Benzene: Encyclopedia - Benzene

Benzene

Benzene, also known as C₆H₆, PhH, and benzol, is an organic chemical compound which is a colorless and flammable liquid with a pleasant, sweet smell. Benzene is a known carcinogen. It is a component of gasoline. It is an important industrial solvent and precursor in the production of drugs, plastics, gasoline, synthetic rubber, and dyes. Benzene is a natural constituent of crude oil, but it is usually synthesized from other compounds present in petroleum. Benzene is an aromatic hydrocarbon, and the second [n]-annulene ([6]-annulene).

Benzene - History

Benzene was discovered in 1825 by the English scientist Michael Faraday, who isolated it from oil gas and gave it the name bicarburet of hydrogen. In 1833, the German chemist Eilhard Mitscherlich produced it via the distillation of benzoic acid (from gum benzoin) and lime. Mitscherlich gave the compound the name benzin. In 1845, the English chemist Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar. Four years later, Mansfield began the first industrial-scale production of benzene, based on the coal-tar method.

Benzene - Structure

The formula of benzene (C₆H₆) caused a mystery for some time after its discovery, as no explanation had been found that could account for all the bonds — carbon usually forms four single bonds and hydrogen one.

The chemist Friedrich August Kekulé von Stradonitz was the first to deduce the ring structure of benzene. An often-repeated story claims that after years of studying carbon bonding, benzene and related molecules, he dreamt one night of the Ouroboros, a snake eating its own tail, and that upon waking he was inspired to deduce the ring structure of benzene. However, the story first appeared in the Berichte der Durstigen Chemischen Gesellschaft (Journal of the Thirsty Chemical Society), a parody of the Berichte der Deutschen Chemischen Gesellschaft which appeared annually in the late 19th century on the occasion of the congress of German chemists; as such, it is probably to be treated with circumspection.

While his (more formal) claims were well publicized and accepted, by the early 1920s Kekulé's biographer came to the conclusion that Kekulé's understanding of the tetravalent nature of carbon bonding depended on the previous research of Archibald Scott Couper (1831-1892); further, the Austrian chemist Josef Loschmidt (1821-1895) had earlier posited a cyclic structure for benzene as early as 1862. The cyclic nature of benzene was finally confirmed by the eminent crystallographer Kathleen Lonsdale.

Benzene presents a special problem in that, to account for all the bonds, there must be alternating double carbon bonds:

Using X-ray diffraction, researchers discovered that all of the carbon-carbon bonds in benzene are of the same length, and it is known that a single bond is longer than a double bond. In addition, the bond length, the distance between the two bonded atoms in benzene is greater than a double bond, but shorter than a single bond. There seemed to be in effect, a bond and a half between each carbon.

This is explained by electron delocalization. In order to visualise this, one should consider the position of electrons in the bonds of benzene.

One representation is that the structure exists as a superposition of the forms below, rather than either form individually. This type of structure is called a resonance hybrid.

In reality, neither form really exists. Delocalisation must be explained using a higher level of theory than single and double bonds. The single bonds are formed with electrons in line between the carbon atoms - this is called σ (sigma) symmetry. Double bonds consist of a sigma bond and another, π bond. This second bond has electrons orbiting in paths above and below the plane of the ring at each bonded carbon atom. The π-bonds are formed from atomic p-orbitals above and below the plane of ring. The following diagram shows the positions of these p-orbitals:

Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalised. This means that instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry.

This delocalisation of electrons is known as aromaticity, and gives benzene great stability. This is the fundamental property of aromatic chemicals which differentiates them from non-aromatics.

To reflect the delocalised nature of the bonding, benzene may be depicted as a circle inside a hexagon in chemical structure diagrams:

As is common in diagrams of organic structures, the carbon atoms in the diagram above have been left unlabeled.

Benzene occurs sufficiently often as a component of organic molecules that there is a Unicode symbol with the code 232C to represent it: ⌬

Note: Many fonts do not have this Unicode character, so your browser may not be able to display it correctly.

