Question

A carbocation is a group of atoms with a positively charged carbon atom having six electrons in the valence shell after sharing. Carbocations are formed in the heterolysis of a bond and are planar species. Stability of carbocation is determined by inductive effect, hyperconjugation and resonance effect. Greater the number of contributing structures more is the stability of a Carbocation. Electron releasing groups (+I effect) increases the stability of a carbocation whereas the electron withdrawing groups (I effect) have an opposite effect.

Which of the following is most stable carbocation?

• A. $\stackrel{+}{C}{H}_3$
• B. $C{H}_3\stackrel{+}{C}HC{H}_3$
• C. $C{H}_3\stackrel{+}{C}{H}_2$
• D. $C{H}_3-\stackrel{+}{\underset{\underset{C{H}_3}{|}}{C}}-C{H}_3$

Answer 1

The correct option is D $C{H}_3-\stackrel{+}{\underset{\underset{C{H}_3}{|}}{C}}-C{H}_3$

The stability order of carbocations is $M{e}_3{C}^{+}>M{e}_2{C}^{+}H>Me{C}^{+}{H}_2>M{e}^{+}$.

This is due to hyperconjugation or no bond resonance of adjacent hydrogens with the positive charge. Tertiary carbocation has 3 methyl groups with 9 hyperconjugative hydrogens which makes it most stable.