12.2 Chemical reactions of the alkanes

• Alkanes have a low reactivity
• Fuels are substances that can be burnt in oxygen to release heat energy
• Alkanes burn completely in oxygen to produce carbon dioxide and water
• In a limited supply of oxygen, incomplete combustion occurs
• Incomplete combustion produces carbon monoxide
• Carbon monoxide is a toxic gas that binds to haemoglobin in red blood cells
• Methane reacts with chlorine in the presence of ultraviolet light radiation by radical substitution
• Radical substitution mechanisms has three steps: initiation, propagation and termination
• In initiation, UV radiation provides energy for homolytic fission of chlorine molecules, producing highly reactive chlorine radicals

  Cl2 -> 2Cl.
• Propagation proceeds by two reaction which produces HCl and CH3Cl

  • Chlorine radicals react with methane to form HCl and methyl radicals
  • Methyl radicals react with chlorine to form CH3Cl and a chlorine radical
• In each termination step, two radicals react together to form a new molecule
• Radical substitution has limitations for organic synthesis because a large mixture of products can be formed