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Carbanions are organic ions with a negative charge on a carbon atom. They are intermediates in many chemical reactions, particularly those involving the addition of nucleophiles to carbon-carbon double or triple bonds. The carbon atom in a carbanion is sp2 or sp hybridized, and it has a lone pair of electrons, which makes it a good nucleophile. Carbanions are often stabilized by resonance, inductive effects, or hyperconjugation. For example, in the conjugate base of acetone, the negative charge is delocalized over three carbon atoms, making it a stable carbanion. Carbanions can also be stabilized by adjacent electron-withdrawing groups, such as in the conjugate base of acetic acid, where the carbonyl group withdraws electron density from the carbon atom bearing the negative charge. Carbanions are important in organic synthesis, as they can be used to form new carbon-carbon bonds or to activate substrates for further reactions. However, they are often highly reactive and can undergo side reactions, so their use requires careful control of reaction conditions.