Question

Arrange the carbanions $(C{H}_3{)}_3{C}^{-},C{l}_3{C}^{-},(C{H}_3{)}_{2}H{C}^{-},C_6{H}_5{H}_2{C}^{-}$ in order of their decreasing stability :

• A. $(C{H}_3{)}_{2}H{C}^{-}>C{l}_3{C}^{-}>C_6{H}_5{H}_2{C}^{-}>(C{H}_3{)}_3{C}^{-}$
• B. $C{l}_3{C}^{-}>C_6{H}_5{H}_2{C}^{-}>(C{H}_3{)}_{2}H{C}^{-}>(C{H}_3{)}_3{C}^{-}$
• C. $(C{H}_3{)}_3{C}^{-}>(C{H}_3{)}_{2}H{C}^{-}>C_6{H}_5{H}_2{C}^{-}>C{l}_3{C}^{-}$
• D. $C_6{H}_5{H}_2{C}^{-}>C{l}_3{C}^{-}>(C{H}_3{)}_3{C}^{-}>(C{H}_3{)}_{2}H{C}^{-}$

Answer 1

The correct option is B $C{l}_3{C}^{-}>C_6{H}_5{H}_2{C}^{-}>(C{H}_3{)}_{2}H{C}^{-}>(C{H}_3{)}_3{C}^{-}$

1. $(C{H}_3{)}_3{C}^{-}$ is the least stable as it supply more electrons $+I$ to the carbanion.

2. $(C{H}_3{)}_{2}H{C}^{-}$ is stable than $(C{H}_3{)}_3{C}^{-}$ as it has less $+I$ effect compared to the other. But still it is less stable than the others, the reason being the same as above.

3.$C_6{H}_5{H}_2{C}^{-}$ is a resonating structure. Normally resonance effect is more compared to other effects. But since $C{l}_3{C}^{-}$ has $3Cl$ group $-I$ effect, inductive effect overruns resonance effect.