Question

Which of the following compounds undergoes replacement of –Cl by –OH by merely warming the compound with aqueous NaOH?
(a)-

(b)-

(c)-

(d)-

Answer 1

Hint: Both the –Cl and –OH are nucleophiles, so when –Cl is exchanged with –OH ion then it is known as Aromatic Nucleophilic substitution reaction. As the electron-withdrawing group increases on the benzene ring then the replacement will be easy.

Complete answer:
Both the –Cl (Chloride ion) and –OH (hydroxyl ion) are nucleophiles, so when –Cl is exchanged with –OH ion then it is known as an Aromatic Nucleophilic substitution reaction. Nucleophilic substitution reaction is not easy in the aromatic compounds.
Hence, there must be some electron-withdrawing group on the benzene ring, so that there can be substitution of the group because the electron-withdrawing increases the activity of the benzene ring and increases the reactivity. As the number of an electron-withdrawing group increases on the benzene ring then the replacement will be easy.
In option (a) there is no electron-withdrawing group, in option (b) there is one nitro group present at the ortho position, in option (c) there is two nitro group present at ortho and para position, and in option (d) there is three nitro group present at two orthos and one para position. As we know that the nitro group is a strong electron-withdrawing group, therefore, the compound with three nitro groups will have the highest reactivity towards the Nucleophilic substitution reaction.
Hence, the correct answer is an option (d)-

Note:
As compared to the nucleophilic substitution reaction, the aromatic compounds undergo Electrophilic substitution reactions easily. If the number of electron-donating groups increases on the benzene ring then the reactivity will decrease.