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Benzene

|Section2={{Chembox Properties | C=6 | H=6 | Appearance = Colorless liquid | Odour = Aromatic, gasoline-like | Density = 0.8765(20) g/cm3 | Solubility = 1.53 g/L (0 °C) 1.81 g/L (9 °C) 1.79 g/L (15 °C) 1.84 g/L (30 °C) 2.26 g/L (61 °C) 3.94 g/L (100 °C) 21.7 g/kg (200 °C, 6.5 MPa) 17.8 g/kg (200 °C, 40 MPa)http://chemister.ru/Database/properties-en.php?dbid=1&id=644 | SolubleOther = Soluble in alcohol, CHCl3, CCl4, diethyl ether, acetone, acetic acid | Solubility1 = 5.83 g/100 g (20 °C) 6.61 g/100 g (40 °C) 7.61 g/100 g (60 °C) | Solvent1 = ethanediol | Solubility2 = 20 °C, solution in water: 1.2 mL/L (20% v/v) | Solvent2 = ethanol | Solubility3 = 20 °C, solution in water: 7.69 mL/L (38.46% v/v) 49.4 mL/L (62.5% v/v) | Solvent3 = acetone | Solubility4 = 52 g/100 g (20 °C) | Solvent4 = diethylene glycol | MeltingPtC = 5.53 | BoilingPtC = 80.1 | Viscosity = 0.7528 cP (10 °C) 0.6076 cP (25 °C) 0.4965 cP (40 °C) 0.3075 cP (80 °C) | LambdaMax = 255 nm | RefractIndex = 1.5011 (20 °C) 1.4948 (30 °C) | LogP = 2.13 | MagSus = -54.8·10−6 cm3/mol | VaporPressure = 12.7 kPa (25 °C) 24.4 kPa (40 °C) 181 kPa (100 °C) }} |Section3={{Chembox Structure | MolShape = Trigonal planar | Dipole = 0 D }} |Section4={{Chembox Thermochemistry | DeltaHf = 48.7 kJ/mol | HeatCapacity = 134.8 J/mol·K | Entropy = 173.26 J/mol·K | DeltaHc = 3267.6 kJ/mol }} |Section7={{Chembox Hazards | ExternalSDS = HMDB | GHSPictograms = Sigma-Aldrich Co., Benzene. Retrieved on 2014-05-29. | GHSSignalWord = Danger | HPhrases = | PPhrases = | FlashPtC = −11.63 | AutoignitionPtC = 497.78 | ExploLimits = 1.2–7.8% | NFPA-H = 2 | NFPA-F = 3 | NFPA-R = 0 | LD50 = 930 mg/kg (rat, oral) | PEL = TWA 1 ppm, ST 5 ppm | REL = Ca TWA 0.1 ppm ST 1 ppm | IDLH = 500 ppm | MainHazards = potential occupational carcinogen, flammable | LCLo = 44,000 ppm (rabbit, 30 min)44,923 ppm (dog)52,308 ppm (cat)20,000 ppm (human, 5 min) }} |Section8={{Chembox Related | OtherCompounds = Toluene Borazine }} }} Benzene is an important organic chemical compound with the chemical formula C6 H6. The benzene molecule is composed of 6 carbon atoms joined in a ring with 1 hydrogen atom attached to each. As it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon. Benzene is a natural constituent of crude oil and is one of the elementary petrochemicals. Due to the cyclic continuous pi bond between the carbon atoms, benzene is classed as an aromatic hydrocarbon, the second n- annulene ( 6-annulene). It is sometimes abbreviated Ph–H. Benzene is a colorless and highly flammable liquid with a sweet smell, and is responsible for the aroma around petrol stations. It is used primarily as a precursor to the manufacture of chemicals with more complex structure, such as ethylbenzene and cumene, of which billions of kilograms are produced. As benzene has a high octane number, it is an important component of gasoline. As benzene is a human carcinogen, most non-industrial applications have been limited.

