Carbanions are best described as?

• A. Neutral carbon
• B. Negatively charged species with a negative formal charge on carbon
• C. Positively charged carbon
• D. Carbon with octet expansion

Answer: (B)

At the heart of a carbanion is a negatively charged species with the charge located on carbon. This happens when carbon has one extra electron beyond its neutral count. For example, in the methyl anion CH3−, carbon forms three C–H bonds and has a lone pair. Using formal charge counting, carbon’s valence electrons (4) minus the nonbonding electrons (2) minus half the bonding electrons (6/2 = 3) gives a formal charge of −1 on carbon. That negative charge on carbon is what defines a carbanion. This rules out a neutral carbon or a positively charged carbon, which describe different species (neutral carbon has no charge, and a carbocation would be positively charged). It also avoids octet expansion, since carbon in carbanions typically satisfies the octet with a lone pair rather than expanding to more than eight electrons.