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Typical Composition of Unleaded Petrol

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The table below gives the typical composition of unleaded petrol. If you don't recognise some of the names, take this quick link to our organic chemistry nomenclature trail. It will open in a new window so you can get back to this trail more easily.

n-hexane to n-nonane12%
isomeric alkanes and n-butane11%
cyclohexane and derivatives5%
butene to hexene25%
higher aromatics11%

Why should we care what's in unleaded petrol?

Not too many years ago, in an unnamed part of Australia where one of us was a postgraduate student, there was an item on the news about a dangerous chemical spill that had closed a building.

The offending chemical? 4 litres of xylene!

para-xylene, meta-xylene and ortho-xylene

All the while, mechanics throughout the country were washing their hands and tools in the stuff...

toluene, a derivative of benzene

In another example, benzene is rapidly being banned from most schools and universities, due to the carcinogenic and toxic effects of its vapour. It is (quite rightly) treated with great caution and paranoia.

You might be interested to know that regulations in most countries stipulate that unleaded petrol can contain up to 5% benzene.

Everyone knows that Leaded (or Super) petrol contains lead, and is a bad thing to breathe, drink, or get on your skin. What we are saying is, treat unleaded petrol with the same care - it is worse than most organic chemicals you will encounter in the laboratory.

Further information about toluene can be found at the Howard Hughes Medical Institute. A more comprehensive, but not quite as easy guide can also be found at the US Center for Disease Control and Prevention.