Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

Objectives

After completing this section, you should be able to explain the “unusual” products formed in certain reactions in terms of the rearrangement of an intermediate carbocation.

Key TERMS

Make certain that you can define, and use in context, the key terms below.

alkyl shift
hydride shift

Study Notes

Whenever possible, carbocations will rearrange from a less stable isomer to a more stable isomer. This rearrangement can be achieved by either a hydride shift, where a hydrogen atom migrates from one carbon atom to the next, taking a pair of electrons with it; or an alkyl shift, in which an alkyl group undergoes a similar migration, again taking a bonding pair of electrons with it. These migrations usually occur between neighbouring carbon atoms, and hence are termed 1,2-hydride shifts or 1,2-alkyl shifts.

[A hydride ion consists of a proton and two electrons, that is, [H:]⁻. Hydride ions exist in compounds such as sodium hydride, NaH, and calcium hydride, CaH₂.]

An electrophilic reaction such as HX with an alkene will often yield more than one product. This is strong evidence that the mechanism includes intermediate rearrangement steps of the cation.

1,2-Hydride Shift

A 1,2-hydride shift is a carbocation rearrangement in which a hydrogen atom in a carbocation migrates to the carbon atom bearing the formal charge of +1 (carbon 2) from an adjacent carbon (carbon 1).

eg:

1,2-Alkyl Shift

A 1,2-alkyl shift is a carbocation rearrangement in which an alkyl group migrates to the carbon atom bearing the formal charge of +1 (carbon 2) from an adjacent carbon atom (carbon 1), e.g.