History

Discovery

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.The word "benzoin" is derived from the Arabic expression "luban jawi", or " frankincense of Java". 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. On pages 443–450, Faraday discusses "bicarburet of hydrogen" (benzene). On pages 449–450, he shows that benzene's empirical formula is C6H6, although he doesn't realize it because he (like most chemists at that time) used the wrong atomic mass for carbon (6 instead of 12). In 1833, Eilhard Mitscherlich produced it by distilling benzoic acid (from gum benzoin) and lime. He gave the compound the name benzin. In a footnote on page 43, Liebig, the journal's editor, suggested changing Mitscherlich's original name for benzene (namely, "benzin") to "benzol", because the suffix "-in" suggested that it was an alkaloid (e.g., Chinin (quinine)), which benzene isn't, whereas the suffix "-ol" suggested that it was oily, which benzene is. Thus on page 44, Mitscherlich states: "Da diese Flüssigkeit aus der Benzoësäure gewonnen wird, und wahrscheinlich mit den Benzoylverbindungen im Zusammenhang steht, so gibt man ihr am besten den Namen Benzol, da der Name Benzoïn schon für die mit dem Bittermandelöl isomerische Verbindung von Liebig und Wöhler gewählt worden ist." (Since this liquid benzene is obtained from benzoic acid and probably is related to benzoyl compounds, the best name for it is "benzol", since the name "benzoïn" has already been chosen, by Liebig and Wöhler, for the compound that's isomeric with the oil of bitter almonds benzaldehyde.) In 1836, the French chemist Auguste Laurent named the substance "phène";Laurent, Auguste (1836) "Sur la chlorophénise et les acides chlorophénisique et chlorophénèsique," Annales de Chemie et de Physique, vol. 63, pp. 27–45, see p. 44: "Je donne le nom de phène au radical fondamental des acides précédens (φαινω, j'éclaire), puisque la benzine se trouve dans le gaz de l'éclairage." (I give the name of "phène" (φαινω, I illuminate) to the fundamental radical of the preceding acids, because benzene is found in illuminating gas.) 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. . In 1845, Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar.Hofmann, A. W. (1845) "Ueber eine sichere Reaction auf Benzol" (On a reliable test for benzene), Annalen der Chemie und Pharmacie, vol. 55, pp. 200–205; on pp. 204–205, Hofmann found benzene in coal tar oil. Four years later, Mansfield began the first industrial-scale production of benzene, based on the coal-tar method.Charles Mansfield filed for (November 11, 1847) and received (May 1848) a patent (no. 11,960) for the fractional distillation of coal tar. 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. The empirical formulas of organic compounds that appear in Hofmann's article (p. 3) are based upon an atomic mass of carbon of 6 (instead of 12) and an atomic mass of oxygen of 8 (instead of 16). In 1997, benzene was detected in deep space.

Ring formula

(1867),Claus, Adolph K.L. (1867) "Theoretische Betrachtungen und deren Anwendungen zur Systematik der organischen Chemie" (Theoretical considerations and their applications to the classification scheme of organic chemistry), Berichte über die Verhandlungen der Naturforschenden Gesellschaft zu Freiburg im Breisgau (Reports of the Proceedings of the Scientific Society of Freiburg in Breisgau), 4 : 116-381. In the section Aromatischen Verbindungen (aromatic compounds), pp. 315-347, Claus presents Kekulé's hypothetical structure for benzene ( p. 317), presents objections to it, presents an alternative geometry ( p. 320), and concludes that his alternative is correct ( p.326). See also figures on p. 354 or p. 379. Dewar (1867),Dewar, James (1867) "On the oxidation of phenyl alcohol, and a mechanical arrangement adapted to illustrate structure in the non-saturated hydrocarbons," Proceedings of the Royal Society of Edinburgh 6: 82–86. Ladenburg (1869),Ladenburg, Albert (1869) "Bemerkungen zur aromatischen Theorie" (Observations on the aromatic theory), Berichte der Deutschen Chemischen Gesellschaft 2: 140–142. Armstrong (1887),Armstrong, Henry E. (1887) "An explanation of the laws which govern substitution in the case of benzenoid compounds," Journal of the Chemical Society, 51, 258–268; see p. 264. Thiele (1899)Thiele, Johannes (1899) "Zur Kenntnis der ungesättigten Verbindungen" (On our knowledge of unsaturated compounds), Justus Liebig’s Annalen der Chemie,306: 87–142; see: "VIII. Die aromatischen Verbindungen. Das Benzol." (VIII. The aromatic compounds. Benzene.), pp. 125–129. See further: Thiele (1901) "Zur Kenntnis der ungesättigen Verbindungen," Justus Liebig’s Annalen der Chemie, 319: 129–143. 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 1861J. Loschmidt, Chemische Studien (Vienna, Austria-Hungary: Carl Gerold's Sohn, 1861), pp. 30, 65. 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. On p. 100, Kekulé suggests that the carbon atoms of benzene could form a "chaîne fermée" (a closed chain, a loop). 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.Rocke, A. J. (2010) Image and Reality: Kekule, Kopp, and the Scientific Imagination, University of Chicago Press, pp. 186–227, . 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 Ouroboros or Endless knot). This vision, he said, came to him after years of studying the nature of carbon-carbon bonds. This was 7 years after he had solved the problem of how carbon atoms could bond to up to four other atoms at the same time. Curiously, a similar, humorous depiction of benzene had appeared in 1886 in a pamphlet entitled Berichte der Durstigen Chemischen Gesellschaft (Journal of the Thirsty Chemical Society), a parody of the Berichte der Deutschen Chemischen Gesellschaft, only the parody had monkeys seizing each other in a circle, rather than snakes as in Kekulé's anecdote.English translation Some historians have suggested that the parody was a lampoon of the snake anecdote, possibly already well known through oral transmission even if it had not yet appeared in print. Kekulé's 1890 speech in which this anecdote appeared has been translated into English. If the anecdote is the memory of a real event, circumstances mentioned in the story suggest that it must have happened early in 1862.Gillis, Jean "Auguste Kekulé et son oeuvre, realisee a Gand de 1858 a 1867," Memoires de l'Academie Royale de Belgique, 37:1 (1866), 1–40. The cyclic nature of benzene was finally confirmed by the crystallographer Kathleen Lonsdale in 1929.

