis

Cells use amino acids to build their own proteins.

The sequence of amino acids in a protein is ultimately determined by genetic information in:

DNA

A simplified flow is:

DNA → RNA → amino-acid sequence → protein

This creates a major connection between:

• chemistry
• genetics
• cell biology

The properties of proteins ultimately depend on the chemistry of their amino acids.

---

Amines in Biology

Amines also occur in many biologically important compounds.

Nitrogen-containing organic molecules are found in:

• neurotransmitters
• hormones
• medicines
• natural products

Examples of biologically important molecules containing amine-related functional groups include compounds involved in communication between nerve cells.

This demonstrates why nitrogen chemistry is extremely important in biochemistry and medicine.

---

Amines in Medicines

Many pharmaceutical molecules contain amine groups.

The basic properties of amines can influence:

• water solubility
• interaction with biological molecules
• absorption
• how medicines are formulated

Amines can also react with acids to form salts.

Some medicines are therefore manufactured or supplied as amine salts, which can have useful physical properties.

---

Comparing Amines and Carboxylic Acids

Amines and carboxylic acids often show opposite acid-base behaviour.

Amine

Can accept H⁺.

Therefore:

basic

Carboxylic Acid

Can donate H⁺.

Therefore:

acidic

An amino acid contains both functional groups.

This explains why amino acids can behave as both acids and bases.

---

Comparing the Organic Families

This illustrates why identifying functional groups is so useful.

Once the functional group is recognized, many properties of the molecule can be predicted.

---

Worked Example: Identify the Functional Group

Compound:

CH₃CH₂NH₂

Look for:

–NH₂

Functional group:

amino group

Family:

amine

---

Worked Example: Amine or Alcohol?

Compound A:

CH₃CH₂OH

contains:

–OH

Therefore:

alcohol

Compound B:

CH₃CH₂NH₂

contains:

–NH₂

Therefore:

amine

---

Worked Example: Identify an Amino Acid

Consider:

H₂N–CH₂–COOH

It contains:

–NH₂

and:

–COOH

Therefore, it is an:

amino acid

Specifically, it is:

glycine

---

Worked Example: Identify Both Functional Groups

Consider:

H₂N–CH(CH₃)–COOH

Functional group 1:

–NH₂ → amino group

Functional group 2:

–COOH → carboxyl group

Therefore:

amino acid

This particular amino acid is:

alanine

---

Worked Example: Predict Acid-Base Behaviour

Consider:

CH₃NH₂

Would you expect the compound to behave as an acid or a base?

The molecule contains an amine group.

The nitrogen can accept H⁺.

Therefore:

CH₃NH₂ behaves as a weak base.

---

Worked Example: Forming a Peptide

Two amino acids react.

What type of bond forms?

Peptide bond

What small molecule is produced?

Water

What type of reaction is this?

Condensation reaction

Therefore:

amino acid + amino acid → dipeptide + water

---

Amino Acid Sequence Matters

Imagine two polypeptides containing the same amino acids but arranged in different sequences.

Protein A:

A–B–C–D–E

Protein B:

A–C–B–E–D

Although they contain the same types of amino acids, their different sequences can cause different folding patterns.

Different folding can result in:

different shapes → different functions

This is why the order of amino acids is critical.

---

Denaturation

Protein structure can be disrupted by conditions such as:

• high temperatures
• extreme pH
• certain chemicals

This process is called:

denaturation

When a protein is denatured, its three-dimensional shape changes.

If the shape is important for function—as it usually is—the protein may stop working properly.

For example, an enzyme's active site may change shape.

Importantly, denaturation does not usually mean that every peptide bond in the protein has been broken.

---

Common Mistakes

Confusing –NH₂ and –OH

Amine:

–NH₂

Alcohol:

–OH

Confusing an Amine with an Amino Acid

An amine contains an amine functional group.

An amino acid contains both:

–NH₂ and –COOH

Thinking Amines Are Acids

Many simple amines behave as:

weak bases

Thinking Weak Means Harmless

A weak base can still be concentrated, corrosive, toxic, or otherwise hazardous.

Forgetting the Carboxyl Group in Amino Acids

A typical amino acid contains both:

amino group + carboxyl group

Thinking the R Group Is Always the Same

The R group varies between different amino acids.

Thinking All Amino Acids Have Identical Properties

Different R groups give amino acids different properties.

