JEE MAIN - Chemistry (2024 - 9th April Morning Shift - No. 18)
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : $$\mathrm{S}_{\mathrm{N}} 2$$ reaction of $$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br}$$ occurs more readily than the $$\mathrm{S}_{\mathrm{N}} 2$$ reaction of $$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}$$.
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 :
Explanation
To determine the correct answer, we need to analyze both the Assertion (A) and the Reason (R) independently, as well as see if the Reason (R) provides the correct explanation for the Assertion (A).
Assertion (A) Analysis:
Assertion (A) states that $$\mathrm{S}_{\mathrm{N}} 2$$ reaction of $$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br}$$ occurs more readily than the $$\mathrm{S}_{\mathrm{N}} 2$$ reaction of $$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}$$. In an $$\mathrm{S}_{\mathrm{N}} 2$$ reaction, the rate depends on the nucleophile as well as the substrate. Specifically, $$\mathrm{S}_{\mathrm{N}} 2$$ reactions proceed through a backside attack mechanism, leading to the formation of a transition state where the nucleophile and the leaving group are simultaneously bonded to the carbon atom. The benzyl group $$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2-$$ is more stable for $$\mathrm{S}_{\mathrm{N}} 2$$ reactions compared to the ethyl group $$\mathrm{CH}_3 \mathrm{CH}_2-$$ because the benzyl carbon's partial positive charge in the transition state is stabilized by the resonance of the phenyl ring.
Reason (R) Analysis:
Reason (R) explains that the partially bonded unhybridized p-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring. This means that the presence of the phenyl ring can delocalize and stabilize the charge in the transition state, facilitating the $$\mathrm{S}_{\mathrm{N}} 2$$ reaction.
Combining Assertion (A) and Reason (R):
Given that the phenyl ring in $$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br}$$ can indeed stabilize the transition state through conjugation more effectively than an ethyl group, both the Assertion (A) and Reason (R) are correct. Furthermore, Reason (R) provides a valid explanation for Assertion (A), reinforcing why the $$\mathrm{S}_{\mathrm{N}} 2$$ reaction occurs more readily for $$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br}$$.
Thus, the most appropriate answer is:
Option B: Both (A) and (R) are correct and (R) is the correct explanation of (A)
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