Q.Arrange the methyl carbanion and the primary, secondary and tertiary carbanions in decreasing order of stability, and explain why this order is the reverse of the order for carbocations.

Carbanion: The Carbon That Wants to Give

Imagine a carbon atom that has grown tired of sharing. Normally, carbon forms four bonds, each electron pair shared with a neighbour. But sometimes, carbon ends up with three bonds and an extra electron pair all to itself — a lone pair. That carbon now carries a full negative charge. That is a carbanion.

The intuition is simple: a carbanion is a carbon atom that has gained an electron (or lost a proton) and now has a surplus of negative charge. It is electron-rich, nucleophilic, and eager to donate that lone pair to something electron-deficient. Think of it as a carbon with a "gift" it wants to give away.

The Precise Definition

A carbanion is a reactive intermediate in which a carbon atom bears a formal negative charge and possesses an unshared pair of electrons (a lone pair). The carbon is sp³ hybridised (or nearly so), with the lone pair occupying one of the four hybrid orbitals. The three substituents and the lone pair arrange themselves in a trigonal pyramidal geometry — exactly like ammonia.

The negative charge makes the carbon a strong base and a strong nucleophile. It will readily attack any electrophile — a proton, a carbonyl carbon, an alkyl halide — to form a new bond.

Stability: What Makes a Carbanion Happy or Miserable

A carbanion is not a happy species. It carries a full negative charge on carbon, which is not particularly electronegative. So stability depends heavily on what is attached to that carbon.

Electron-withdrawing groups stabilise a carbanion. They pull electron density away from the negative charge, spreading it out and reducing the energy. Common stabilising groups include:

• Nitro ($-NO_2$)
• Cyano ($-CN$)
• Carbonyl ($-C=O$)
• Sulfonyl ($-SO_2R$)
• Halogens (weakly, by induction)

The more such groups, the more stable the carbanion. A triphenylmethyl carbanion ($Ph_3C^-$) is famously stable because the three phenyl rings delocalise the negative charge through resonance.

Resonance is the most powerful stabiliser. If the lone pair can be delocalised into a neighbouring $\pi$ system, the charge spreads over several atoms. For example, the enolate ion (from a carbonyl compound) is a resonance-stabilised carbanion:

The negative charge is shared between carbon and oxygen — much more stable than a bare carbanion.

Inductive effects also matter. Alkyl groups are electron-donating, so they destabilise a carbanion. A methyl carbanion ($CH_3^-$) is less stable than a primary carbanion ($RCH_2^-$), which is less stable than a secondary ($R_2CH^-$), which is less stable than a tertiary ($R_3C^-$). Wait — that is the opposite of carbocations. For carbanions, more alkyl substitution means less stability because alkyl groups push electron density onto an already negative carbon.

Hybridisation and Geometry

The carbanion carbon is sp³ hybridised. The three substituents occupy three of the four hybrid orbitals; the lone pair sits in the fourth. This gives a trigonal pyramidal shape, with bond angles close to 109.5°.

However, if the carbanion is conjugated with a $\pi$ system (like in an allyl or benzyl carbanion), the carbon may adopt sp² hybridisation to allow the lone pair to participate in resonance. In that case, the geometry becomes planar, and the lone pair occupies a p orbital.

Formation: How Carbanions Are Born

Carbanions are usually generated in situ — they are too reactive to isolate (except for a few exceptionally stable ones). Common methods:

1. Deprotonation of a C–H acid: A strong base abstracts a proton from a carbon that is slightly acidic (e.g., $\alpha$-hydrogen next to a carbonyl). …