Reactivity 3.3 – Electron sharing reactions

R3.3.3 – Radical substitution reactions of alkanes

📌 Substitution

  • Substitution is the chemical reaction in which one functional group in a compound is replaced by another
  • Alkanes are highly unreactive as the C-C and C-H bonds are extremely strong and they do not attract reactive species as they are non-polar.
  • Alkanes also have high activation energies associated with reactions. This makes them kinetically stable.

📌 Stages of substitution

  • Substitution occurs via a 3 step chain reaction (known as a reaction mechanism)
  • Reactions between alkanes and halogens (substitution) produce halogenoalkanes and hydrogen halides (XH). These reactions require high energy provided by UV light.
  1. Initiation : The formation of radicals that are later used in the chain reaction
  2. Propagation : The radicals formed in the initiation step react with other species to form new radicals
  3. Termination : Radicals react with other radicals to re-form covalent bonds and become more energetically stable

Example : Formation of chloromethane

  1. Initiation : Cl2 → 2Cl•
  2. Propagation : Cl• + CH4 → CH3• + HCl | CH3• + Cl2 → CH3Cl + Cl•
  3. Termination : Cl• + Cl• → Cl2