Question

Which amongst the following is the most stable carbocation?

• A.

  

• B.

  

• C. $^{CH_3}_{+}$
• D. $CH_3 \, \, _{CH_3}^{+}$

Answer 1

The Correct Option is B

The most stable carbocation is t-alkyl carbocation because the order of stability of alkyl carbocation is t-alkyl >s-alkyl> p-alkyl *> CH3 carbocation. This stability order is described with the help of hyperconjugation and inductive effect. On the basis of hyperconjugation, (CH$_3)_2 \, \, ^{+}_{CH}$ CH shows six resonating structures due to the presence of six a-C - H bonds, Greater the $\alpha$ H-atom greater will be the hyper conjugation resonating structure and therefore, greater will be the stability. $^{+}_{CH_3}$does not show the property of resonance while $CH_3 - \, ^{+}{CH}$shows three resonating structures due to presence of three a-C - H bonds. Hence, larger number of resonating structures are possible in (b), so it is most stable. The above order of stability is also explained with the help of (+) /-effect of -CH$_3$ group. More the number of - CH$_3$ group more will be tendency to displace the electrons towards positive charged carbon of carbocation. Thus, positive charge is decreased or compensated and stability of carbocation is increased.