Carbocation:
Carbocation is a group of atoms which contain positively charged carbon having only six electrons and is obtained by heterolytic fission of a covalent bond involving carbon atom.
The carbocations are classified as primary (1°), secondary (2°) or tertiary (3°) depending upon whether one, two or three alkyl groups are attached to carbon bearing the positive charge as:
Relative stability of carbocation:
We know that alkyl group has + I inductive effect i.e. electron releasing. The alkyl group attached to +vely charged carbon atom tends to release electrons towards carbon. As a result, it decreases the +ve charge on the carbon atom but itself becomes somewhat positive. As a result, the positive charge on the carbon atom gets dispersed. The dispersal of charge results into stability. Therefore, more the number of alkyl groups, the greater will be the dispersal of charge and therefore, more stable will be the carbocation.
Thus, tertiary carbocation in which there are three alkyl groups attached to positive carbon is more stable than a secondary carbocation with two alkyl groups. Similarly, a secondary carbocation is more stable than a primary carbocation which in turn is more stable than methyl carbocation.