Carboxylic Acids

Carboxylic acids contain the carboxyl group, COOH. The carboxyl carbon has a C=O bond and an O-H bond. The general formula is RCOOH, e.g. ethanoic acid, CH₃COOH, the acid in vinegar.Making them: carboxylic acids are made by oxidising primary alcohols or aldehydes with acidified potassium dichromate(VI), heating under reflux.Hydrogen bonding: the carboxyl group has a polar O-H bond and a polar C=O bond, so carboxylic acid molecules hydrogen bond strongly to each other. This gives them high boiling points, higher than alkanes and aldehydes of similar M_r.Solubility: the smaller carboxylic acids (up to about four carbons) mix completely with water because they form hydrogen bonds with water molecules. As the hydrocarbon chain gets longer, the non-polar part of the molecule gets bigger and solubility decreases.Acidity: carboxylic acids are weak acids. They partly dissociate in water to release H⁺ ions, and they react with bases and carbonates to form salts. They also react with alcohols to form esters. These reactions are covered in the following lessons.Example: oxidising ethanol, CH₃CH₂OH, under reflux with acidified potassium dichromate(VI) gives ethanoic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.