Read Electron-Rich vs Electron-Poor Before Naming a Species
Electrophiles seek electrons (empty orbital or partial positive charge), nucleophiles donate them (lone pair or π electrons available) — and carbanion stability follows the OPPOSITE trend from carbocation stability, since electron-withdrawing groups now help rather than hurt.
Carbanion Stability is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Identifying electrophiles and nucleophiles from structure
SO3 (sulfur has an empty/accessible orbital and is electron-poor from three attached oxygens) is a classic electrophile; a carbene like :CCl2 (has a lone pair available to donate despite also having an empty orbital) acts as a nucleophile in most reactions via that lone pair — always check whether a lone pair or π system is AVAILABLE to donate (nucleophile) versus whether an empty/accessible orbital seeks electrons (electrophile), rather than judging from overall charge alone.
Carbanion stability rewards electron-withdrawing groups — the reverse of cation stability
A carbanion adjacent to an electron-withdrawing group (like -NO2) is MORE stable, because that group helps disperse the negative charge — this is the exact opposite of carbocation stability, where electron-withdrawing groups destabilise. A simple methyl carbanion is less stable than one stabilised by an adjacent -NO2 group specifically because of this charge-dispersal effect.
Benzylic and allylic carbanions are resonance-stabilised, just like their cation counterparts
A benzyl carbanion is more stable than a simple alkyl carbanion via the same resonance-delocalisation mechanism seen in benzylic cations — ring substituents shift stability the same direction as for cations too (electron-withdrawing para substituents further stabilise a benzylic carbanion, since they help accept the extra electron density).
Vinyl and alkynyl carbanions are destabilised by higher s-character
An sp-hybridised carbanion (like HC≡C-) holds its lone pair in an orbital with more s-character, closer to the nucleus, which is actually MORE stable than an sp3 carbanion's lone pair — this is why acetylide anions form relatively readily (terminal alkynes are more acidic than simple alkanes), the same s-character logic that governs bond lengths elsewhere in GOC.
OlympIQ exam lens
- ✓Nucleophiles have an available lone pair or π-system to donate; electrophiles have an empty or electron-poor orbital seeking electrons.
- ✓Carbanion stability is HELPED by electron-withdrawing groups — the opposite of carbocation stability.
- ✓Benzylic and allylic carbanions are resonance-stabilised, just like the corresponding cations, and ring substituents shift stability the same direction.
- ✓Higher s-character (sp > sp2 > sp3) stabilises a carbanion's lone pair, which is why terminal alkynes are relatively acidic.
Don't apply the carbocation stability rules to carbanions by habit — electron-withdrawing groups stabilise carbanions but destabilise carbocations; the two systems respond to substituents in opposite directions.
Ready to try the pattern?
100% coverage across the 14 linked questions.
Practise 14 linked questions