Confusing Amino Acids with Proteins

Amino acids are the building blocks.

Proteins are large molecules made from amino-acid residues.

Saying Amino Acids Join Directly with No Product

Peptide bond formation releases:

water

Confusing Peptide Bonds with Ester Bonds

Peptide bond:

–CO–NH–

Ester linkage:

–COO–

Thinking Protein Shape Is Unimportant

Protein shape is closely connected to function.

Thinking Denaturation Always Breaks the Protein into Amino Acids

Denaturation mainly changes protein folding and shape. Breaking a protein completely into amino acids requires hydrolysis of peptide bonds.

---

Key Terms

Amine — A nitrogen-containing organic compound related to ammonia.

Primary amine — An amine in which nitrogen is attached to one carbon-containing group and retains two hydrogens.

Amino group — The –NH₂ functional group emphasized in simple primary amines and amino acids.

Nitrogen — The element present in the amino functional group.

Ammonia — NH₃, a nitrogen compound closely related structurally to amines.

Base — A substance capable of accepting H⁺ according to the Brønsted–Lowry definition.

Weak base — A base that reacts only partially with water or accepts H⁺ to a limited extent under given conditions.

Lone pair — A pair of electrons not shared in a covalent bond.

Hydrogen bonding — An intermolecular attraction involving hydrogen bonded to an electronegative atom such as nitrogen or oxygen.

Amino acid — An organic molecule containing amino and carboxyl functional groups.

Carboxyl group — The –COOH functional group.

Alpha carbon — The central carbon in the basic structure of most protein-forming amino acids.

R group — The variable side chain that distinguishes one amino acid from another.

Side chain — Another term for the R group.

Glycine — The simplest amino acid, with H as its side chain.

Alanine — An amino acid with CH₃ as its side chain.

Amphoteric — Able to behave as either an acid or a base.

Zwitterion — A molecule containing both positive and negative charges while potentially having zero overall charge.

Peptide bond — The –CO–NH– linkage joining amino-acid residues.

Condensation reaction — A reaction in which molecules join while a small molecule such as water is produced.

Dipeptide — A molecule formed when two amino acids join by a peptide bond.

Polypeptide — A chain containing many amino-acid residues joined by peptide bonds.

Protein — A biological macromolecule consisting of one or more folded polypeptide chains.

Protein synthesis — The biological process by which cells assemble amino acids into proteins according to genetic information.

Essential amino acid — An amino acid that must be obtained in sufficient amounts through the diet because the body cannot synthesize enough of it.

Protease — An enzyme that catalyses the breakdown of proteins or peptides.

Hydrolysis — A reaction involving water that can break chemical bonds such as peptide bonds.

Denaturation — Disruption of a protein's normal three-dimensional structure.

Enzyme — A biological catalyst, usually a protein.

Functional group — An atom or group of atoms responsible for characteristic properties and reactions of an organic compound.

---

Key Takeaways

• Simple primary amines contain the –NH₂ amino functional group.
• Amines are nitrogen-containing organic compounds related to ammonia.
• Methylamine, ethylamine, and propylamine are simple examples.
• Many simple amines behave as weak bases.
• The nitrogen atom can accept H⁺.
• Amines can react with acids to form salts.
• Small primary amines can participate in hydrogen bonding.
• Many small amines are soluble in water.
• Some small amines have strong characteristic odours.
• Amino acids contain both an amino group, –NH₂, and a carboxyl group, –COOH.
• A general amino-acid structure can be represented as H₂N–CH(R)–COOH.
• The central carbon is bonded to an amino group, carboxyl group, hydrogen, and R group.
• The R group distinguishes different amino acids.
• Glycine has H as its R group.
• Alanine has CH₃ as its R group.
• Proteins commonly use 20 standard amino acids encoded by the genetic code.
• Amino acids can behave as both acids and bases.
• This behaviour is described as amphoteric.
• Amino acids can exist as zwitterions containing both positive and negative charges.
• Amino acids are the building blocks of proteins.
• Two amino acids can join by a peptide bond.
• Peptide bond formation produces water.
• It is therefore a condensation reaction.
• Two amino acids form a dipeptide.
• Many amino acids form a polypeptide.
• Polypeptide chains can fold into functional proteins.
• Amino-acid sequence influences protein shape.
• Protein shape strongly influences protein function.
• Proteins function as enzymes, antibodies, structural materials, transport molecules, receptors, and more.
• Dietary proteins can be digested into amino acids.
• Essential amino acids must be obtained in sufficient amounts from food.
• Functional groups allow us to connect organic chemistry with biological chemistry.

