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Carbanions: Stability & Reactions | Organic Chemistry PDF Download

Anion of carbon is known as carbanion. Carbanion carriers three bond pairs and one lone pair, thus making the carbon atom negatively charged. So carbanion may be represented as

Carbanions: Stability & Reactions | Organic Chemistry

(A) What are Carboanions:

Hybridisation and gemometry: Alkyl carbanion has three bond pairs and one lone pair. Thus hybridisation is spand geometry is tetrahedral (shape: pyramidal).

Carbanions: Stability & Reactions | Organic Chemistry

Note: Geometry of allyl and benzyl carbanion is almost planar and hybridisation is sp2.

(i) There are eight electrons in the outermost orbit of carbanionic carbon hence its octet is complete.

(ii) It behaves as charged nucleophile.

(iii) It is diamagnetic in character because all eight electrons are paired.

(iv) it is formed by heterolytic bond fission.

(vi) It reacts with electrophiles.

(B) Generation of Carbanions

Three principal ways in which carbanions are generated are

(a) Action of base on carbon acids.

Carbanions: Stability & Reactions | Organic Chemistry

(b) A negative ion adds to C—C multiple bond.

Carbanions: Stability & Reactions | Organic Chemistry

(c) Action of active metals (Li, Na, K, Mg, etc.) on alkyl halide generates organometallic comopunds which behave as a carbanion.

Carbanions: Stability & Reactions | Organic Chemistry

Note: Compunds with direct carbon-metal bond are known as organometallic compounds.

Reactions in which product formation takes place by formation of carbanion as reaction intermediate:

In the following reactions formation takes place by the formation of carbanion as reaction intermediates.

(i) Condensation reactions of carbonyl compounds, i.e., Aldol condensation, Perkin reaction, Reformatsky reaction etc.

(ii) Condensation reactions of ester; Claisen condensation.

(iii) Witting reaction.


(C) Fate of Carbanion

(a) As a base it pickup acidic proton.

Carbanions: Stability & Reactions | Organic Chemistry

(b) As a nucleophile: This is one of the best method by the which C—C bond formation takes place.

e.g.,

Carbanions: Stability & Reactions | Organic Chemistry

(iii) It may pickup β -H to give elimination product

Carbanions: Stability & Reactions | Organic Chemistry


(D) Stability of carbanions: The stability of carbanion may be explained by


(i) Electronegativity of carbanionic carbon:

Stability ∝ Electronegativity of carbonic carbon

∝ % s-character of carbanionic carbon

Carbanions: Stability & Reactions | Organic Chemistry


(ii) Stability of Aromatic Carbanions:

(a) Anions in which negative charge is present on carbon of aromatic system is known as aromatic carbanions.

(b) Aromatic carbanions are most stable carbanions.

(c) Anions obeying Huckel rule are stable because they are aromatic and there is complete delocalisation of negative charge.

Carbanions: Stability & Reactions | Organic Chemistry

With aromaticity as the factor for stability, few carbanions and their stability order is:

Carbanions: Stability & Reactions | Organic Chemistry

Aromatic carbanion benzyl carbanion Allyl carbanionCarbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry


(iii) Delocalisation or Resonance: Allyl and benzyl carbanions are stabilising by delocalisation of negative charge.

Carbanions: Stability & Reactions | Organic Chemistry


(iv) Inductive effect: Stability of carbanions depends on the +I or –I groups as follows:

(a) +I group

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

(b) –I group: Stability of  carbanions∝– I power of the group

For example:

Carbanions: Stability & Reactions | Organic Chemistry

(v) Stablisation by Sulphur and Phosphorus: Attachment of carbanionic carbon of a sulphur and phosphorus atom causes an increase in carbanion stability.

The cause of stability is due to the delocalisation of negative charge of carbanion by vacant d-groups stabilise carbanion by resonance effect.

Carbanions: Stability & Reactions | Organic Chemistry

Contribution of structure (II) is more than (I) because in (II) negative charge is present on electronegative oxygen.

Some examples for illustration:

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

Carbanions: Stability & Reactions | Organic Chemistry

The document Carbanions: Stability & Reactions | Organic Chemistry is a part of the Chemistry Course Organic Chemistry.
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FAQs on Carbanions: Stability & Reactions - Organic Chemistry

1. What is a carbanion?
A carbanion is an organic ion that has a negatively charged carbon atom. It is formed when a carbon atom gains an extra electron, giving it a negative charge. Carbanions are highly reactive due to the presence of the negative charge and can participate in various chemical reactions.
2. How is the stability of carbanions determined?
The stability of carbanions is determined by the presence of electron-donating or electron-withdrawing groups attached to the carbon atom carrying the negative charge. Electron-donating groups, such as alkyl groups, stabilize the carbanion by dispersing the negative charge. On the other hand, electron-withdrawing groups, such as halogens or carbonyl groups, destabilize the carbanion by withdrawing electron density from the carbon atom.
3. What are the reactions of carbanions?
Carbanions can undergo various reactions, including nucleophilic substitution, elimination, and addition reactions. In nucleophilic substitution reactions, the carbanion acts as a nucleophile and replaces a leaving group. In elimination reactions, the carbanion acts as a base and removes a proton from a neighboring carbon atom. In addition reactions, the carbanion reacts with an electrophile to form a new bond.
4. How are carbanions formed?
Carbanions can be formed through different methods, such as deprotonation of a carbon-hydrogen bond, deprotonation of a carbon-halogen bond, or through electron transfer from a metal in organometallic compounds. Common reagents used for carbanion formation include strong bases like hydroxide ions (OH-) or alkoxides (RO-) that can abstract a proton from a carbon atom.
5. How does the stability of carbanions affect their reactivity?
The stability of carbanions directly affects their reactivity. More stable carbanions, which have a greater electron density on the carbon atom, are less reactive compared to less stable carbanions. This is because the higher stability allows the carbanion to better accommodate the negative charge, reducing its tendency to react with other species. Conversely, less stable carbanions are more reactive and tend to undergo reactions more readily.
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