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What is Carbanion?

A carbanion definition can be described as a negatively charged ion. A carbon atom exhibits the trivalence (indicating that it forms three bonds total) and contains a formal negative charge whose magnitude can be given as -1 at least. When the pi delocalization does not happen in the organic molecule (because it does in aromatic compounds), carbanions typically assume linear, bent, or a trigonal pyramidal molecular geometry. It is so important to note that all the carbanions are conjugate bases of some carbon acids.

Carbanion Explained

Electron density is highly concentrated in all the carbanions at the negatively charged carbon atom. Thus, this carbon becomes an ideal point of attack for most electrophiles and a few electron-deficient species. Moreover, this carbon atom can also be given as the site where the molecule reacts with proton donors, including the halogenating reagents like diiodine.

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A detailed illustration that explains the possible resonance structures of a carbanion (carbanion structure), where carbon, holding the negative charge, is bound to 3 different R groups, is given above. It is to be noted that every R-group in the given illustration can either donate a hydrogen atom, an aryl, or alkyl group. A Carbanion example is a circle of carbon atoms that join bi pi and Sigma bonds.

Occurrence and Properties

Carbanions typically behave as nucleophiles, and they are basic in nature (usually, their pH is above 7). Usually, the basicity and nucleophilicity of carbanion is described by the substituent groups attached to the negatively charged carbon. In the case of many charged carbon species, the substituent groups can increase or decrease the carbanion stability entirely. 

The effects while considering the carbanions' stability depending on the substituent groups attached to them are given below:

  • Inductive Effect  

Through this highly electronegative substituent groups attached to the carbanion help subdue its negative charge and make the molecule more stable. On the other side, the highly electropositive substituent groups increase the negative charge existing on the carbanion and, thus, decrease the overall molecule's stability.

  • Resonance Effect  

Through this electron's delocalization distributes the negative charge over the total carbanion by adding the stability in the process. The aromatic systems add great stability deal to the carbanions when they exist as a substituent group as the resonance effect result (and the greater delocalization extent of electrons over the aromatic system)

  • Conjugation of Carbanion

Carbanions are detected in the solution's phase via proton nuclear magnetic resonance, an NMR spectroscopy application. Carbanions, which are present in the condensed phase, can be isolated as an ionic species only if the molecule is sufficiently stabilized as a whole by the electron delocalization.

Carbon Acids

Any compound having the ability to undergo deprotonation can, in principle, can be considered as an acid. The compound, which is formed after deprotonation, is called the conjugate base of an acid. Acid is called a carbon acid when the deprotonation positively charged hydrogen ion loss, attached to the carbon atom. Thus, the deprotonated carbon acid will hold a negative charge (resulting from the electron's bond pair retain from the carbon-hydrogen bond) and is considered a carbanion.

Carbon acids are considered extremely weak acids. When the pH values of carbon acids are compared to the powerful mineral acids such as hydrochloric acid, sulfuric acid, it can be noticed that the carbon acid's ph values are lower by various multitudes. These acids are also weaker than carboxylic acids (organic compounds that hold the carboxyl functional group and are denoted as R-COOH).

When the corresponding conjugate base (the carbanion) negative charge is delocalized, the acidity of the carbon acid increases. Thus, if highly electronegative substituent groups are attached to the negatively charged carbon atom present in the conjugate base, the corresponding carbon acid's acidity becomes high. In the same way, usually, the weakly acidic carbon acids hold highly electropositive species attached to the negatively charged carbon in the conjugate base.

Chirality of Carbanions

The molecular geometry assumed by a carbanion depends on the substituent group count attached to the negatively charged carbon. Whereas if the negatively charged carbon is attached to 3 substituent groups, the overall molecular geometry becomes trigonal pyramidal. 

The trigonal pyramidal geometry's activation barrier is low enough that the chirality introduction to the molecule may result in the carbanion racemization (in a process similar to nitrogen inversion). However, it is noted that carbanions have the possibility to exhibit chirality. In some experiments involving dry ice, 2-methyl octanoic acid with the optically active properties can be obtained as a product.

Preparation of Carbanion

Any preparation of the organic-alkali-metal compounds can be given as a carbanion source. The organic compound reaction containing atoms of bromine, iodine, or chlorine with the alkali metals is often used. This reaction can be expressed as follows.

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Here, X is an atom of bromine, chlorine, or iodine; M is an alkali metal's atom; R is an organic group.

The conversion of one carbanion into others is accomplished either with organic halides or hydrocarbons, as given by the below equations.

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FAQ (Frequently Asked Questions)

1. Why are the Primary Carbanions More Stable?

Ans: The carbanion is rich in electrons. It also becomes more sterically hindered with more neighboring or surrounding electrons present around it.

The carbon atom wants to donate its additional electron to become neutral, thereby releasing "stress" - having more electrons from the neighboring carbons only adds to the "stress". Whereas for a carbocation, the problem is the electron deficiency, and in that case, the neighboring electrons help stabilize the carbocation.

A tertiary carbon will feel the inductive effect of the 3 neighboring carbons' electron densities more than a primary carbon due to the reason there is a more spread out and separation.

2. Explain the Factors that Affect the Stability of the Carbanion?

Ans: A carbanion is meant by the anion, where the carbon is tervalent, which forms 3 bonds and bears a formal negative charge in one significant mesomeric contributor. The stability depends on the inductive hybridization effect of the charge extent of the anion's conjugation. A few factors that affect carbanion are the c-c of multiple bonds and neighboring carbon atom lone pairs. A carbocation is positively charged, and the positive atom count decreases, so the carbon's stability decreases.

3. Why is Carbanion Ion an sp3?

A carbanion holds an electron free pair in one of the tetrahedron’s apices and it is isoelectronic with ammonia. So, the sp3 hybridization is corresponding to that configuration.

4. Explain if Carbanion is Paramagnetic?

Carbanion has eight electrons in their valence shell. Where, 4 own valence electrons, 1 electron from the electropositive atom and the remaining 3 electrons are from other three bonded atoms. So, the carbanion is paramagnetic.