1.9.5Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Free Radicals and Fission — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 35 of 36 Unit quiz
Part 1 of 5Breaking Covalent Bonds
The way a bond breaks decides what type of reactive species forms.
Homolytic fission
- The bonding pair is split equally, one electron going to each atom, to give two free radicals.
Heterolytic fission
- Both bonding electrons go to the same atom, producing a positive ion and a negative ion.
Conditions
- Homolytic fission usually needs ultraviolet light or high temperature, while heterolytic fission is common in polar bonds in solution.
Example of homolytic
- Cl–Cl → 2Cl• in sunlight, with a bond enthalpy of 243 kJ mol⁻¹ supplied by the light.
Example of heterolytic
- HBr → H⁺ + Br⁻, where the electron pair stays with the more electronegative bromine atom.
Part 2 of 5Free Radicals
Free Radicals
Definition
- A free radical is a species with an unpaired electron, shown by a dot, such as Cl• or •CH₃.
Reactivity
- The unpaired electron makes radicals highly reactive and short-lived, so they react quickly with other species.
Formation
- They arise in the initiation step of reactions started by light or heat.
Importance
- Free radicals are involved in the chlorination of alkanes, in polymerisation and in the breakdown of ozone by CFCs.
Ozone link
- UV light breaks C–Cl bonds in CFCs, releasing chlorine radicals that catalyse ozone destruction.
Part 3 of 5Showing Movement of Electrons
1
A curly arrow shows the movement of electrons, with the tail starting at the bond or lone pair.
2
A full double-headed arrow shows movement of a pair of electrons, as in heterolytic fission.
3
A half-headed single arrow, often called a fish-hook, shows movement of a single electron in homolytic fission.
4
For Cl–Cl, two fish-hook arrows start in the bond and point to each chlorine atom.
5
For HBr heterolytic fission, one full arrow starts from the bond and points to the bromine atom.
Part 4 of 5Comparing the Two
Comparing the Two
Products of homolytic
- Two radicals, each with one unpaired electron.
Products of heterolytic
- One cation and one anion, each with a full set of paired electrons.
Bond polarity
- Heterolytic fission is more likely when the bond is polar, because the pair moves towards the more electronegative atom.
Exam tip
- Always draw the arrow heads accurately and show which atom each electron ends up on.
| Type | Electron movement | Products |
|---|---|---|
| Homolytic | One electron to each atom | Two radicals |
| Heterolytic | Both electrons to one atom | Cation and anion |
Part 5 of 5Exam Checklist & Tips
1.9.5 – Key Points for the Exam
- Free radical A species with an unpaired electron, which is very reactive.
- Homolytic fission Breaking of a covalent bond so that each atom takes one electron.
- Heterolytic fission Breaking of a covalent bond so that one atom takes both electrons.
- Curly arrow An arrow in a mechanism showing the movement of electrons.
- Initiation The first step in a radical reaction, in which radicals are formed.
Common Mistakes
- Using a full-headed arrow to show homolytic fission.
- Forgetting the dot to show the unpaired electron in a radical.
- Saying radicals carry a charge.
- Stating that homolytic fission produces ions.
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