Benzene

• 1. Introduction

  ›

  Benzene is a colorless liquid that smells sweet and helps give gasoline its aroma. It is six carbon atoms locked in a flat hexagonal ring, each carbon holding a single hydrogen, and nothing else. That simple arrangement, just carbon and hydrogen, hides a long argument among chemists about how those atoms could possibly fit together. It hides a darker fact too. The same compound that flavors the smell of a fuel station is classified as a carcinogen, a notorious cause of bone marrow failure, with no exposure level considered safe. How did a substance first scraped from an oily residue become one of the elementary petrochemicals, produced and processed by the billions of kilograms? Why did a German chemist say he found its shape in a daydream of a snake biting its own tail? And how did a molecule celebrated for its stability come to be linked with leukemia, with poisoned rivers, and with one of history's worst crimes? The answers begin with a word borrowed from an aromatic resin traded into Europe centuries ago.

• 2. A Name From Resin

  ›

  Gum benzoin, an aromatic resin known since ancient times in Southeast Asia, gave benzene its name. European pharmacists and perfumers met it through trade routes in the 16th century. From this resin, chemists drew an acidic material by sublimation and called it flowers of benzoin, or benzoic acid. The hydrocarbon later traced back to benzoic acid inherited the related names benzin, benzol, and benzene. Michael Faraday first isolated and identified the compound in 1825, pulling it from the oily residue left by the production of illuminating gas. He gave it the cumbersome name bicarburet of hydrogen. In 1833, Eilhard Mitscherlich made it by distilling benzoic acid from gum benzoin together with lime, and named it benzin. The French chemist Auguste Laurent offered another label in 1836, calling the substance phene. That word survives in two everyday chemical terms: phenol, which is hydroxylated benzene, and phenyl, the radical left when a hydrogen atom is removed from the ring. The naming was settling, but the substance was about to leave the laboratory bench for the factory floor.

• 3. From Coal Tar To Industry

  ›

  In 1845, Charles Blachford Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar. Four years later he began the first industrial-scale production, built on that coal-tar method. As more chemists handled related substances, a sense grew that they formed a single chemical family. In 1855, Hofmann was the first to apply the word aromatic to this group, naming it after a property many of its members shared. For commercial supply, the source shifted with the times. Until World War II, much benzene came as a by-product of coke production for the steel industry, known as coke-oven light oil. In the 1950s, rising demand, especially from the growing polymers industry, pushed producers to draw benzene from petroleum instead. Today most benzene comes from the petrochemical industry, with only a small fraction made from coal. The compound has also turned up far from any factory. Benzene was detected in deep space in 1997, and it has been found on Mars.

• 4. The Riddle Of The Ring

  ›

  Just one hydrogen atom for each carbon atom made benzene's structure a puzzle that resisted easy answers. The empirical formula was known, but that highly polyunsaturated shape was hard to pin down. Archibald Scott Couper in 1858 and Johann Josef Loschmidt in 1861 each proposed structures with multiple double bonds or multiple rings. With so little understood about aromatic chemistry, no one could marshal evidence to favor one formula over another. In 1865, Friedrich August Kekule published a paper in French, since he was then teaching in Francophone Belgium, proposing a ring of six carbon atoms with alternating single and double bonds. He followed it the next year with a longer paper in German. Kekule pointed to a striking pattern: every monoderivative of benzene seemed to have only one isomer, and every disubstituted derivative seemed to have exactly three, now understood as the ortho, meta, and para patterns. His symmetrical ring explained those facts and benzene's one-to-one carbon-hydrogen ratio. The discovery mattered enough that in 1890 the German Chemical Society held an elaborate tribute, marking twenty-five years since his first benzene paper. There Kekule told the story that would follow the molecule ever after.

