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A carbanion is an anion, a negatively charged molecule, that includes a carbon atom with a negative charge. The carbon atom's charge is caused by a lone pair of electrons that are unshared, as opposed to its three other pairs that are bonded with other atoms. Carbanions are known as reactive intermediates due to their instability and tendency to form stable compounds rather than remaining in original form. They are studied as part of organic chemistry.
The negative charge of the carbanion causes its to react with other compounds as a nucleophile, meaning it readily donates its electrons. It bonds to another atom by donating both electrons — in this case, the lone pair. This action defines the carbanion as a Lewis base, as opposed to a Lewis acid, which would accept a lone pair of electrons in a chemical bond.
Structurally, carbanions have a tetrahedral molecular geometry if the lone pair is counted. This means that carbon, the central atom, is symmetrically surrounded by the electron orbitals in the shape of a tetrahedron. If only the three bonds are included in the geometry, the molecule is trigonal pyramidal, with the carbon atom at the apex of the pyramid, the three bonded atoms forming the base, and the lone pair floating on top.
Depending on the substituents that are bonded to the carbon atom, the carbanion structure can invert, flipping the molecule. Usually the energy barrier that must be exceeded for carbanion inversion to take place is fairly low, and the reaction easily proceeds. Various factors can raise this energy barrier. For example, the ring structure of cyclopropyl makes inversion difficult, and carbanions in such structures behave more stably than they would in a more open system.
Electronegativity also plays a role in stabilizing carbanions. An atom or group that is electronegative tends to attract electrons. When a carbanion is surrounded by electronegative atoms, its lone pair is attracted to them and it becomes stabilized. This transmission of charge is more broadly known as the inductive effect.
Carbanions are important in organic chemistry and are seen as intermediates in many reactions. They are involved in the formation of Grignard reagents, which are organic derivatives of magnesium that behave as carbanions. The Grignard reaction is an example of organometallic chemistry, the study of compounds in which a metal is bonded to a carbon. In these reactions, the nucleophilic properties of carbanions are used to attack and modify other compounds to create chemical products.