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

---

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.

---

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

---

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.

---

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

---

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

---

Worked Example: Name CH₃OH

There is one carbon.

Prefix:

meth-

It contains –OH.

Ending:

-anol

Therefore:

methanol

---

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.

---

Worked Example: Name CH₃CH₂CH₂OH

There are three carbon atoms.

Parent chain:

propane

The –OH group is attached to carbon 1.

Therefore:

propan-1-ol

---

Worked Example: Name CH₃CH(OH)CH₃

There are three carbon atoms.

The –OH group is attached to the middle carbon.

Therefore:

propan-2-ol

---

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.

---

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.

---

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.

---

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.

---

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.

---

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

---

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.

---

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."

---

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.

---

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.

---

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.

---

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.

---

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.

---

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.

---

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.

---

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.

---

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

---

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.

---

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.

---

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.

---

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

---

Comparing an Alkane and an Alcohol

Compare ethane and ethanol.

The –OH group produces major differences in physical behaviour.

---

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.

---

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.

---

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.

---

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.

---

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

---

Worked Example: Identify the Functional Group

Consider:

CH₃CH₂CH₂CH₂OH

The functional group is:

–OH

Name:

hydroxyl group

Organic family:

alcohol

---

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

---

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.

---

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.

---

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.

---

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.

---

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

---

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.