Which of these alkyl halides will undergo $S_{N} 1$ reaction at the fastest rate?

C l - C H_{2} - C N

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C l - C H_{2} - N O_{2}

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C l - C H_{2} - O M e

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C l - C H_{2} - C H_{3}

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Solution

The correct option is C $C l - C H_{2} - O M e$
The rate of $S_{N} 1$ reactions is governed by the stability of the carbocation intermediate. More stable is the carbocation, faster will be the rate.

From the options, we can see $- C N$ and $- N O_{2}$ have the $- I$ effect and will destabilize the carbocation.
$a . H_{2} ⨁ C - C N$

$b . H_{2} ⨁ C - N O_{2}$

$- O M e$ will stabilize the carbocation through resonance hence, it will have the fastest rate.
$c . H_{2} ⨁ C - O M e ⟷ H_{2} C = ⨁ O M e$

$- C H_{3}$ will stabilize the carbocation by hyperconjugation and inductive effect.
$d . H_{2} ⨁ C - C H_{3}$

Out of all these, resonance effect dominates. Hence, correct option is (c).

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