The carboxyl group
Carboxylic acids contain the carboxyl group, –COOH — a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. They are the end point of oxidising a primary alcohol, and include familiar substances: ethanoic acid (vinegar), methanoic acid (in ant stings), and the long-chain fatty acids in fats.
Why they are acidic
Carboxylic acids are weak acids: they partly ionise in water, releasing H⁺ from the –OH. They are acidic (unlike alcohols) because the resulting carboxylate ion is stabilised — its negative charge is spread over both oxygen atoms by delocalisation, making the H⁺ easier to lose. Being weak, they only partly dissociate, so a solution of ethanoic acid is far less acidic than the same concentration of hydrochloric acid.
Reactions as acids
They show the usual acid reactions, each giving a salt (a carboxylate, e.g. ethanoate): with reactive metals → salt + hydrogen; with bases/alkalis → salt + water; with carbonates → salt + water + carbon dioxide (the fizzing that confirms a carboxylic acid).
Esters and other derivatives
The signature reaction is esterification: a carboxylic acid + an alcohol, with a little concentrated sulfuric acid catalyst, gives an ester + water. Esters have sweet, fruity smells and are used as flavourings, perfumes and solvents; fats and oils are esters of glycerol.
Esters can be hydrolysed back to the acid and alcohol (or, with alkali, to the carboxylate salt and alcohol — the basis of soap-making, saponification). Other derivatives include amides (the linkage in proteins).
The fizz test
A carboxylic acid added to a carbonate fizzes, releasing CO₂ that turns limewater milky — a quick confirmation that you have a carboxylic acid rather than, say, a phenol (which is too weak to do this).
A worked example
Ethanoic acid is warmed with ethanol and a little concentrated sulfuric acid. What forms, and what is the reaction called?
- A carboxylic acid + an alcohol with an acid catalyst undergoes esterification.The concentrated H₂SO₄ catalyses ester formation and removes water.
- The –OH of the acid and the H of the alcohol's –OH leave as water; the fragments join.Esterification eliminates water between the two –OH-containing groups.
- Product: ethyl ethanoate (a sweet-smelling ester) + water.Ethanoic acid + ethanol → ethyl ethanoate + H₂O.
Ethyl ethanoate (a fragrant ester) plus water — the reaction is esterification.
Preparation and the family of derivatives
Carboxylic acids are made by fully oxidising a primary alcohol (through the aldehyde) or by hydrolysing a nitrile. From the acid springs a whole family of derivatives in which the –OH is replaced: esters (–OR), acyl chlorides (–Cl) and amides (–NH₂). Acyl chlorides are the most reactive and are used to make the others.
All the derivatives can be hydrolysed back to the parent carboxylic acid, so the family interconverts.
Esterification and hydrolysis
Warming a carboxylic acid with an alcohol and a little concentrated sulfuric acid gives a sweet-smelling ester plus water — esterification, a reversible reaction the acid catalyses. Esters are used as flavourings, perfumes and solvents, and fats and oils are simply esters of glycerol.
Running it backwards, esters are hydrolysed by water: with acid it reverses to the carboxylic acid and alcohol; with alkali it gives the carboxylate salt and alcohol — and when the ester is a fat, that alkaline hydrolysis is exactly how soap is made (saponification).
Soap from fat
Boiling a fat (an ester of glycerol) with sodium hydroxide hydrolyses it into glycerol and sodium salts of long-chain fatty acids — the salts are soap. This alkaline hydrolysis is called saponification.