Question

3-Bromo-1-butene and 1-bromo-2-butene undergo${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$ reaction at nearly the same rate, even though one is a secondary halide and the other is primary. Explain.

Short answer

The ${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$ reaction is a nucleophilic substitution reaction where carbon-halogen bond is broken which results in positively charged carbon, and then nucleophile attacks the carbocation and forms a new bond. Only one reacting species is involved in the rate determining step of the reaction. The term${\mathrm{S}}_{\mathrm{N}}1$ stands for substitution nucleophilic unimolecular. The hydrolysis of tert-butyl bromide with aqueous NaOH solution is an example of${\mathrm{S}}_{\mathrm{N}}1$ reaction.

Step-by-step solution

Nucleophilic unimolecular substitution reaction SN1

The ${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$ reaction is a nucleophilic substitution reaction where carbon-halogen bond is broken which results in positively charged carbon, and then nucleophile attacks the carbocation and forms a new bond. Only one reacting species is involved in the rate determining step of the reaction. The term${\mathrm{S}}_{\mathrm{N}}1$ stands for substitution nucleophilic unimolecular. The hydrolysis of tert-butyl bromide with aqueous NaOH solution is an example of${\mathrm{S}}_{\mathrm{N}}1$ reaction.

3-Bromo-1-butene and 1-bromo-2-butene undergo SN1 reaction

The two bromobutene from the same allylic carbocation in the rate limiting step is shown below.