What this chapter covers18.1, 18.2

Carboxylic acids contain the –COOH functional group: a carbonyl group with an –OH on the same carbon. The hydrogen on that –OH is acidic enough to react with metals, alkalis and carbonates. This page covers two related topics: 18.1 deals with the production and reactions of carboxylic acids, while 18.2 covers esters — the products of the condensation reaction between a carboxylic acid and an alcohol — and their hydrolysis.

The verbs tell you how much detail to give. 18.1.1 says recall: reagents, conditions and equations. 18.1.2 says describe: equations, reagents, conditions and observations. 18.2.1 says recall: the reaction, its reagents and conditions. 18.2.2 says describe: equations, reagents, conditions and the difference between acid hydrolysis and alkaline hydrolysis.

The page is in two parts. The first covers carboxylic acids and esters: how they are made, how carboxylic acids behave as acids, how esters are formed and hydrolysed. The second is a review section with a data summary and a self-test.

Carboxylic acids: the –COOH group18.1

A carboxylic acid contains the functional group –COOH: a carbonyl group, C=O, with an –OH attached to the same carbon. This combination gives the group its distinctive behaviour — the hydrogen on the –OH is acidic enough to be lost as H⁺ to water, metals, alkalis and carbonates.

Name a carboxylic acid from the longest chain that includes the –COOH carbon. Drop the –e of the parent alkane and add –oic acid. The –COOH is always C1, so no locant is needed.

Physical properties

Carboxylic acids have high boiling points — much higher than aldehydes or ketones of similar M_r. This is because the –COOH group can form two hydrogen bonds to another –COOH group at once, giving a cyclic dimer held by two O–H···O=C hydrogen bonds. In the liquid and in the vapour at temperatures just above the boiling point, most molecules exist as these dimers.

The short-chain acids are soluble in water (ethanoic acid mixes in all proportions). As the chain lengthens, the non-polar hydrocarbon tail dominates and solubility falls.

Making carboxylic acids18.1.1

The syllabus lists three routes to carboxylic acids. Two of them have already been met in earlier topics; the third is new here.

(a) Oxidation of primary alcohols and aldehydes

Heat a primary alcohol under reflux with excess acidified K₂Cr₂O₇ or acidified KMnO₄. The alcohol is oxidised first to the aldehyde and then to the carboxylic acid. Starting from the aldehyde itself, a single [O] step suffices.

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O    ethanol → ethanoic acid

CH₃CHO + [O] → CH₃COOH    ethanal → ethanoic acid

Reflux, not distillation: the aldehyde must stay in contact with the oxidising agent. The colour change is orange (Cr₂O₇²⁻) to green (Cr³⁺).

(b) Hydrolysis of nitriles

A nitrile, RCN, is hydrolysed by heating under reflux with dilute hydrochloric acid or with dilute sodium hydroxide followed by acidification. The –CN group is converted to –COOH (or –COO⁻ with alkali, which must be acidified to release the free acid).

CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl    (acid hydrolysis)

CH₃CN + NaOH + H₂O → CH₃COO⁻Na⁺ + NH₃    (alkaline hydrolysis, then acidify)

This is synthetically important because adding HCN to a carbonyl compound (17.1.2(b)) gives a hydroxynitrile with one extra carbon, and hydrolysing it gives a hydroxy acid — a two-step chain extension.

(c) Hydrolysis of esters

An ester is hydrolysed by heating under reflux with dilute acid (reversible, catalysed by H⁺) or with dilute alkali (irreversible, because the acid is trapped as its carboxylate salt). Alkaline hydrolysis must be followed by acidification to obtain the free carboxylic acid.

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH    (dilute acid, reflux)

CH₃COOCH₂CH₃ + NaOH → CH₃COO⁻Na⁺ + CH₃CH₂OH    (dilute alkali, reflux)

Carboxylic acids as acids18.1.2

Carboxylic acids are weak acids. In water, ethanoic acid partially dissociates:

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

The carboxylate ion, RCOO⁻, is stabilised by delocalisation: the negative charge is spread equally over the two oxygen atoms, making the ion less willing to take the proton back. Even so, the equilibrium lies well to the left — only about 1 in 250 ethanoic acid molecules is dissociated in a 1.0 mol dm⁻³ solution. Carboxylic acids are much weaker than mineral acids such as HCl.

Despite being weak, they are strong enough to react with metals, alkalis and carbonates. The five reactions below are all in outcome 18.1.2.

(a) Reaction with reactive metals

Carboxylic acids react with reactive metals such as magnesium or zinc. The acid donates H⁺, the metal is oxidised, a salt and hydrogen gas are produced. The reaction is slower and gentler than the same metals with hydrochloric acid.

