Question

Aryl halides are less reactive towards nucleophilic substitution reaction when compared to alkyl halides. This is because
• [I.] If aryl cation is formed, it is not stabilized by resonance
• [II.] $\text{C}-\text{X}$ has partial double bond character due to resonance}
• [III.] $\text{sp}^3$-hybridized carbon is attached to the halogen}
• [IV.] $\text{C}-\text{X}$ bond length is more} The correct reasons are

• A. $\text{I \& II only}$
• B. $\text{II \& III only}$
• C. $\text{III \& IV only}$
• D. $\text{I \& IV only}$

Hint:

Aryl halides resist both SN1 (unstable carbocation) and SN2 (partial double bond character) mechanisms, making them much less reactive than alkyl halides.

Answer 1

The Correct Option is A

Concept: Aryl halides show unusually low reactivity toward nucleophilic substitution reactions due to a combination of electronic and structural factors. The key reasons involve resonance stabilization of the C–X bond and the instability of the aryl carbocation intermediate.

Step 1: Evaluating Reason I (aryl carbocation stability).
In an $\text{S}_\text{N}1$ mechanism, bond cleavage would form a phenyl (aryl) carbocation. However, this carbocation is extremely unstable because the positive charge is located in an orbital that cannot overlap effectively with the aromatic $\pi$ system. Hence, resonance stabilization is not possible, making carbocation formation highly unfavorable. Therefore, Statement I is correct.

Step 2: Evaluating Reason II (resonance in C–X bond).
In aryl halides, lone pairs on the halogen atom participate in resonance with the benzene ring. This delocalization introduces partial double bond character in the C–X bond. As a result, the bond becomes shorter, stronger, and more difficult to break during nucleophilic substitution. Hence, Statement II is correct.

Step 3: Evaluating Reason III (hybridization).
In aryl halides, the halogen is attached to an $\text{sp}^2$-hybridized carbon atom, not an $\text{sp}^3$ carbon. Therefore, Statement III is incorrect.

Step 4: Evaluating Reason IV (bond length).
Due to $\text{sp}^2$ hybridization and resonance effects, the C–X bond in aryl halides is actually shorter, not longer. Hence, Statement IV is incorrect. Conclusion:
Only Statements I and II correctly explain the low reactivity of aryl halides. Therefore, the correct answer is Option (1).