Benzene is an organic chemical with the molecular formula C₆H₆. The benzene molecule has six carbon atoms arranged in a flat, six-sided ring, with one hydrogen atom attached to each carbon. Since it contains only carbon and hydrogen atoms, benzene is a hydrocarbon.

Benzene is found naturally in petroleum and is one of the basic chemicals used in petroleum processing. Because the carbon atoms in benzene share electrons in a special way that follows Hückel's rule, it is considered an aromatic hydrocarbon. Benzene is a colorless, highly flammable liquid with a sweet smell. It contributes to the scent of gasoline. It is mainly used to make other chemicals, such as ethylbenzene and cumene, which are produced in large amounts each year. Even though benzene is widely used in industry, it is rarely used in consumer products because it is harmful to health. Benzene is a volatile organic compound.

Benzene is classified as a carcinogen. Its effects on human health, including long-term risks from accidental exposure, have been reported by news organizations like The New York Times. For example, a 2022 article mentioned that benzene contamination in the Boston area created dangerous conditions in several locations. The article noted that the chemical could eventually lead to leukemia in some people.

History

The word "benzene" comes from "gum benzoin," an aromatic resin used in Southeast Asia since ancient times. European scientists and perfumers learned about it in the 16th century through trade. From gum benzoin, an acidic substance called "flowers of benzoin" or benzoic acid was made by sublimation. A hydrocarbon linked to benzoic acid was named benzin, benzol, and finally benzene. Michael Faraday first isolated and identified benzene in 1825 from the oily residue of illuminating gas production. He called it "bicarburet of hydrogen." In 1833, Eilhard Mitscherlich made benzene by distilling benzoic acid and lime, naming it "benzin." In 1836, French chemist Auguste Laurent called the substance "phène," a root for the words "phenol" and "phenyl," which relate to benzene.

In 1845, Charles Blachford Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar. Four years later, Mansfield began the first large-scale industrial production of benzene using coal tar. Chemists gradually realized that many substances were chemically related to benzene, forming a family of compounds. In 1855, Hofmann first used the word "aromatic" to describe this family, based on a shared property of its members. In 1997, benzene was found in deep space.

The formula for benzene was known for a long time, but its structure was hard to determine. Archibald Scott Couper in 1858 and Johann Josef Loschmidt in 1861 proposed structures with multiple double bonds or rings, but little was known about aromatic chemistry then. As research on aromatic substances grew, especially in Germany, more data became available. In 1865, German chemist Friedrich August Kekulé published a paper suggesting benzene had a ring of six carbon atoms with alternating single and double bonds. He supported this idea using evidence that benzene derivatives had only one isomer when substituted once and three isomers when substituted twice, matching the patterns known as ortho, meta, and para. Kekulé’s ring structure explained these patterns and the 1:1 carbon-hydrogen ratio in benzene.

In 1872, Kekulé refined his theory, describing rapid alternation of double bonds in the ring. His work was so important that in 1890, the German Chemical Society honored him for his benzene research. Kekulé described a dream of a snake biting its own tail (a symbol called the ouroboros) that inspired his discovery of the ring shape. This vision came after years of studying carbon-carbon bonds. A similar joke about monkeys in a circle appeared in a parody of a chemistry journal in 1886, possibly mocking Kekulé’s story.

In 1929, crystallographer Kathleen Lonsdale confirmed benzene’s ring structure using X-ray diffraction on hexamethylbenzene crystals. She calculated over thirty parameters and proved the ring was a flat hexagon, with equal carbon-carbon bond lengths.

In 1867, Wilhelm Körner suggested using the prefixes ortho-, meta-, and para- to describe di-substituted benzene derivatives, but he did not use them to show substituent positions. Carl Gräbe first applied these prefixes to describe substituent locations on naphthalene in 1869. Viktor Meyer later used Gräbe’s system for benzene in 1870.

