Carbanions: Explanation, Occurrence, Properties and Examples

Carbanions are members of the group of organic compounds in which a negative electrical charge is primarily located on the carbon. So basically, a carbanion is an anion with carbon that is trivalent and carries a formal negative charge on it. They are produced as the intermediate of the chemical reactions involving the removal of positively charged atoms and groups. These carbanions are used in various industrial procedures including the production of plastics.

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Carbanions: Explained

As it is mentioned that carbanion is a negatively charged ion where the carbon atom exhibits trivalence and the negative charge is predominantly on the carbon atom (in at least one resonance form). Here, it is important to mention that all carbanions are the conjugate bases of some carbon acid.

R₃CH + :B⁻ → R₃C:⁻ + HB (Here, B stands for a base)

The formation of carbanions takes place by deprotonation of alkanes (at sp³ carbon), alkenes (at sp² carbon), arenes (at sp² carbon), and alkynes (at sp carbon) are known as alkyl, alkenyl (vinyl), aryl, and alkynyl (acetylide) anions, respectively.

Carbanions have 8 electrons in its valence cell. So, it is not electron deficient. As there is a high electron density on the carbon atom it becomes an ideal target of the electrophiles and electron-deficient species of various strength including carbonyl groups, halogenating reagents, imines/iminium salts and other proton donors.

Formation of Carbanion

Carbanions Molecular Structure

While discussing the molecular structure we have to distinguish between the localized ions and delocalized ions as their structure will be significantly different.

Localized ions- Here, the negatively charged electrons are largely concentrated around one carbon atom. As a result, the localized carbanion atoms assume trigonal pyramidal, bent and linear geometries.

For example, the localized carbanion Methide ion (CH₃^-) assumes a geometry of a pyramid with a carbon atom at the apex. The structure is similar to Ammonia molecular structure.

(solid lines = bonds between the atoms; Dotted lines= base of the pyramid)

Delocalized ions- Delocalized ions are those in which the negatively charged electrons are distributed over several atoms. the carbanionic lone pair may occupy a p orbital. A p orbital has a more suitable shape and orientation to overlap with the neighbouring π system, resulting in more effective charge delocalization. Thus, the molecular structure is rather a planner than a pyramidal configuration.

For example, in the case of the Benzylic anion, the negative charge is distributed over an extended pi-bond system which includes an aromatic ring.

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Occurrence and Properties of Carbanions

Carbanions typically behave as nucleophiles and are basic in nature. However, the degree of the basicity or nucleophilicity is largely determined by the substituents of carbon atoms. These include-

Inductive Effect

In this case, the transmission of a bond is unequally shared which means when a highly electronegative substituent group is attached to the carbanion it helps to subdue the negative charge on it resulting in a more stable molecule. On the other hand, in presence of a highly electropositive substituent group increases the negative charge on the carbanion, therefore decreasing the overall stability of the molecule.

Resonance Effect

Through the delocalization of electrons, the negative charge is distributed all over the carbanion augmenting the stability of the molecule. Aromatic groups add stability to the carbanions through this resonance effect.]

Conjugation of Carbanions

Carbanions are detected in the solution phase by proton NMR spectroscopy. Carbanions are present in the condensed phase only if the carbanions are stabilized by electron delocalization.

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Preparation of Carbanions

Any preparation containing organic-alkali-metal is a source of carbanions. The reactions of organic substances containing bromine, chlorine, iodine atoms with alkali metals are commonly used methods of generation of carbanions.

This reaction can be expressed as-

[ R= organic group, X= halide atoms, M= atom of alkali metal]

The conversion of one carbanion atom into another carbanion can be achieved with hydrocarbons or organic halides. The conversion reaction is given below-

Carbon Acids

In principle, any compound having the ability to undergo deprotonation is considered acid. The compound which is formed after deprotonation is called the conjugate base of the acid. When the deprotonation causes a loss of positively charged hydrogen ions attached to the carbon atom, it is referred to as a carbon acid. So, the deprotonated carbon acid will have a negative charge (due to retention of the bond pair of electrons from the carbon-hydrogen bond) and can be considered as a carbanion.

Carbon acids are extremely weak acids. They are much weaker than carboxylic acids and other strong acids like sulphuric and hydrochloric acid. The acidity of the carbon acid increases when the negative charge on the corresponding negative base is delocalized.

Chirality of Carbon

The molecular geometry assumed by a carbanion depends on the number of substituent groups attached to the negatively charged carbon. If the negatively charged carbon is attached to three substituent groups, the overall molecular geometry will be trigonal pyramidal. The activation barrier for such trigonal pyramidal structure is low enough that the introduction of chirality to the molecule can result in the racemization of the carbanion. The carbanions possibly exhibit chirality. In certain experiments with dry ice, 2-methyl octanoic acid optically active properties have been obtained.

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Things to Remember

• Carbanions are members of the group of organic compounds in which a negative electrical charge is primarily located on the carbon atom.
• A carbanion is an anion with carbon that is trivalent and carries a formal negative charge on it.
• All carbanions are the conjugate bases of some carbon acid.
• The molecular structure depends on the localization of the negatively charged electrons. In the case of localized ions, the negative charge is restricted around the carbon atom. On the other hand, in case of the localized ions, the negatively charged electron is distributed amongst different atoms.
• Carbanions typically behave as nucleophiles and are basic in nature.
• When the deprotonation causes a loss of positively charged hydrogen ion attached to the carbon atom, it is referred to as a carbon acid. Carbon acids are extremely weak acids.

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