Identify the best suitable reagent for the following reaction.

Step-by-step solution

Halogen atoms are ortho- para directing but deactivating groups(due to - I   effect ). The ortho product is the minor product and the para product is the major product. In presence of Concentrated sulphuric acid chlorobenzene undergoes aromatic electrophilic substitution reaction to form 2-Chlorobenzenesulphonic acid and 4-chlorobenzenesulphonic acid(major product), where the electrophile is sulphurtrioxide Reaction:- Hence, concentrated Sulphuric acid is the best suitable reagent for the above reaction.