I have been asked to compare the feasibility of Unimolecular Nucleophilic Substitution ($\mathrm{S_N1}$) in the following question.

t-butyl chloride and chloro-1-methylcyclopropane

I have received instruction that the reaction will be feasible only in the first compound (which results in a 3° stable carbocation) since 1-chloro-1-methylcyclopropane is angle strained and the resulting carbocation in this case will be unstable.

However, if we consider the possibility of hydride rearrangement in the second molecule, after formation of the carbocation, the following rearrangement should stabilise the carbocation formed, through the delocalisation of sigma electron pair as is seen in the case of cyclopropyl methyl carbocations.

Cyclopropyl methyl cation

Where exactly am I going wrong?


The proposed rearrangement would produce a primary carbocation which, relatively lacking in hyperconjugation, is itself unstable. The proposed rearrangement therefore is not really plausible.

The "instruction" that the highly strained 1-methyl-cyclopropyl cation cannot be formed may be wrong anyway. This paper a substituted cyclopropyl cation as an intermediate in a reaction between a more complex chlorocyclopropane and an alcohol.

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