Heterolytic Cleavage - Carbocations

Description:

Organic reactions proceeding via heterolytic cleavage is called ‘ionic’ or ‘polar’ reactions.

Bond is broken such that shared pair of electrons is retained by one of the fragments. A positive and

negatively charged species are formed.

Carbocation

Species having carbon atom with positive charge is called carbocation (or carbonium ion).

It is electron deficient as there are only 6 e⁻ in its valence shell.

CH₃⁺ ion is called methyl cation.

It has vacant 2p_z orbital giving sp² hybridization and has trigonal planar structure.

Carbocations can be

• primary (1⁰) : CH₃CH₂⁺
• secondary (2⁰): (CH₃)₂CH⁺
• tertiary (3⁰): (CH₃)₃C⁺

Stability is determined by −

• Hyperconjugation effects −

  In carbocations, the conjugation is between σ bond of C − H and the positive charge on carbon.

  CH₃CH₂⁺ (3α) < (CH₃)₂CH⁺ (6α) < (CH₃)₃C⁺ (9α)

  More the number of α −hydrogens, more will be the stability of carbocation.

• Inductive effects −

  Since carbocation is electron-deficient, electron donating groups (EDGs) will stabilize it.

  More the no. of alkyl groups (EDGs), greater is the stability.

  Thus, order of stability of carbocations: 3⁰ > 2⁰ > 1⁰

• Resonance effects −

  Adjacent π −bond stabilizes a carbocation. Carbocation has planar structure and empty p −orbital can overlap parallel with the adjacent π −bond resulting in delocalization of positive charge.



• Adjacent Lone pair of electrons −

  Atoms with lone pair of electrons adjacent to carbocation stabilizes the electron deficient species by donation of electrons.

  Electron-withdrawing species such as nitrogen, oxygen, halogens also stabilize carbocations by donation of electrons forming π −bonds, which is a stabilizing influence.

  For example,

𝐶𝑙 exhibits −

• −I effect: destabilizes carbocation.
• +M/+R effect: stabilizes carbocation.

However, the priority of electronic effects −

Resonance > Hyperconjugation > Inductive effect

Thus, although Cl exhibits –I effect due to its electron-withdrawing inductive effect, priority is given to its stabilization of carbocations via resonance (+R) effect.