Organic chemistry: carboxylic acids — structure, properties and reactions

What you'll learn

Carboxylic acids form an important homologous series in organic chemistry, recognisable by their distinctive functional group. This revision guide covers their structure, nomenclature, physical and chemical properties, and key reactions including neutralisation and esterification. You'll learn to identify carboxylic acids from their formulae, predict their properties, and write balanced equations for their reactions—all essential skills for AQA GCSE Chemistry examinations.

Key terms and definitions

Carboxylic acid — an organic compound containing the functional group -COOH (a carbon atom double-bonded to one oxygen and single-bonded to a hydroxyl group)

Functional group — a specific atom or group of atoms within a molecule that determines the characteristic chemical reactions of that compound

Homologous series — a family of organic compounds with the same general formula and similar chemical properties, differing by CH₂ units

Esterification — the reaction between a carboxylic acid and an alcohol to form an ester and water

Aqueous solution — a solution in which water is the solvent

Weak acid — an acid that only partially ionises (dissociates) in aqueous solution

Ester — an organic compound formed from the reaction between a carboxylic acid and an alcohol, containing the functional group -COO-

Ionisation — the process by which a molecule splits into ions when dissolved in water

Core concepts

Structure and general formula

Carboxylic acids contain the carboxyl functional group (-COOH), which consists of a carbonyl group (C=O) and a hydroxyl group (O-H) bonded to the same carbon atom. This functional group must be located at the end of the carbon chain.

The general formula for carboxylic acids is CₙH₂ₙ₊₁COOH or CₙH₂ₙO₂ (where n ≥ 0).

Common examples include:

• Methanoic acid: HCOOH (n=0, simplest carboxylic acid)
• Ethanoic acid: CH₃COOH (n=1, found in vinegar)
• Propanoic acid: CH₃CH₂COOH or C₂H₅COOH (n=2)
• Butanoic acid: CH₃CH₂CH₂COOH or C₃H₇COOH (n=3)

When drawing structural formulae, always show the -COOH group clearly. The displayed formula must show all bonds, particularly the C=O double bond and the O-H single bond.

Naming carboxylic acids

The systematic naming follows IUPAC conventions:

1. Count the number of carbon atoms in the longest chain including the carbon in the -COOH group
2. Use the appropriate stem (meth-, eth-, prop-, but-)
3. Add the suffix -oic acid

The carboxylic acid carbon is always carbon-1, so positional numbers are unnecessary for simple carboxylic acids at GCSE level.

Examples:

• 1 carbon atom total = methanoic acid
• 2 carbon atoms total = ethanoic acid
• 3 carbon atoms total = propanoic acid
• 4 carbon atoms total = butanoic acid

Physical properties

Carboxylic acids exhibit distinctive physical properties due to their functional group:

Solubility in water:

• Short-chain carboxylic acids (methanoic, ethanoic, propanoic) are completely soluble in water
• The -COOH group forms hydrogen bonds with water molecules
• Solubility decreases as carbon chain length increases
• Longer-chain carboxylic acids have large non-polar hydrocarbon sections that cannot form hydrogen bonds with water

Boiling points:

• Carboxylic acids have relatively high boiling points compared to alkanes of similar molecular mass
• Hydrogen bonding occurs between carboxylic acid molecules (between O-H of one molecule and C=O of another)
• More energy is required to overcome these intermolecular forces
• Boiling point increases with increasing carbon chain length

Odour:

• Many carboxylic acids have distinctive, often unpleasant odours
• Ethanoic acid has a vinegar-like smell
• Butanoic acid smells like rancid butter

Acidic properties and reactions

Carboxylic acids are weak acids. When dissolved in water, they only partially ionise:

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

The reversible arrow (⇌) indicates incomplete ionisation. At equilibrium, most molecules remain as CH₃COOH rather than dissociating into ions.

Typical pH values:

• Aqueous solutions of carboxylic acids have pH values of 3-6
• This is higher than strong acids (pH 0-2) of the same concentration
• For example, vinegar (dilute ethanoic acid) has a pH of approximately 3

Reactions with metals:

Carboxylic acids react with reactive metals to produce a salt and hydrogen gas:

2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g)

The salt formed is a metal carboxylate (e.g., magnesium ethanoate).

Reactions with metal carbonates:

Carboxylic acids react with carbonates and hydrogencarbonates to produce a salt, water and carbon dioxide:

2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g)

CH₃COOH(aq) + NaHCO₃(aq) → CH₃COONa(aq) + H₂O(l) + CO₂(g)

This reaction provides a test for carboxylic acids: fizzing occurs and the gas produced turns limewater milky (confirming CO₂).

Reactions with metal oxides:

Like other acids, carboxylic acids react with metal oxides to produce a salt and water:

2CH₃COOH(aq) + CuO(s) → (CH₃COO)₂Cu(aq) + H₂O(l)

Reactions with alkalis (neutralisation):

Carboxylic acids neutralise alkalis to form a salt and water only:

CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)

Esterification reaction

Carboxylic acids react with alcohols in the presence of an acid catalyst (usually concentrated sulfuric acid) to form esters and water.

