Carboxylic Acids (AQA GCSE Chemistry)

Carboxylic acids

• Carboxylic acids are a homologous series of compounds that have the functional group -COOH

• They have similar chemical properties

Carboxylic acid functional group

Diagram of the general structure of a carboxylic acid. The R- represents a varying hydrocarbon chain

Carboxylic acid structure

• The names and structure of the first four carboxylic acids are shown below

• In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘oic’ and then the word acid

Carboxylic acid structure

The structures and formulae of the first four carboxylic acids

Reactions of carboxylic acids

Reactions with water

• Carboxylic acids are soluble in water

• When carboxylic acids are placed in water, they dissolve

• A weakly acidic solution is formed

Reactions with carbonates

• In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced

• For example in reaction with potassium carbonate the salt potassium butanoate is formed by reaction with butanoic acid:

butanoic acid + potassium carbonate → potassium butanoate + water + carbon dioxide

Reactions with alcohols: Esterification

• Alcohols and carboxylic acids react to make esters in esterification reactions in the presence of an acid catalyst, usually concentrated sulfuric acid

• Esters are compounds with the functional group R-COO-R

• Esters are sweet smelling oily liquids used in food flavourings and perfumes

• They are volatile, meaning they vapourise easily

• Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

Esterification

Diagram showing the formation of ethyl ethanoate