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂(g)

ethanoic acid + magnesium → magnesium ethanoate + hydrogen

Observation: The metal dissolves and effervescence is seen (hydrogen). A lighted splint gives a squeaky pop.

(b) Neutralisation with alkalis

With a base such as NaOH or KOH, the reaction is a standard neutralisation: acid + base → salt + water.

CH₃COOH + NaOH → CH₃COONa + H₂O

ethanoic acid + sodium hydroxide → sodium ethanoate + water

There is no visible change (unless an indicator is present), because both the acid and the salt are colourless solutions.

(c) Reaction with carbonates

Carboxylic acids react with carbonates (e.g. Na₂CO₃) and hydrogencarbonates (e.g. NaHCO₃) to give a salt, water and carbon dioxide.

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

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

Observation: Effervescence (carbon dioxide). The gas turns limewater milky. This is a quick test to confirm that an unknown liquid is a carboxylic acid: add sodium carbonate or sodium hydrogencarbonate and look for fizzing.

(d) Esterification with alcohols

A carboxylic acid reacts with an alcohol, with concentrated H₂SO₄ as catalyst, to form an ester and water. The reaction is reversible and reaches equilibrium.

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

ethanoic acid + ethanol ⇌ ethyl ethanoate + water

Heat under reflux. The concentrated sulfuric acid both catalyses the reaction (providing H⁺) and absorbs water, shifting the equilibrium to the right.

(e) Reduction by LiAlH₄

LiAlH₄ in dry ether reduces a carboxylic acid to a primary alcohol. NaBH₄ is not strong enough.

CH₃COOH + 4[H] → CH₃CH₂OH + H₂O

ethanoic acid → ethanol

The product has the same number of carbon atoms. The –COOH carbon becomes the –CH₂OH carbon of a primary alcohol.

Esters: formation by condensation18.2.1(a)

An ester is formed when a carboxylic acid reacts with an alcohol. The reaction is a condensation: two molecules join together and a small molecule — water — is lost. The link that forms between the acid and the alcohol is the ester linkage, –COO–.

The esterification reaction

Heat a carboxylic acid with an alcohol and a few drops of concentrated H₂SO₄ as catalyst, under reflux. The reaction is reversible and reaches equilibrium.

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

ethanoic acid + ethanol ⇌ ethyl ethanoate + water

CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O

ethanoic acid + methanol ⇌ methyl ethanoate + water

CH₃CH₂COOH + CH₃CH₂OH ⇌ CH₃CH₂COOCH₂CH₃ + H₂O

propanoic acid + ethanol ⇌ ethyl propanoate + water

The concentrated sulfuric acid serves two roles: it provides H⁺ ions that catalyse the reaction, and it absorbs water, shifting the equilibrium towards the ester.

Naming esters

The name of an ester has two parts. The first part comes from the alcohol (the alkyl group): methyl from methanol, ethyl from ethanol, propyl from propan-1-ol. The second part comes from the acid, with –ic acid replaced by –ate: ethanoic acid becomes ethanoate, propanoic acid becomes propanoate.

Esters often have a sweet or fruity smell. In the lab, warming an acid with an alcohol and a drop of concentrated sulfuric acid and then pouring the mixture into water lets you smell the ester — it sits on top of the water as a thin, fragrant layer.

Hydrolysis of esters18.2.2

Hydrolysis is the breaking of a bond by water. In an ester, the bond that breaks is the C–O bond of the ester linkage, splitting the molecule back into the carboxylic acid (or its salt) and the alcohol. There are two methods.

Acid hydrolysis

Heat the ester under reflux with dilute hydrochloric acid (or dilute sulfuric acid). The H⁺ ions catalyse the reverse of esterification. This is a reversible reaction — it reaches equilibrium, so hydrolysis is not complete.

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

ethyl ethanoate + water ⇌ ethanoic acid + ethanol

Alkaline hydrolysis (saponification)

Heat the ester under reflux with dilute sodium hydroxide (or potassium hydroxide). The hydroxide ion attacks the ester and the reaction goes to completion — it is irreversible, because the carboxylic acid produced is immediately neutralised to its carboxylate salt.

CH₃COOCH₂CH₃ + NaOH → CH₃COO⁻Na⁺ + CH₃CH₂OH

ethyl ethanoate + sodium hydroxide → sodium ethanoate + ethanol

To obtain the free carboxylic acid from alkaline hydrolysis, the solution must be acidified afterwards (e.g. with dilute HCl).

Reaction map18.1, 18.2

The interactive map below connects the reactions of carboxylic acids. Pick a starting compound and a target to see the multi-step route with reagents and conditions for each step.

Data summary18.1, 18.2

Self-test18.1, 18.2

Twenty questions covering topics 18.1 and 18.2. Each question has one correct answer and an explanation. Reset to start again.