Why doesn't Brady's reagent react with carboxylic acids?

Igloo replies In common with ketones and aldehydes, carboxylic acids contain a carbonyl group (C=O), but in addition they have an electron withdrawing hydroxyl group (OH) attached to the carbonyl carbon atom, and this has a significant effect on the chemical properties of the carboxyl group (COOH). This means that the carboxylic acid group has quite distinct properties from the carbonyl group. Addition reactions are characteristic of carbonyl compounds while substitution is the norm for carboxylic acids and derivatives. Nucleophilic attack on carbonyl compounds results in addition, since the alkyl groups are obviously not good leaving groups (R- is not a feasible product). However, the hydroxyl group in acids is capable of leaving (as OH-), so the consequence of nucleophilic attack on acids results in substitution, not addition. 2,4-dinitrophenylhydrazine (Bradys Reagent) reacts by adding to the carbonyl group, and this stage is immediately followed by the elimination of water. 2,4- ...  more