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.

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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.

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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.

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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

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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.

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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

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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.

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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.

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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.

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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.

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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

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Worked Example: CH₃CH₂OH

Structure:

CH₃CH₂OH

Look for the characteristic group:

–OH

Therefore:

Functional group:

hydroxyl

Family:

alcohol

Name:

ethanol

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Worked Example: CH₃COOH

Structure:

CH₃COOH

Look for:

–COOH

Therefore:

Functional group:

carboxyl

Family:

carboxylic acid

Name:

ethanoic acid

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Worked Example: CH₃COOCH₃

Structure:

CH₃COOCH₃

Look for:

–COO–

Therefore:

Family:

ester

Name:

methyl ethanoate

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Worked Example: CH₃CH₂NH₂

Structure:

CH₃CH₂NH₂

Look for:

–NH₂

Therefore:

Functional group:

amino

Family:

amine

Name:

ethylamine

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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.

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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.

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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

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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.

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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.

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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.