Alkanes as saturated hydrocarbons from crude oil, fractional distillation, cracking. Combustion of alkanes and the formation of pollutants. Free-radical substitution of alkanes by halogens, including initiation, propagation and termination.

What this dot point is asking

AQA wants you to explain how alkanes are separated from crude oil and cracked, write equations for complete and incomplete combustion, describe the pollutants formed and how catalytic converters remove them, and give the full free-radical substitution mechanism.

Crude oil, fractional distillation and cracking

Crude oil is separated by fractional distillation: it is heated and the vapour rises up a column with a temperature gradient, so each fraction condenses at its boiling point. Boiling point rises with chain length because of stronger van der Waals (London) forces between larger molecules.

Cracking breaks long-chain alkanes into shorter, more useful molecules.

• Thermal cracking: high temperature and pressure, produces a high proportion of alkenes.
• Catalytic cracking: zeolite catalyst, slight pressure, lower temperature, produces branched alkanes, cycloalkanes and aromatics for motor fuels.

Combustion

Incomplete combustion (limited oxygen) gives toxic carbon monoxide ($CO$) and carbon (soot).

Alkanes are saturated (only single $C-C$ and $C-H$ bonds), non-polar and have only weak van der Waals forces between molecules, so they are generally unreactive. They burn readily and react with halogens only under ultraviolet light. Their lack of polarity means they are not attacked by nucleophiles or electrophiles, which is why their main reactions are combustion and radical substitution.

Free-radical substitution

The reaction of an alkane with a halogen needs UV light to start it, and proceeds through three stages with single-electron (fishhook) movement.

Because each propagation step regenerates a radical, a single initiation event can trigger thousands of product molecules. The reaction is hard to control: further substitution gives a mixture of $\text{CH}_3\text{Cl}$, $\text{CH}_2\text{Cl}_2$, $\text{CHCl}_3$ and $\text{CCl}_4$, and termination of different radicals gives traces of ethane and other products. This poor selectivity is why radical substitution is rarely used to make a single pure product in industry.

Try this

Q1. Write an equation for the incomplete combustion of propane forming carbon monoxide. [2 marks]

• Cue. $2C_3H_8 + 7O_2 \rightarrow 6CO + 8H_2O$.

Q2. Name the three stages of free-radical substitution. [3 marks]

• Cue. Initiation, propagation, termination.