The most important structural distinction is:

Amine: –NH₂

Carboxylic acid: –COOH

Amino acid: contains both –NH₂ and –COOH

And the key biological sequence is:

amino acids → peptide bonds → polypeptides → folded proteins → biological functions

Check Your Understanding

1. What functional group is characteristic of simple primary amines?

2. What element distinguishes amines from alcohols and hydrocarbons?

3. Identify the functional group in CH₃CH₂NH₂.

4. Name CH₃NH₂.

5. Name CH₃CH₂NH₂.

6. Are simple amines generally acidic or basic?

7. Explain why an amine can accept H⁺.

8. What does it mean to describe an amine as a weak base?

9. Why does "weak base" not mean "dilute base"?

10. What two important functional groups are present in an amino acid?

11. Write the general structure of a simple amino acid.

12. What is the R group?

13. Why are R groups important?

14. What is the R group in glycine?

15. What is the R group in alanine?

16. Explain why amino acids can behave as both acids and bases.

17. What does amphoteric mean?

18. What is a zwitterion?

19. Explain how a zwitterion can contain charges but have zero overall charge.

20. What type of bond joins amino acids together?

21. What small molecule is produced when a peptide bond forms?

22. Why is peptide formation a condensation reaction?

23. What is a dipeptide?

24. What is a polypeptide?

25. Explain the relationship between amino acids, polypeptides, and proteins.

26. Why is the sequence of amino acids in a protein important?

27. Give four biological functions performed by proteins.

28. What are essential amino acids?

29. What happens to dietary proteins during digestion?

30. Challenge: Consider the compounds:

A: CH₃CH₂OH

B: CH₃CH₂NH₂

C: CH₃COOH

D: H₂N–CH₂–COOH

a. Identify the functional group in A.
b. State the organic family of A.
c. Identify the functional group in B.
d. State the organic family of B.
e. Explain why B behaves as a base.
f. Identify the functional group in C.
g. State the organic family of C.
h. Identify both functional groups in D.
i. Explain why D is classified as an amino acid.
j. Name amino acid D.
k. Explain why D can behave as both an acid and a base.
l. Describe the zwitterionic form of D.
m. Explain how two amino acids can join together.
n. Name the bond formed between them.
o. Identify the other product of the reaction.
p. Explain why this is a condensation reaction.
q. Explain how many amino acids can form a polypeptide.
r. Explain why changing the order of amino acids can alter a protein's properties.
s. Explain why protein shape is important for biological function.
t. Describe how this topic connects organic chemistry to biology.

Comparing Functional Groups

Organic molecules can contain the same basic carbon skeleton but behave very differently because of their functional groups.

A functional group is a specific atom or group of atoms that gives an organic molecule many of its characteristic physical and chemical properties.

The major functional groups we have studied are:

In this topic, the main comparison is between alcohols, carboxylic acids, esters, and amines.

Learning to recognize these functional groups allows us to look at an unfamiliar molecule and make useful predictions about how it might behave.

Why Functional Groups Matter

Consider these four compounds:

Ethanol: CH₃CH₂OH

Ethanoic acid: CH₃COOH

Ethyl ethanoate: CH₃COOCH₂CH₃

Ethylamine: CH₃CH₂NH₂

All contain carbon and hydrogen.

However, they have very different properties.

Ethanol is an alcohol.

Ethanoic acid is acidic.

Ethyl ethanoate is an ester and is commonly used as a solvent.

Ethylamine is an amine and behaves as a base.

The reason for these differences is largely their functional groups.

---

The Carbon Skeleton and Functional Group

An organic molecule can be thought of as having two important structural features:

carbon skeleton + functional group

The carbon skeleton affects properties such as:

• molecular size
• shape
• boiling point
• water solubility

The functional group strongly affects:

• polarity
• intermolecular forces
• acidity or basicity
• chemical reactions
• solubility
• characteristic uses

Both parts of the molecule matter.

---

Alcohols

Alcohols contain the:

–OH

functional group.

This is called the:

hydroxyl group

Examples include:

methanol: CH₃OH

ethanol: CH₃CH₂OH

propan-1-ol: CH₃CH₂CH₂OH

Alcohols contain carbon, hydrogen, and oxygen.