Nomenclature

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.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 concise dictionary of chemistry (Braunschweig, Germany: Friedrich Vieweg und Sohn, 1874), vol. 1, p. 1142. 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).Graebe (1869) "Ueber die Constitution des Naphthalins" (On the structure of naphthalene), Annalen der Chemie und Pharmacie, 149 : 20–28 ; see especially p. 26. In 1870, the German chemist Viktor Meyer first applied Gräbe's nomenclature to benzene.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.

Early applications

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. In 1903, Ludwig Roselius popularized the use of benzene to decaffeinate coffee. This discovery led to the production of Sanka. This process was later discontinued. Benzene was historically used as a significant component in many consumer products such as Liquid Wrench, several paint strippers, rubber cements, spot removers, and other products. Manufacture of some of these benzene-containing formulations ceased in about 1950, although Liquid Wrench continued to contain significant amounts of benzene until the late 1970s.

Occurrence

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, most benzene was obtained as a by-product of coke production (or "coke-oven light oil") for the steel industry. However, in the 1950s, increased demand for benzene, especially from the growing polymers 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.

Structure

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. Benzene has 6 hydrogen atoms – fewer than the corresponding parent alkane, hexane. The molecule is planar. The MO description involves the formation of three delocalized π orbitals spanning all six carbon atoms, while the VB description involves a superposition of resonance structures. It is likely that this stability contributes to the peculiar molecular and chemical properties known as aromaticity. To accurately reflect the nature of the bonding, benzene is often depicted with a circle inside a hexagonal arrangement of 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.

Benzene derivatives

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.

Production

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.Hillis O. Folkins "Benzene" Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.

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 (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

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 + 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.

Toluene disproportionation

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

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).

Other methods

Although of no commercial significance, many other routes to benzene exist. Phenol and halobenzenes can be reduced with metals. Benzoic acid and its salts undergo decarboxylation to benzene. Via the reaction the diazonium compound with hypophosphorus acid aniline gives benzene. Trimerization of acetylene gives benzene.

Uses

Benzene is used mainly as an intermediate to make other chemicals, above all ethylbenzene, cumene, cyclohexane, nitrobenzene, and alkylbenzene. More than half of the entire benzene production is processed into ethylbenzene, a precursor to styrene, which is used to make polymers and plastics like polystyrene and EPS. Some 20% of the benzene production is used to manufacture cumene, which is needed to produce phenol and acetone for resins and adhesives. Cyclohexane consumes ca. 10% of the world's benzene production; it is primarily used in the manufacture of nylon fibers, which are processed into textiles and engineering plastics. Smaller amounts of benzene are used to make some types of rubbers, lubricants, dyes, detergents, drugs, explosives, and pesticides. In 2013, the biggest consumer country of benzene was China, followed by the USA. Benzene production is currently expanding in the Middle East and in Africa, whereas production capacities in Western Europe and North America are stagnating. 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. File:Benzene_uses.png|center|Major commodity chemicals and polymers derived from benzene. Clicking on the image loads the appropriate article|600px|thumb rect 39 660 435 807 Benzene rect 665 60 1062 207 Ethylbenzene rect 665 426 1062 579 Cumene rect 665 795 1062 942 Cyclohexane rect 665 1161 1062 1317 Aniline rect 665 1533 1062 1686 Chlorobenzene rect 1215 345 1614 495 Acetone rect 1215 636 1614 783 Phenol rect 1764 57 2163 210 Styrene rect 1764 432 2163 585 Bisphenol A rect 1764 1083 2163 1233 Adipic acid rect 1764 1332 2163 1482 Caprolactam rect 2313 57 2712 207 Polystyrene rect 2313 315 2712 462 Polycarbonate rect 2313 570 2712 717 Epoxy resin rect 2313 822 2712 975 Phenolic resin rect 2313 1083 2712 1233 Nylon 6-6 rect 2313 1335 2712 1485 Nylon 6 desc bottom-left

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%.Kolmetz, Gentry, Guidelines for BTX Revamps, AIChE 2007 Spring Conference 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%.

Reactions

The most common reactions of benzene involve substitution of a proton by other groups.
"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Benzene, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Benzene&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany