Key Definition
Heterolytic fission is the breaking of a covalent bond where both bonding electrons remain with one of the two fragments formed. This produces two oppositely charged ions.
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 Fission
Both electrons go to one fragment.
A:B → A⁺ + :B⁻
Produces ions. Shown with a double-headed curly arrow.
Homolytic Fission
One electron goes to each fragment.
A:B → A· + ·B
Produces radicals. Shown with single-headed fish-hook arrows (R3.3).
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
- The arrow starts from the source of the electron pair (a lone pair or the centre of a bond)
- The arrow points to the destination (the atom accepting the electron pair)
- The arrow must be double-headed (showing a pair of electrons, not a single electron)
- Do not confuse with fish-hook arrows (single-headed), which show single electron movement in radical reactions (R3.3)
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 |
Think About It
How do you know which fragment ends up with the negative charge after heterolytic fission?
The more electronegative atom takes both bonding electrons. Since it gains the electron pair, it becomes the negatively charged ion. The atom that loses the electrons becomes positively charged.
Common Exam Mistakes
- Using fish-hook (single-headed) arrows instead of curly (double-headed) arrows. Heterolytic fission involves pairs of electrons.
- Drawing the curly arrow pointing the wrong way. It must go from the bond to the more electronegative atom.
- Confusing heterolytic fission (produces ions) with homolytic fission (produces radicals).
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