Which of these alkyl halides will undergo SN1 reaction at the fastest rate?
A
Cl−CH2−CN
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B
Cl−CH2−NO2
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C
Cl−CH2−OMe
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D
Cl−CH2−CH3
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Solution
The correct option is CCl−CH2−OMe The rate of SN1 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 −CN and −NO2 have the −I effect and will destabilize the carbocation.
−OMe will stabilize the carbocation through resonance hence, it will have the fastest rate.
−CH3 will stabilize the carbocation by hyperconjugation and inductive effect.
Out of all these, resonance effect dominates. Hence, correct option is (c).