List the following carbocations in decreasing order of their stability.

A carbocation is an intermediate that has a positive charge because it is an electron deficient species. It will be destabilized by an electron withdrawing group and will be stabilized by an electron releasing group. The order of stability of carbocation is as follows: methyl < primary < secondary < tertiary

Carbocation stability is influenced by three factors:

a) Hyperconjugation: Increasing the number of alkyl substituents increases the stability of the carbocation.

b) Inductive Effects: It will be destabilized by an electron withdrawing group and will be stabilized by an electron releasing group.

c) Resonance Effects: Conjugation with a multiple bond or lone pairs of electrons increases the stability of a carbocation.

Keeping in mind all the conditions and factors of the stability of carbocation, every carbocation given in the question can be analysed. Carbocation I is a secondary carbocation, carbocation II is a primary carbocation, carbocation III is a primary carbocation with conjugation along with a double bond, carbocation IV is a tertiary carbocation and carbocation V is a tertiary carbocation with conjugation. Therefore, the order of the decreasing stability of carbocation is V>IV>I>III>II.