Carbanion Stability

Carbanion stability increases when the lone pair carrying the negative charge can be delocalised or withdrawn by adjacent electron-attracting groups, and it decreases when electron-donating groups intensify the charge. A carbanion alpha to a nitro group is exceptionally stable because the nitro group withdraws the negative charge by both a strong -I effect and powerful -M resonance, spreading it onto the nitro oxygens and forming a delocalised, low-energy anion. The carbanion alpha to a methyl group is destabilised because the electron-donating methyl group pushes electron density toward the already negative carbon. The tertiary butyl carbanion is the least stable of all, since three electron-donating alkyl groups maximally intensify the negative charge. An ethyl carbanion stabilised only inductively lacks any resonance delocalisation and cannot compete with the nitro-stabilised species. Because resonance withdrawal by nitro is the strongest available stabilising influence, that carbanion is the most stable, consistent with NCERT reasoning on intermediates. This same principle explains why protons alpha to nitro, carbonyl, or cyano groups are unusually acidic, since removing them generates resonance-stabilised carbanions. The stability order here is essentially the reverse of the carbocation order, because what stabilises a positive centre destabilises a negative one. Plausibility check: the group that best removes negative charge gives the most stable anion, and nitro is the strongest -I, -M group here, confirming the choice.