Benzene
<p ><b>Benzene</b> is an <a href="http://en.wikipedia.org/wiki/Organic_compound" title="Organic compound" >organic</a> <a href="http://en.wikipedia.org/wiki/Chemical_compound" title="Chemical compound" >chemical compound</a> with the molecular formula <a href="http://en.wikipedia.org/wiki/Carbon" title="Carbon" >C</a><sub >6</sub><a href="http://en.wikipedia.org/wiki/Hydrogen" title="Hydrogen" >H</a><sub >6</sub>. Its molecule is composed of 6 carbon atoms joined in a ring, with 1 hydrogen atom attached to each carbon atom. Because its molecules contain only carbon and hydrogen atoms, benzene is classed as a <a href="http://en.wikipedia.org/wiki/Hydrocarbon" title="Hydrocarbon" >hydrocarbon</a>.</p>
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Benzene is a natural constituent of <a href="http://en.wikipedia.org/wiki/Petroleum" title="Petroleum" >crude oil</a>, and is one of the most basic <a href="http://en.wikipedia.org/wiki/Petrochemical" title="Petrochemical" >petrochemicals</a>. Benzene is an <a href="http://en.wikipedia.org/wiki/Aromatic_hydrocarbon" title="Aromatic hydrocarbon" >aromatic hydrocarbon</a>and the second [<i>n</i>]-<a href="http://en.wikipedia.org/wiki/Annulene" title="Annulene" >annulene</a> ([6]-annulene), a cyclic hydrocarbon with a continuous <a href="http://en.wikipedia.org/wiki/Pi_bond" title="Pi bond" >pi bond</a>. It is sometimes abbreviated <a href="http://en.wikipedia.org/wiki/Phenyl" title="Phenyl" class="mw-redirect" >Ph</a>–H. Benzene is a <a href="http://en.wikipedia.org/wiki/Color" title="Color" >colorless</a> and highly <a href="http://en.wikipedia.org/wiki/Flammable" title="Flammable" class="mw-redirect" >flammable</a> liquid with a sweet smell. It is mainly used as a precursor to heavy chemicals, such as <a href="http://en.wikipedia.org/wiki/Ethylbenzene" title="Ethylbenzene" >ethylbenzene</a> and <a href="http://en.wikipedia.org/wiki/Cumene" title="Cumene" >cumene</a>, which are produced on a billion kilogram scale. Because it has a high <a href="http://en.wikipedia.org/wiki/Octane_number" title="Octane number" class="mw-redirect" >octane number</a>, it is an important component of <a href="http://en.wikipedia.org/wiki/Gasoline" title="Gasoline" >gasoline</a>, comprising a few percent of its mass. Its <a href="http://en.wikipedia.org/wiki/Carcinogen" title="Carcinogen" >carcinogenicity</a> has limited most non-industrial applications.</p>
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The word "benzene" derives historically from "gum benzoin", sometimes called "benjamin" (i.e., <a href="http://en.wikipedia.org/wiki/Benzoin_resin" title="Benzoin resin" >benzoin resin</a>), an aromatic resin known to European pharmacists and perfumers since the 15th century as a product of southeast Asia. "Benzoin" is itself a corruption of the Arabic expression "<i >luban jawi</i>", or "<a href="http://en.wikipedia.org/wiki/Frankincense" title="Frankincense" >frankincense</a>of <a href="http://en.wikipedia.org/wiki/Java" title="Java" >Java</a>". 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.<sup id="cite_ref-rocke_4-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Benzene#cite_note-rocke-4" >[5]</a></sup>
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Four chemical processes contribute to industrial benzene production: <a href="http://en.wikipedia.org/wiki/Catalytic_reforming" title="Catalytic reforming" >catalytic reforming</a>, <a href="http://en.wikipedia.org/wiki/Toluene" title="Toluene" >toluene</a>hydrodealkylation, toluene disproportionation, and <a href="http://en.wikipedia.org/wiki/Steam_cracking" title="Steam cracking" class="mw-redirect" >steam cracking</a>. According to the <a href="http://en.wikipedia.org/wiki/ATSDR" title="ATSDR" class="mw-redirect" >ATSDR</a> Toxicological Profile for benzene, between 1978 and 1981, catalytic reformats accounted for approximately 44–50% of the total U.S benzene production.
