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Carbanions

Carbanions

Shivani PooniaUpdated on 02 Jul 2025, 06:29 PM IST

In organic chemistry, a carbanion is an anion (negatively charged ion) derived from a carbon atom. Carbanions are usually unstable, and they are often difficult to study because they tend to react with other molecules quickly. A carbanion is an ion with a negatively charged carbon atom. The most stable carbanions have six electrons in the valence shell of the carbon atom. Carbanions are important in organic chemistry because they can act as nucleophiles, which means they can donate electrons to other molecules. Carbanions are also important in biochemistry because they can be used to transfer electrons between molecules.

This Story also Contains

  1. Carbanions
  2. Solved Examples Based On Carboanions
  3. Conclusion
Carbanions
Cabanions

In this article, we will cover the topic (carbocation). This topic falls under the broader category of (Some Basic Principles of Organic Chemistry), which is a crucial chapter in (Class 11 Chemistry). It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more.

Carbanions

The carbon species carrying a negative charge on carbon atoms is called carbanion. Carbon in carbanion is generally sp3 hybridised and its structure is a distorted tetrahedron as shown in the figure given below. Carbanions are also unstable and reactive species.

The stability order of carbanions:-

1. The stability of carbanions is influenced by resonance, inductive effect and s-character of orbitals. The group having +I effect decreases the stability while groups having -I effect increase the stability of carbanions.

2. The groups like -NO2,−CN,−COOC2H5, halogens and C6H5(electron attracting) increase the stability of carbanions.


Recommended topic video on(Cabanions)



Solved Examples Based On Carboanions

Q.1 The shape of methyl anion (CH3−)is :

(1) bent

(2) trigonal planar

(3) tetrahedral

(4) trigonal pyramidal

Solution:

As we learned -

The carbon species carrying a negative charge on carbon atoms is called carbanion. Carbon in carbanion is generally sp3 hybridised and its structure is distorted tetrahedron Carbanions are also unstable and reactive species.

CH3− is sp3 hybridised having tetrahedral electronic geometry and trigonal pyramidal shape.

Hence, the answer is the option (4).

Q.2 Which one of the following Grignard reagents is least stable or most reactive?

(1) CH3MgBr

(2) CH3−CH2MgBr

(3)

(4)

Solution:

As we learned -

Stability order of carbanion:-

3∘<2∘<1∘<CH3

is the most reactive among the given options because it is a very unstable tertiary carbanion.

Therefore, option (4) is correct

Conclusion

Carbanions are important in many different areas of chemistry, including organic synthesis and catalysis. Carbanions can be used to make new bonds or to break existing bonds. Carbanions are also used as catalysts in many different reactions. Carbanions are relatively unstable, but they can be stabilized by hybridization or by using special stabilizing groups. Carbanions are an important part of organic chemistry and play a vital role in many different reactions.


Frequently Asked Questions (FAQs)

Q: How do carbanions participate in the Knoevenagel condensation?
A:
In the Knoevenagel condensation, a carbanion formed from an active methylene compound (like malonic ester) attacks the carbonyl group of an aldehyde or ketone. This leads to the formation of an α,β-unsaturated carbonyl compound after dehydration.
Q: What is the role of carbanions in the synthesis of acetoacetic ester?
A:
In the synthesis of acetoacetic ester, a carbanion intermediate is formed by deprotonation of ethyl acetate. This carbanion then attacks another molecule of ethyl acetate, leading to the formation of ethyl acetoacetate through a Claisen condensation.
Q: How do carbanions behave in the Dieckmann condensation?
A:
In the Dieckmann condensation, an intramolecular version of the Claisen condensation, a carbanion formed at one end of a diester attacks the other ester group, forming a cyclic β-keto ester. The carbanion acts as the nucleophile in this ring-forming reaction.
Q: What is the significance of carbanions in asymmetric synthesis?
A:
Chiral carbanions or carbanion-like species can be used in asymmetric synthesis to create new stereogenic centers. The stereochemistry of the product can be controlled by the geometry of the carbanion approach to the electrophile.
Q: How do carbanions relate to the concept of tautomerism?
A:
Carbanions are often intermediates in tautomerization reactions, particularly keto-enol tautomerism. The carbanion formed by deprotonation can re-protonate at a different site, leading to the tautomeric form.
Q: What is the role of carbanions in the synthesis of Grignard reagents?
A:
While Grignard reagents are not true carbanions, their formation involves a step where the organic halide behaves like a carbanion, attacking the magnesium metal. The resulting organomagnesium compound then behaves similarly to a carbanion in subsequent reactions.
Q: How do carbanions participate in the Cannizzaro reaction?
A:
In the Cannizzaro reaction, a hydride (which can be thought of as a simple carbanion) is transferred from one aldehyde molecule to another. This results in the simultaneous oxidation and reduction of the aldehyde to a carboxylic acid and an alcohol, respectively.
Q: What is the significance of carbanions in biochemistry?
A:
While true carbanions are rare in biochemical systems due to the aqueous environment, carbanion-like intermediates play crucial roles in many enzymatic reactions, particularly those involving carbon-carbon bond formation or cleavage.
Q: What is the significance of carbanions in organic synthesis planning?
A:
Carbanions are important synthetic tools in retrosynthetic analysis. They allow for the reversal of polarity at carbon centers (umpolung), enabling novel disconnections and synthetic routes that might not be obvious when considering only electrophilic carbon species.
Q: How do carbanions behave in rearrangement reactions?
A:
Carbanions can undergo rearrangements to form more stable carbanion intermediates. For example, in the Favorskii rearrangement, a cyclic carbanion intermediate undergoes ring contraction to form a more stable species.
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