hydrocarbon part ) α - Halogenation Carboxylic acids having an α - hydrogen are halogenated at the α - position on treatment with chlorine or bromine in the presence of small amount of red posphorus to form α halo carboxylic acids. This reaction is known as Hell – Volhard – Zelinsky reaction (HVZ reaction) The α - Halogenated acids are convenient starting materials for preparing α - substituted acids. COOH Cl / red P H O COOH Cl Acetic acid Mono Chloro acetic acid ) Electrophilic substitution in aromatic carboxylic acids Aromatic carboxylic acid undergoes electrophilic substitution reactions. The carboxyl group is a deactivating and meta directing group.
Some common electrophilic substitution reactions of benzoic acid are given below i) Halogenation COOH COOH Br Br FeBr + HBr Benzoic acid m - bromo benzoic acid ii) Nitration + H O Benzoic acid m-Nitrobenzoic acid COOH COOH NO Conc. HNO Conc. H SO iii) Sulphonation + H O Benzoic acid COOH COOH SO H Fuming Conc. H SO m-Sulphobenzoic acid XII U12-Carbonyl XII U12-Carbonyl - - - - iv) Benzoic acid does not undergo friedal craft's reaction.
This is due to the strong deactivating nature of the carboxyl group. E) Reducing action of Formic acid Formic acid contains both an aldehyde as well as an acid group. Hence, like other aldehydes, formic acid can easily be oxidised and therefore acts as a strong reducing agent Aldehyde group Carboxylic acid group i) Formic acid reduces Tollens reagent (ammonical silver nitrate solution) to metallic silver. HCOO - + 2Ag + + 3OH - Ag + CO - + 2H O (Tollens reagent) Silver mirror ii) Formic acid reduces Fehling's solution.
It reduces blue coloured cupric ions to red coloured cuprous ions. HCOO - + 2Cu + + OH - Cu O + CO - + H O (Fehlings solution) red precipitate Tests for carboxylic acid group i) In aqueous solution carboxylic acid turn blue