BENZENE is an important organic chemical compound with the chemical formula C 6H 6. The benzene molecule is composed of 6 carbon atoms joined in a ring with 1 hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon .
Because benzene is a human carcinogen , most non-industrial applications have been limited.
* 1 History
* 1.1 Discovery * 1.2 Ring formula * 1.3 Nomenclature * 1.4 Early applications * 1.5 Occurrence
* 2 Structure
* 4 Production
* 5 Uses
* 5.1 Component of gasoline
* 6 Reactions
* 6.1 Sulfonation, chlorination, nitration
* 7 Health effects
* 8 Exposure to benzene
* 8.2 Toxicology
* 8.2.1 Biomarkers of exposure * 8.2.2 Biotransformations * 8.2.3 Molecular toxicology * 8.2.4 Biological oxidation and carcinogenic activity
* 8.3 Routes of exposure
* 8.3.1 Inhalation * 8.3.2 Exposure from soft drinks * 8.3.3 Contamination of water supply
* 9 See also * 10 References * 11 External links
The word "benzene" derives historically from "gum benzoin" (benzoin resin ), an aromatic resin known to European pharmacists and perfumers since the 15th century as a product of southeast Asia. An acidic material was derived from benzoin by sublimation , and named "flowers of benzoin", or benzoic acid. The hydrocarbon derived from benzoic acid thus acquired the name benzin, benzol, or benzene. Michael Faraday first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, giving it the name bicarburet of hydrogen. In 1833, Eilhard Mitscherlich produced it by distilling benzoic acid (from gum benzoin ) and lime . He gave the compound the name benzin. In 1836, the French chemist Auguste Laurent named the substance "phène"; this word has become the root of the English word "phenol ", which is hydroxylated benzene, and "phenyl ", the radical formed by abstraction of a hydrogen atom (free radical H•) from benzene. Kekulé's 1872 modification of his 1865 theory, illustrating rapid alternation of double bonds
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. Gradually, the sense developed among chemists that a number of substances were chemically related to benzene, comprising a diverse chemical family. In 1855, Hofmann used the word "aromatic " to designate this family relationship, after a characteristic property of many of its members. In 1997, benzene was detected in deep space .
Historic benzene structures (from left to right) by Claus (1867), Dewar (1867), Ladenburg (1869), Armstrong (1887), Thiele (1899) and Kekulé (1865). Dewar benzene and prismane are different chemicals that have Dewar's and Ladenburg's structures. Thiele and Kekulé's structures are used today.
The empirical formula for benzene was long known, but its highly polyunsaturated structure, with just one hydrogen atom for each carbon atom, was challenging to determine. Archibald Scott Couper in 1858 and Joseph Loschmidt in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but too little evidence was then available to help chemists decide on any particular structure.
In 1865, the German chemist Friedrich August Kekulé published a paper in French (for he was then teaching in Francophone Belgium) suggesting that the structure contained a ring of six carbon atoms with alternating single and double bonds. The next year he published a much longer paper in German on the same subject. Kekulé used evidence that had accumulated in the intervening years—namely, that there always appeared to be only one isomer of any monoderivative of benzene, and that there always appeared to be exactly three isomers of every disubstituted derivative—now understood to correspond to the ortho, meta, and para patterns of arene substitution —to argue in support of his proposed structure. Kekulé's symmetrical ring could explain these curious facts, as well as benzene's 1:1 carbon-hydrogen ratio.
The new understanding of benzene, and hence of all aromatic
compounds, proved to be so important for both pure and applied
chemistry that in 1890 the German Chemical Society organized an
elaborate appreciation in Kekulé's honor, celebrating the
twenty-fifth anniversary of his first benzene paper. Here Kekulé
spoke of the creation of the theory. He said that he had discovered
the ring shape of the benzene molecule after having a reverie or
day-dream of a snake seizing its own tail (this is a common symbol in
many ancient cultures known as the
The cyclic nature of benzene was finally confirmed by the crystallographer Kathleen Lonsdale in 1929.
The German chemist Wilhelm Körner suggested the prefixes ortho-, meta-, para- to distinguish di-substituted benzene derivatives in 1867; however, he did not use the prefixes to distinguish the relative positions of the substituents on a benzene ring. It was the German chemist Karl Gräbe who, in 1869, first used the prefixes ortho-, meta-, para- to denote specific relative locations of the substituents on a di-substituted aromatic ring (viz, naphthalene). In 1870, the German chemist Viktor Meyer first applied Gräbe's nomenclature to benzene.
In the 19th and early 20th centuries, benzene was used as an after-shave lotion 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, especially toluene (methylbenzene), which has similar physical properties but is not as carcinogenic.
Ludwig Roselius popularized the use of benzene to
decaffeinate coffee . This discovery led to the production of
This process was later discontinued.
