Question

When does a hydride shift occur?

Answer 1

To specify, you meant to ask about a 1,2-hydride shift. Those are taught in organic chemistry, first semester.

When you have a reaction that contains a carbocation intermediate (such as an `"S"_N1` reaction of tert-butyl bromide with water), you have to question whether you can achieve a more stable carbocation or not.

CARBOCATION HYPERCONJUGATION

Carbocation stability goes as follows:

`3^@ > 2^@ > 1^@ > "methyl"`

This is due to the `"C"-"H"` bonds on the alkyl groups surrounding the central, cationic carbon.

These `"C"-"H"` bonds can spread their electron density out and share it with the central empty `p` orbital on the cationic carbon, thus stabilizing it. This is called hyperconjugation.

The more alkyl groups surrounding the cationic carbon, the more stable it thus becomes, because an additional `"C"-"H"` bond is available to share more negative charge density and offset the positivity in the central cationic carbon.

1,2-HYDRIDE SHIFTS

Whenever you have a more substituted carbon adjacent to a cationic carbon, a 1,2-hydride shift would shift the positive charge over to a carbon that can better stabilize itself.