In 1903, Ludwig Roselius helped make benzene use popular for decaffeinating coffee, leading to the creation of Sanka. This process was later stopped. Benzene was once used in many consumer products, such as Liquid Wrench, paint strippers, and rubber cements. Production of some benzene-containing products stopped around 1950, though Liquid Wrench still contained benzene until the 1970s.

Trace amounts of benzene are found in petroleum and coal. It forms when materials burn incompletely. Before World War II, benzene was mostly obtained as a byproduct of coke production for the steel industry. However, rising demand in the 1950s, especially from the polymer industry, led to benzene production from petroleum. Today, most benzene comes from the petrochemical industry, with only a small amount from coal. Benzene has also been detected on Mars.

Structure

X-ray diffraction shows that all six carbon-carbon bonds in benzene are the same length, measuring 140 picometers (pm). These bond lengths are longer than those in a double bond (135 pm) but shorter than those in a single bond (147 pm). This middle length happens because electrons involved in double bonds are shared equally among all six carbon atoms. Benzene has six hydrogen atoms, which is fewer than the 14 hydrogen atoms found in hexane, a related compound. Benzene and cyclohexane have similar structures, but benzene differs because it has a ring of shared electrons and loses one hydrogen atom per carbon. The molecule is flat, or planar. In terms of bonding, the molecular orbital model describes three π orbitals that extend across all six carbon atoms, while the valence bond model explains bonding as a mix of different possible structures. This stability likely explains the special properties of benzene called aromaticity. To show the shared nature of the bonds, benzene is often drawn with a circle inside a hexagon of carbon atoms.

Symbols for benzene have been created for use in writing. The Unicode Consortium assigned the symbol ⌬ (U+232C) to represent benzene with three double bonds and ⏣ (U+23E3) for a version showing shared electrons.

Benzene derivatives

Many important chemical compounds are made from benzene by replacing one or more of its hydrogen atoms with another functional group. Examples of simple benzene derivatives include phenol, toluene, and aniline, abbreviated as PhOH, PhMe, and PhNH₂, respectively. Connecting benzene rings forms biphenyl, C₆H₅–C₆H₅. Removing more hydrogen atoms creates "fused" aromatic hydrocarbons, such as naphthalene, anthracene, phenanthrene, and pyrene. The final stage of this process is graphite, which is a form of carbon without hydrogen.

In heterocycles, carbon atoms in the benzene ring are replaced with other elements. The most important variations include nitrogen. Replacing one CH with N forms the compound pyridine, C₅H₅N. Although benzene and pyridine are structurally related, benzene cannot be converted into pyridine. Replacing a second CH bond with N creates, depending on where the second N is placed, pyridazine, pyrimidine, or pyrazine.

Production

Four chemical processes help make benzene in industry: catalytic reforming, toluene hydrodealkylation, toluene disproportionation, and steam cracking. According to the ATSDR Toxicological Profile for benzene, between 1978 and 1981, catalytic reformates accounted for about 44–50% of the total US benzene production.

In catalytic reforming, a mixture of hydrocarbons with boiling points between 60 and 200 °C is mixed with hydrogen gas and then heated to 500–525 °C under pressures of 8–50 atm. A special catalyst made of platinum or rhenium helps aliphatic hydrocarbons form rings and lose hydrogen to become aromatic hydrocarbons. The aromatic products are then separated from the reaction mixture using solvents like diethylene glycol or sulfolane. Benzene is further separated from other aromatics through distillation. This step aims to produce aromatics with the least non-aromatic components. Recovery of aromatics, known as BTX (benzene, toluene, and xylene isomers), involves extraction and distillation.

UOP and BP developed a method using LPG (mainly propane and butane) to make aromatics in a way similar to catalytic reforming.

