Benzoic acid (C6H5COOH) can be modified by attaching various substituents to the para…
CHEMISTRY · Multi-part · A Level · Chemistry · Carboxylic acids and derivatives · Carboxylic acids and derivatives
Problem
Benzoic acid (C6H5COOH) can be modified by attaching various substituents to the para position of the aromatic ring. Two derivatives are shown below.
• p-Methoxybenzoic acid: CH3O–C6H4–COOH
• p-Nitrobenzoic acid: O2N–C6H4–COOH
A chemist compares their acid strengths in aqueous solution.
(a) Predict which of the two para-substituted benzoic acids is stronger. Clearly state your choice.
(b) Provide a detailed explanation for your prediction in part (a) that accounts for both electron donation and electron withdrawal effects operating through the aromatic ring.
(c) m-Nitrobenzoic acid (O2N–C6H4–COOH with the –NO2 group at the meta position) is also studied. Predict whether its pKa is higher or lower than that of p-nitrobenzoic acid, and justify your answer in terms of the ways the substituent can influence the carboxylate anion.