What is a Carbanion?
A carbanion can be defined as a negatively charged ion in which a carbon atom exhibits trivalence (implying it forms a total of three bonds) and holds a formal negative charge whose magnitude is at least -1. When pi delocalization does not occur in the organic molecule (as it does in the case of aromatic compounds), carbanions typically assume a bent, linear, or a trigonal pyramidal molecular geometry. It is important to note that all carbanions are conjugate bases of some carbon acids.
In all carbanions, the electron density is highly concentrated at the negatively charged carbon atom. Therefore, this carbon becomes an ideal point of attack for many electrophiles and other electron-deficient species. Furthermore, this carbon atom is also the site at which the molecule reacts with proton donors and halogenating reagents such as diiodine.
An illustration detailing the possible resonance structures of a carbanion in which the carbon holding the negative charge is bound to three different R groups is provided above. It can be noted that each of the R-groups in this illustration can either denote an alkyl group, an aryl group, or a hydrogen atom.
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Occurrence and Properties
Typically, carbanions behave as nucleophiles and are basic in nature (their pH is usually above 7). The nucleophilicity and the basicity of the carbanion is usually determined by the substituent groups that are attached to the negatively charged carbon. As is the case with most charged carbon species, the substituent groups can either increase or decrease the stability of the molecule as a whole. The effects that come into play while considering the stability of carbanions based on the substituent groups attached to them include:
- The inductive effect, via which highly electronegative substituent groups attached to the carbanion help subdue the negative charge on it and make the molecule more stable. On the other hand, highly electropositive substituent groups can increase the negative charge on the carbanion and, therefore, decrease the overall stability of the molecule.
- The resonance effect, via which the delocalization of the electrons distributes the negative charge all over the carbanion, adds stability to the process. Aromatic systems add a great deal of stability to carbanions when they are present as a substituent group as a result of the resonance effect (and the greater extent of delocalization of electrons over the aromatic system)
- The conjugation of the carbanion.
Carbanions can be detected in the solution phase via proton nuclear magnetic resonance, which is an application of NMR spectroscopy. Carbanions present in the condensed phase can be isolated as an ionic species only if the molecule (as a whole) is stabilized enough by the delocalization of electrons.
Any compound that has the ability to undergo deprotonation can, in principle, be considered as an acid. The compound formed after the deprotonation is referred to as the conjugate base of the acid. An acid is referred to as a carbon acid when the deprotonation causes the loss of a positively charged hydrogen ion which was attached to the carbon atom. Therefore, the deprotonated carbon acid will have a negative charge (as a result of retaining the bond pair of electrons from the carbon-hydrogen bond) and can be considered as a carbanion.
Carbon acids are considered to be extremely weak acids. When their pH values are compared to the pH values of powerful mineral acids like sulfuric acid and hydrochloric acid, it can be observed that the pH values of carbon acids are lower by several multitudes. These acids are also weaker than carboxylic acids (organic compounds which contain the carboxyl functional group, denoted by R-COOH).
The acidity of carbon acids increases when the negative charge on the corresponding conjugate base (the carbanion) is delocalized. Therefore, if highly electronegative substituent groups are attached to the negatively charged carbon atom in the conjugate base, the acidity of the corresponding carbon acid will be high. Similarly, weakly acidic carbon acids usually contain highly electropositive species attached to the negatively charged carbon in the conjugate base.
Chirality of Carbanions
The molecular geometry assumed by a carbanion is dependent 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 the trigonal pyramidal geometry is low enough that the introduction of chirality to the molecule can result in the racemization of the carbanion (in a process that is similar to nitrogen inversion). However, it can be noted that it is possible for carbanions to exhibit chirality. In certain experiments involving dry ice, 2-methyl octanoic acid with optically active properties has been obtained as a product.
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