R3.3 Electron sharing reactionsIB Chemistry HL: Revision notes

A radical is a molecular entity with an unpaired electron, shown by a dot on the atom that carries it: Cl•, •CH₃, •C₂H₅. Radicals are highly reactive because the unpaired electron readily pairs up by forming a new bond. Radicals can be atoms (Cl•) or fragments of molecules (•CH₃); they are uncharged unless stated otherwise.

In homolytic fission a covalent bond breaks so that each atom takes one electron from the shared pair, forming two radicals. (In heterolytic fission one atom takes both electrons, forming ions.) Halogens undergo homolytic fission in UV light or on heating, because the X–X bond is weak (Cl–Cl 242, Br–Br 193 kJ mol⁻¹) compared with C–H (414 kJ mol⁻¹).

This is the initiation step of a chain reaction: Cl₂ → 2Cl• (UV).

In propagation, a radical reacts with a molecule to produce a new radical, so the number of radicals stays the same. For methane: Cl• + CH₄ → •CH₃ + HCl •CH₃ + Cl₂ → CH₃Cl + Cl• The Cl• produced in the second step starts the cycle again, so one initiation event causes thousands of cycles: a chain reaction.

In termination, two radicals combine to form a molecule, removing radicals and ending the chain: 2Cl• → Cl₂ •CH₃ + Cl• → CH₃Cl 2•CH₃ → C₂H₆ The last step explains why small amounts of ethane appear when methane is chlorinated.

Radical substitution of alkanes is unselective:

• Further substitution: Cl• attacks the product (CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄).
• Different positions: in larger alkanes any hydrogen can be replaced, giving isomers (propane → 1-chloropropane and 2-chloropropane).
• Termination products: longer alkanes, e.g. butane from ethane. Using excess alkane favours monosubstitution; excess halogen favours polysubstitution. Products must be separated, for example by fractional distillation.