Trace amounts of benzene are found in petroleum and coal. It is a
byproduct of the incomplete combustion of many materials. For
commercial use, until
World War II
Main article: Aromaticity The various representations of benzene
X-ray diffraction shows that all six carbon-carbon bonds in benzene
are of the same length, at 140 picometres (pm). The C–C bond lengths
are greater than a double bond (135 pm) but shorter than a single bond
(147 pm). This intermediate distance is consistent with electron
delocalization : the electrons for C–C bonding are distributed
equally between each of the six carbon atoms.
Derivatives of benzene occur sufficiently often as a component of organic molecules that the Unicode Consortium has allocated a symbol in the Miscellaneous Technical block with the code U+232C (⌬) to represent it with three double bonds, and U+23E3 (⏣) for a delocalized version.
Many important chemical compounds are derived from benzene by replacing one or more of its hydrogen atoms 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 as naphthalene and anthracene . The limit of the fusion process is the hydrogen-free allotrope of carbon, graphite .
In heterocycles , carbon atoms in the benzene ring are replaced with other elements. The most important variations contain 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 .
Four chemical processes contribute to industrial benzene production: catalytic reforming , toluene hydrodealkylation, toluene disproportionation, and steam cracking . According to the ATSDR Toxicological Profile for benzene, between 1978 and 1981, catalytic reformats accounted for approximately 44–50% of the total U.S benzene production.
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 (or reformate) 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. The extraction step of aromatics from the reformate is designed to produce aromatics with lowest non-aromatic components. Recovery of the aromatics, commonly referred to as BTX (benzene, toluene and xylene isomers), involves such extraction and distillation steps. There are a good many licensed processes available for extraction of the aromatics.
In similar fashion to this catalytic reforming, UOP and BP commercialized a method from LPG (mainly propane and butane) to aromatics.
Toluene hydrodealkylation converts toluene to benzene. In this hydrogen-intensive 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 (at the similar reaction condition). Under these conditions, toluene undergoes dealkylation to benzene and methane : C6H5CH3 + H2 → C6H6 + CH4
This irreversible reaction is accompanied by an equilibrium side reaction that produces biphenyl (aka diphenyl) at higher temperature: 2 C 6H 6 ⇌ H 2 + C 6H 5–C 6H 5
If the raw material stream contains much non-aromatic components (paraffins or naphthenes), those are likely decomposed to lower hydrocarbons such as methane, which increases the consumption of hydrogen.
A typical reaction yield exceeds 95%. Sometimes, xylenes and heavier aromatics are used in place of toluene, with similar efficiency.
This is often called "on-purpose" methodology to produce benzene, compared to conventional BTX (benzene-toluene-xylene) extraction processes.
Where a chemical complex has similar demands for both benzene and xylene , then toluene disproportionation (TDP) may be an attractive alternative to the toluene hydrodealkylation. In the broad sense, 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. In some current catalytic systems, even the benzene-to-xylenes ratio is decreased (more xylenes) when the demand of xylenes is higher.
Steam cracking is the process for producing ethylene and other alkenes from aliphatic hydrocarbons . Depending on the feedstock used to produce the olefins, steam cracking can produce a benzene-rich liquid by-product called pyrolysis gasoline . Pyrolysis gasoline can be blended with other hydrocarbons as a gasoline additive, or routed through an extraction process to recover BTX aromatics (benzene, toluene and xylenes).
Although of no commercial significance, many other routes to benzene
Toluene is now often used as a substitute for benzene, for instance as a fuel additive. The solvent-properties of the two are similar, but toluene is less toxic and has a wider liquid range. Toluene is also processed into benzene. Major commodity chemicals and polymers derived from benzene. Clicking on the image loads the appropriate article
COMPONENT OF GASOLINE
As a gasoline (petrol) additive, benzene increases the octane rating and reduces knocking . As a consequence, gasoline 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 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's entering the groundwater have led to stringent regulation of gasoline's benzene content, with limits typically around 1%. European petrol specifications now contain the same 1% limit on benzene content. The United States Environmental Protection Agency introduced new regulations in 2011 that lowered the benzene content in gasoline to 0.62%.
The most common reactions of benzene involve substitution of a proton
by other groups.
Electrophilic aromatic substitution is a general
method of derivatizing benzene.
The most widely practiced example of this reaction is the ethylation of benzene.
Approximately 24,700,000 tons were produced in 1999. Highly instructive but of far less industrial significance is the Friedel-Crafts alkylation of benzene (and many other aromatic rings) using an alkyl halide in the presence of a strong Lewis acid catalyst. Similarly, the Friedel-Crafts acylation is a related 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 or Iron(III) chloride . Friedel-Crafts acylation of benzene by acetyl chloride
SULFONATION, CHLORINATION, NITRATION
Using electrophilic aromatic substitution, many functional groups are introduced onto the benzene framework. Sulfonation of benzene involves the use of oleum , a mixture of sulfuric acid with sulfur trioxide . Sulfonated benzene derivatives are useful detergents . In nitration , benzene reacts with nitronium ions (NO2+), which is a strong electrophile produced by combining sulfuric and nitric acids. Nitrobenzene is the precursor to aniline . Chlorination is achieved with chlorine to give chlorobenzene in the presence of a catalyst such as aluminium tri-chloride.
