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. $\left( CH _{3}\right)_{2} HC ^{-}> Cl _{3} C ^{-}> C _{6} H _{5} H _{2} C ^{-}>\left( CH _{3}\right)_{3} C ^{-}$
• B. $Cl _{3} C ^{-}> C _{6} H _{5} H _{2} C ^{-}>\left( CH _{3}\right)_{2} HC ^{-}>\left( CH _{3}\right)_{3} C ^{-}$
• C. $\left( CH _{3}\right)_{3} C ^{-}>\left( CH _{3}\right)_{2} HC ^{-}> C _{6} H _{5} H _{2} C ^{-}> Cl _{3} C ^{-}$
• D. $C _{6} H _{5} H _{2} C ^{-}> Cl _{3} C ^{-}>\left( CH _{3}\right)_{3} C ^{-}>\left( CH _{3}\right)_{2} HC ^{-}$

Answer 1

Answer: (B)

1. $\left( CH _{3}\right)_{3} C ^{-}$ is the least stable as it supply more electrons $+ I$ to the carbanion.
2. $\left( CH _{3}\right)_{2} H C ^{-}$ is stable than $\left( CH _{3}\right)_{3} C ^{-}$ 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 $Cl _{3} C ^{-}$ has $3 Cl$ group - I effect, inductive effect overruns resonance effect.