7.2.4 Carboxylic Acids

Carboxylic acids have the functional group -COOH, also known as the carboxyl group. The first four members of the homologous series of carboxylic acids are methanoic acid, ethanoic acid, propanoic acid, and butanoic acid.

The structures of carboxylic acids can be represented as follows:

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

When carboxylic acids react with carbonates, they produce a salt, carbon dioxide gas, and water. For example, ethanoic acid reacting with sodium carbonate:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O

Carboxylic acids readily dissolve in water, forming acidic solutions. The carboxyl group can ionise, releasing H⁺ ions, making carboxylic acids weak acids. The pH of carboxylic acid solutions is typically below 7.

Carboxylic acids can react with alcohols in the presence of an acid catalyst to form esters. This reaction is called esterification. For example, ethanoic acid reacting with ethanol:

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

The weak acidity of carboxylic acids is due to the partial ionisationThe process of turning an uncharged molecule into an ion. In MS this is done so the ionised fragment can be analysed as it is affected by electric and magnetic fields. of the carboxyl group. The carboxyl group can donate a proton (H⁺) to water, resulting in the formation of hydronium ions (H₃O⁺). However, carboxylic acids do not completely ionise in water, which accounts for their weaker acidity compared to strong mineral acids.