Rearrangements in SN1 Reactions

In SN1 reactions, an intermediate carbenuim ion appears. If a secondary carbenium ion is formed, a rearrangement may occur that results in a more stable tertiary carbenium ion. If 2-bromo-3-methylbutane, for instance, is treated with water under SN1 conditions, the secondary carbenium ion is initially fashioned. However, this secondary carbenium ion can then be rearranged by a 1,2-hydride shift that yields a tertiary carbenium ion.

Fig.11,2-Hydride shift.

By analogy, a 1,2-methyl shift takes place if 3-bromo-2,2-dimethyl butane is applied to the same reaction.

Fig.21,2-Methyl shift.