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Benzene

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

          You may be looking for benzine or Benzin, which are pronounced
          the same as benzene.

                               Benzene
                              Benzene
                               General
         Systematic name                                      Benzene
             Other names                                       Benzol
                                                1,3,5-cyclohexatriene
       Molecular formula                                     C[6]H[6]
                  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                                        D[6h]
           Dipole moment                                          0 D
                               Hazards
                    MSDS                                External MSDS
       EU classification              Flammable (F)
                                            Carc. Cat. 1
                                            Muta. Cat. 2
                                                            Toxic (T)
                NFPA 704

                         3
                         2
                         0

               R-phrases                       R45, R46, R11, R36/38,
                                                    R48/23/24/25, R65
               S-phrases                                     S53, 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 C[6]H[6]. It is sometimes abbreviated PhH. 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 (-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

   The formula of benzene (C[6]H[6]) 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
   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 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); 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:

   Benzene with alternating double 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:

   Benzene structure with a circle inside the hexagon

   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

   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 PhNH[2], respectively. Linking
   benzene rings gives biphenyl, C[6]H[5]-C[6]H[5]. 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,
   C[5]H[5]N. 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:

          C[6]H[5]CH[3] + H[2] → C[6]H[6] + CH[4]

   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 has been supplanted by other solvents.

   In 1903, Lugwig Roselius popularized the use of benzene to decaffeinate
   coffee. This discovery lead to the production of Sanka, -ka 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. However, 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 values
   around 1% typical. 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
     *
          + 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
            (NO[2]^+) as the electrophile. Thus, warming benzene with a
            combination of concentrated sulphuric and nitric acid gives
            nitrobenzene.
     * Hydrogenation: Benzene and derivatives convert to cyclohexane and
       derivatives when treated with hydrogen at high hydrogen pressures.
     * Benzene is an excellent ligand in the organometallic chemistry of
       low-valent metals. Important examples include the sandwich and
       half-sandwich complexes respectively Cr(C[6]H[6])[2] and
       [RuCl[2](C[6]H[6])][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 anaemia. 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, 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.
   Retrieved from " http://en.wikipedia.org/wiki/Benzene"
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   with only minor checks and changes (see www.wikipedia.org for details
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