Via hydrogenation , benzene and its derivatives convert to cyclohexane and derivatives. This reaction is achieved by the use of high pressures of hydrogen at high temperatures in the presence of a finely divided nickel , which serves as a catalyst . In the absence of the catalyst, benzene is impervious to hydrogen. This reaction is practiced on a very large scale industrially.
A bottle of benzene. The warnings show benzene is a toxic and flammable liquid.
Petroleum Institute (API) stated in 1948 that "it is
generally considered that the only absolutely safe concentration for
benzene is zero". There is no safe exposure level; even tiny amounts
can cause harm. The
US Department of Health and Human Services (DHHS)
classifies benzene as a human carcinogen . Long-term exposure to
excessive levels of benzene in the air causes leukemia , a potentially
fatal cancer of the blood-forming organs. In particular, acute myeloid
leukemia or acute nonlymphocytic leukemia (AML however, ingestion and
dermal absorption of benzene can also occur through contact with
OSHA regulates levels of benzene in the workplace. The maximum allowable amount of benzene in workroom air during an 8-hour workday, 40-hour workweek is 1 ppm. Because benzene can cause cancer , NIOSH recommends that all workers wear special breathing equipment when they are likely to be exposed to benzene at levels exceeding the recommended (8-hour) exposure limit of 0.1 ppm.
BENZENE EXPOSURE LIMITS
The United States Environmental Protection Agency has set a maximum contaminant level (MCL) for benzene in drinking water at 0.005 mg/L (5 ppb), as promulgated via the U.S. National Primary Drinking Water Regulations. This regulation is based on preventing benzene leukemogenesis. The maximum contaminant level goal (MCLG), a nonenforceable health goal that would allow an adequate margin of safety for the prevention of adverse effects, is zero benzene concentration in drinking water. The EPA requires that spills or accidental releases into the environment of 10 pounds (4.5 kg) or more of benzene be reported.
The U.S. 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. These legal limits were based on studies demonstrating compelling evidence of health risk to workers exposed to benzene. The risk from exposure to 1 ppm for a working lifetime has been estimated as 5 excess leukemia deaths per 1,000 employees exposed. (This estimate assumes no threshold for benzene's carcinogenic effects.) OSHA has also established an action level of 0.5 ppm to encourage even lower exposures in the workplace.
The U.S. National Institute for Occupational Safety and Health (NIOSH) revised the Immediately Dangerous to Life and Health (IDLH) concentration for benzene to 500 ppm. The current NIOSH definition for an IDLH condition, as given in the NIOSH Respirator Selection Logic, is one that poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment . The purpose of establishing an IDLH value is (1) to ensure that the worker can escape from a given contaminated environment in the event of failure of the respiratory protection equipment and (2) is considered a maximum level above which only a highly reliable breathing apparatus providing maximum worker protection is permitted ]. In September 1995, NIOSH issued a new policy for developing recommended exposure limits (RELs) for substances, including carcinogens. Because benzene can cause cancer, NIOSH recommends that all workers wear special breathing equipment when they are likely to be exposed to benzene at levels exceeding the REL (10-hour) of 0.1 ppm. The NIOSH short-term exposure limit (STEL – 15 min) is 1 ppm.
American Conference of Governmental Industrial Hygienists (ACGIH) adopted Threshold Limit Values (TLVs) for benzene at 0.5 ppm TWA and 2.5 ppm STEL.
Biomarkers Of Exposure
Several tests can determine exposure to benzene.
Even if it is not a common substrate for metabolism, benzene can be oxidized by both bacteria and eukaryotes . In bacteria, dioxygenase enzyme can add an oxygen to the ring, and the unstable product is immediately reduced (by NADH ) to a cyclic diol with two double bonds, breaking the aromaticity. Next, the diol is newly reduced by NADH to catechol . The catechol is then metabolized to acetyl CoA and succinyl CoA , used by organisms mainly in the Krebs Cycle for energy production.
