Trends And occurrence Of Carbanions

Carbanions are normally nucleophilic and basic, with the exception of few. 

Carbanions’ basicity and nucleophilicity are determined by the substituents attached to their carbon atoms.

The presence of electronegative atoms next to the charge will stabilise the charge.

The amount to which the anion is conjugated.

This is referred to as the inductive effect. 

The anion can be stabilised by resonance effects. 

This is especially true when the anion has been stabilised as a result of the presence of aromatic compounds.

Carbanions are nucleophilic compounds that are found in nature in large quantities. 

The degree of basicity or nucleophilicity, on the other hand, is mostly regulated by the substituents attached to the carbon atoms. 

Occurrence

Carbanions are nucleophiles in nature and are hence basic in their behaviour (their pH is usually above 7). 

Most of the time, the carbanion’s nucleophilicity and basicity are determined by the substituent groups that are connected to the negatively charged carbon. 

Like most charged carbon species, the substituent groups can either boost or decrease the overall stability of the molecule. 

This is true for most charged carbon species as well.

Following are some of the factors that come into play while determining the stability of carbanions based on the substituent groups that are connected to them:–

A mechanism by which highly electronegative substituent groups linked to the carbanion assist in subduing the negative charge on it and making the molecule more stable is called the inductive effect. 

High electropositive substituent groups, on the other hand, can increase the negative charge on the carbanion and, as a result, impair the overall stability of the molecule as a whole.

The resonance effect, which occurs as a result of the delocalization of electrons, spreads the negative charge across the carbanion, increasing the stability of the reaction.

As a result of the resonance effect, aromatic systems significantly increase the stability of carbanions when they are present as a substituent group in carbanions (and the greater extent of delocalization of electrons over the aromatic system)

The carbanion is conjugated in this reaction.

Carbanions can be detected in the solution phase using proton nuclear magnetic resonance, which is a type of NMR spectroscopy that is used in the detection of carbanions. 

Carbanions existing in the condensed phase can only be isolated as an ionic species if the molecule (as a whole) is sufficiently stabilised by the delocalization of electrons, which is not always the case.

Features of Carbanion

1. Carbanion is composed of a negative, trivalent carbon atom with eight electrons in its valence shell, giving it the name.
2. Carbanion does not contain any electrons.
3. Carbanions are Lewis’ bases (donors of electron pairs) that have electronic components that are comparable to amines in their electronic structure.
4. Due to the fact that the facility already has an octet of electrons, electron donation groups coupled to an anionic centre are required to activate a carbanion. 
5. As a result, the carbanion has a stable order that is diametrically opposed to that of carbocations and radicals. 

Given our observation that alkyl groups release electrons when they come into contact with hydrogen, we can also assert that electron-releasing groups stabilise carbocation while disrupting carbanion.

Structure

The negatively charged carbon atom of carbanion sp3 hybridised with the positively charged carbon atom of carbanion sp3.

 Four orbitals of the tetrahedron are oriented toward the corners of the tetrahedron, similar to the other sp3-hybridised specimens . 

One of the sp3 hybrid orbitals contains a pair of electrons that have not been allocated.

Because of this, carbanions have a pyramidal geometrical structure.

Based on the structure Carbanion types are as follows:

• When a single carbon atom is attacked directly by an anionic carbon molecule, this is referred to as primary carbonation

• If two atoms of carbon are bonded to an anionic carbon, this is referred to as secondary carbanion formation

• If three carbon atoms are evenly distributed on the anionic carbon, the result is a positive charge

After obtaining the carbon atom, the tertiary stage begins.

They are referred to as tertiary carbanions at this point.

• If there is no carbon atom connected to anionic carbon, the compound is referred to as methyl carbanion

Conclusion

Due to the fact that in aqueous solution, a C single bond H bond can disintegrate appreciably only if substituted by two or three highly electron-withdrawing groups, the presence of carbanions in moderate to high concentration in aqueous solution is restricted. 

However, dipolar aprotic solvents such as acetonitrile, hexamethylphosphoramide (HMPA), or DMSO can support substantial quantities of carbanions that are less strongly stabilised than the aforementioned solvents.

Carbanions can exist as free ions in such solvents on occasion, but they are most commonly found in the form of solvent-separated or contact ion pairs in these environments. 

Strong ion pairing of a carbanion with its gegenion will be expected in less acidic solvents such as THF, 2-Me-THF, DME, and the higher glymes such as diethyl ether and pentane.

If the anion is not stabilised by an electron-delocalisation group, it is likely to establish a covalent bond with the cation, resulting in the formation of a polar covalent compound.