Carboxylic Acids

Carboxylic acids are a family of organic compounds that all contain the carboxyl functional group, written as –COOH. Vinegar is a dilute solution of ethanoic acid, which gives it a sharp smell and sour taste.

The Family and Formula

Carboxylic acids form a homologous series. Their general formula is:

$$\mathrm{C}_n\mathrm{H}_{2n+1}\mathrm{COOH}$$

• Methanoic acid HCOOH
• Ethanoic acid CH₃COOH
• Propanoic acid CH₃CH₂COOH
• Butanoic acid CH₃CH₂CH₂COOH

Key Properties

• Acidic in water: turn blue litmus red and react like other acids.
• Ethanoic acid is a weak acid. It only partly forms ions in water:

$$\mathrm{CH_3COOH\;(aq)\;\rightleftharpoons\;H^+\;(aq) + CH_3COO^-\;(aq)}$$

Reactions of Ethanoic Acid

• With metals (e.g. Mg) → salt + hydrogen gas (fizzing).
  $$2\,\mathrm{CH_3COOH} + \mathrm{Mg} \rightarrow (\mathrm{CH_3COO})_2\mathrm{Mg} + \mathrm{H_2}$$
• With bases (alkalis) → salt + water.
  $$\mathrm{CH_3COOH} + \mathrm{NaOH} \rightarrow \mathrm{CH_3COONa} + \mathrm{H_2O}$$
• With carbonates → salt + water + carbon dioxide (fizzing).
  $$2\,\mathrm{CH_3COOH} + \mathrm{Na_2CO_3} \rightarrow 2\,\mathrm{CH_3COONa} + \mathrm{H_2O} + \mathrm{CO_2}$$

Salts are named “metal + ethanoate”, e.g. sodium ethanoate (CH₃COONa), magnesium ethanoate ((CH₃COO)₂Mg).

Making Ethanoic Acid

• Oxidising ethanol with acidified potassium manganate(VII): ethanol → ethanoic acid.
• Bacterial oxidation of ethanol during vinegar production.

Forming Esters

Carboxylic acid + alcohol with an acid catalyst (e.g. concentrated sulfuric acid) and warming → ester + water. Esters often smell fruity.

Example: ethanol + ethanoic acid → ethyl ethanoate + water.