Toluene hydrodealkylation changes toluene into benzene. This process uses a lot of hydrogen. Toluene is mixed with hydrogen and passed over a catalyst made of chromium, molybdenum, or platinum oxide at 500–650 °C and 20–60 atm. Sometimes, higher temperatures are used instead of a catalyst. Under these conditions, toluene becomes benzene and methane. This reaction is irreversible but may also produce biphenyl (diphenyl) at higher temperatures. If the raw material has many non-aromatic components, such as paraffins or naphthenes, they may break down into smaller hydrocarbons like methane, increasing hydrogen use. The reaction usually produces more than 95% benzene. Sometimes, xylenes and heavier aromatics are used instead of toluene, with similar results. This method is called "on-purpose" because it is specifically designed to make benzene, unlike conventional BTX extraction processes.

Toluene disproportionation (TDP) converts toluene into benzene and xylene. Since demand for para-xylene (p-xylene) is much higher than for other xylene isomers, a modified version of TDP called Selective TDP (STDP) may be used. This process produces a xylene stream that is about 90% p-xylene. In some systems, the ratio of benzene to xylenes is adjusted to favor xylenes.

Steam cracking is a process that makes ethylene and other alkenes from aliphatic hydrocarbons. Depending on the feedstock, steam cracking can produce a benzene-rich by-product called pyrolysis gasoline. This gasoline can be blended with other hydrocarbons as a gasoline additive or processed to recover BTX aromatics (benzene, toluene, and xylenes).

Even though they are not used in industry, other methods to make benzene exist. These include reducing phenol and halobenzenes with metals, decarboxylating benzoic acid and its salts, reacting diazonium compounds from aniline with hypophosphorus acid, trimerizing acetylene, and fully decarboxylating mellitic acid.

Uses

Benzene is mainly used as a starting material to create other chemicals, including ethylbenzene (and other alkylbenzenes), cumene, cyclohexane, and nitrobenzene. In 1988, it was reported that two-thirds of all chemicals listed by the American Chemical Society contained at least one benzene ring. Over half of all benzene produced is used to make ethylbenzene, which is used to create styrene. Styrene is used to make polymers and plastics like polystyrene. About 20% of benzene is used to make cumene, which is needed to produce phenol and acetone for resins and adhesives. Cyclohexane uses about 10% of the world’s benzene production. It is mainly used to make nylon fibers, which are turned into textiles and engineering plastics. Smaller amounts of benzene are used to make certain types of rubbers, lubricants, dyes, detergents, drugs, explosives, and pesticides. In 2013, China was the largest consumer of benzene, followed by the United States. Benzene production is growing in the Middle East and Africa, while production in Western Europe and North America is not increasing.

Toluene is often used as a substitute for benzene, such as in fuel additives. Toluene has similar solvent properties to benzene but is less toxic and has a wider liquid range. Toluene is also processed into benzene.

As a gasoline additive, benzene increases the octane rating and reduces engine knocking. Before the 1950s, gasoline often contained several percent benzene until tetraethyl lead replaced it as the main antiknock additive. After leaded gasoline was phased out globally in the 1970s–1990s, benzene was used again as a gasoline additive, but it is now strictly limited in most countries. For example, in the United States, strict rules limit benzene in gasoline to about 1%. European gasoline also has a 1% limit. In 2011, the U.S. Environmental Protection Agency reduced the benzene content in gasoline to 0.62%.

In some European languages, the word for petroleum or gasoline is closely related to "benzene." For example, Germanic and Scandinavian languages use "bensin" or "benzin" for gasoline. Turkish and many Turkic languages use "benzin" for gasoline. In Catalan and Italian, "benzina" can refer to gasoline. In Polish and Greek, "benzyna" is used for gasoline or petrol.

Reactions

The most common reactions of benzene involve replacing a hydrogen atom with other groups. A method called electrophilic aromatic substitution is often used to change benzene into other compounds. Benzene is reactive enough to react with acylium ions and alkyl carbocations, forming substituted derivatives.

