Question

Why do aryl halides not show nucleophilic substitution reaction?

• A. The C--X bond has partial double bond character due to resonance.
• B. There is very high steric hindrance in the benzene ring.
• C. The size of the halogen atom is very small.
• D. Aryl carbocation is highly stable.

Hint:

Remember why aryl halides are less reactive: \[ \boxed{\text{Resonance} \Rightarrow \text{Partial double bond character}} \] Therefore, \[ \boxed{\text{Stronger C--X bond}} \] which resists nucleophilic substitution. This is one of the most frequently asked concepts in Haloarenes.

Answer 1

The Correct Option is A

Concept: Aryl halides are much less reactive towards nucleophilic substitution than alkyl halides. The main reason is the resonance between the halogen atom and the benzene ring. The lone pair of electrons on the halogen atom participates in resonance with the aromatic ring, giving the carbon-halogen bond partial double bond character. As a result,

• The C--X bond becomes shorter and stronger.

• Bond cleavage becomes difficult.

• Nucleophilic substitution does not occur easily.

Step 1: Understand the effect of resonance.
In aryl halides, the halogen atom donates its lone pair into the benzene ring. Therefore, \[ \boxed{\text{C--X bond acquires partial double bond character}.} \] This strengthens the bond considerably.

Step 2: Why does substitution become difficult?
Because of the partial double bond character, \[ \boxed{\text{The C--X bond is difficult to break}.} \] Hence, neither the \(S_N1\) nor the \(S_N2\) mechanism proceeds easily.

Step 3: Examine the options.
Option (A): \[ \boxed{\text{Correct}} \] because resonance gives partial double bond character. Option (B): Steric hindrance is not the primary reason. Option (C): The size of the halogen atom is not responsible. Option (D): Aryl carbocations are highly unstable, not stable. Therefore, \[ \boxed{\textbf{Option (A)}} \] is the correct answer.