Benzene

Benzene is the cornerstone of aromatic organic molecules. It has a long and checkered history—much too long to cover here—so the focus will be on the very old and very new.

Legendary British scientist Michael Faraday is primarily known for his discoveries in electricity and electromagnetism; but it was he who first isolated benzene in 1825 from coal-derived “illuminating gas”, which was used for lighting in the early 19th century.

Chemists soon determined that the benzene molecule contains six carbon and six hydrogen atoms; but for decades, they struggled to determine its precise structure. In 1865, German chemist Friedrich August Kekulé published a paper in which he described benzene as consisting of a ring of six carbon atoms, each bonded to a hydrogen atom. The story goes that Kekulé dreamt of a snake biting its tail, which inspired him to conceive of the structure.

Kekulé’s original concept was that the carbon atoms are attached to each other by alternating single and double bonds. But in the years following his original paper, his experiments showed that all six bonds are equivalent, which meant that each one oscillates between single and double bonding. Eventually, researchers, including Linus Pauling, concluded that this phenomenon is the basis of aromaticity, in which the carbon atoms are connected to each other via σ- and π-bonding rather than discrete double bonds.

In the 20th century, benzene, derived mostly from petroleum, came into widespread production for use as a solvent and as a starting material for manufacturing other organic compounds, especially ethylbenzene, which is converted to styrene. The use of benzene for making a large array of products continues today; but its use as a solvent declined abruptly in the 1980s, when it was established as a human carcinogen.

People nonetheless continue to be exposed to benzene. Gasoline contains 0.5–2.0 vol% benzene, which is one reason that gasoline dispensers now connect tightly to automobiles’ fuel tanks. In the past year, studies have shown that household stoves that use natural gas or propane can expose residents to harmful concentrations of benzene.

Last month, Robert B. Jackson at Stanford University (CA) and colleagues there and at other institutions in the San Francisco Bay area reported that gas and propane combustion from stoves emits benzene and increases indoor air pollution. They reported that the mean benzene emissions from gas and propane burners set on high and ovens set to 350 °F [177 °C] ranged from 2.8 to 6.5 μg/min, 10–25 times higher than emissions from electric coil and radiant alternatives.

Jackson et al.’s findings verify others’ reports that gas stovetops and ovens could be health hazards. The issue has become political: Some jurisdictions have already banned new installations of gas-fueled cooking equipment; in other areas, protesters are loudly crying, “Save our gas stoves!”