Benzene

Benzene
General
Systematic name	Benzene
Other names	Benzol 1,3,5-cyclohexatriene
Molecular formula	C6H6
SMILES	c1ccccc1 C1=CC=CC=C1
InChI	InChI=1/C6H6 /c1-2-4-6-5-3-1/h1-6H
Molar mass	78.1121 g/mol
Appearance	Colorless liquid
CAS number	[71-43-2]
Properties
Density and phase	0.8786 g/cm³, liquid
Solubility in water	1.79 g/L (25 °C)
Melting point	5.5 °C (278.6 K)
Boiling point	80.1 °C (353.2 K)
Viscosity	0.652 cP at 20 °C
Structure
Molecular shape	Planar
Symmetry group	D6h
Dipole moment	0 D
Hazards
MSDS	External MSDS
EU classification	Flammable (F) Carc. Cat. 1 Muta. Cat. 2 Toxic (T)
NFPA 704	NFPA 704 safety square 2 3 0
R-phrases	Template:R45, Template:R46, Template:R11, Template:R36/38, Template:R48/23/24/25, Template:R65
S-phrases	Template:S53, Template:S45
Flash point	−11 °C
Autoignition temperature	561 °C
RTECS number	CY1400000
Related compounds
Related hydrocarbons	cyclohexane naphthalene
Related compounds	toluene borazine
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references

Benzene, also known as benzol, is an organic chemical compound with the formula C₆H₆. It is sometimes abbreviated Ph. Benzene is a colorless and flammable liquid with a sweet smell and a relatively high melting point. It is carcinogenic and its use as additive in gasoline is now limited, but it is an important industrial solvent and precursor in the production of drugs, plastics, 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).

History

Benzene has been the subject of many studies by many famous scientists ranging from Michael Faraday to Linus Pauling. In 1825 Faraday reported its isolation from oil gas and gave it the name bicarburet of hydrogen. In 1833, Eilhard Mitscherlich produced it via the distillation of benzoic acid (from gum benzoin) and lime. Mitscherlich gave the compound the name benzin. In 1845, 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.

Structure

Main article: aromaticity

The formula of benzene (C₆H₆) mystified scientists who could not figure out its structure. 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 and structure of benzene. Other common tale is he obtained the inspiration from the figure of an hexagon in a tabern sign in Germany. 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; accordingly, it should probably 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); furthermore, Josef Loschmidt (1821-1895) had earlier posited a cyclic structure for benzene as early as 1861. 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 of 140 picometres (pm). The C-C bond lengths are greater than a double bond (134pm) but shorter than a single bond (147pm). This intermediate distance is explained by electron delocalization: the electrons for C-C bonding are distributed equally between each of the six carbon atoms. One representation is that the structure exists as a superposition of so-called resonance structures, rather than either form individually. This delocalisation of electrons is known as aromaticity, and gives benzene great stability. This enhanced stability is the fundamental property of aromatic molecules that differentiates them from molecules that are non-aromatic. To reflect the delocalised nature of the bonding, benzene is often depicted with a circle inside a hexagonal arrangement of carbon atoms:

As is common in organic chemistry, 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:

⌬

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

Substituted benzene derivatives

Main article: Aromatic hydrocarbons

Many important chemicals are derived from benzene, wherein with one or more of the hydrogen atoms is replaced with another functional group. Examples of simple benzene derivatives are phenol, toluene, and aniline, abbreviated PhOH,PhMe, and PhNH₂, respectively. Linking benzene rings gives biphenyl, C₆H₅-C₆H₅. Further loss of hydrogen gives "fused" aromatic hydrocarbons, such naphthalene and anthracene. The limit of the fusion process is the hydrogen-free material graphite.

In heterocycles, carbon atoms in the benzene ring are replaced with other elements. The most important derivatives are the rings containing nitrogen. Replacing one CH with N gives the compound pyridine, C₅H₅N. Although benzene and pyridine are structurally related, benzene cannot be converted into pyridine. Replacement of a second CH bond with N gives, depending on the location of the second N, pyridazine, pyrimidine, and pyrazine.

Production

Trace amounts of benzene may result whenever carbon-rich materials undergo incomplete combustion. It is produced 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.

Catalytic reforming

In catalytic reforming, a mixture of hydrocarbons with boiling points between 60-200 °C is blended with hydrogen gas and then exposed to a bifunctional 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.

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:

C₆H₅CH₃ + H₂ → C₆H₆ + CH₄

This irreversible reaction is accompanied by an equilibrium side reaction that produces biphenyl (aka diphenyl): 2C6H6 ↔ H2 + C12H10

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

Toluene disproportionation

Where a chemical complex has similar demands for both benzene and xylene, then toluene disproportionation (TDP) may be an attractive alternative. Broadly speaking 2 toluene molecules are reacted and the methyl groups rearranged from one toluene molecule to the other, yielding one benzene molecule and one xylene molecule.

Given that demand for para-xylene (p-xylene) substantially exceeds demand for other xylene isomers, a refinement of the TDP process called Selective TDP (STDP) may be used. In this process, the xylene stream exiting the TDP unit is approximately 90% paraxylene.

Steam cracking

Steam cracking is the process for producing 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.

