Learning Objective: To learn about the structural feature that stabilizes also destabilizes the carbocation intermediate.
Skill Level - Intermediate
Prerequisites:
Hyperconjugation (Chapter - Electronic Displacements in a Covalent Bond)
Rearrangement Reaction (Chapter – Common Reaction Types)
Chapter: Reactive Intermediates
Sub-topic: Carbocation
Author's Note: The reaction speed is a major difference separating the free radical reactions from other electrophilic or nucleophilic reactions. Free radicals are highly unstable and undergo faster, uncontrolled reactions, generating undesired side products. In the case of carbocation, a reaction intermediate just like the free radicals, the reaction speed can be controlled by stabilizing the carbocation that comparatively slows the reactions and leads to the formation of desired products. I have discussed five structural features that stabilize a carbocation and one that destabilizes it. Understanding this is very crucial for designing molecules or also while predicting a reaction outcome.
Structural Features Stabilizing and Destabilizing the Carbocations
As a rule, the more concentrated the charge on any atom, the less stable the species carrying the charge.
Less stable and more reactive species have lower control over a reaction’s or product’s outcome, leading to the formation of undesired or side-products. Therefore, charge stabilization of the carbocation intermediate facilitates faster, smoother, and more fruitful formation of the desired product.
There are five ways to stabilize a carbocation –
a) Inductive Stabilization of the Carbocation
The carbocation is an electron-deficient positively charged carbon centre. The carbocation out of deprivation seeks electrons and tries to pull them from the attached substituents. The attached substituents push their electrons to the carbocation. Due to the pull and the push, the electron density is higher near the carbocation, and the bonds linked to the carbocation become polarised. This effect is called the inductive effect.
Alkyl groups are electron donors when placed next....
b) Resonance Stabilization of the Carbocation
When carbocation is in conjugation with double or triple bonds,...
c) Hyperconjugation Effect
The alkyl groups reduce the positive charge concentration on the carbocation by spreading the charges....
.
.
d) Heteroatom Adjacent to the Carbocation
A structural feature increasing carbocations stability is having adjacent lone pairs containing heteroatoms.
.
.
e) Rearrangements Increase Carbocation Stability
If the carbocation formed is less stable, the molecule undergoes structural rearrangement to generate the most stable carbocation form.
.
.