Heterolytic Fission and Ion Formation
Quick Notes
- Heterolytic fission occurs when a covalent bond breaks and both electrons from the bond go to one atom.
- This forms a positive ion (cation) and a negative ion (anion).
- Use curly arrows (double-headed) to show movement of electron pairs.
- Different to homolytic fission, which produces radicals.
- Heterolytic fission is the key bond-breaking process in nucleophilic substitution.
Full Notes
What Is Heterolytic Fission?
Heterolytic fission is when a covalent bond breaks unevenly.
Both electrons from the bond are taken by one atom, producing ions: one atom becomes negatively charged, the other positively charged.
The atom with the highest electronegativity usually takes the bonding electron pair.
Drawing Heterolytic Fission
We can show heterolytic fission in mechanisms by drawing curly arrows. A full arrowhead represents a pair of electrons.
The arrows start from the bond and point to the atom that receives both bonding electrons. The use of curly arrows in mechanisms allows us to show the proposed movement of electron pairs during reactions.
Comparison of Homolytic vs Heterolytic Fission
| Feature | Heterolytic Fission | Homolytic Fission |
|---|---|---|
| Electron distribution | Both electrons go to one atom | One electron goes to each atom |
| Products | Ions (cation + anion) | Radicals (neutral species with unpaired e⁻) |
| Arrow type | Double-headed arrow (→) | Single-headed arrow (fish hook) |
| Example | CH₃Br → CH₃⁺ + Br⁻ | Cl₂ → Cl• + Cl• |
| Occurs in | Nucleophilic substitution, ionisation | Radical substitution, photochemical reactions |
Summary
- Heterolytic fission produces ions; homolytic fission produces radicals.
- Curly arrows with double heads show electron pairs in heterolytic fission.
- Important in mechanisms like nucleophilic substitution.
Linked Course Question
What is the difference between the bond-breaking that forms a radical and the bond-breaking that occurs in nucleophilic substitution reactions?
Radical formation involves homolytic fission: the bond breaks evenly and each atom takes one electron → forms radicals (e.g. Cl₂ → 2Cl•).
Nucleophilic substitution involves heterolytic fission: the bond breaks unevenly and both electrons go to one atom → forms ions (e.g. CH₃Br → CH₃⁺ + Br⁻).