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MCQEasyJEE 2023SN1 & SN2 Reactions

JEE Chemistry 2023 Question with Solution

Decreasing order towards SN1SN_1 reaction for the following compounds is:

Four benzyl chloride derivatives labeled a, b, c and d with para substituents NO2, OMe, Cl and H respectively on the benzene ring.
  • A

    a >> c >> d >> b

  • B

    a >> b >> c >> d

  • C

    b >> d >> c >> a

  • D

    d >> b >> c >> a

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Four substituted benzyl chlorides a, b, c, d are compared for their tendency towards SN1SN_1 reaction.

Find: The decreasing order of reactivity.

In an SN1SN_1 reaction, the rate depends on the stability of the carbocation formed after the leaving group departs.

The benzylic carbocation is stabilized or destabilized by the para substituent on the ring. The order shown in the solution is

b>d>c>ab > d > c > a

Thus, the reactivity order towards SN1SN_1 reaction is b>d>c>ab > d > c > a.

Therefore, the correct option is C.

Comparison of four para-substituted benzylic carbocations showing OMe most stabilizing, then unsubstituted, then Cl, and NO2 least stabilizing, with reactivity order b greater than d greater than c greater than a.

Carbocation Stability Basis

Given: SN1SN_1 rate follows carbocation stability.

Find: Which substituted benzyl chloride forms the most stable benzylic carbocation.

  • b has OMe, an electron-donating group, so it stabilizes the benzylic carbocation most.
  • d is unsubstituted and has the normal benzylic carbocation stability.
  • c has Cl, which overall stabilizes less effectively than OMe here.
  • a has NO2, a strong electron-withdrawing group, so it destabilizes the carbocation most.

Hence the decreasing order is

b>d>c>ab > d > c > a

So the correct option is C.

Common mistakes

  • Assuming SN1SN_1 reactivity depends mainly on the strength of the C-Cl\text{C-Cl} bond is incorrect here. The dominant factor is carbocation stability after chloride leaves. Compare the substituted benzylic carbocations first.

  • Treating NO2 as if it stabilizes the benzylic carbocation is wrong. NO2 is strongly electron-withdrawing and destabilizes positive charge. Therefore compound a should be least reactive.

  • Ignoring the effect of the para substituent on resonance is a conceptual error. In benzylic systems, ring substituents strongly affect the cation through resonance and inductive effects. Use substituent effects to rank the intermediates.

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