Carboxylic acids

Carboxylic acids are weak acids but show all the usual acid reactions:

• Metal: $2\text{CH}_3\text{COOH} + \text{Mg} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2$
• Base: $\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$
• Carbonate: $2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

The effervescence (CO₂) with a carbonate is a key identifying reaction — and it distinguishes a carboxylic acid from a phenol, which is too weak to release CO₂.

Reduction. Carboxylic acids are reduced to primary alcohols by the strong reducing agent LiAlH₄ (in dry ether). NaBH₄ is too weak to reduce a carboxylic acid (it only reduces aldehydes/ketones). $\text{CH}_3\text{COOH} \xrightarrow{\text{LiAlH}_4} \text{CH}_3\text{CH}_2\text{OH}$

Esterification. A carboxylic acid reacts with an alcohol (conc H₂SO₄ catalyst) in a reversible reaction to give an ester + water: $\text{CH}_3\text{COOH} + \text{CH}_3\text{OH} \rightleftharpoons \text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O}$ (A faster, irreversible route uses the acyl chloride instead of the acid.)

Acid strength depends on the stability of the carboxylate anion formed when the acid loses H⁺. The more stable the anion, the stronger the acid.

Electron-withdrawing groups (e.g. –Cl, which has a −I effect) pull electron density away from the –COO⁻, spreading and stabilising the negative charge. So chloroethanoic acid (ClCH₂COOH) is a stronger acid than ethanoic acid, and dichloroethanoic acid is stronger still.

Conversely, electron-donating alkyl groups (+I) intensify the negative charge, destabilising the anion and giving a weaker acid. (This is also why carboxylic acids are weak overall — and why an electron-withdrawing ring makes benzoic acid stronger than a simple aliphatic acid.)