General equation:

Carboxylic acid + Alcohol → Ester + Water

Specific example:

CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O

ethanoic acid + methanol → methyl ethanoate + water

Key features of esterification:

• Requires heating under reflux (to prevent loss of volatile reactants/products while maintaining temperature)
• Requires an acid catalyst (concentrated H₂SO₄ or HCl)
• The reaction is reversible (shown by ⇌)
• The -OH group comes from the carboxylic acid, -H from the alcohol
• Water is always a product

Naming esters:

Esters have the general structure R-COO-R'. The name consists of two parts:

1. First part: from the alcohol (ending in -yl)
2. Second part: from the carboxylic acid (ending in -oate)

Examples:

• Methanol + ethanoic acid → methyl ethanoate
• Ethanol + methanoic acid → ethyl methanoate
• Ethanol + propanoic acid → ethyl propanoate

Properties and uses of esters:

• Pleasant, fruity smells
• Used as flavourings and fragrances
• Used as solvents
• Lower boiling points than corresponding carboxylic acids (no hydrogen bonding between ester molecules)

Worked examples

Example 1: Identifying and naming carboxylic acids

Question: A carboxylic acid has the molecular formula C₃H₆O₂.

(a) Give the displayed formula of this carboxylic acid. [2 marks]

(b) Name this carboxylic acid. [1 mark]

(c) Explain why this carboxylic acid is soluble in water. [2 marks]

Answer:

(a)

    H   H   O
    |   |   ||
H—C—C—C—O—H
    |   |
    H   H

[1 mark for correct carbon skeleton with -COOH group; 1 mark for all bonds and hydrogen atoms shown correctly]

(b) Propanoic acid [1 mark]

(c) The carboxylic acid molecule contains a -COOH group [1 mark] which can form hydrogen bonds with water molecules [1 mark].

Example 2: Reaction with sodium carbonate

Question: A student adds sodium carbonate to a solution of ethanoic acid.

(a) Describe what the student would observe. [1 mark]

(b) Write a balanced symbol equation for this reaction. [3 marks]

(c) How could the student prove that carbon dioxide has been produced? [1 mark]

Answer:

(a) Fizzing/effervescence/bubbles produced [1 mark]

(b) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ [1 mark for correct formulae, 1 mark for correct products, 1 mark for balanced equation]

(c) Bubble the gas through limewater—it will turn milky/cloudy [1 mark]

Example 3: Esterification reaction

Question: A student heats a mixture of propanoic acid and methanol with a few drops of concentrated sulfuric acid.

(a) Name the type of reaction taking place. [1 mark]

(b) Name the organic product formed. [1 mark]

(c) Write a balanced symbol equation for this reaction. [2 marks]

(d) Explain the role of concentrated sulfuric acid in this reaction. [1 mark]

Answer:

(a) Esterification [1 mark]

(b) Methyl propanoate [1 mark]

(c) C₂H₅COOH + CH₃OH ⇌ C₂H₅COOCH₃ + H₂O or CH₃CH₂COOH + CH₃OH ⇌ CH₃CH₂COOCH₃ + H₂O [1 mark for correct formulae, 1 mark for reversible arrow and balanced equation]

(d) (Acid) catalyst / speeds up the reaction [1 mark]

Common mistakes and how to avoid them

• Writing the wrong general formula: Remember CₙH₂ₙ₊₁COOH or CₙH₂ₙO₂, not CₙH₂ₙ. Count the oxygens in the functional group carefully.

• Confusing weak acids with dilute acids: "Weak" refers to the extent of ionisation, not concentration. Ethanoic acid is always a weak acid, whether concentrated or dilute. This is a chemical property, not dependent on how much water is added.

• Missing the reversible arrow in esterification equations: Esterification is reversible, so always use ⇌ not →. You will lose marks for using a single arrow.

• Incorrect ester naming: The alcohol part comes first (ending in -yl), then the acid part (ending in -oate). Methanol + ethanoic acid gives methyl ethanoate, NOT ethyl methanoate.

• Forgetting to show all bonds in displayed formulae: The C=O double bond must be clearly shown. Draw two lines between C and O. The O-H bond should be shown as a single line.

• Writing incorrect products for carboxylic acid reactions: Remember the pattern: metal → salt + hydrogen; carbonate → salt + water + carbon dioxide; alkali → salt + water only. Don't add extra products or forget water.

Exam technique for "Organic chemistry: carboxylic acids — structure, properties and reactions"

• Balancing equations with carboxylic acids: Count atoms systematically. Remember that carboxylate salts like CH₃COONa are ionic, so the formula is different from the molecular acid. Metal carbonates often require a coefficient of 2 for the carboxylic acid.

• "Explain" questions about solubility or boiling points: Always mention the specific intermolecular force (hydrogen bonding) and link it to the functional group present. State that more/less energy is required to overcome these forces for 2 marks.

• Drawing displayed formulae: Show every bond and every atom. Don't use condensed formulae like -COOH unless the question specifically asks for a structural formula. Check you haven't missed any hydrogen atoms by counting bonds to each carbon (should total 4).

• Esterification practical questions: If asked about experimental procedure, mention heating under reflux, acid catalyst, and that the reaction is slow/reversible/reaches equilibrium. Know that esters have fruity smells and can be used to identify the product.

Quick revision summary

Carboxylic acids contain the -COOH functional group and follow the general formula CₙH₂ₙO₂. They are weak acids that partially ionise in water, producing pH 3-6 solutions. Short-chain carboxylic acids dissolve in water due to hydrogen bonding. They react with metals, carbonates, oxides and alkalis like typical acids. Esterification occurs when carboxylic acids react with alcohols in the presence of an acid catalyst, producing sweet-smelling esters and water. Know how to name carboxylic acids, write equations for their reactions, and explain their physical properties.