Properties of Alcohols

The –OH group is polar.

This allows alcohol molecules to form hydrogen bonds.

As a result, small alcohols often:

• have relatively high boiling points compared with similar-sized hydrocarbons
• dissolve well in water
• act as useful solvents
• are liquids at room temperature
• are volatile
• are flammable

Water solubility generally decreases as the hydrocarbon chain becomes longer.

---

Chemical Behaviour of Alcohols

Alcohols can undergo several important reactions.

They can:

• burn in oxygen
• undergo oxidation
• react with carboxylic acids to form esters

For example:

ethanol + oxygen → carbon dioxide + water

during complete combustion.

Ethanol can also be oxidized:

ethanol → ethanoic acid

And it can react with ethanoic acid:

ethanol + ethanoic acid ⇌ ethyl ethanoate + water

---

Carboxylic Acids

Carboxylic acids contain:

–COOH

This is called the:

carboxyl group

Examples include:

methanoic acid: HCOOH

ethanoic acid: CH₃COOH

propanoic acid: CH₃CH₂COOH

The carboxyl group contains both:

C=O

and:

O–H

within the same functional group.

Properties of Carboxylic Acids

Carboxylic acids are generally:

• polar
• capable of hydrogen bonding
• acidic
• relatively high-boiling compared with similar-sized hydrocarbons

Small carboxylic acids can be very soluble in water.

As their carbon chains become longer, their water solubility generally decreases.

---

Chemical Behaviour of Carboxylic Acids

Carboxylic acids behave as weak acids.

They can partially ionize in water:

RCOOH ⇌ H⁺ + RCOO⁻

They react with bases:

carboxylic acid + base → salt + water

They react with carbonates:

carboxylic acid + carbonate → salt + water + carbon dioxide

They can react with reactive metals:

carboxylic acid + metal → salt + hydrogen

They also react with alcohols:

carboxylic acid + alcohol ⇌ ester + water

---

Esters

Esters contain:

–COO–

A simple ester can be represented as:

R–COO–R′

An example is:

ethyl ethanoate: CH₃COOCH₂CH₃

Esters are formed when carboxylic acids react with alcohols.

Properties of Esters

Many small esters are:

• volatile
• colourless liquids
• only moderately or slightly soluble in water
• useful organic solvents

Many also have distinctive aromas that may be described as:

• fruity
• sweet
• floral

However, not every ester smells pleasant.

---

Why Esters Differ from Alcohols

Alcohols contain an:

O–H bond

Simple esters do not.

Therefore, alcohol molecules can form strong hydrogen bonds with one another more readily than ester molecules can.

This affects:

• boiling point
• volatility
• water solubility

As a result, many small esters are more volatile than comparable alcohols or carboxylic acids.

---

Amines

Simple primary amines contain:

–NH₂

This is called the:

amino group

Examples include:

methylamine: CH₃NH₂

ethylamine: CH₃CH₂NH₂

propylamine: CH₃CH₂CH₂NH₂

Amines contain nitrogen, which distinguishes them from the oxygen-containing functional groups studied above.

Properties of Amines

Many small amines are:

• polar
• soluble in water
• volatile
• basic
• capable of hydrogen bonding when N–H bonds are present

Some small amines have strong characteristic odours.

The amino group also gives many amines an important chemical property:

they can act as bases.

---

Why Amines Are Basic

The nitrogen atom has a lone pair of electrons.

This allows it to accept:

H⁺

For example:

RNH₂ + H⁺ → RNH₃⁺

Therefore, many amines behave as:

weak bases

This provides a useful contrast:

carboxylic acids → acidic

amines → basic

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Side-by-Side Comparison

This table is useful, but understanding why these differences occur is more important than memorizing them.

---

Recognizing Functional Groups from Structures

When given an unfamiliar organic molecule, search for characteristic arrangements of atoms.

