Homolytic Fission, UV Activation, and Radical Chain Reactions
Quick Notes
- Homolytic fission is the breaking of a covalent bond so that each atom gets one electron, forming two radicals.
- Requires UV light or heat to initiate – e.g., Cl2 → 2Cl•
- This is the initiation step in radical chain reactions (e.g. halogenation of alkanes).
- Use a single-barbed arrow (fish hook) to show movement of one electron.
- CFCs break down in the stratosphere under UV light to release chlorine radicals, not fluorine, due to bond strength differences.
- Chlorine radicals break down ozone (O3) but not O2 → implies the O–O bond in O3 is weaker than in O2.
- The reverse of homolytic fission is radical recombination, forming a covalent bond.
Full Notes
Homolytic Fission
Homolytic fission is a type of bond breaking where each bonded atom takes one of the shared electrons, forming two radicals.
It occurs under UV radiation or heat, with a specific amount of energy required to homolytically break the bond.
Homolytic fission is often the initiation step in a chain reaction, as it produces radicals that can react with stable molecules to form new radicals that can then go on and keep reacting – propagating the process. A key example is free radical substitution in alkanes (see below).
Drawing Homolytic Fission
We can show homolytic fission in mechanisms by drawing single-barbed arrows (sometimes called fish hooks).
The arrows start from the bond and point to each species that each get one electron from the bond.
Example Chlorine
Cl2 → 2Cl•
- Initiated by UV light
- Each Cl atom receives one electron from the shared pair
Summary
- Homolytic fission splits a bond evenly to form radicals.
- UV or heat provides energy for initiation.
- Single-barbed arrows track single-electron movement.
- CFCs release Cl• under UV because C–Cl is weaker than C–F.
- Ozone is cleaved by Cl• whereas O2 is not indicating weaker O–O bonding in O3.
- Radical recombination is the reverse of homolysis forming a covalent bond.
Linked Course Questions
Why do chlorofluorocarbons (CFCs) in the atmosphere break down to release chlorine radicals but typically not fluorine radicals?
CFCs (chlorofluorocarbons) contain carbon–chlorine (C–Cl) and carbon–fluorine (C–F) bonds. In the upper atmosphere, ultraviolet (UV) radiation provides enough energy to break the weaker C–Cl bonds but not the stronger C–F bonds.
Bond enthalpy (approximate values):
- C–Cl: ~340 kJ mol⁻¹
- C–F: ~485 kJ mol⁻¹
This means:
- C–Cl bonds undergo homolytic fission under UV light → release Cl• radicals.
- C–F bonds remain intact because typical UV wavelengths in the stratosphere do not carry enough energy to break them.
What is the reverse process of homolytic fission?
The reverse of homolytic fission is called radical recombination.
- Two radicals combine
- Each donates one unpaired electron to form a covalent bond
Example Cl• + Cl• → Cl2 (two chlorine radicals recombine to form a chlorine molecule)
Chlorine radicals released from CFCs are able to break down ozone (O3), but not oxygen (O2), in the stratosphere. What does this suggest about the relative strengths of bonds in the two allotropes?
This suggests that the bonds in ozone (O3) are weaker than the bonds in oxygen (O2).
Chlorine radicals (Cl•) are highly reactive but still require a certain amount of energy to break a bond.
The fact that Cl• can break the O–O bonds in ozone but not the O=O double bond in O2 means O3 has weaker bonds (lower bond enthalpy) than O2.