Question

When does a hydride shift occur?

Answer 1

A 1,2-shift is an organic process in which a substituent in a chemical molecule transfers from one atom to another. The movement of two neighbouring atoms is involved in a 1,2 shift, however bigger distances can be covered. The substituent R transfers from carbon atom ${{C}_{2}}$ to carbon atom ${{C}_{3}}$ generally.

Complete answer:
The rearrangement is intramolecular, with structural isomers for the starting molecule and reaction result. The 1,2-rearrangement is a type of chemical reaction known as a rearrangement reaction. A 1,2-hydride shift is a rearrangement that involves a hydrogen atom. If the rearranged substituent is an alkyl group, the shift is called after the anion of the alkyl group: 1,2-methanide shift, 1,2-ethanide shift, and so on.
The creation of a reactive intermediate, such as, is frequently used to initiate a 1,2-rearrangement.
a carbocation formed by heterolysis in a nucleophilic or anionotropic rearrangement
In an electrophilic or cationotropic rearrangement, a carbanion is used.
Homolysis produces a free radical.
a kind of nitrene
The creation of a more stable intermediate is the driving factor for the actual migration of a substituent in step two of the rearrangement. Because a tertiary carbocation is more stable than a secondary carbocation, the SN1 reaction between neopentyl bromide and ethanol produces tert-pentyl ethyl ether.
Carbocation rearrangements occur more frequently than carbanion or radical rearrangements. The rule of Hückel can be used to explain this finding. Because it retains two electrons, a cyclic carbocation transition state is aromatic and stable. An anionic transition state, on the other hand, has four electrons, making it antiaromatic and unstable.

Note:
There is no way to stabilise or destabilise a radical transition state. The Wagner–Meerwein rearrangement is the most important carbocation 1,2-shift. The benzilic acid rearrangement involves a carbanionic 1,2-shift.