Soundbite molecules - benzene
Simon Cotton, teacher at Uppingham School, takes a look at those compounds that find themselves in the news or relate to our everyday lives.
In this issue: benzene.
Isn't that the chemical which was recently found to be in soft drinks?
Not all soft drinks - supermarkets removed only a few bottles of soft drinks from their shelves. This was because unacceptable levels of benzene were found in them.
How much is unacceptable?
The World Health Organisation (WHO) recommends a top limit of 10 parts per billion (ppb), while the UK's upper limit for benzene in drinking water is even less, at 1 ppb. The US Environmental Protection Agency (EPA) limits the concentration of benzene in drinking water to 5 ppb, and ultimately plans to get down to 0 ppb. There doesn't seem to be any recommended figure in EU legislation. The majority of the 230 drinks tested were below the WHO limit, but the highest level was 28 ppb.
In 1993, Professor Glenn Lawrence working at the Long Island University published research showing that benzene found in soft drinks could form by a reaction between ascorbic acid (vitamin C) and sodium benzoate, a food preservative added to drinks to extend their shelf lives. This reaction is assisted by heat and light, and may also be catalysed by traces of metals.
Is benzene dangerous?
Benzene is a carcinogen, ie a cancer-causing chemical. Continuous exposure to even small amounts of the chemical lowers white cell levels in blood. By interfering with blood cell production, anaemia can result, and the ability of the body to fight infections is decreased. At high levels of exposure, bone cancer (leukaemia) can occur.
So these drinks are dangerous to us?
No. An occasional drink does not pose much of a hazard. What matters more is continual exposure, for example if you were doing a job where benzene was in the air. There are many other ways of being exposed to benzene. Petrol fumes, especially in areas of heavy traffic and around petrol stations, contain benzene. Benzene is added - up to 1 per cent - to unleaded petrol as an anti-knock agent and helps improve the fuel's 'octane rating'. Smokers are particularly at risk from benzene because smoking a cigarette releases between 50 and 150 micrograms of benzene, which means 'passive' smokers are also exposed to the chemical. Smokers can get almost 90 per cent of their benzene exposure from their cigarettes. Some benzene is also formed when wood is burned.
How long has benzene been known to be dangerous?
People have known about benzene for nearly 200 years. Michael Faraday first isolated the chemical in 1825. He heated oil gas, a fuel derived from whale fat and used for lighting, and found a compound with the formula C6H6in the distillate. Benzene used to be obtained from the distillation of coal, but nowadays it is made by 'reforming' hydrocarbons over 'precious metal' catalysts at 500°C. No one had much idea about the structure until German chemist Friedrich Kekulé is said to have had a dream in 1865 that led him to suggest the cyclic structure. Only in the second half of the 20th century did people become aware of the health hazards from continuous exposure to benzene and start to take precautions in its handling.
Is benzene useful?
Many important compounds contain the benzene ring; lots of them have characteristic smells (aromas), which led to the chemistry of benzene and other ring-containing compounds to be called aromatic chemistry. Benzene has a rather sweet smell and in the 19th and early 20th centuries people used the chemical as an aftershave. Even today, benzene is used as the starting material for a huge number of chemicals, such as dyes, explosives and many polymers, such as nylon and polystyrene. A lot of pharmaceuticals are also derived from benzene.