Carboxylic Acids: Structure, Properties, and Formation Revision Notes | A Level AQA Chemistry

1. The Carboxyl Group: Structure and Naming

Carboxylic acids are built around one of the most recognisable functional groups in organic chemistry: the carboxyl group, written as -COOH. Picture a carbonyl group (C=O) and a hydroxyl group (O-H) fused onto the same carbon atom — that combination is what gives carboxylic acids their distinctive character.

The general molecular formula for a straight-chain carboxylic acid can be written as $C_nH_{2n}O_2$ (where $n$ is the total number of carbon atoms; e.g. $n=2$ gives ethanoic acid, $C_2H_4O_2$) or alternatively as $C_nH_{2n+1}COOH$ (where $n$ is the number of carbons in the alkyl chain). Note that $n$ refers to different quantities in each notation, so the two expressions are not numerically identical for the same value of $n$. The carboxyl carbon is always carbon-1, so it is never given a locant in the name — the suffix -oic acid tells you it is there.

The table below shows the first four members of the homologous series. You will notice that the names follow directly from the parent alkane chain, replacing the terminal -e with -oic acid.

2. Physical Properties: Why Carboxylic Acids Behave the Way They Do

The boiling points in the table above are strikingly high for such small molecules. Ethanoic acid (Mr = 60) boils at 118 °C — far above what you would predict from its size alone. The reason lies in hydrogen bonding.

In a dimer, each carboxylic acid molecule forms two simultaneous hydrogen bonds with one partner molecule: the O-H group donates to the partner carbonyl oxygen, and the carbonyl oxygen accepts from the partner O-H group.

Solubility follows a clear trend: short-chain carboxylic acids (up to about four carbons) are miscible with water in all proportions. The -COOH group forms hydrogen bonds with water molecules, releasing enough energy to overcome the lattice-like structure of water. As the non-polar hydrocarbon chain lengthens, it dominates the molecule's character and solubility decreases markedly.

3. Carboxylic Acids as Weak Acids

Here is where the chemistry gets genuinely interesting. Carboxylic acids donate protons (H+) to water — that is what makes them acids. But unlike hydrochloric acid, which dissociates completely, a carboxylic acid only partially dissociates. An equilibrium is established between the undissociated acid and its ions.

The carboxylate ion ($CH_3COO^-$) is stabilised by delocalisation: the negative charge is spread across both oxygen atoms rather than sitting on one. This makes the ion relatively stable, which is why the acid does dissociate to some extent — but the equilibrium still strongly favours the undissociated form, giving a Ka value typically around $10^{-5}$ mol dm$^{-3}$ for ethanoic acid.

A practical consequence: a solution of ethanoic acid at the same concentration as hydrochloric acid will have a higher pH (less acidic) because fewer H+ ions are present in solution. This is a key distinction examiners test regularly.

3a. Reaction with Carbonates

Although carboxylic acids are weak, they are still acidic enough to react with metal carbonates and hydrogencarbonates, liberating carbon dioxide gas. This is a reliable test for the presence of a carboxylic acid: add solid sodium carbonate and observe effervescence.

The same pattern applies with sodium hydrogencarbonate ($NaHCO_3$): the acid donates a proton to the hydrogencarbonate ion, producing $CO_2$, water, and the sodium salt. This reaction is used analytically to distinguish carboxylic acids from phenols — phenols do not react with sodium hydrogencarbonate to give $CO_2$, even though they are also weakly acidic.

Notice that the carboxylate ion (RCOO-) appears on the right-hand side of the equation. This is the conjugate base formed when the carboxylic acid donates a proton.

4. Formation of Esters: Esterification

Carboxylic acids react with alcohols in a reaction called esterification. The product is an ester — a compound containing the -COO- linkage — and water. This reaction requires a concentrated acid catalyst, typically concentrated sulphuric acid, and is reversible.

The naming convention for esters is worth fixing firmly: the alcohol-derived part comes first (ending in -yl), followed by the acid-derived part (ending in -anoate). So ethanol + ethanoic acid gives ethyl ethanoate. Methanol + propanoic acid gives methyl propanoate.

The concentrated sulphuric acid acts as a catalyst in two ways: it protonates the carbonyl oxygen to activate the carboxylic acid, and it is regenerated at the end of the mechanism. Because the reaction is reversible, the equilibrium yield of ester is typically around 65–70% under standard conditions.

Worked example: Naming an ester from its reactants

Identify the ester formed when methanol reacts with butanoic acid in the presence of concentrated H2SO4. Write the balanced equation with state symbols.

1. Alcohol: methanol ($CH_3OH$) → gives the 'methyl' part of the ester name — The alcohol always provides the first part of the ester name, ending in -yl.
2. Acid: butanoic acid ($CH_3CH_2CH_2COOH$) → gives the 'butanoate' part of the ester name — The carboxylic acid provides the second part of the name, replacing -ic acid with -ate.
3. Ester name: methyl butanoate — Alcohol-derived name first (methyl), acid-derived name second (butanoate).
4. $CH_3CH_2CH_2COOCH_3$ — The -COO- linkage connects the butanoyl chain to the methyl group. The oxygen from the alcohol is the one that bonds to the carbonyl carbon.
5. $CH_3CH_2CH_2COOH(l) + CH_3OH(l) \rightleftharpoons CH_3CH_2CH_2COOCH_3(l) + H_2O(l)$ — The reversible arrow is essential — esterification is an equilibrium reaction. Concentrated H2SO4 is written above the arrow as the catalyst, not as a reactant.

Answer: Methyl butanoate, $CH_3CH_2CH_2COOCH_3$. Balanced equation: $CH_3CH_2CH_2COOH(l) + CH_3OH(l) \rightleftharpoons CH_3CH_2CH_2COOCH_3(l) + H_2O(l)$ (concentrated $H_2SO_4$ catalyst).

5. Exam Technique and Common Mistakes

Carboxylic acid questions appear across multiple paper sections — from short-answer structure questions to extended 'explain' and 'evaluate' items. The marks are often lost not through lack of knowledge, but through imprecise language or missing conditions.

These four specification points — structure, physical properties, weak acid behaviour with carbonates, and esterification — form a tightly connected set of ideas. The -COOH group explains everything: its polarity drives hydrogen bonding and high boiling points, its O-H bond enables partial proton donation (weak acidity), and its carbonyl carbon is the electrophilic site attacked during esterification.

For AQA, focus on three linked ideas: the acidic proton is on the O-H group, the conjugate base is the carboxylate ion, and strong intermolecular hydrogen bonding raises the boiling point because dimers can form.