Question

Most stable carbanion is:
(A) $HC\equiv {{C}^{-}}$
(B) ${{C}_{6}}H_{5}^{-}$
(C) ${{(C{{H}_{3}})}_{3}}CCH_{2}^{-}$
(D) ${{(C{{H}_{3}})}_{3}}CC{{H}^{-}}$

Answer 1

Hint: Recollect what a carbanion is. We know anion is an ionic species which is electron-rich and has a negative charge. Think how is the negative charge on a carbon atom stabilized in organic compounds. Think about the electronic effects which are present in organic compounds. Look at each of the given options and then choose the most stable carbanion.

Complete step by step solution:
- Carbanion is a negatively charged electron-rich species in which the carbon atom is trivalent and has an unshared pair of electrons.
- Carbon in carbanion is $s{{p}^{3}}$ hybridized. It has a trigonal pyramidal geometry due to the presence of paired electrons in one of the hybrid orbitals inducing lone-pair-bond-pair repulsion and distorting trigonal planar geometry.
- Order of stability of carbanion is:
Methyl Carbanion > Primary Carbanion > Secondary Carbanion > Tertiary Carbanion
- Let’s now take a look at each of the options.
(A) $HC\equiv {{C}^{-}}$ - Here, acetylene has lost a proton to form acetylene carbanion. Here the carbon is sp hybridized and therefore, it has linear geometry. We know that acetylene is highly acidic in nature and easily donate protons. The carbon atom in acetylene has 50% s-character and 50% p-character. More the s-character, more stable is the molecule/ion. Therefore, this carbanion is highly stable.
(B) ${{C}_{6}}H_{5}^{-}$ - This is a phenyl carbanion. In phenyl carbanion, benzene has a negative charge on one of the carbon atoms. Here, due to the presence of negative charge on one the carbon atoms in the ring, the resonance is destroyed. Therefore, this is less stable.
(C) ${{(C{{H}_{3}})}_{3}}CCH_{2}^{-}$- This carbanion is a primary carbanion which is bonded to a tertiary carbon atom or t-butyl group. Here the carbon atom in carbanion has 25% s-character because it is $s{{p}^{3}}$ hybridized. But it has the presence of a bulky group on adjacent carbon atoms and so, compared to option A it is less stable.
(D) ${{(C{{H}_{3}})}_{3}}CC{{H}^{-}}$- This carbanion is doubly bonded ad has 33.33% s-character. Here, also due to lower s-character than option A and presence of adjacent bulky groups, it is less stable than option A.
- Therefore, the most stable carbanion is $HC\equiv {{C}^{-}}$.

- Therefore, the answer is option A.

Note: Remember primary carbanion and methyl carbanion are the most stable carbanions. If electron-withdrawing groups are present in the molecule then, carbanion stability increases due to more stabilization of negative charge in the molecule. The presence of an electron-donating group reduces stability of the carbanion.