In its crude state, petroleum is a virtually useless material. However when refined, the hydrocarbons it contains supply almost half the world’s current energy needs and are the starting chemicals from which about 90% of the worlds organic chemicals are made. Fractional distillation is the process by which the different fractions in crude petroleum are separated according to their boiling points.
One of the fractions obtained is naphtha. Further fractionation of naphtha yields petrol (C – C alkanes). In the internal combustion engine, a piston compresses a mixture of air and petrol vapour. At the point of maximum compression, an electrical spark ignites the petrol/air mixture and rapid combustion occurs. A typical reaction would be the combustion of octane:
The hot gaseous products expand against the piston and force it downwards. This mechanical energy is transmitted to the drive wheels of the car, enabling it to move.
Petrol also contains various additives such as lubricants, rust inhibitors and anti-knock agent. Some hydrocarbons have a tendency to ignite spontaneously before maximum compression is achieved. This premature explosion, known as knocking, still forces the piston downwards and powers the vehicle. However, the chemical energy in the petrol is less efficiently converted into mechanical energy. As a result, the vehicle will do fewer miles per gallon. Knocking also causes a rough ride and excessive engine wear as the explosion is occurring as the piston is still moving up in its cylinder.
The anti-knock properties of petrol must conform to an international standard known as the octane scale. Branched- chain alkanes have been found to be much more resistant to knocking than their straight-chain isomers. In particular, 2,2,4-trimethylpentane has been identified as having an exceptionally high resistance to knocking. It is said that pure 2,2,4-trimethylpentane has an octane number of 100 and pure heptane is given an octane number of 0 because it is the straight chain alkane with the lowest resistance to knocking. To determine the octane rating of petrol a sample is taken and its anti-knock performance is compared to that of 2,2,4-trimethylpentane/heptane mixtures.
There are 2 ways of increasing the petrol’s octane number. The anti-knock agent tetraethyl lead can be added. This breaks up during combustion to form tiny lead(IV)oxide particles which ensure even combustion of the fuel at a moderate rate. To prevent a build up of lead(IV)oxide in the cylinder, a small amount of 1,2-dibromoethane is added to the petrol. This reacts with the residual lead oxide to produce lead bromide, a fairly volatile compound. As it is volatile, lead bromide passes with the other exhaust gases into the atmosphere.
There is a danger with lead compounds as they are very toxic and lead to atmospheric pollution. Inhalation of lead-polluted air causes a build up of lead in the blood, and this is thought to impair the brains activity. In acute cases, lead poisoning can cause permanent brain damage. There is such concern over the effects of lead pollution that many countries are phasing out the use of tetraethyl lead. In Britain, all new petrol driven cars have to be run on unleaded petrol.
The second way of increasing petrol’s octane number is by using unleaded petrol. Unleaded petrol contains an increased proportion o branched alkanes, cycloalkanes and aromatic hydrocarbons. These compounds have excellent anti-knock properties but they only occur in relatively small amounts in crude oil.
Whenever alkanes are burnt in a limited supply of oxygen, some incomplete combustion occurs. In a petrol engine, for example, incomplete combustion causes a deposit of carbon to build up on the valves and pistons, and this eventually decreases the engines efficiency. Another product of incomplete combustion is carbon monoxide, a very poisonous gas which bonds irreversibly with the haemoglobin in the blood, thereby blocking the uptake of oxygen and preventing its transport around the body. In an enclosed space, exposure to carbon monoxide can be fatal. Car exhaust fumes may contain up to 2% by volume of carbon monoxide, together with other pollutants.
There are other concerns over the use of petrol to fuel our transport systems. Unburnt hydrocarbons, escape in exhaust fumes, and from petrol tanks when they are being filled at petrol stations. These may contain benzene, which is a known carcinogen. The oxides of nitrogen are present in the exhaust gases. These cause several pollution problems once they enter the atmosphere. One of these is acid rain. In the presence of sunlight the nitrogen dioxide present in exhaust gases also reacts with the unburnt hydrocarbons to form photochemical smog. The smog contains a variety of chemicals that are irritants, and may be responsible for the increase in asthma attacks during the summer.