Final answer:
A tertiary carbocation is more reactive than secondary and primary carbocations due to increased resonance stabilization and greater steric hindrance.
Step-by-step explanation:
A tertiary carbocation is more reactive than secondary and primary carbocations due to increased resonance stabilization. The delocalization of electrons in the tertiary carbocation results in more bonding and lowers the potential energy, making it more stable. Additionally, tertiary carbocations also have greater steric hindrance, which makes them more reactive compared to primary and secondary carbocations.
For example, in the case of alkyl halide substitutions and allylic phosphates, the stabilization through inductive effect by vicinal carbons enhances the formation of tertiary carbocations. This type of mechanism is typical of tertiary alkyl halide substitutions and allylic phosphates.
Therefore, the reactivity of carbocations increases in the order of primary < secondary < tertiary due to a combination of factors including resonance stabilization and steric hindrance.