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Q.
Which of the following is the most stable carbocation?
CMC MedicalCMC Medical 2009
Solution:
As the number of electron releasing groups ( $ +I $ showing groups such as $ -C{{H}_{3}}, $ $ -{{C}_{2}}{{H}_{5}} $ etc), attached with positively charged carbon, increases, stability of carbocations increases. Further, more the number of alkyl groups attached with $ {{C}^{+}}, $ more will be the hyper conjugative forms and higher will be the stability. Benzyl carbocation being resonance stabilised, is almost as stable as secondary alkyl carbocation. Therefore, the order of stability of given carbocations is as $ \underset{\begin{smallmatrix} 9\,hyperconjugative \\ \,\,\,\,\,\,\,\,\,\,structures \end{smallmatrix}}{\mathop{{{(C{{H}_{3}})}_{3}}{{C}^{+}}}}\,>\underset{\begin{smallmatrix} resonance \\ stabilised \end{smallmatrix}}{\mathop{{{C}_{6}}{{H}_{5}}CH_{2}^{+}}}\,>\underset{\begin{smallmatrix} 6\,hyperconjugative \\ \,\,\,\,\,\,\,\,\,\,structures \end{smallmatrix}}{\mathop{{{(C{{H}_{3}})}_{2}}\overset{+}{\mathop{CH}}\,}}\, $ $ >\underset{\begin{smallmatrix} 3\,\,hyperconjugative \\ \,\,\,\,\,\,\,\,\,\,structures \end{smallmatrix}}{\mathop{C{{H}_{3}}\overset{+}{\mathop{C{{H}_{2}}}}\,}}\, $ Hence, $ {{(C{{H}_{3}})}_{3}}{{C}^{+}} $ is the most stable carbocation among the given.