One widely used example of this reaction is ethylation, where benzene gains an ethyl group. In 1999, about 24,700,000 tons of ethylbenzene were produced. Another example is Friedel-Crafts alkylation, which uses an alkyl halide and a strong Lewis acid catalyst to add an alkyl group to benzene. A similar process, Friedel-Crafts acylation, uses acyl chloride and a Lewis acid catalyst like aluminum chloride or iron(III) chloride to add an acyl group to benzene.

Through electrophilic aromatic substitution, many functional groups can be added to benzene. Sulfonation involves using oleum, a mixture of sulfuric acid and sulfur trioxide, to attach a sulfonic acid group to benzene. These sulfonated compounds are useful as detergents. Nitration occurs when benzene reacts with nitronium ions (NO₂), which are made by mixing sulfuric and nitric acids. Nitrobenzene is used to make aniline. Chlorination adds chlorine atoms to benzene, producing chlorobenzene with a Lewis acid catalyst like aluminum chloride.

Hydrogenation converts benzene into cyclohexane by adding hydrogen under high pressure and using catalysts like nickel. This reaction is used in industry to fully saturate benzene into cyclohexane. However, under specific conditions, benzene can be partially hydrogenated to form cyclohexene or cyclohexadienes. A related process called Birch reduction, which does not use a catalyst, converts benzene into cyclohexadiene.

Benzene is a strong ligand in organometallic chemistry. It forms complexes with low-valent metals, such as the sandwich compound Cr(C₆H₆)₂ and the half-sandwich compound [RuCl₂(C₆H₆)]₂.

Health effects

Benzene is a substance that increases the risk of cancer and other serious health problems. It is also known to cause bone marrow failure, which affects the body's ability to produce blood cells. Many studies have shown that benzene is linked to aplastic anemia, acute leukemia, problems in the bone marrow, and heart and blood vessel diseases. Specific blood-related cancers connected to benzene include acute myeloid leukemia (AML), aplastic anemia, myelodysplastic syndrome (MDS), acute lymphoblastic leukemia (ALL), and chronic myeloid leukemia (CML).

In 1897, Swedish scientist C. G. Santesson discovered that benzene causes cancer in female workers at a tire factory. In 1948, the American Petroleum Institute stated that "the only absolutely safe level of benzene is zero." No amount of benzene exposure is considered safe, even very small amounts can cause harm. The US Department of Health and Human Services classifies benzene as a human carcinogen. Prolonged exposure to high levels of benzene in the air can lead to leukemia, a deadly cancer of the blood-forming organs. Benzene is especially linked to acute myeloid leukemia, also called acute nonlymphocytic leukemia (AML & ANLL). The International Agency for Research on Cancer (IARC) has classified benzene as "known to be carcinogenic to humans" (Group 1).

Benzene is found in gasoline and other fuels used worldwide, making it a global health concern. It affects the liver, kidneys, lungs, heart, and brain, and can cause breaks in DNA strands and damage to chromosomes, making it mutagenic. Benzene causes cancer in both animals and humans. Studies have shown that it can cause cancer in both male and female laboratory animals exposed through different methods.

Exposure to benzene

Benzene is a chemical that can be made by humans or found naturally in the environment. It comes from natural events like volcanic eruptions and wildfires, as well as human activities such as making chemicals like phenol, synthetic fibers, rubbers, lubricants, pesticides, medications, and dyes. The main ways people are exposed to benzene are through tobacco smoke, gas stations, car exhaust, and industrial emissions. Benzene can also enter the body through drinking water that is polluted. In the body, benzene is processed in the liver and then removed through urine. Scientists measure benzene in the air and water using activated charcoal tubes and a tool called a gas chromatograph. In humans, benzene can be tested in urine, blood, or breath, but these methods are limited because the body quickly breaks down benzene.

Exposure to benzene can lead to serious health problems, including aplastic anemia, leukemia, and multiple myeloma. The Occupational Safety and Health Administration (OSHA) sets rules for benzene in workplaces. The maximum allowed level of benzene in the air during an 8-hour workday is 1 part per million (ppm). Because benzene can cause cancer, the National Institute for Occupational Safety and Health (NIOSH) recommends that workers wear special breathing equipment if they might be exposed to benzene at levels above 0.1 ppm.

