Question

What is the action of following reagents on chlorobenzene? (i) conc. $HNO_3$, (ii) fuming $H_2SO_4$

Hint:

For haloarenes, the para isomer is almost always the major product due to less steric hindrance compared to the ortho isomer.

Answer 1

Step 1: Understanding the Concept:
Chlorobenzene undergoes electrophilic substitution reactions. The Chlorine atom is ortho-para directing due to its +M effect (resonance effect).
Step 2: Detailed Explanation:
(i) Action of conc. \( HNO_3 \) (Nitration):
When chlorobenzene is heated with nitrating mixture (conc. \( HNO_3 + \) conc. \( H_2SO_4 \)), it forms a mixture of 1-chloro-2-nitrobenzene (minor) and 1-chloro-4-nitrobenzene (major).
Reaction: \( C_6H_5Cl \xrightarrow{conc. HNO_3 / H_2SO_4} Cl-C_6H_4-NO_2 \)

(ii) Action of fuming \( H_2SO_4 \) (Sulfonation):
When chlorobenzene is heated with fuming sulfuric acid, it undergoes sulfonation to form 2-chlorobenzene sulfonic acid (minor) and 4-chlorobenzene sulfonic acid (major).
Reaction: \( C_6H_5Cl \xrightarrow{\text{fuming } H_2SO_4, \Delta} Cl-C_6H_4-SO_3H \)
Step 3: Final Answer:
Nitration gives nitro derivatives, and sulfonation gives sulfonic acid derivatives, primarily at the para position.