Question

Sulphonation differs from most of the electrophilic aromatic substitution reactions with the fact that the reaction:
A. is reversible
B. requires lewis acid as catalyst
C. is explosive
D. take place at high temperature

Answer 1

Hint:When a sulphur group is introduced in a benzene ring that process is called sulphonation. It occurs when benzene is heated with fuming sulphuric acid or concentrated sulphuric acid as a product it yields benzene sulphonic acid. It is followed by electrophilic substitution reaction.

Complete answer:
Sulphonation is unusually different from all the electrophilic aromatic substitution reactions because it is a reversible reaction. Removal of water molecules from the system favours the formation of sulfonation products. But when sulphuric acid is heated with aqueous sulphuric acid then the reaction will get reversed. That reverse reaction causes desulphonation. In other words it is a reversible reaction because Sulfur trioxide readily reacts with water and produces sulfuric acid and heat. So, by applying heat to benzenesulfonic acid in diluted
aqueous sulfuric acid the reaction is reversed.

Hence, option A is correct.

Note:
Electrophilic substitution reactions are those chemical reactions in which displacement of a functional group takes place by an electrophile, which is generally, but not always, a hydrogen atom. Aromatic compounds mostly go under electrophilic substitution because the aromatic structure of the compound provides significant additional stability to the molecule. Substitution reactions allow the aromatic structure to remain but on the other hand additions would change the carbon-carbon bonding away from being aromatic. In a series of activating groups alcohol comes first and then ketone towards electrophilic substitution reaction so phenol is most reactive towards electrophilic substitution reaction. The rate determining step in the electrophilic aromatic substitution is the step where formation of cationic intermediate takes place. Benzene sulphonic acid is least reactive towards electrophilic aromatic substitution reaction due to the –M effect.