Alkanes — common mistakes

In exams, you must be precise. State that UV light is needed for initiation. Use dots (•) to clearly show the unpaired electron on free radicals. Remember that propagation involves one radical on each side of the equation, while termination involves two radicals reacting to form zero radicals.

Alkanes are unreactive because they contain strong C-C and C-H sigma bonds which require a large amount of energy to break. Also, these bonds are essentially non-polar, so they do not attract electrophiles or nucleophiles.

In homolytic fission, a covalent bond breaks and each atom gets one of the bonding electrons, forming two radicals (e.g., $Cl_2 \rightarrow 2Cl\cdot$). In heterolytic fission, the bond breaks and one atom gets both bonding electrons, forming a positive and a negative ion (e.g., $H-Cl \rightarrow H^+ + Cl^-$).

Several factors lead to a mixture. Firstly, substitution can occur at any position on the carbon chain, leading to isomers (e.g., 1-chloropropane and 2-chloropropane). Secondly, further substitution can occur, where a second hydrogen on the same molecule is replaced (e.g., forming dichloromethane from chloromethane). Finally, different termination steps produce various side-products (e.g., two alkyl radicals joining).

Cracking is vital for the oil industry. It converts the surplus of long-chain hydrocarbons from fractional distillation, which have low demand, into more valuable products. These include shorter-chain alkanes for petrol (gasoline) and alkenes, which are essential starting materials for the production of polymers (plastics) and other chemicals.