question_answer

Which amongst the following is the most stable carbocation?                   [AIPMT (S) 2005]

A)

B)

C)

D)

Correct Answer: B

Solution :

[b] The most stable carbocation is t-alkyl carbocation because the order of stability of alkyl carbocation is t-alkyl> s-alkyl > p-alkyl >  carbocation. This stability order is described with the help of hyper conjugation and inductive effect. On the Basis of hyper conjugation  shows six resonating structures due to presence of six  C — H bonds,

shows nine resonating structures

due to presence of nine  bonds.

does not show the property of resonance while  shows three resonating structures due to presence of three  bonds. Hence, larger number of resonating structures are possible in (2), so it is most stable. The above order of stability is also explained with the help of (+) effect of -CH3 group. More the number of -CH3 group more will be tendency to displace the electrons towards positive charged carbon of carbocation. Thus (+) charge is decreased or compensated i.e., positive charge is decreased and stability of carbocation is increased.