Rearrangement of Carbon Cations

Another feature of E1 reactions (and also of SN1 reactions) is the tendency of the initially formed carbocation to rearrange, especially if a more stable carbocation is formed thereby. For example, the very slow SN1 solvolysis of neopentyl iodide in methanoic acid leads predominantly to 2-methyl-2-butene:

In this reaction, ionization results in migration of a methyl group with its bonding pair of electrons from the β to the α carbon, thereby transforming an unstable primary carbocation to a relatively stable tertiary carbocation. Elimination of a proton completes the reaction.

Rearrangements involving shifts of hydrogen (as H:^⊖) occur with comparable ease if a more stable carbocation can be formed thereby:

Rearrangements of carbocations are among the fastest organic reactions known and must be reckoned with as a possibility whenever carbocation intermediates are involved.