7.9 Carboxylic acids and esters

The -COOH functional group, the names and formulae of methanoic to butanoic acid, their reactions with carbonates and metals, why they are weak acids, and how esters such as ethyl ethanoate are made and used.

Separate only AQA Edexcel OCR 1 min read

Carboxylic acids

Key terms

Carboxylic acid: an organic compound containing the functional group –COOH (carboxyl).
Ester: a compound with the functional group –COO–, made from a carboxylic acid and an alcohol.

Displayed formulae of methanoic, ethanoic, propanoic and butanoic acids, and ethanol reacting with ethanoic acid to make the ester ethyl ethanoate and water

The first four carboxylic acids
Name	Formula
methanoic acid	HCOOH
ethanoic acid	CH₃COOH
propanoic acid	C₂H₅COOH
butanoic acid	C₃H₇COOH

Note that the carbon in the –COOH group is counted in the name: ethanoic acid has 2 carbon atoms.

Vinegar is a solution of ethanoic acid in water.

Reactions

Carboxylic acids dissolve in water to form acidic solutions (pH below 7). They react like other acids:

with carbonates: fizzing, giving a salt, water and carbon dioxide ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide
with reactive metals: a salt and hydrogen
with alcohols: an ester and water

Weak acids

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

CH₃COOH ⇌ \mathrm{CH_3COO^-} + \mathrm{H^+}

So a solution of ethanoic acid has a higher pH than hydrochloric acid of the same concentration, and reacts more slowly with carbonates and metals.

Esters

alcohol + carboxylic acid ⇌ ester + water

ethanol + ethanoic acid ⇌ ethyl ethanoate + water (with an acid catalyst, such as concentrated sulfuric acid)

Esters:

are volatile (evaporate easily) and have fruity smells
are used in perfumes and food flavourings
ethyl ethanoate is a common solvent

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