The pathway for the metabolism of benzene is complex and begins in
the liver. Several enzymes are involved. These include cytochrome P450
2E1 (CYP2E1), quinine oxidoreductase (NQ01), GSH, and myeloperoxidase
(MPO). CYP2E1 is involved at multiple steps: converting benzene to
oxepin (benzene oxide), phenol to hydroquinone , and hydroquinone to
both benzenetriol and catechol . Hydroquinone, benzenetriol and
catechol are converted to polyphenols. In the bone marrow, MPO
converts these polyphenols to benzoquinones. These intermediates and
metabolites induce genotoxicity by multiple mechanisms including
inhibition of topoisomerase II (which maintains chromosome structure),
disruption of microtubules (which maintains cellular structure and
organization), generation of oxygen free radicals (unstable species)
that may lead to point mutations, increasing oxidative stress,
Genetic polymorphisms in these enzymes may induce loss of function or gain of function. For example, mutations in CYP2E1 increase activity and result in increased generation of toxic metabolites. NQ01 mutations result in loss of function and may result in decreased detoxification. Myeloperoxidase mutations result in loss of function and may result in decreased generation of toxic metabolites. GSH mutations or deletions result in loss of function and result in decreased detoxification. These genes may be targets for genetic screening for susceptibility to benzene toxicity.
The paradigm of toxicological assessment of benzene is shifting
towards the domain of molecular toxicology as it allows understanding
of fundamental biological mechanisms in a better way. Glutathione
seems to play an important role by protecting against benzene-induced
Biological Oxidation And Carcinogenic Activity
One way of understanding the carcinogenic effects of benzene is by
examining the products of biological oxidation. Pure benzene, for
example, oxidizes in the body to produce an epoxide, benzene oxide ,
which is not excreted readily and can interact with
ROUTES OF EXPOSURE
Outdoor air may contain low levels of benzene from automobile service stations, wood smoke, tobacco smoke, the transfer of gasoline, exhaust from motor vehicles, and industrial emissions. About 50% of the entire nationwide (United States) exposure to benzene results from smoking tobacco or from exposure to tobacco smoke. After smoking 32 cigarettes per day, the smoker would take in about 1.8 milligrams (mg) of benzene. This amount is about 10 times the average daily intake of benzene by nonsmokers.
Inhaled benzene is primarily expelled unchanged through exhalation. In a human study 16.4 to 41.6% of retained benzene was eliminated through the lungs within five to seven hours after a two- to three-hour exposure to 47 to 110 ppm and only 0.07 to 0.2% of the remaining benzene was excreted unchanged in the urine. After exposure to 63 to 405 mg/m3 of benzene for 1 to 5 hours, 51 to 87% was excreted in the urine as phenol over a period of 23 to 50 hours. In another human study, 30% of absorbed dermally applied benzene, which is primarily metabolized in the liver, was excreted as phenol in the urine.
Exposure From Soft Drinks
Main article: Benzene in soft drinks
Under specific conditions and in the presence of other chemicals benzoic acid (a preservative) and ascorbic acid (Vitamin C) may interact to produce benzene. 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. Similar problems were reported by the FDA in the United States.
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.
* 6-membered aromatic rings with one carbon replaced by another group: borabenzene , BENZENE, silabenzene , germabenzene , stannabenzene , pyridine , phosphorine , arsabenzene , pyrylium salt * Industrial Union Department v. American Petroleum Institute * BTEX
* ^ Nomenclature of Organic Chemistry : IUPAC Recommendations and
Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of
Chemistry . 2014. pp. 138, 577. ISBN 978-0-85404-182-4 . doi
* ^ Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics
(86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5 .
* ^ Arnold, D.; Plank, C.; Erickson, E.; Pike, F. (1958).
* ^ See:
* Wilhelm Körner (1867) "Faits pour servir à la détermination du lieu chimique dans la série aromatique" (Facts to be used in determining chemical location in the aromatic series), Bulletins de l'Académie royale des sciences, des lettres et des beaux-arts de Belgique, 2nd series, 24 : 166–185 ; see especially p. 169. From p. 169: "On distingue facilement ces trois séries, dans lesquelles les dérivés bihydroxyliques ont leurs terms correspondants, par les préfixes ortho-, para- et mêta-." (One easily distinguishes these three series – in which the dihydroxy derivatives have their corresponding terms – by the prefixes ortho-, para- and meta-.) * Hermann von Fehling, ed., Neues Handwörterbuch der Chemie (Braunschweig, Germany: Friedrich Vieweg und Sohn, 1874), vol. 1, p. 1142.
* ^ Graebe (1869) "Ueber die Constitution des Naphthalins" (On the
structure of naphthalene), Annalen der Chemie und Pharmacie, 149 :
20–28 ; see especially p. 26.
* ^ Victor Meyer (1870) "Untersuchungen über die Constitution der
zweifach-substituirten Benzole" (Investigations into the structure of
di-substituted benzenes), Annalen der Chemie und Pharmacie, 156 :
265–301 ; see especially pp. 299–300.
* ^ A B Hillis O. Folkins "Benzene" Ullmann’s Encyclopedia of
Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi
* ^ Moran D, Simmonett AC, Leach FE, Allen WD, Schleyer PV,
Schaefer HF (2006). "Popular Theoretical Methods Predict
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