Science: Molecule of the Month: A fuel-enhancing anaesthetic with explosive potential: John Emsley looks at the gas ethyl chloride, a catalyst that requires careful handling

It is 8.30pm on 1 February at Associated Octel's chemical plant in Ellesmere Port, Cheshire. A pressurised reactor vessel begins to leak highly flammable ethyl chloride mixed with corrosive hydrogen chloride gas. The alarm is raised, the emergency services alerted, and the local fire brigade is quickly on the scene. Then, at 10pm, the dreaded event occurs: a spark ignites the ethyl chloride vapour and flames engulf the reactor, which still contains more than 20,000 litres of ethyl chloride under pressure. A major disaster threatens, but thanks to modern chemical plant design there is no devastating fireball. The blaze is brought under control without people being injured or homes put at risk, though it causes severe damage to the plant.

Ethyl chloride, also known as chloroethane, is a gas with an ether-like smell. It condenses to a colourless liquid at 12C and will remain liquid under pressure at much higher temperatures. In vapour form it is twice as dense as air, so that any leakage tends to seep along the ground rather than disperse. It may well have been this which allowed the leaking material to find a source of ignition, perhaps something as tiny as an electrostatic spark.

The Health and Safety Executive (HSE) is investigating the fire, and meanwhile has banned the manufacture of ethyl chloride at Ellesmere Port. 'We have served a prohibition notice on Octel and this will remain in force until we are satisfied that they are taking all necessary precautions to limit the consequences of a release of flammable substances on workers, the environment and those living nearby,' says Mark Wheeler of the HSE.

In 1993 Octel manufactured more than 45,000 tonnes of ethyl chloride by reacting ethylene and hydrogen chloride, with the latter being made on site from chlorine gas. The chlorine is made by passing electricity through a salt solution. Only a year ago the company spent pounds 10m to become the first UK chemical company to make all its chlorine by an environmentally friendly process that does not use mercury. According to Dr John Little, managing director, the new process also cuts energy consumption by 25 per cent and meets the requirements of the Paris Convention, which aims to phase out the mercury process in Europe by 2010.

Octel does not sell much ethyl chloride as such but converts most of it to tetraethyl lead (TEL), the anti-knock additive put into petrol, which it supplies to more than 50 refineries around the world. A little TEL will turn a low-grade fuel into a high-octane one by acting as a catalyst.

The accident at Ellesmere Port demonstrated the importance of modern design and plant layout. When gas under pressure escapes from a ruptured pipe or tank it can lead to spectacular disasters, of which Piper Alpha is the worst example. Octel's ethyl chloride reactor vessel was specially designed to avert such an outcome. According to Brian Swithenbank, company safety officer, it was sheathed in a coating that automatically protects the contents when the tank is engulfed in flames. Gases within the coating cause it to expand, forming an insulating layer that can withstand heat and flames for at least two hours.

Ethyl chloride has other uses as a chemical reagent. At one time it was used to manufacture an early type of sleeping pill called sulphonal. Today it is reacted with wood pulp or cotton to make ethyl cellulose. This is used as a plastic, a coating for paper and wire, a binder for tablets and as a hot-melt adhesive. Ethyl chloride is also used by itself, in local anaesthetic sprays, as a refrigerant and as a speciality solvent for fats, oils, resins and waxes. It is non-toxic and the safe working level in air is as high as 0.1 per cent, although at such concentrations it irritates the eyes.

Ethyl chloride's main outlet is still for manufacturing TEL, although less and less of this is now produced. Lead in petrol interferes with catalytic converters, and is also undesirable for health reasons. However, in the years when most petrol was leaded with TEL, some plants in the US made up to a billion litres of ethyl chloride a year.

Another reason for the decline in production is that it is an organochlorine compound, and some environmentalists are worried about the release of such materials into the atmosphere. However, ethyl chloride poses no threat to the ozone layer because it quickly reacts with oxygen and is carried away by rain. As yet it has not been identified as a natural chemical, though its counterparts, methyl chloride and ethyl bromide, have been discovered in marine algae.

The author is science writer in residence in the Department of Chemistry, Imperial College, London.