Question

Which of the following alkyl halides undergo rearrangement in ${{S}_{{{N}^{1}}}}$ reaction?
A.

B.


C.


  D. All of these

Answer 1

Hint: When reactants like alkyl halides undergo a reaction to form carbocation, carbocations are subject to a phenomenon named carbocation rearrangement. Carbocations are very reactive and consist of a positive charge on the central carbon atom with sextet instead of the octet. They are highly stabilised either by alkyl shift or hydride shift to form the most stable carbocation.

Complete step by step solution:
When the leaving group$(I)$ from the given alkyl halide, a carbocation is generated. A rearrangement can occur to form the most stable carbocation. Carbocation stability increases from tertiary to primary. These carbocations are stabilised by hyperconjugation and inductive effect. It is seen that a pair of electrons is transferred to the vacant p orbital on the carbocation through hydride or alkyl shift.

In the case of A, an alkyl group ($-C{{H}_{3}}$) is shifted to form a tertiary $({{3}^{{\mathrm O}}})$ carbocation, which is shown below:

In the case of B, a hydride shift$(-H)$ takes place to form a stable ${{3}^{{\mathrm O}}}$ carbocation.


In the case of C, after shifting of the hydride group a secondary carbocation is generated which is then further stabilised by a nearest double bond and forms a most stable carbocation one which undergoes a rapid rearrangement reaction.

So, all of these compounds undergo rearrangement reactions via ${{S}_{{{N}^{1}}}}$
Hence, Option (D) is correct.

Note: The rate of ${{S}_{{{N}^{1}}}}$ reaction affects the stability of intermediate,strong leaving group and dielectric constants of solvent. The first step is ${{S}_{{{N}^{1}}}}$ reaction is carbocation formation and the rate of ${{S}_{{{N}^{1}}}}$ reaction is faster when the most stable carbocation is formed.