Look for –OH

If you see:

–OH

attached appropriately to a carbon skeleton, the compound may be an:

alcohol

Look for –COOH

If you see:

–COOH

the compound is a:

carboxylic acid

Look for –COO–

If you see:

–COO–

connecting carbon-containing groups, the compound may be an:

ester

Look for –NH₂

If you see:

–NH₂

the compound may be a:

primary amine

---

Worked Example: CH₃CH₂OH

Structure:

CH₃CH₂OH

Look for the characteristic group:

–OH

Therefore:

Functional group:

hydroxyl

Family:

alcohol

Name:

ethanol

---

Worked Example: CH₃COOH

Structure:

CH₃COOH

Look for:

–COOH

Therefore:

Functional group:

carboxyl

Family:

carboxylic acid

Name:

ethanoic acid

---

Worked Example: CH₃COOCH₃

Structure:

CH₃COOCH₃

Look for:

–COO–

Therefore:

Family:

ester

Name:

methyl ethanoate

---

Worked Example: CH₃CH₂NH₂

Structure:

CH₃CH₂NH₂

Look for:

–NH₂

Therefore:

Functional group:

amino

Family:

amine

Name:

ethylamine

---

Functional Groups and Polarity

A molecule is polar when its electrical charge is distributed unevenly.

Oxygen and nitrogen attract bonding electrons relatively strongly.

Therefore, functional groups containing these atoms often create polar regions.

For example:

–OH

–COOH

–COO–

–NH₂

can all influence molecular polarity.

Polarity affects how molecules interact with:

• water
• other polar molecules
• biological molecules
• solvents

Functional Groups and Water Solubility

Water is a polar solvent.

Polar functional groups can interact with water molecules.

Therefore, small molecules containing:

• –OH
• –COOH
• –NH₂

often have significant water solubility.

Esters can also interact with water through their oxygen atoms, although their behaviour differs because they lack an O–H group.

---

Carbon-Chain Length Also Matters

Functional group alone does not determine solubility.

Compare:

ethanol

and:

hexanol

Both contain:

–OH

However, hexanol contains a much longer non-polar hydrocarbon chain.

As the carbon chain grows:

non-polar portion becomes more important

Therefore:

water solubility generally decreases.

The same general idea applies to many organic families.

---

Functional Groups and Boiling Point

Boiling requires molecules to separate from one another.

Stronger intermolecular attractions generally require more energy to overcome.

Therefore:

stronger intermolecular forces → generally higher boiling point

Alcohols and carboxylic acids can form strong hydrogen bonds.

Primary amines can also form hydrogen bonds.

Esters have polar interactions but lack an O–H or N–H hydrogen-bond donor.

This produces important differences in boiling points.

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Comparing Similar-Sized Molecules

Suppose four organic molecules have similar molecular sizes.

If one is an alcohol, another a carboxylic acid, another an ester, and another an amine, they may have very different boiling points.

Why?

Because their functional groups create different:

• polarities
• intermolecular forces
• hydrogen-bonding abilities

Therefore, molecular formula or size alone does not determine physical properties.

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Functional Groups and Acidity

The:

–COOH

group gives carboxylic acids their acidic behaviour.

A carboxylic acid can donate:

H⁺

Simplified:

RCOOH ⇌ H⁺ + RCOO⁻

Therefore, recognizing –COOH allows us to predict:

• pH below 7 in aqueous solution
• reaction with bases
• reaction with carbonates
• possible reaction with reactive metals

This is an example of predicting chemical behaviour from structure.

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Functional Groups and Basicity

The:

–NH₂

group in simple amines contains nitrogen with a lone pair.

The nitrogen can accept:

H⁺

Therefore:

RNH₂ + H⁺ → RNH₃⁺

Recognizing an amine group allows us to predict that the molecule may behave as a:

base

Again:

structure → predicted chemical behaviour

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Functional Groups and Characteristic Reactions

Different organic families undergo characteristic reactions.

Alcohol

Can undergo:

• combustion
• oxidation
• esterification

Carboxylic Acid

Can undergo:

• neutralization
• reactions with carbonates
• reactions with metals
• esterification

Ester

Can undergo:

• hydrolysis

Amine

Can undergo:

• acid-base reactions with acids

Functional groups therefore help chemists predict which reactions are likely.

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Connecting the Families

Organic families are not isolated.

Chemical reactions can convert one family into another.

For example:

alkene → alcohol

through hydration.

alcohol → carboxylic acid

through oxidation for suitable alcohols.

alcohol + carboxylic acid ⇌ ester + water

through esterification.

ester + water → alcohol + carboxylic acid

through hydrolysis.

This creates a network of related organic reactions.

