Question

Consider the following reaction,

Which of the following statements via $S_N2$ is correct?

• A. Formation of (I) and (II) proceeds via $S_N1$
• B. Formation of (I) proceeds via $S_N1$ while (II)
• C. Formation of (I) and (II) proceed via $S_N2$
• D. Formation of (I) proceed via, $S_N2$ while (II) via $S_N1$

Answer 1

Answer: (D)

d. Formation of (I) proceed via, $S_N2$ while (II) via $S_N1$

Answer 2

The reaction involves a primary allylic halide reacting with a strong nucleophile (CN⁻). The substrate is 1-chloro-3-pentene: CH₃CH=CHCH₂Cl. Product (I) is 1-cyano-3-pentene: CH₃CH=CHCH₂CN. This is the direct substitution product. Product (II) is 3-cyano-1-pentene: CH₃CH(CN)CH=CH₂. This is the rearranged substitution product.

Let's consider the possible mechanisms:

1. $S_N2$: The nucleophile attacks the carbon bearing the leaving group from the backside. This leads to the formation of product (I).

   CH₃CH=CHCH₂Cl + CN⁻ → CH₃CH=CHCH₂CN + Cl⁻

2. $S_N1$: The leaving group departs to form a carbocation. Since the substrate is allylic, a resonance-stabilized carbocation is formed.

   CH₃CH=CHCH₂Cl → CH₃CH=CHCH₂⁺ + Cl⁻ ↔ CH₃CH⁺CH=CH₂

   The nucleophile can attack either carbon with a partial positive charge.

   Attack at the primary carbon (original position of Cl): CH₃CH=CHCH₂⁺ + CN⁻ → CH₃CH=CHCH₂CN (Product I)

   Attack at the secondary carbon (rearranged position): CH₃CH⁺CH=CH₂ + CN⁻ → CH₃CH(CN)CH=CH₂ (Product II)

3. $S_N2'$: The nucleophile attacks the carbon two positions away from the leaving group (γ-carbon), the double bond shifts, and the leaving group departs from the original carbon (α-carbon).

   CH₃CH=CHCH₂Cl + CN⁻ → [Transition state] → CH₃CH(CN)CH=CH₂ + Cl⁻ (Product II)

With a primary allylic halide and a strong nucleophile, $S_N2$ and $S_N2'$ are significant pathways. $S_N1$ is also possible due to the resonance-stabilized allylic carbocation.

Considering the options, option d provides a reasonable description of possible pathways for the formation of the products. Product (I) is the direct substitution product, which is typically formed by $S_N2$ with primary halides and strong nucleophiles. Product (II) is the rearranged product, which can be formed via $S_N1$ (attack on the resonance-stabilized carbocation) or $S_N2'$. Since $S_N1$ is listed as a possibility for (II), and $S_N2$ for (I), option d is the most likely correct answer among the given choices, assuming these are significant pathways.