Benzene
Michael Faraday first isolated benzene in 1825 from the oily residue left over during the production of illuminating gas. He called this new substance bicarburet of hydrogen and published his findings on pages 443 through 450 of a scientific journal. Faraday determined that the empirical formula was C6H6, though he used an incorrect atomic mass for carbon at the time. The word itself traces back to gum benzoin, an aromatic resin known since ancient times in Southeast Asia. European pharmacists encountered this resin via trade routes in the 16th century. They derived an acidic material named flowers of benzoin or benzoic acid from it. Eilhard Mitscherlich produced benzene by distilling benzoic acid with lime in 1833. He initially gave the compound the name benzin. A footnote by editor Justus von Liebig suggested changing the suffix to -ol because the original ending implied an alkaloid. Mitscherlich accepted this advice and renamed the liquid benzol. Auguste Laurent later called the substance phène in 1836, creating roots for words like phenol and phenyl. Charles Blachford Mansfield isolated benzene from coal tar in 1845 while working under August Wilhelm von Hofmann. Four years after that discovery, Mansfield began the first industrial-scale production using the coal-tar method.
Friedrich August Kekulé published a paper in French in 1865 suggesting the structure contained a ring of six carbon atoms. He proposed alternating single and double bonds within this closed chain. The next year he released a longer version of his theory in German. Kekulé used evidence showing only one isomer existed for any monoderivative of benzene. He also noted exactly three isomers appeared for every disubstituted derivative. These patterns corresponded to ortho, meta, and para substitution positions understood today. In 1890, the German Chemical Society organized an elaborate appreciation celebrating twenty-five years since his first paper. During this event, Kekulé described discovering the ring shape after having a reverie or day-dream. He said a snake biting its own tail appeared before him as a symbol known as the ouroboros. This vision came after years of studying carbon-carbon bonds. Kathleen Lonsdale confirmed the cyclic nature of benzene in 1929 using X-ray diffraction methods. She worked with large crystals of hexamethylbenzene which shared the same core of six carbon atoms. Lonsdale calculated more than thirty parameters from her diffraction patterns. Her work demonstrated that the benzene ring could not be anything but a flat hexagon. She provided accurate distances for all carbon-carbon bonds in the molecule.
Four chemical processes contribute to industrial benzene production including catalytic reforming and steam cracking. Between 1978 and 1981, catalytic reformates accounted for approximately 44 to 50 percent of total US benzene production. Catalytic reforming blends hydrocarbons with boiling points between 60 and 200 degrees Celsius with hydrogen gas. The mixture then exposes to platinum chloride or rhenium chloride catalysts at temperatures ranging from 500 to 525 degrees Celsius. Pressures during this process range from 8 to 50 atmospheres. Toluene hydrodealkylation converts toluene into benzene through a hydrogen-intensive process. This reaction occurs over chromium, molybdenum, or platinum oxide catalysts at 500 to 650 degrees Celsius. Pressures reach 20 to 60 atmospheres during these conversions. Steam cracking produces ethylene and other alkenes from aliphatic hydrocarbons. Depending on feedstock used, steam cracking creates a benzene-rich liquid by-product called pyrolysis gasoline. Pyrolysis gasoline can be blended as a gasoline additive or routed through extraction to recover BTX aromatics. UOP and BP commercialized methods converting LPG mainly propane and butane into aromatics.
Benzene serves mainly as an intermediate to make other chemicals like ethylbenzene and cumene. In 1988, two-thirds of all chemicals on the American Chemical Society lists contained at least one benzene ring. Approximately 24,700,000 tons of ethylbenzene were produced in 1999 alone. More than half of entire benzene production processes into ethylbenzene which becomes styrene for polymers. Some 20 percent of benzene production manufactures cumene needed for phenol and acetone resins. Cyclohexane consumes around 10 percent of world benzene production for nylon fibers. Electrophilic aromatic substitution allows substitution of a proton by other groups onto the benzene framework. Sulfonation involves oleum a mixture of sulfuric acid with sulfur trioxide to create useful detergents. Nitration reacts benzene with nitronium ions produced by combining sulfuric and nitric acids. Chlorination achieves chlorobenzene using chlorine in presence of aluminium chloride catalysts. Hydrogenation introduces hydrogen under high pressure with finely divided nickel catalysts to convert benzene into cyclohexane. Benzene derivatives include biphenyl formed by linking rings together.
Benzene is classified as a carcinogen increasing risk of cancer and bone marrow failure. Swedish pharmacologist discovered carcinogenic activity in 1897 among female workers at a tire-making factory. The American Petroleum Institute stated in 1948 that only absolutely safe concentration was zero. Long-term exposure causes leukemia potentially fatal cancer of blood-forming organs. Acute myeloid leukemia or acute nonlymphocytic leukemia results from benzene exposure. IARC rated benzene known to be carcinogenic to humans in Group 1 classifications. Benzene targets liver, kidney, lung, heart and brain causing DNA strand breaks and chromosomal damage. About 50 percent of entire nationwide United States exposure results from smoking tobacco or secondhand smoke. After smoking 32 cigarettes per day, the smoker takes in about 1.8 milligrams of benzene. This amount equals roughly ten times average daily intake by nonsmokers. In March 2006, Food Standards Agency surveyed 150 brands of soft drinks finding four contained levels above World Health Organization limits. Benzene oxidizes in body to produce epoxide benzene oxide which interacts with DNA producing harmful mutations.
Occupational Safety and Health Administration regulates benzene levels in workplace air during an eight-hour workday. Maximum allowable amount reaches one part per million parts of air for forty-hour workweeks. National Institute for Occupational Safety and Health recommends special breathing equipment when exposure exceeds 0.1 ppm. United States Environmental Protection Agency set maximum contaminant level at 0.005 mg/L for drinking water. EPA requires reporting spills or accidental releases of 10 pounds or more into environment. Short term exposure limit for airborne benzene is five ppm for fifteen minutes. Risk from exposure to 1 ppm over working lifetime estimated as five excess leukemia deaths per thousand employees. US National Institute for Occupational Safety and Health revised Immediately Dangerous to Life and Health concentration to 500 ppm. American Conference of Governmental Industrial Hygienists adopted Threshold Limit Values at 0.02 ppm TWA in 2024. Germany’s Technical Rule For Hazardous Substances sets workspace exposure limits through TRGS 900 regulations. European Union exposure limit stands about 0.66 milligrams per cubic meter. In 2005, city of Harbin China cut off water supply due to major benzene exposure after explosion at CNPC factory on November 13.
Continue Browsing
Common questions
When did Michael Faraday first isolate benzene and what was his initial name for it?
Michael Faraday first isolated benzene in 1825 from the oily residue left over during the production of illuminating gas. He called this new substance bicarburet of hydrogen.
Who proposed the ring structure of benzene and when did he publish his theory?
Friedrich August Kekulé published a paper in French in 1865 suggesting the structure contained a ring of six carbon atoms. He released a longer version of his theory in German the next year.
What percentage of US benzene production came from catalytic reformates between 1978 and 1981?
Between 1978 and 1881, catalytic reformates accounted for approximately 44 to 50 percent of total US benzene production. This process blends hydrocarbons with boiling points between 60 and 200 degrees Celsius with hydrogen gas.
How much benzene does a person consume by smoking 32 cigarettes per day?
After smoking 32 cigarettes per day, the smoker takes in about 1.8 milligrams of benzene. This amount equals roughly ten times average daily intake by nonsmokers.
When was the city of Harbin China forced to cut off water supply due to benzene exposure?
In 2005, city of Harbin China cut off water supply due to major benzene exposure after explosion at CNPC factory on November 13.