Question

The carbanion which is most stable is ?
(A) \[HC \equiv {C^ - }\]
(B) \[C{H_2} = CH - C{H_2}^ - \]
(C) \[{C_6}{H_5} - C{H_2}^ - \]
(D) \[^ - C - {(C{H_3})_3}\]

Answer 1

Hint: Strength of the conjugate acid decides the stability of a carbanion. The conjugate base which is weak results in higher stability of carbanion. Other factors are aromaticity, resonance and inductive effect.

Complete step-by-step solution:
Aromatic carbanions are considered to be most stable. Although Benzyl carbanion (\[{C_6}{H_5} - C{H_2}^ - \]) is resonance stabilized, yet in three out of five resonating structures, aromatic stabilisation of the ring is lost.
Another factor that can determine the stability is:
\[ - I{\text{ }}and{\text{ }} - M\]groups are used to define the stability of carbanions. Those carbanions which are stabilised by conjugated highly electronegative atoms are less stable than the aromatic ones. Like option (B).
Stability order decreases as we approach tertiary anion from primary one. This is because of the inductive effect of methyl groups which increases the intensity of negative charge on central carbon in tertiary carbanion. This makes it less stable as in option (D).
Therefore, the carbanion bonded to triple bond is most stable here because it has 50% s-character being sp hybridised.

Hence, the correct option is (A).

Note: Electronegativity and hybridisation are the two important factors along with these to find the most stable carbanion. The more polarizable the atom, the more stable the anion. Carbanions prefer a lesser degree of alkyl group substitution.