The United States Environmental Protection Agency (EPA) requires that benzene in drinking water not exceed 0.005 mg/L (5 parts per billion, or ppb). This rule is based on preventing cancer caused by benzene. The goal of keeping benzene out of drinking water completely is considered ideal, but it is not legally enforceable. The EPA also requires that spills of 10 pounds (4.5 kg) or more of benzene be reported.

OSHA allows up to 1 ppm of benzene in the air during an 8-hour workday and 5 ppm for 15 minutes. These limits are based on research showing health risks from benzene exposure. The risk of developing leukemia from exposure to 1 ppm over a lifetime is estimated to be 5 additional cases per 1,000 workers. OSHA also sets an action level of 0.5 ppm to encourage even lower exposure.

NIOSH updated the level of benzene that is immediately dangerous to life and health (IDLH) to 500 ppm. This level is used to ensure workers can escape from dangerous environments if their breathing equipment fails. NIOSH recommends that workers wear special breathing equipment if benzene levels exceed 0.1 ppm for 10 hours. The short-term exposure limit for benzene is 1 ppm for 15 minutes.

The American Conference of Governmental Industrial Hygienists (ACGIH) set a threshold limit value (TLV) for benzene at 0.02 ppm in 2024.

In Germany, benzene is regulated by "Technische Regeln für Gefahrstoffe" (Technical Rules for Hazardous Substances). The workspace exposure limit in Germany is TRGS 900. TRGS 910 sets the risk level for cancer-causing chemicals and explains how much exposure should be reduced. In the European Union, the exposure limit is about 0.66 mg/m³. According to Germany’s rules, the target concentration is 0.2 mg/m³, and the temporary upper limit is 1.9 mg/m³.

Scientific studies show a connection between leukemia and benzene exposure. Lowering benzene levels below 5 μg/m³ has been linked to a reduced risk of leukemia. This information highlights the importance of controlling benzene levels, as it has caused health issues in some areas. This research was supported by Germany’s Federal Ministry of Environment, Nature Conservation, and Nuclear Safety.

Germany is one of the top three benzene producers in the European Union. The company BASF is responsible for overseeing benzene and other chemicals. Germany is also among the top six countries for benzene exports and imports globally. In the EU, Germany is a major trader of benzene.

Several tests can detect benzene exposure. Benzene can be found in breath, blood, or urine, but these tests are usually only useful within the first 24 hours after exposure because the body quickly removes it. Most people in developed countries have small amounts of benzene and other petroleum chemicals in their blood. In the body, benzene is broken down into compounds like muconic acid, phenylmercapturic acid, phenol, catechol, hydroquinone, and 1,2,4-trihydroxybenzene. These substances can be measured in urine and are used to determine how much benzene someone has been exposed to. The current ACGIH biological exposure limits for occupational exposure are 500 μg/g creatinine for muconic acid and 25 μg/g creatinine for phenylmercapturic acid in urine collected at the end of a work shift.

Benzene can be broken down by bacteria and other living organisms. In bacteria, an enzyme called dioxygenase adds oxygen to benzene, forming a compound that is then reduced to a substance called catechol. Catechol is further broken down into acetyl CoA and succinyl CoA, which are used for energy production.

In humans, benzene is processed in the liver. Key enzymes involved include cytochrome P450 2E1 (CYP2E1), quinine oxidoreductase (NQ01), glutathione (GSH), and myeloperoxidase (MPO). CYP2E1 helps convert benzene into oxepin (benzene oxide), phenol into hydroquinone, and hydroquinone into benzenetriol and catechol. These compounds are then turned into polyphenols. In the bone marrow, MPO converts these polyphenols into benzoquinones. These substances can damage DNA by interfering with topoisom