Question

Give reasons for the following:
Benzoic acid does not undergo Friedel-Crafts reaction.
(b) HCHO is more reactive than $CH_3\text{CHO towards addition of HCN}$
(c) Vinyl group directly attached with carboxylic acid should decrease the acidity of corresponding carboxylic acid due to resonance, but on the contrary it increases the acidity.

Hint:

When analyzing reactions, always consider the electron-donating or electron-withdrawing effects of substituents on functional groups, as they can significantly alter the reactivity.

Answer 1

(a) Benzoic acid does not undergo Friedel-Crafts reaction because of the presence of the electron-withdrawing carboxyl group which deactivates the benzene ring, making it less reactive towards electrophilic aromatic substitution.
(b) HCHO is more reactive than $CH_3\text{CHO}$ towards addition of HCN because formaldehyde has a smaller alkyl group (hydrogen) compared to acetaldehyde, which makes the carbonyl carbon more electrophilic and hence more reactive towards nucleophilic addition.
(c) The vinyl group directly attached to carboxylic acid increases the acidity of the corresponding carboxylic acid due to the ability of the vinyl group to stabilize the negative charge on the conjugate base through resonance, making the carboxylate ion more stable and thus increasing acidity.