If there is any possibility of the formation of more than one carbocation. Then the less stable carbocation is transformed into the more stable one. This is known as carbocation rearrangement. In this rearrangement, mostly secondary carbocation is rearranged into tertiary carbocation. Hence, benzylic or allylic allows the formation of a primary carbocation. This is because of resonance stability.
Types of carbocation rearrangement
There are three types of carbocation rearrangement Hydride shift, methyl shift, and phenyl shift.
“The carbocation rearrangement involves the migration of hydrogen is known as hydride shift.”
When a secondary carbocation is adjacent to the tertiary carbon-bearing hydrogen. Hence, a 1,2 hydride shift will occur.
Mechanism of 1,2 hydride shift
In the 1,2 hydride shift, the arrow starts from a tertiary carbon. This shows the movement of hydrogen to the secondary carbocation. It results in the formation of a more stable carbocation.
There is no possibility for the 1,3 or 1,4 hydride shift.
“A rearrangement in which alkyl group is shifted from less stable carbocation to more stable is known as alkyl shift.”
Mechanism of 1,2 methyl shift
The mechanism of the alkyl shift is similar to the hydride shift.
“The conversion of less stable carbocation to the more stable one by transfer of phenyl group is known as phenyl shift.”
Mechanism of 1,2 phenyl shift
Migratory aptitude of groups in carbocation rearrangement
“The ability of the migratory group to migrate in a rearrangement reaction is known as migratory aptitude.”
If more than one type of group is present on the tertiary carbon adjacent to the secondary carbocation. Hence, migratory aptitude depends on the electron density or electron-donating ability of the migratory group. The order of migratory aptitude is the following:
Carbocation rearrangement in SN1 reaction
Some evidence for carbocation rearrangement comes from nucleophilic substitution reaction. For example, hydrolysis of secondary alkyl bromide produces the rearranged tertiary alcohol product.
The mechanism of this reaction involves the hydride shift. Therefore, secondary carbocation is converted into more stable tertiary carbocation. However, this type of rearrangement is not observed in the SN2 reaction of alkyl halide.
Carbocation rearrangement in alcohol dehydration
Some primary or secondary alcohol also undergo carbonium ion rearrangement.
In the above example, the secondary carbonium ion is converted into a more stable tertiary carbonium ion. Hence, 1,2 methyl shifts occur.
The reaction of hydrogen chloride with 3-Methyl-1-butene is expected to yield 2-Chloro-3-methyl butane. Instead, a mixture of two alkenes is formed. This is because of the hydride shift of secondary carbonium into tertiary.
What is carbocation explained?
A molecule that has a positive charge on the carbon atom is known as carbocation. It is also known as carbonium ion.
How carbocation is formed?
When one of the bonds of a carbon atom is broken. It results in the incomplete valence shell of carbon. Hence, it carries a +1 charge.
How does carbocation rearrangement reaction occur?
The rearrangement is occur by shifting the hydrogen and methyl groups. This shifting occurs from secondary carbocation to a more stable one.
Which carbocation is more stable?
The carbocation which contains more alkyl groups is more stable. The alkyl group stabilizes it by donating the electrons.
What is the migratory order in carbocation rearrangements?
The migratory order depends on the electron-donating ability of the migratory group. The migratory order is as follows:
Tert alkyl > sec alkyl ~ benzyl ~ phenyl l> pri alkyl > methyl
Why is an unsaturated carbocation rearrangement not possible?
The unsaturated carbocation rearrangement is not possible. This is because unsaturated carbocation is highly unstable. It may decay in a very short time.
- Organic Chemistry tenth edition by Francis A, Carey(University of Virginia) and Robert M.Giuliano (Villanova University)
- Essentials of Organic Chemistry by Paul M Dewick (University of Nottingham, Uk)
- Rearrangements reactions of alkyl carbocation (byjus.com)