Carboxylic Acids, Acyl Chlorides and Esters

Carboxylic acids, acyl chlorides and esters all contain a carbon atom double bonded to oxygen and singly bonded to a second oxygen. This carbonyl-plus-oxygen arrangement is called the carboxyl-type group, and its exact identity changes depending on what is attached to that second oxygen (or, for acyl chlorides, a chlorine instead of oxygen).

These three groups sit alongside aldehydes and ketones in the family of carbonyl-containing compounds, but the extra oxygen (or chlorine) attached to the carbonyl carbon gives carboxylic acids, acyl chlorides and esters their own distinctive chemistry.

Carboxylic acids are named by counting the longest carbon chain including the \( -COOH \) carbon and adding the suffix "-oic acid", so \( CH_3CH_2COOH \) is propanoic acid. Replacing the \( -OH \) with chlorine gives the acyl chloride, named with the ending "-oyl chloride": propanoic acid becomes propanoyl chloride, \( CH_3CH_2COCl \).

Esters are named as two words. The first word comes from the alcohol part (the group attached to the ester oxygen) and the second word comes from the acid part, ending in "-oate". For example, reacting ethanol with ethanoic acid produces ethyl ethanoate, \( CH_3COOC_2H_5 \).

Carboxylic acids are weak acids that undergo the typical reactions of an acid, plus one very important extra reaction with alcohols.

• With reactive metals: \( 2CH_3COOH + Mg \)→\( (CH_3COO)_2Mg + H_2 \)
• With bases: \( CH_3COOH + NaOH \)→\( CH_3COONa + H_2O \)
• With carbonates: \( 2CH_3COOH + Na_2CO_3 \)→\( 2CH_3COONa + H_2O + CO_2 \), which fizzes and is a useful test for the \( -COOH \) group
• Esterification with an alcohol, catalysed by concentrated sulfuric acid: \( CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O \)
• Conversion into an acyl chloride using \( SOCl_2 \) or \( PCl_5 \): \( CH_3COOH + SOCl_2 \)→\( CH_3COCl + SO_2 + HCl \)

The esterification reaction is an equilibrium, so yields are often modest unless the water produced is removed or an excess of one reactant is used. This is exactly why chemists prefer acyl chlorides when a clean, high-yield ester or amide synthesis is needed.

Chlorine is a much better leaving group than the hydroxide-type oxygen in a carboxylic acid, so acyl chlorides react rapidly and irreversibly with nucleophiles such as water, alcohols and amines. Each reaction proceeds by nucleophilic addition-elimination at the carbonyl carbon, with chloride ion leaving.

• Hydrolysis (even by cold water, often producing fumes of \( HCl \)): \( CH_3COCl + H_2O \)→\( CH_3COOH + HCl \)
• Reaction with alcohols to give esters, with no acid catalyst needed: \( CH_3COCl + C_2H_5OH \)→\( CH_3COOC_2H_5 + HCl \)
• Reaction with ammonia or amines to give amides: \( CH_3COCl + NH_3 \)→\( CH_3CONH_2 + HCl \)

Because these reactions go essentially to completion rather than reaching an equilibrium, acyl chlorides are the reagent of choice whenever a reliable, near-quantitative ester or amide preparation is required, including the industrial routes used to make some amines, amides and amino acids.

Beyond direct esterification of a carboxylic acid, esters can be made from acyl chlorides (as shown above) or from acid anhydrides reacting with alcohols. Once formed, an ester can be hydrolysed back to its parent acid and alcohol in two contrasting ways.

• Acid hydrolysis (dilute acid, reflux) is the reverse of esterification and is an equilibrium: \( CH_3COOC_2H_5 + H_2O \rightleftharpoons CH_3COOH + C_2H_5OH \)
• Alkaline hydrolysis (aqueous \( NaOH \), reflux) goes to completion because the carboxylate ion formed cannot re-esterify: \( CH_3COOC_2H_5 + NaOH \)→\( CH_3COONa + C_2H_5OH \)

Alkaline hydrolysis of an ester is sometimes called saponification, since it is the reaction used to make soap from fats and oils. The same repeated ester linkage, formed between many diol and diacid units, is also the basis of condensation polymers, which is explored fully in polyesters and polyamides.

Question: Methyl propanoate, \( CH_3CH_2COOCH_3 \), is heated with excess aqueous sodium hydroxide. What are the organic products?

Since alkaline hydrolysis breaks the ester into a carboxylate salt and an alcohol, the products are sodium propanoate, \( CH_3CH_2COONa \), and methanol, \( CH_3OH \). No equilibrium is set up, so the reaction is driven to completion and the ester is fully converted, unlike the reversible acid hydrolysis case.

This map summarises the key point of the topic: acyl chlorides sit at the reactive centre, converting quickly and completely into esters, amides or the acid itself, while the direct acid-to-ester route through esterification is the only one of these that is a true equilibrium.

Questions on carboxylic acid reactions at A Level frequently test whether you can distinguish an equilibrium esterification from an irreversible acyl chloride reaction, and whether you can predict the correct hydrolysis products under acidic versus alkaline conditions. Being confident with the mechanism at the carbonyl carbon also supports later topics on increasing carbon chain length using Grignard reagents, where a similar addition-type attack on a carbonyl group builds new carbon-carbon bonds.

Related lessons

• Uk A Level Chemistry