Carbocations | Stability of Carbocations

What are Carbocations?

Carbocation or Carbonium ions are species with positive charge on carbon atom. In carbocation, carbon is trivalent and sp2 hybridized. Therefore, it has a planar geometry.

As carbon in carbocation has six electrons in its outmost orbital, it is an electron deficient species. Therefore, carbocation acts as elctrophile.
Types of Carbocations 
There are most common types of carbocations 
a. Alkyl carbocations
b. Allyl Carbocation 
c. Benzyl Carbocation 

Alkyl carbocations 

Alkyl carbocation contains alkyl groups attached to the +ve carbon atom. According to the number of alkyl groups present, alkyl carbocations are three types; 
a. Primay carbocation 
b. Secondary carbocation 
c. Tertiary carbocation 

Their structures are shown in Figure.

Stabilty of Alkyl carbocations

Alkyl carbocations are stable except methyl carbocation. The stabilization is due to the two main effects, those are ; 
1. Inductive effect ( I-effect) 
2. Hyperconjugation

Inductive effect ( I-effect) 
Carbocations are stabilized by positive Inductive effect ( +I-effect). This is the electrons pushing effect by alkyl groups towards the +ve centre. When attached alkyl groups are increases, stability increases due to increase of +I effect. As methyl carbocation has no any alkyl group attached, its stability is very low.

Hyperconjugation is the sharing of sigma electrons with orbital containing positive charge or radical. This is a hypothetical π (pi) bond between sigma orbital (from H of C-H bond) and porbital of carbocation 

When number of methyl groups attached to the carbocation increases, number of hyperconjugations increase, hence stability increases. Therefore, tertiary carbocation is the most stable due to more hyperconjugation and higher +I effect.

Allylic Carbocation 

Allylic carbon is the carbon just next to the double -bonded carbon. The cation at the allylic carbon is named as allylic carbocation
Allylic carbocation
Allylic carbocation

Stabilty of allylic carbocation

Allylic carbocation is stable. The stabilty arises due to the delocalization of +ve charge with pi-orbital next to allylic centre. This is called resonance stabilization.

Stabilization of Allylic carbocation by resonance
Stabilization of Allylic carbocation by resonance

Benzylic carbocation 

The carbon directly attached to the benzene carbon is called as benzylic carbon. The cation on the benzylic carbon is called as benzylic cation
Benzylic carbocation
Benzylic carbocation

Stability of Benzylic Carbocation

Benzylic carbocation is stable and stabilized by delocalization of +ve charge with pi-orbitals in benzene ring. This is resonance stabilization.

Stabilization of Benzylic carbocation
Stabilization of Benzylic carbocation

Generating of Carbocations :

There are different ways of generating cabocations using different substrates. 

1. Solvolysis of alkyl halides:

Solvent used for the solvolysis must be polar solvents such as water or alcohol. The solvent play dual role in solvolysis reaction; 

a. Act as solvent, consequently breaking of C-X bond due to polarization 
b. Act as nucleophile to attack on carbocation formed

2. Using Diazonium salts 

  Alkyl Diazonium ions are unstable and decomposed readily to give carbocations.

3. Protonation of alcohols 

Alcohols are easily protonated using dilute acids and carbocations are formed after loosing water molecule.

4. From alkyl halides when the presence of Lewis acid catalysts , eg: AlCl3

5. From acyl chloride when the presence of Lewis acid catalysts

Rearrangement of Carbocations:

Sometime, unexpected products are formed when the reactions proceed through carbocation intermediates. This is due to the rearrangement of first formed carbocation into most stable carbocation and undergoes the second reaction to form the product. Rearrangement is most common in carbocations.

 Example-1 : Pinacole-Pinacolone Rearrnagement

This is due to the rearrangement of carbocation as shown here


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