Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R):
Assertion (A): SN$_2$ reaction of C$_6$H$_5$CH$_2$Br occurs more readily than the SN$_2$ reaction of CH$_3$CH$_2$Br.
Reason (R): The partially bonded unhybridized p-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring.
In the light of the above statements, choose the most appropriate answer from the options given below:

• A. (A) is not correct but (R) is correct
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• D. (A) is correct but (R) is not correct

Answer 1

The Correct Option is C

The given question involves understanding the mechanism and stability of SN₂ reactions in organic chemistry, specifically comparing the reactivities between benzyl bromide (C₆H₅CH₂Br) and ethyl bromide (CH₃CH₂Br).

SN₂ Reaction Overview:

• SN₂ stands for substitution nucleophilic bimolecular reaction.
• It proceeds with a backside attack by the nucleophile, resulting in inversion of configuration at the carbon center.
• The reaction proceeds via a single transition state, which is a trigonal bipyramidal structure.

Explanation:

1. Assertion (A): SN₂ reaction of C₆H₅CH₂Br occurs more readily than the SN₂ reaction of CH₃CH₂Br.
   The statement requires analyzing the SN₂ reactivity of benzyl bromide and ethyl bromide. Benzyl bromide undergoes SN₂ reactions more readily due to the stability provided by resonance. The benzyl group (C₆H₅CH₂-) allows the transition state to stabilize through resonance with the phenyl ring, which lowers the activation energy necessary for the reaction.
2. Reason (R): The partially bonded unhybridized p-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring.
   This statement accurately explains why the transition state of the benzyl bromide SN₂ reaction is stabilized. The conjugation of the unhybridized p-orbital in the transition state with the phenyl ring provides resonance stabilization.
3. Conclusion: Given both the assertion and reason align with the fact that the benzyl bromide SN₂ reaction is facilitated by stability through conjugation, both (A) and (R) are correct and (R) is the correct explanation of (A).

Therefore, the correct answer is: Both (A) and (R) are correct and (R) is the correct explanation of (A).

Answer 2

Explanation:

1. Assertion (A): Correct. - In \( C_6H_5CH_2Br \), the \( CH_2-Br \) bond is connected to a benzyl group. The phenyl ring allows for stabilization of the transition state via resonance, facilitating the \( S_N2 \) reaction. This makes the reaction proceed more readily compared to \( CH_3CH_2Br \), where no such stabilization exists.

2. Reason (R): Correct. - The unhybridized \( p \)-orbital formed during the trigonal bipyramidal transition state interacts with the conjugated system of the phenyl ring, providing extra stabilization.

3. Conclusion: Both (A) and (R) are correct, and (R) is the correct explanation for (A).

Final Answer is option (3).