Benzene - Substituted benzenes

Many important chemicals are essentially benzene, with one or more of the hydrogen atoms replaced with another functional group:

Benzene - Alkyl substituents alkylbenzenes

• Ethylbenzene C₆H₅-CH₂-CH₃
• Mesitylene C₆H₃(-CH₃)₃
• Toluene C₆H₅-CH₃
• Xylene C₆H₄(-CH₃)₂

Benzene - Other substituents

• Aniline C₆H₅-NH₂
• Acetylsalicylic acid C₆H₄(-O-C(=O)-CH₃)(-COOH)
• Benzoic acid C₆H₅-COOH
• Biphenyl (C₆H₅)₂
• Chlorobenzene C₆H₅-Cl
• Nitrobenzene C₆H₅-NO₂
• Paracetamol C₆H₄(-NH-C(=O)-CH₃)(-OH)
• Phenacetin C₆H₄(-NH-C(=O)-CH₃)(-O-CH₂-CH₃)
• Phenol C₆H₅-OH
• Picric acid C₆H₂(-OH)(-NO₂)₃
• Salicylic acid C₆H₄(-OH)(-COOH)
• Trinitrotoluene C₆H₂(-CH₃)(-NO₂)₃

Benzene - Fused aromatic rings

• Anthracene
• Benzofuran
• Indole
• Isoquinoline
• Naphthalene
• Phenanthrene
• Polycyclic aromatic hydrocarbons (PAH)
• Quinoline

Benzene - Heterocyclic analogs

In heterocycles, carbon atoms in the benzene ring are replaced with another element:

• Pyrazine
• Pyridazine
• Pyridine
• Pyrimidine

See Simple aromatic ring for analogs of benzene.

Benzene - Production

Benzene may result whenever carbon-rich materials undergo incomplete combustion. It is produced naturally in volcanoes and forest fires, and is also a component of cigarette smoke.

Up until World War II, most benzene was produced as a byproduct of coke production in the steel industry. However, in the 1950s, increased demand for benzene, especially from the growing plastics industry, necessitated the production of benzene from petroleum. Today, most benzene comes from the petrochemical industry, with only a small fraction being produced from coal.

Three chemical processes contribute equally to industrial benzene production: catalytic reforming, toluene hydrodealkylation, and steam cracking.

Benzene - Catalytic reforming

In catalytic reforming, a mixture of hydrocarbons with boiling points between 60-200°C is blended with hydrogen gas, then exposed to a platinum chloride or rhenium chloride catalyst at 500-525°C and pressures ranging from 8-50 atm. Under these conditions, aliphatic hydrocarbons form rings and lose hydrogen to become aromatic hydrocarbons. The aromatic products of the reaction are then separated from the reaction mixture by extraction with any one of a number of solvents, including diethylene glycol or sulfolane, and benzene is then separated from the other aromatics by distillation.

Benzene - Toluene hydrodealkylation

Toluene hydrodealkylation converts toluene to benzene. In this process, toluene is mixed with hydrogen, then passed over a chromium, molybdenum, or platinum oxide catalyst at 500-600°C and 40-60 atm pressure. Sometimes, higher temperatures are used instead of a catalyst. Under these conditions, toluene undergoes dealkylation according to the chemical equation:

Typical reaction yields exceed 95%. Sometimes, xylene and heavier aromatics are used in place of toluene, with similar efficiency.

Benzene - Steam cracking

Steam cracking is the process used to produce ethylene and other olefins from aliphatic hydrocarbons. Depending on the feedstock used to produce the olefins, steam cracking can produce a benzene-rich liquid byproduct called pyrolysis gasoline. Pyrolysis gasoline can be blended with other hydrocarbons as a gasoline additive, or distilled to separate it into its components, including benzene.

Benzene - Uses

In the 19th and early 20th centuries, benzene was used as an aftershave because of its pleasant smell. Prior to the 1920s, benzene was frequently used as an industrial solvent, especially for degreasing metal. As its toxicity became obvious, other solvents replaced benzene in applications that directly exposed the user to benzene.

Benzene was also used to initially decaffeinate coffee by German importer Lugwig Roselius in 1903. This lead to the production of Sanka, -ka for kaffein, but later discontinued the use of benzene.