Uses

Early uses

In the 19th and early-20th centuries, benzene was used as an after-shave lotion and for douches 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, benzene was supplanted by other solvents.

In 1903, Ludwig Roselius popularized the use of benzene to decaffeinate coffee. This discovery lead to the production of Sanka (the letters "ka" in the brand name stand for kaffein). This process was later discontinued.

As a petrol additive, benzene increases the octane rating and reduces knocking. Consequently, petrol often contained several percent benzene before the 1950s, when tetraethyl lead replaced it as the most widely-used antiknock additive. With the global phaseout of leaded petrol, 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 petrol's benzene content, with limits typically around 1%. European petrol specifications now contain the same 1% limit on benzene content.

Current uses of benzene

Today benzene is mainly used as an intermediate to make other chemicals. Its most widely-produced derivatives include styrene, which is used to make polymers and plastics, phenol for resins and adhesives (via cumene), and cyclohexane, which is used in the manufacture of Nylon. 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 used as a substitute for benzene. The solvent-properties of the two are similar but toluene is less toxic and has a wider liquid range.

Benzene has been used as a basic research tool in a variety of experiments including analysis of a two-dimensional gas.

Reactions of benzene

Electrophilic aromatic substitution of benzene

• Electrophilic aromatic substitution is a general method of derivatizing benzene. Benzene is sufficiently nucleophilic that it undergoes substitution by acylium ions or alkyl carbocations to afford give substituted derivatives.

Friedel-Crafts acylation of benzene by acetyl chloride

Friedel-Crafts acylation of benzene by acetyl chloride

• • The Friedel-Crafts acylation is a specific example of electrophilic aromatic substitution. The reaction involves the acylation of benzene (or many other aromatic rings) with an acyl chloride using a strong Lewis acid catalyst such as aluminium chloride..

Friedel-Crafts alkylation of benzene with methyl chloride

• • Like the Friedel-Crafts acylation, the Friedel-Crafts alkylation involves the alkylation of benzene (and many other aromatic rings) usng an alkyl halide in the presence of a strong Lewis acid catalyst.

• • sulfonation.
  • Nitration: Benzene undergoes nitration with nitronioum ions (NO₂⁺) as the electrophile. Thus, warming benzene at 50-55 degrees Celsius, with a combination of concentrated sulphuric and nitric acid to produce the electrophile, gives nitrobenzene.
• Hydrogenation: Benzene and derivatives convert to cyclohexane and derivatives when treated with hydrogen at 450K and 10atm of pressure with a finely divided nickel catalyst.
• Benzene is an excellent ligand in the organometallic chemistry of low-valent metals. Important examples include the sandwich and half-sandwich complexes respectively Cr(C₆H₆)₂ and [RuCl₂(C₆H₆)]₂.

Health effects

Benzene exposure has serious health effects. Breathing high levels of benzene can result in death, while low 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 and acute lymphoblastic leukaemia (AML & ALL) 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, because 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. The short term exposure limit for airborne benzene is 5 ppm for 15 minutes.

In March 2006, the official Food Standards Agency in Britain conducted a survey of 150 brands of soft drinks. It found that four contained benzene levels above World Health Organization limits. The affected batches were removed from sale.

In recent history there have been many examples of the harmful health effects of benzene and its derivatives. Toxic Oil Syndrome caused localised immune-suppression in Madrid in 1981 from people ingesting benzene-contaminated olive-oil. Chronic Fatigue Syndrome has also been highly correlated with people who eat "denatured" food that use solvents to remove fat or contain benzoic acid.

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.

Water and soil contamination are important pathways of concern for transmission of benzene contact. In the U.S. alone there are approximately 100,000 different sites which have benzene soil or groundwater contamination. 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.

Main article: benzene in soft drinks

References

• Archibald Scott Couper, On a New Chemical Theory, Philosophical Magazine 16, 104-116 (1858)
• Josef Loschmidt, Chemische Studien I, Carl Gerold's Sohn, Vienna (1861),
• Josef Loschmidt, Chemische Studien I, Aldrich Chemical Co, Milwaukee (1989), catalog no. Z-18576-0, and (1913) catalog no. Z-18577-9
• Kathleen Lonsdale, "The Structure of the Benzene Ring in Hexamethylbenzene," Proceedings of the Royal Society 123A: 494 (1929).
• Kathleen Lonsdale, "An X-Ray Analysis of the Structure of Hexachlorobenzene, Using the Fourier Method," Proceedings o

.

External links

• ATSDR - Case Studies in Environmental Medicine: Benzene Toxicity
• Benzene
• Benzene Material Safety Data Sheet
• International Chemical Safety Card 0015
• National Pollutant Inventory - Benzene
• NIOSH Pocket Guide to Chemical Hazards
• IARC Monograph: "Benzene"

• CID 241 from PubChem
• Computational Chemistry Wiki
• Couper and Carbon bonds
• Dept. of Health and Human Services: TR-289: Toxicology and Carcinogenesis Studies of Benzene
• Kekule, Couper and dreams of Benzene
• Loschmidt's Benzene structure
• video podcast (Sir john Cadogan giving a lecture on Benzene since Faraday, in 1991)
• benzene 3D view and pdb-file