Rearrangement Reactions Of Alkyl Carbocation

Carbocation rearrangements are prevalent in organic chemistry and are described as the transition of a carbocation from an unstable to a more stable state via different structural reorganizational “shifts” within the molecule.

When an alkyl halide, alcohol, or alkene is converted into a carbocation, it may undergo rearrangement. Carbocation rearrangements are classified into two types: hydride shifts and alkyl shifts. When the ensuing carbocation is rearranged, it will react further to generate a final product with a different alkyl skeleton than the initial material.

Reactions observed in Carbocations

• Carbocation reacts with nucleophilic compounds because they have a tendency to extract electrons to produce alcohol.
• Protons from carbocation are removed in this procedure to form alkene molecules.
• Carbocation undergoes a rearrangement reaction to generate more stable carbocation in this process.

Also Check: Bohr's Theory of Hydrogen Atoms 

The alkyl shift

• Not all carbocations contain acceptable hydrogen atoms (secondary or tertiary) on nearby carbon atoms that can be rearranged.
• In this situation, the reaction might undergo an alkyl shift, which is a distinct way of rearrangement (or alkyl group migration).
• The alkyl shift works similarly to the hydride shift.
• The electron pair carried by the moving alkyl group forms a bond with the surrounding carbocation.
• The moving alkyl group and the positive charge of the carbocation cause the molecule’s location to change.
• Tertiary carbocations are substantially more stable than primary or secondary carbocations, hence an alkyl shift on a primary or secondary carbocation to generate a tertiary carbocation occurs often.

Alkyl Shift Rearrangement Reaction Mechanism

• During the first phase of the rearrangement process, electrons align themselves with the C—-C bond. When a pair of electrons is given into the vacant p-orbitals, one C—-C bond begins to dissolve, forming a new C—-C bond.
• The pair of electrons from the C-C bond must align with the carbocation’s empty p orbital ( this means they have to be aligned in the same plane). Then, when the pair of electrons from the C–C bond is given into the vacant p-orbital, one C–C bond breaks, and a new C–C bond forms.
• In the transition state, partial bonds exist between the carbon being transferred and each of the two nearby carbon atoms. A tertiary carbocation is produced as an end product in the final phase of the reaction.

Any reaction that involves a carbocation intermediate is susceptible to rearrangement. During some electrophilic additions, carbocation rearrangements may occur.