We know that methyl group is electron releasing i.e. has +I inductive effect. The alkyl group attached to positively charged carbon atom tends to release electrons towards carbon. This decreases the positive charge on the carbon atom but the carbon itself is somewhat positive. As a result, the positive charge on the carbon atom gets dispersed, giving it 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:
Thus, the order of stability is: CH3+ < 1o < 2o < 3o
Other carbocations are benzyl carbocation, which is primary in nature but is highly stable because of resonance stabilization.
Triphenyl carbocation, Ph3C+ is the most stable because of resonance stabilization among three phenyl groups.
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B.Tech, IIT Delhi