Hydride Shift

What is Hydride Shift?

Hydride shift is a rearrangement of a hydrogen atom in a carbocation to make the intermediate structure more stable. It is generally observed in rearrangement reactions. After shifting a hydrogen atom from one carbon to another, structural isomers of compounds are formed. This rearrangement happens to make carbocation more stable.

Table of Content

• Rearrangement Reaction
• Reaction Mechanism
• Wagner-Meerwein Rearrangement
• Conditions of Hydride Shift
• 1,2 Hydride Shift
• 1,3 Hydride Shift
• Frequently Asked Questions – FAQs

Introduction

Hydride shift is a rearrangement of a hydrogen atom in a carbocation to make the intermediate structure more stable. It is generally observed in rearrangement reactions. After shifting a hydrogen atom from one carbon to another, structural isomers of compounds are formed. This rearrangement happens to make carbocation more stable.

When the less stable carbocation is transformed into more stable carbocation, the C-H bond in the intermediate states is rearranged to proceed with the reaction.

What is Rearrangement Reaction?

A rearrangement reaction is a type of intermolecular substitution reaction in which the carbon skeleton of a molecule is rearranged to form a structural isomer of the original molecule.

The substituent moves from one atom to another atom within the same molecule.

Based on atom or group rearranging, rearrangement reactions are of two types.

1. Hydride Shift
2. Alkyl Shift

Based on the site of rearrangement, rearrangement reactions are of two types.

1. 1,2 Rearrangement Reaction
2. 1,3 Rearrangement Reaction

Reaction Mechanism

• Hydride shift is often initialised by forming a reactive intermediate carbocation.
• The driving force for the migration of hydrogen atoms in step two of arrangement is the formation of a more stable intermediate carbocation.
• A more substituted carbocation is found to be more stable.
• A tertiary carbocation is more stable than secondary carbocation, followed by primary carbocation.
• Thus, a rearrangement takes place to stabilise the intermediate transition state.
• The most crucial carbocation 1,2 hydride shift is the Wagner-Meerwein rearrangement.

What is Wagner-Meerwein Rearrangement?

Wagner- Meerwein rearrangement is a type of 1,2- rearrangement reaction in which hydrogen, an alkyl or an aryl migrates from one carbon to the adjacent carbon.

It is a cationic [1,2]-sigmatropic rearrangement.

• Rearrangement occurs because a tertiary carbocation is more stable than secondary carbocation, followed by primary carbocation.

This reaction is named after a Russian chemist Yegor Yegorovich Vagner. He was born in Germany and has also published German journals as Georg Wagner and Hans Meerwein.

Conditions of Hydride Shift

A hydride shift will occur whenever we have a more substituted carbon around a cationic carbon. It would shift the positive charge over to carbon, stabilising it better, thus stabilising the intermediate state.

1,2 Hydride Shift

• Heinrich Otto Wieland reported the first 1,2 hydride shift in 1911.
• 1,2 Hydride Shift is a rearrangement reaction in which hydrogen moves from one carbon atom to another carbon in a chemical compound.
• In a 1,2 hydride shift, the movement involves two adjacent atoms, but movement over more considerable distances is also possible.

1,3 Hydride Shift

• 1,3 Hydride Shift is a rearrangement reaction in which hydrogen moves from one carbon atom to the third carbon atom in a chemical compound.