• 5. A Snake And A Crystal

  ›

  A snake biting its own tail, the ouroboros of ancient cultures, was how Kekule described the moment he saw the ring. He said the vision came after years of studying carbon-carbon bonds, seven years after he had worked out how a carbon atom could bond to as many as four others at once. If the anecdote records a real event, details in the story suggest it happened early in 1862. The tale has a curious companion. In 1886, a pamphlet titled Berichte der Durstigen Chemischen Gesellschaft, the Journal of the Thirsty Chemical Society, parodied a serious chemistry journal with a drawing of monkeys seizing one another in a circle rather than snakes. Some historians think the parody mocked Kekule's snake story, perhaps already passed around by word of mouth before it appeared in print. Proof of the ring's shape took decades more and came not from reverie but from radiation. In 1929, the crystallographer Kathleen Lonsdale confirmed benzene's cyclic nature using X-ray diffraction. Working with large crystals of hexamethylbenzene, a derivative sharing the same six-carbon core, she calculated more than thirty parameters. Her work showed the ring could be nothing but a flat hexagon and gave accurate distances for every carbon-carbon bond.

• 6. What Benzene Becomes

  ›

  Two-thirds of all chemicals on the American Chemical Society's lists contained at least one benzene ring, according to a 1988 report. The molecule is used mainly as an intermediate, a starting point for other chemicals. Replacing one or more of its hydrogen atoms with another group yields familiar derivatives such as phenol, toluene, and aniline. More than half of all benzene production becomes ethylbenzene, a precursor to styrene, which makes polymers and plastics like polystyrene. About 20% goes to cumene, needed to produce phenol and acetone for resins and adhesives. Cyclohexane takes roughly 10% of the world's benzene, used mainly to make nylon fibers for textiles and engineering plastics. Smaller amounts feed rubbers, lubricants, dyes, detergents, drugs, explosives, and pesticides. The chemistry behind this rests on substitution, swapping a proton on the ring for another group. Around 24,700,000 tons of ethylbenzene were produced in 1999, the most widely practiced example. Other transformations introduce different groups: sulfonation with oleum gives useful detergents, nitration produces nitrobenzene as a precursor to aniline, and hydrogenation over finely divided nickel converts benzene into cyclohexane. In 2013, the biggest consumer of benzene was China, followed by the United States.

• 7. The Hidden Hazard

  ›

  Zero is the only absolutely safe concentration for benzene, the American Petroleum Institute stated in 1948. There is no safe exposure level, and even tiny amounts can cause harm. The compound's carcinogenic activity was discovered by a Swedish pharmacologist in 1897, found in female workers at a tire-making factory. Benzene is linked to aplastic anemia, acute leukemia, bone marrow abnormalities, and cardiovascular disease, and the International Agency for Research on Cancer rates it as known to be carcinogenic to humans, Group 1. It targets the liver, kidney, lung, heart, and brain, and can break DNA strands and damage chromosomes. The harm traces back to how the body handles it. Benzene metabolism begins in the liver and involves several enzymes, including cytochrome P450 2E1, which converts benzene to benzene oxide and drives further changes. In the bone marrow, the enzyme myeloperoxidase turns certain polyphenols into benzoquinones, and these metabolites damage cells in multiple ways. Pure benzene oxidizes in the body into an epoxide, benzene oxide, which is not excreted readily and can interact with DNA to produce harmful mutations. Because benzene is everywhere in gasoline and hydrocarbon fuels, human exposure is a global health problem.

• 8. Living With Exposure

  ›

  About 50% of all nationwide exposure to benzene in the United States comes from smoking tobacco or breathing tobacco smoke. After smoking 32 cigarettes a day, a smoker takes in about 1.8 mg of benzene, roughly ten times the average daily intake of a nonsmoker. Other major sources are automobile service stations, motor vehicle exhaust, and industrial emissions, with contaminated water adding a further route. Regulators have drawn firm lines around the chemical. The United States Environmental Protection Agency set a maximum contaminant level for benzene in drinking water at 0.005 mg per liter, or 5 ppb, while its health goal for the same water is zero. The Occupational Safety and Health Administration limits workplace air to 1 part per million over an 8-hour workday and a 40-hour workweek, with a short-term limit of 5 ppm for 15 minutes. The risk from a working lifetime at 1 ppm has been estimated at 5 excess leukemia deaths per 1,000 exposed employees. The dangers turn concrete in disasters and crimes. In 2005, the water supply to Harbin, a Chinese city of almost nine million people, was cut off after benzene leaked into the Songhua River following an explosion at a factory in Jilin on the 13th of November 2005. Nazi Germany used benzene injected into the body as one of its methods of killing.