Definition of Heterolytic Fission
Heterolytic fission is the breaking of a covalent bond where both bonding electrons remain with one of the fragments, generating positive and negative ions:
\[\text{A-B} \rightarrow \text{A}^+ + \text{:B}^-\]Heterolytic vs Homolytic Fission
There are two ways a covalent bond can break. In R3.4 we focus on heterolytic fission, where electron pairs stay together.
Heterolytic vs Homolytic Fission
Heterolytic Fission (Forms Ions)
\(\text{A:B} \rightarrow \text{A}^+ + \text{:B}^-\)
Both electrons go to the more electronegative atom. Shown with a double-headed curly arrow.
Homolytic Fission (Forms Radicals)
\(\text{A:B} \rightarrow \text{A}^\bullet + \text{B}^\bullet\)
One electron goes to each fragment. Shown with single-headed fish-hook arrows.
Forming Ions by Heterolytic Fission
When a bond between two atoms of different electronegativity breaks heterolytically, the more electronegative atom takes both electrons.
Curly Arrow Rules
A curly arrow (double-headed) represents the movement of an electron pair. You must follow these rules when drawing them.
Rules for Drawing Curly Arrows in Mechanisms
- Start: Starts precisely from the source of the electron pair (a lone pair or the centre of a covalent bond).
- End: Points directly to the atom or bond destination receiving the electron pair.
- Arrowhead: Double-headed arrow (\(\curvearrowright\)) indicates movement of two electrons.
Examples of Ion Formation
| Bond | Heterolytic fission produces | Why? |
|---|---|---|
| C-Cl | C⁺ + Cl⁻ | Cl is more electronegative |
| C-Br | C⁺ + Br⁻ | Br is more electronegative |
| C-I | C⁺ + I⁻ | I is more electronegative |
| H-Cl | H⁺ + Cl⁻ | Cl is more electronegative |
Electronegativity Dictates Fission Direction
The fragment that ends up with the negative charge is the more electronegative atom, as it exerts a stronger electrostatic pull on the shared bonding pair.
Arrow Conventions and Radical Confusion
- Arrowhead type: Never use single-headed (fish-hook) arrows for heterolytic mechanisms (fish-hook arrows are reserved exclusively for radical processes).
- Arrow direction: Arrows represent electron movement, never movement of positive charges or atomic nuclei.
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