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A Simple Organic Reaction Map

One useful way to organize the chemistry is:

alkene → alcohol → carboxylic acid

and:

alcohol + carboxylic acid ⇌ ester + water

Meanwhile:

amine + acid → ammonium-type salt

These relationships show how identifying a functional group can help predict both the reactants and possible products of a reaction.

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Molecules Can Have More Than One Functional Group

Some organic molecules contain multiple functional groups.

An important example is an amino acid.

General structure:

H₂N–CH(R)–COOH

It contains:

–NH₂ → amino group

and:

–COOH → carboxyl group

Therefore, amino acids can display properties associated with both groups.

They can behave as:

• acids
• bases

This is why amino acids are described as:

amphoteric

Worked Example: Multiple Functional Groups

Consider:

H₂N–CH₂–COOH

Look for:

–NH₂

and:

–COOH

Therefore, this molecule contains:

• an amino group
• a carboxyl group

It is an:

amino acid

Specifically:

glycine

Because it contains both acidic and basic functional groups, it can show both types of behaviour.

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Functional Groups Influence Smell

Functional groups can influence how molecules interact with receptors in our noses.

For example:

Many small esters have odours described as:

• fruity
• sweet
• floral

Some small amines have odours often described as:

• fishy
• ammonia-like

Some carboxylic acids have:

• sharp
• sour
• pungent

odours.

However, smell depends on the entire molecular structure, not just the functional group.

It should never be used as the sole method for identifying an unknown chemical.

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Functional Groups Influence Uses

Because functional groups affect properties, they also affect how compounds are used.

Alcohols

Useful as:

• fuels
• solvents
• disinfecting agents
• chemical feedstocks

Carboxylic Acids

Used in:

• foods
• preservatives
• chemical manufacture
• polymers
• pharmaceuticals

Esters

Used in:

• fragrances
• flavourings
• solvents
• polymers
• fuels

Amines

Found in or used for:

• medicines
• dyes
• polymers
• biological molecules
• industrial chemicals

---

Functional Groups in Biology

Functional groups are extremely important in biological molecules.

For example:

amino acids contain amino and carboxyl groups.

proteins contain peptide linkages.

fats and oils contain ester linkages.

Many biological molecules contain hydroxyl groups.

Therefore, functional-group chemistry provides a bridge between:

organic chemistry and biology

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Functional Groups in Materials

Functional groups also influence the properties of materials.

For example:

polyesters

contain repeated ester linkages.

proteins

contain repeated peptide linkages.

Functional groups influence:

• flexibility
• strength
• interactions with water
• chemical resistance
• biodegradability
• melting behaviour

Understanding functional groups therefore helps chemists design new materials.

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Predicting Properties from Structure

Suppose you are given an unknown molecule:

CH₃CH₂CH₂OH

You identify:

–OH

You can predict that it:

• is an alcohol
• is polar
• can form hydrogen bonds
• has some water solubility
• can undergo combustion
• may undergo oxidation
• can react with a carboxylic acid to form an ester

You have learned a great deal from identifying only one structural feature.

---

Worked Example: Predicting Properties of an Acid

Unknown compound:

CH₃CH₂COOH

Functional group:

–COOH

Therefore, predict:

• carboxylic acid
• acidic aqueous solution
• weak acid
• reacts with bases
• reacts with carbonates
• can form an ester with an alcohol
• can hydrogen bond
• likely significant water solubility because it is relatively small

Name:

propanoic acid

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Worked Example: Predicting Properties of an Ester

Unknown compound:

CH₃CH₂COOCH₃

Functional group:

–COO–

Therefore:

• it is an ester
• it may be relatively volatile
• it may have a distinctive odour
• it can act as an organic solvent
• it can undergo hydrolysis

Its name is:

methyl propanoate

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Worked Example: Predicting Properties of an Amine

Unknown compound:

CH₃CH₂CH₂NH₂

Functional group:

–NH₂

Therefore:

• it is a primary amine
• it contains nitrogen
• it is likely basic
• it can accept H⁺
• it can react with acids
• it may have significant water solubility because it is relatively small

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Classification Strategy

When asked to classify an organic compound, use this sequence:

First: Look for nitrogen.

If you see:

–NH₂

consider an amine.

Second: Look carefully at oxygen-containing groups.

If you see:

–COOH

carboxylic acid.

If you see:

–COO–

ester.

If you see:

–OH

without it being part of –COOH:

alcohol.

