Learning Objective: To learn different methods to stabilize a carbanion.
Skill Level - Advanced
Prerequisites:
s-character
Chapter: Reactive Intermediates
Sub-topic: Carbanion
Author's Note: Carbanions are widely used as a base or a nucleophile in organic chemistry. Highly reactive and less stable carbanions function better as a base than a nucleophile. However, stabilizing a carbanion to act as a nucleophile is imperative for functional group interconversion. Only then can various combinations of C-C bonds can be developed. This section covers various methods to stabilize a carbanion, also discussing different groups or substituents that, in particular, improve its stabilization.
Carbanion Stabilization
Carbanions, due to their negative charge, are a reactive species.
High reactivity and less stability are needed in some organic reactions- for example, as a strong base.
These highly basic carbanions abstract protons in such reactions and are commonly associated with metal- for example, butyl lithium (Bu-Li+). The tert-butyllithium, an unstable carbanion, can abstract protons from many carbon molecules, including benzene. These organometallic carbanions in solution possess a higher concentration and a long lifetime.
Other carbanions are short-lived intermediates that are generated in basic reaction conditions where another base abstracts a proton to form a carbanion. In such situations, the carbanion, also being a base, must be weaker than the base employed for proton removal; otherwise, it will take up a proton and again create a C-H bond.
Forming a stabilized carbanion is a must for the reaction to proceed and form other products. In such reactions where the carbanion is essential to form other types of bonds (C-C or C-S), it acts as a nucleophile.
The major ways to stabilize the carbanion are-
a) Inductive Stabilization
b) Resonance stabilization
c) Increase in s-character of the carbanion
d) Organometallic stabilization
a) Inductive Stabilization
The presence of substituent groups attached to the charge-containing carbon centre significantly affects the molecule's stability due to the Inductive Effect.
If the carbon centre is electron-deficient, then it is stabilized by the presence of electron-donating groups. Alternatively, if the carbon centre is electron-rich, it needs an electron-withdrawing group to stabilize it. The presence of electron-donating groups around the already electron-rich carbon centre, makes it unstable.
Inductive effect of electron-withdrawing groups on Carbanion
The presence of electron-withdrawing groups .....
b) Resonance stabilization of the carbanion
The negative charge of the carbanion, when in conjugation with....
Role of electron-withdrawing groups in resonance stabilization
The electron-withdrawing functional groups have...
.
.
c) Hybridization effect and % s-character
s-orbitals are closest to the nucleus, and these.....
d) Organometallic Stabilization
An organometallic compound contains a bond between a carbon and a metal atom....
Subscribe to learn about various strategies used to stabilize a carbanion.
