Allylic carbocation are considered to be more stable than substituted alkyl carbocations because delocalisation is associated with the resonance interaction between the positively charged carbon and the adjacent pie (π) bond.
The allyl cation can be represented as a hybrid of two equivalent contributing structures.
A distributed charge in a molecule is more stabilizing than a more localized charge and it is also experimentally determined that the double bond of an adjacent vinyl group provides approximately as much stabilization as two alkyl groups and hence, the allyl cation are comparably more stable.
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