This prevents several common identification mistakes.

---

Structure → Properties → Uses

A powerful way to think about organic chemistry is:

STRUCTURE → PROPERTIES → USES

For example:

Ethanol

Structure:

–OH

leads to:

polarity + hydrogen bonding

which contributes to:

water solubility + solvent behaviour

which helps explain uses in:

solvents and cleaning products

Ethanoic Acid

Structure:

–COOH

leads to:

acidic behaviour

which helps explain its role in:

vinegar and chemical reactions

Ethyl Ethanoate

Structure:

–COO–

contributes to:

volatility + solvent properties

which helps explain uses in:

solvents and fragrances

Ethylamine

Structure:

–NH₂

leads to:

basic behaviour

which influences its:

chemical reactions and industrial uses

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Common Mistakes

Looking Only for an Oxygen Atom

Many organic families contain oxygen.

You must identify the arrangement of atoms, not simply whether oxygen is present.

Confusing –OH and –COOH

Alcohol:

–OH

Carboxylic acid:

–COOH

Confusing –COOH and –COO–

Carboxylic acid:

–COOH

Ester:

–COO–

Calling Every –NH₂-Containing Molecule Simply an Amine

Some molecules contain multiple functional groups.

An amino acid, for example, contains both:

–NH₂ and –COOH

Thinking Functional Group Is the Only Factor Affecting Properties

Carbon-chain length, molecular size, shape, branching, and other functional groups also matter.

Assuming All Esters Smell Fruity

Many small esters have characteristic pleasant aromas, but this is not universal.

Assuming All Amines Have the Same Smell

Odour depends on the entire molecular structure.

Thinking Alcohols Are Strongly Acidic Because They Contain O–H

Alcohols do not normally behave as acids like carboxylic acids do.

Thinking Carboxylic Acids Are Strong Acids

Simple carboxylic acids are generally weak acids.

Thinking Amines Are Strong Bases

Many common amines are weak bases.

Confusing Hydrogen Bonding with Covalent Bonding

Hydrogen bonds are intermolecular attractions in this context, not the covalent bonds holding the molecule itself together.

Predicting Solubility from Functional Group Alone

Carbon-chain length also matters.

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Key Terms

Functional group — A specific atom or arrangement of atoms that gives an organic compound characteristic properties and reactions.

Organic compound — A carbon-based compound belonging to organic chemistry.

Carbon skeleton — The chain or framework of carbon atoms in an organic molecule.

Hydroxyl group — The –OH functional group characteristic of alcohols.

Alcohol — An organic compound containing a hydroxyl functional group.

Carboxyl group — The –COOH functional group characteristic of carboxylic acids.

Carboxylic acid — An organic compound containing a carboxyl group.

Ester group — The –COO– functional arrangement characteristic of esters.

Ester — An organic compound containing an ester functional group.

Amino group — The –NH₂ group emphasized in simple primary amines and amino acids.

Amine — A nitrogen-containing organic compound related to ammonia.

Primary amine — An amine containing one carbon-containing group attached to nitrogen, with two N–H bonds.

Hydrocarbon — A compound containing only carbon and hydrogen.

Alkene — A hydrocarbon containing a C=C double bond.

Polarity — Uneven distribution of electrical charge within a molecule.

Polar molecule — A molecule containing regions of partial positive and negative charge.

Intermolecular force — An attraction between separate molecules.

Hydrogen bonding — A relatively strong intermolecular attraction involving suitable O–H or N–H groups.

Solubility — The extent to which a substance dissolves in a solvent.

Volatility — The tendency of a substance to enter the gas phase.

Weak acid — An acid that only partially ionizes in aqueous solution.

Weak base — A base that accepts H⁺ or reacts with water only to a limited extent.

Lone pair — A pair of electrons on an atom that is not part of a covalent bond.

Esterification — Reaction between a carboxylic acid and an alcohol producing an ester and water.

Oxidation — A chemical process that can convert suitable alcohols into carboxylic acids.

Hydrolysis — A reaction involving water that can break an ester into other compounds.

Amphoteric — Able to behave as both an acid and a base.

Amino acid — An organic compound containing both amino and carboxyl functional groups.

Structure-property relationship — The relationship between molecular structure and the physical or chemical behaviour of a substance.