Until <a href="http://en.wikipedia.org/wiki/World_War_II" title="World War II" >World War II</a>, most benzene was produced as a by-product of <a href="http://en.wikipedia.org/wiki/Coke_(fuel)" title="Coke (fuel)" >coke</a> production (or "coke-oven light oil") in the <a href="http://en.wikipedia.org/wiki/Steel" title="Steel" >steel</a> industry. However, in the 1950s, increased demand for benzene, especially from the growing<a href="http://en.wikipedia.org/wiki/Polymer" title="Polymer" >polymers</a> industry, necessitated the production of benzene from petroleum. Today, most benzene comes from the <a href="http://en.wikipedia.org/wiki/Petrochemical" title="Petrochemical" >petrochemical</a> industry, with only a small fraction being produced from coal.</p>
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Benzene is used mainly as an intermediate to make other chemicals. About 80% of benzene is consumed in the production of three chemicals, <a href="http://en.wikipedia.org/wiki/Ethylbenzene" title="Ethylbenzene" >ethylbenzene</a>, <a href="http://en.wikipedia.org/wiki/Cumene" title="Cumene" >cumene</a>, and <a href="http://en.wikipedia.org/wiki/Cyclohexane" title="Cyclohexane" >cyclohexane</a>. Its most widely produced derivative is ethylbenzene, precursor to <a href="http://en.wikipedia.org/wiki/Styrene" title="Styrene" >styrene</a>, which is used to make polymers and plastics. Cumene is converted <a href="http://en.wikipedia.org/wiki/Phenol" title="Phenol" >phenol</a> for resins and adhesives. <a href="http://en.wikipedia.org/wiki/Cyclohexane" title="Cyclohexane" >Cyclohexane</a> is used in the manufacture of Nylon. Smaller amounts of benzene are used to make some types of <a href="http://en.wikipedia.org/wiki/Rubber" title="Rubber" class="mw-redirect" >rubbers</a>, <a href="http://en.wikipedia.org/wiki/Lubricant" title="Lubricant" >lubricants</a>, <a href="http://en.wikipedia.org/wiki/Dye" title="Dye" >dyes</a>, <a href="http://en.wikipedia.org/wiki/Detergent" title="Detergent" >detergents</a>, <a href="http://en.wikipedia.org/wiki/Drug" title="Drug" >drugs</a>,<a href="http://en.wikipedia.org/wiki/Explosive" title="Explosive" class="mw-redirect" >explosives</a>, and <a href="http://en.wikipedia.org/wiki/Pesticide" title="Pesticide" >pesticides</a>.</p>
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As a <a href="http://en.wikipedia.org/wiki/Gasoline" title="Gasoline" >gasoline</a> (petrol) additive, benzene increases the <a href="http://en.wikipedia.org/wiki/Octane_rating" title="Octane rating" >octane rating</a> and reduces <a href="http://en.wikipedia.org/wiki/Engine_knocking" title="Engine knocking" >knocking</a>. As a consequence, gasoline often contained several percent benzene before the 1950s, when <a href="http://en.wikipedia.org/wiki/Tetraethyl_lead" title="Tetraethyl lead" class="mw-redirect" >tetraethyl lead</a>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 <a href="http://en.wikipedia.org/wiki/United_States" title="United States" >United States</a>, concern over its negative health effects and the possibility of benzene's entering the <a href="http://en.wikipedia.org/wiki/Groundwater" title="Groundwater" >groundwater</a> have led to stringent regulation of gasoline's benzene content, with limits typically around 1%.<sup id="cite_ref-38" class="reference" ><a href="http://en.wikipedia.org/wiki/Benzene#cite_note-38" >[39]</a></sup> European petrol specifications now contain the same 1% limit on benzene content. The <a href="http://en.wikipedia.org/wiki/United_States_Environmental_Protection_Agency" title="United States Environmental Protection Agency" >United States Environmental Protection Agency</a> introduced new regulations in 2011 that lowered the benzene content in gasoline to 0.62%.<sup id="cite_ref-39" class="reference" ><a href="http://en.wikipedia.org/wiki/Benzene#cite_note-39" >[40]</a></sup></p>
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