Question

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

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

Answer 1

Answer: (B)

1. ${\left(C{H}_3\right)}_3{C}^{-}$ is the least stable as it supply more electrons $+I$ to the carbanion.
2. ${\left(C{H}_3\right)}_{2}H{C}^{-}$ is stable than ${\left(C{H}_3\right)}_3{C}^{-}$ 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.