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Key Takeaways

• Functional groups are specific arrangements of atoms that strongly influence organic compounds.
• Identifying a functional group helps classify an organic molecule.
• Functional groups can help predict physical and chemical properties.
• Alcohols contain –OH.
• The –OH group is called the hydroxyl group.
• Carboxylic acids contain –COOH.
• The –COOH group is called the carboxyl group.
• Esters contain –COO–.
• Simple primary amines contain –NH₂.
• The –NH₂ group is called the amino group.
• Alcohols can form hydrogen bonds.
• Small alcohols are often soluble in water.
• Carboxylic acids behave as weak acids.
• Carboxylic acids can react with bases, carbonates, and reactive metals.
• Esters are often relatively volatile.
• Many small esters have distinctive aromas.
• Amines generally behave as weak bases.
• Amines can accept H⁺.
• Primary amines can participate in hydrogen bonding.
• Functional groups affect polarity.
• Polarity influences water solubility.
• Intermolecular forces influence boiling points and volatility.
• Carbon-chain length also affects physical properties.
• Longer hydrocarbon chains generally reduce water solubility.
• Functional groups help predict characteristic chemical reactions.
• Alcohols can undergo oxidation.
• Carboxylic acids and alcohols can undergo esterification.
• Esters can undergo hydrolysis.
• Amines can undergo acid-base reactions.
• Molecules can contain more than one functional group.
• Amino acids contain both –NH₂ and –COOH.
• Functional groups are important in biological molecules and synthetic materials.
• Molecular structure determines many properties.
• Properties help determine how substances can be used.

The central idea is:

STRUCTURE → PROPERTIES → BEHAVIOUR → USES

A useful recognition guide is:

–OH → alcohol

–COOH → carboxylic acid

–COO– → ester

–NH₂ → primary amine

And remember:

Functional groups are extremely important, but the rest of the molecule matters too.

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Check Your Understanding

1. What is a functional group?

2. Identify the functional group in an alcohol.

3. Identify the functional group in a carboxylic acid.

4. Identify the functional group in an ester.

5. Identify the functional group emphasized in a primary amine.

6. Classify CH₃CH₂OH.

7. Classify CH₃COOH.

8. Classify CH₃COOCH₃.

9. Classify CH₃CH₂NH₂.

10. Name CH₃CH₂OH.

11. Name CH₃COOH.

12. Name CH₃COOCH₃.

13. Which family is normally acidic: alcohols, carboxylic acids, esters, or amines?

14. Which family is normally basic?

15. Explain why many small alcohols dissolve well in water.

16. Explain why water solubility generally decreases as a hydrocarbon chain becomes longer.

17. Why do alcohols often have higher boiling points than similar-sized hydrocarbons?

18. Why are many small esters relatively volatile?

19. Explain why carboxylic acids behave as acids.

20. Explain why amines behave as bases.

21. What happens when a carboxylic acid reacts with an alcohol?

22. What happens when an ester undergoes hydrolysis?

23. How can an alcohol be converted into a carboxylic acid?

24. Which two functional groups occur in an amino acid?

25. Why can an amino acid behave as both an acid and a base?

26. Explain why functional group alone does not completely determine water solubility.

27. Give one important use of alcohols, carboxylic acids, esters, and amines.

28. Explain the meaning of:

structure → properties → uses

29. Explain why two molecules with similar numbers of carbon atoms can have very different chemical properties.

30. Challenge: Consider the compounds:

A: CH₃CH₂CH₂OH

B: CH₃CH₂COOH

C: CH₃COOCH₂CH₃

D: CH₃CH₂CH₂NH₂

E: H₂N–CH₂–COOH

For each compound:

a. identify all relevant functional groups
b. classify the organic family or families
c. predict whether it is likely to show acidic, basic, or approximately neutral behaviour
d. predict whether hydrogen bonding is important
e. predict whether a small molecule of this type should have significant interaction with water
f. identify one characteristic chemical reaction it could undergo.

Then:

g. Which compound is an alcohol?
h. Which compound is a carboxylic acid?
i. Which compound is an ester?
j. Which compound is an amine?
k. Which compound contains two different functional groups?
l. Which compound is an amino acid?
m. Which compound could react with A to form an ester?
n. Which compound could be produced by oxidation of a suitable alcohol?
o. Which compound is most clearly expected to behave as a base?
p. Which compound can behave as both an acid and a base?
q. Explain how identifying the functional groups allowed you to make these predictions.