Carboxylic acids

The essentials

Carboxylic acids contain the carboxyl group, –COOH. They are named by replacing the "-e" of the alkane with "-oic acid" (ethanoic acid, CH₃COOH). Their acidity comes from the fact that the carboxylate ion, RCOO⁻, is stabilised by resonance: the negative charge is spread equally over two oxygen atoms. Carboxylic acids are weaker than mineral acids but stronger than alcohols and most phenols. They have higher boiling points than alcohols of similar mass because they form hydrogen-bonded dimers.

Key points

• Preparation: oxidation of primary alcohols or aldehydes; oxidation of alkylbenzenes with KMnO₄ (the whole side chain becomes –COOH, giving benzoic acid); hydrolysis of nitriles and amides; Grignard reagent + dry ice (CO₂) followed by hydrolysis; hydrolysis of acyl halides, anhydrides and esters.
• Acidity: electron-withdrawing groups (–F, –Cl, –NO₂, –CN) increase acidity; electron-donating groups decrease it. CF₃COOH is a very strong carboxylic acid. Order: FCH₂COOH > ClCH₂COOH > BrCH₂COOH; Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH.
• Methanoic acid (pKa about 3.75) is stronger than ethanoic acid (pKa about 4.76); benzoic acid lies between them.
• Carboxylic acids liberate CO₂ from NaHCO₃; phenols do not, which distinguishes them.
• Reactions of the –OH: esterification (with alcohol and acid catalyst), formation of acid chlorides (PCl₅, PCl₃, SOCl₂), amides (with NH₃ on heating), and anhydrides (with P₂O₅ or acid heating).
• Reduction with LiAlH₄ or diborane gives primary alcohols.
• Decarboxylation: sodium salts heated with soda-lime give an alkane with one carbon fewer. Kolbe electrolysis of sodium salts gives alkanes with twice the number of carbons of the alkyl group.
• Hell–Volhard–Zelinsky reaction: acids with α-H + Cl₂ or Br₂ with red phosphorus → α-halo acids.
• The –COOH group on a benzene ring is deactivating and meta-directing; benzoic acid does not undergo Friedel–Crafts reaction.

Exam tips

• For acidity order questions, look at the number, strength and distance of electron-withdrawing groups from –COOH.

Common mistakes

• Thinking the carbonyl group of a carboxylic acid behaves like that of an aldehyde; resonance with –OH reduces its reactivity towards nucleophilic addition.