As a gasoline additive, benzene increases the octane rating and reduces knocking. As a result, gasoline often contained several percent benzene before the 1950s, when tetraethyl lead replaced it as the most widely used antiknock additive. However, with the global phaseout of leaded gasoline, benzene has made a comeback as a gasoline additive in some nations. In the United States, concern over its negative health effects and the possibility of benzene entering the groundwater have led to stringent regulation of gasoline's benzene content, with values around 1% typical. European gasoline specifications now contain the same 1% limit on benzene content.

By far the largest use of benzene is as an intermediate to make other chemicals. The most widely produced derivatives of benzene are styrene, which is used to make polymers and plastics, phenol for resins and adhesives (via cumene), and cyclohexane, which is used in Nylon manufacture. Smaller amounts of benzene are used to make some types of rubbers, lubricants, dyes, detergents, drugs, explosives and pesticides.

In laboratory research, toluene is now often substituted for benzene because of health concerns.

Benzene - Reactions of benzene

Benzene - Electrophilic aromatic substitution

Electrophilic aromatic substitution is a general method of substituting aromatic rings such as benzene. Benzene is nucleophilic enough, so that in the presence of strong electrophiles such as acylium ions or alkyl carbocations, reaction will occur to ultimately give substituted benzenes.

Benzene - Friedel-Crafts acylation

The Friedel-Crafts acylation is a specific example of electrophilic aromatic substitution. The reaction is the acylation of an aromatic ring (such as benzene) with an acyl chloride using a strong Lewis acid catalyst.

Benzene - Friedel-Crafts alkylation

Like the Friedel-Crafts acylation, the Friedel-Crafts alkylation involves the alkylation of an aromatic ring (such as benzene) and an alkyl halide using a strong Lewis acid catalyst.

Benzene - Health effects

Breathing very high levels of benzene can result in death, while high levels can cause drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, and unconsciousness. Eating or drinking foods containing high levels of benzene can cause vomiting, irritation of the stomach, dizziness, sleepiness, convulsions, rapid heart rate, and death.

The major effect of benzene from chronic (long term) exposure is to the blood. Benzene damages the bone marrow and can cause a decrease in red blood cells leading to anemia. It can also cause excessive bleeding and depress the immune system, increasing the chance of infection.

Some women who breathed high levels of benzene for many months had irregular menstrual periods and a decrease in the size of their ovaries. It is not known whether benzene exposure affects the developing fetus in pregnant women or fertility in men.

Animal studies have shown low birth weights, delayed bone formation, and bone marrow damage when pregnant animals breathed benzene.

The US Department of Health and Human Services (DHHS) classifies benzene as a human carcinogen. Long-term exposure to high levels of benzene in the air can cause leukemia, a potentially fatal cancer of the blood-forming organs. In particular, Acute Myeloid Leukemia (AML) may be caused by benzene.

Several tests can show if you have been exposed to benzene. There is a test for measuring benzene in the breath; this test must be done shortly after exposure. Benzene can also be measured in the blood; however, since benzene disappears rapidly from the blood, measurements are accurate only for recent exposures.

In the body, benzene is metabolized. Certain metabolites can be measured in the urine. However, this test must be done shortly after exposure and is not a reliable indicator of how much benzene you have been exposed to, since the same metabolites may be present in urine from other sources.

The US Environmental Protection Agency has set the maximum permissible level of benzene in drinking water at 0.005 milligrams per liter (0.005 mg/L). The EPA requires that spills or accidental releases into the environment of 10 pounds (4.5 kg) or more of benzene be reported to the EPA.

The US Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit of 1 part of benzene per million parts of air (1 ppm) in the workplace during an 8-hour workday, 40-hour workweek.

Benzene - Benzene exposure

Workers in various industries that make or use benzene may be at risk for being exposed to high levels of this carcinogenic chemical. Industries that involve the use of benzene include the rubber industry, oil refineries, chemical plants, shoe manufacturers, and gasoline related industries. In 1987, OSHA estimated that about 237,000 workers in the United States were potentially exposed to benzene, and it is not known if this number has substantially changed since then.

In 2005, the water supply to the city of Harbin in China with a population of almost nine million people, was cut off because of a major benzene exposure. Benzene leaked into the Songhua River which supplies drinking water to the city, after an explosion at a China National Petroleum Corporation (CNPC) factory in the city of Jilin on 13 November.

Adapted from the Wikipedia article "Benzene", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki