Chemical Properties
Benzene is a colorless, flammable liquid with a pleasant odor. It is used as a solvent in
many areas of industries, such as rubber and shoe manufacturing, and in the production
of other important substances, such as styrene, phenol, and cyclohexane. It is essential in
the manufacture of detergents, pesticides, solvents, and paint removers. It is present in
fuels such as gasoline up to the level of 5%.
General Description
A clear colorless liquid with a petroleum-like odor. Flash point less than 0°F. Less dense than water and slightly soluble in water. Hence floats on water. Vapors are heavier than air.
Reactivity Profile
BENZENE(71-43-2) reacts vigorously with allyl chloride or other alkyl halides even at minus 70°C in the presence of ethyl aluminum dichloride or ethyl aluminum sesquichloride. Explosions have been reported [NFPA 491M 1991]. Ignites in contact with powdered chromic anhydride [Mellor 11:235 1946-47]. Incompatible with oxidizing agents such as nitric acid. Mixtures with bromine trifluoride, bromine pentafluoride, iodine pentafluoride, iodine heptafluoride and other interhalogens can ignite upon heating [Bretherick 5th ed. 1995]. BENZENE(71-43-2) and cyanogen halides yield HCl as a byproduct (Hagedorn, F. H. Gelbke, and Federal Republic of Germany. 2002. Nitriles. In Ullman Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA.). The reaction of BENZENE(71-43-2) and trichloroacetonitrile evolves toxic chloroform and HCl gasses. (Hagedorn, F., H.-P. Gelbke, and Federal Republic of Germany. 2002. Nitriles. In Ullman Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA.).
Air & Water Reactions
Highly flammable. Slightly soluble in water.
Hazard
A confirmed carcinogen. Highly toxic.
Flammable, dangerous fire risk. Explosive limits
in air 1.5 to 8% by volume.
Health Hazard
Dizziness, excitation, pallor, followed by flushing, weakness, headache, breathlessness, chest constriction, nausea, and vomiting. Coma and possible death.
Health Hazard
Exposure to low concentrations of benzene vapor or liquid causes dizziness, lightheadedness, headache, loss of appetite, stomach upset, and irritation to the nose and throat.
Prolonged exposure to high concentrations of benzene leads to functional irregularities in
the heart beat and in severe cases to death. Benzene is a known carcinogen to humans. It
causes leukemia and blood disorders such as aplastic anemia. The major types of leukemia
related to benzene exposure are (i) acute myelogenous leukemia (AML); (ii) acute lymphocytic leukemia (ALL); (iii) chronic myelogenous leukemia, also called chronic myeloid leukemia (CML); (iv) chronic lymphocytic leukemia (CLL), and hairy cell leukemia (HCL).
Occupational exposure to benzene is frequent, such as in road-tanker drivers and Chinese
glueand shoe-making factory workers. Exposure to benzene has been linked with the
development of rarer forms of leukemia, such as AML and ALL. It has also been linked to
lymphoma and rare blood diseases
Potential Exposure
Benzene is used as a constituent in
motor fuels; as a solvent for fats; inks, oils, paints, plastics,
and rubber, in the extraction of oils from seeds and
nuts; in photogravure printing. It is also used as a chemical
intermediate. By alkylation, chlorination, nitration, and
sulfonation, chemicals, such as styrene, phenols, and
malefic anhydride are produced. Benzene is also used in
the manufacture of detergents, explosives, pharmaceuticals;
in the manufacture of cyclohexane and ethylbenzene;
and dye-stuffs. Increased concern for benzene as a significant
environmental pollutant arises from public exposure
to the presence of benzene in gasoline and the increased
content in gasoline due to requirements for unleaded fuels
for automobiles equipped with catalytic exhaust
converters.
Fire Hazard
Behavior in Fire: Vapor is heavier than air and may travel considerable distance to a source of ignition and flash back.
First aid
If this chemical gets into the eyes, remove any
contact lenses at once and irrigate immediately for at least
15 minutes, occasionally lifting upper and lower lids. Seek
medical attention immediately. If this chemical contacts the
skin, remove contaminated clothing and wash immediately
with soap and water. Seek medical attention immediately.
If this chemical has been inhaled, remove from exposure,
begin rescue breathing (using universal precautions, including
resuscitation mask) if breathing has stopped and CPR if
heart action has stopped. Transfer promptly to a medical
facility. When this chemical has been swallowed, rinse
mouth, get medical attention. Do not induce vomiting.
Shipping
UN1114 Benzene, Hazard Class: 3; Labels: 3—
Flammable liquid
Incompatibilities
Incompatible with oxidizers (chlorates,
nitrates, peroxides, permanganates, perchlorates, chlorine,
bromine, fluorine, etc.); contact may cause fires or explosions.
Keep away from alkaline materials, strong bases,
strong acids, oxoacids, epoxides, many fluorides and perchlorates,
nitric acid.
Description
Benzene is a colorless, volatile, highly flammable liquid that is used extensively in the chemical industry and received wide interest in the early days of organic chemistry.
Because of its structure, benzene is a very stable organic compound. It does not readily undergo addition reactions. Addition reactions involving benzene require high temperature, pressure, and special catalysts. The most common reactions involving benzene involve substitution reactions. Numerous atoms and groups of atoms may replace a hydrogen atom or several hydrogen atoms in benzene. Th ree important types of substitution reactions involving benzene are alkylation, halogenation, and nitration. In alkylation, an alkyl group or groups substitute for hydrogen(s).
Waste Disposal
Dissolve or mix the material
with a combustible solvent and burn in a chemical incinerator
equipped with an afterburner and scrubber. All federal,
state, and local environmental regulations must be
observed. Dilution with alcohol or acetone to minimize
smoke is recommended. Bacterial degradation is also
possible.
Physical properties
Clear, colorless to light yellow watery liquid with an aromatic, musty, phenolics or gasoline-like
odor. At 40 °C, an odor threshold concentration of 190 μg/L in air was determined by Young et al.
(1996). An odor threshold of 4.68 ppmv was determined by Leonardos et al. (1969). A detection
odor threshold concentration of 108 mg/m3 (34 ppmv) was reported by Punter (1983). The average
least detectable odor threshold concentrations in water at 60 °C and in air at 40 °C were 0.072 and
0.5 mg/L, respectively (Alexander et al., 1982).
History
Benzene was discovered in 1825 by Michael Faraday (1791–1867), who identified it in a liquid residue from heated whale oil. Faraday called the compound bicarburet of hydrogen, and its name was later changed to benzin by Eilhardt Mitscherlich (1794–1863), who isolated the compound from benzoin (C14H12O2).
Definition
ChEBI: A six-carbon aromatic annulene in which each carbon atom donates one of its two 2p electrons into a delocalised pi system. A toxic, flammable liquid byproduct of coal distillation, it is used as an industrial solvent. Benzene is a carcinogen
that also damages bone marrow and the central nervous system.
Production Methods
Today benzene, which is a natural component of petroleum, is obtained from petroleum by several processes. Toluene hydrodealkylation involves mixing toluene (C6H5CH3) and hydrogen in the presence of catalysts and temperatures of approximately 500°C and pressures of about 50 atmospheres to produce benzene and methane: C6H5CH3 + H2 → C6H6 + CH4. Hydrodealkylation strips the methyl group from toluene to produce benzene. Toluene disproportionation involves combining toluene so that the methyl groups bond to one aromatic ring, producing benzene and xylene. Benzene can also be obtained from petroleum reforming in which temperature, pressure, and catalysts are used to convert petroleum components to benzene, which can then be extracted using solvents and distillation processes. Another source of benzene is pyrolysis gasoline or pygas.
Reactions
Benzene reacts (1) with chlorine, to form (a) substitution products (one-half of the chlorine forms hydrogen chloride) such as chlorobenzene, C6H5Cl; dichlorobenzene, C6H4Cl2(1,4) and (1,2); trichlorobenzene, C6H3Cl3(1,2,4); tetrachlorobenzene (1,2,3,5); and (b) addition products, such as benzene dichloride C6H6Cl2; benzene tetrachloride, C6H6Cl4; and benzene hexachloride, C6H6Cl6. The formation of substitution products of the benzene nucleus, whether in benzene or its homologues, is favored by the presence of a catalyzer, e.g., iodine, phosphorus, iron; (2) with concentrated HNO3, to form nitrobenzene, C6H5NO2; 1,3- dinitrobenzene, C6H4(NO2)2 (1,3), 1,3,5-trinitrobenzene, C6H3(NO2)3 (1,3,5); (3) with concentrated H2SO4, to form benzene sulfonic acid, C6H5SO3H, benzene disulfonic acid, C6H4(SO3H)2(1,3), benzene trisulfonic acid, C6H3(SO3H)3 (1,3–5); (4) with methyl chloride plus anhydrous aluminum chloride (Friedel-Crafts reaction) to form toluene, monomethyl benzene, C6H5CH3; dimethyl benzene C6H4(CH3)2; trimethyl benzene, C6H3(CH3)3; (5) with acetyl chloride plus anhydrous aluminum chloride (Friedel-Crafts reaction) to form acetophenone (methylphenyl ketone), C6H5COCH3.
Flammability and Explosibility
Benzene is a highly flammable liquid (NFPA rating = 3), and its vapors may travel a
considerable distance to a source of ignition and "flash back." Vapor-air mixtures are
explosive above the flash point. Carbon dioxide and dry chemical extinguishers
should be used to fight benzene fires.
Chemical Reactivity
Reactivity with Water No reaction; Reactivity with Common Materials: No reactions; Stability During Transport: Stable; Neutralizing Agents for Acids and Caustics: Not pertinent; Polymerization: Not pertinent; Inhibitor of Polymerization: Not pertinent.
Industrial uses
Benzene (C6H6, CAS No. 71-43-2) is an aromatic hydrocarbon compound used extensively in the chemical industry as an intermediate in the manufacture of polymers and other products. It is also a common atmospheric contaminant and is present in motor vehicle exhaust emissions and cigarette smoke.
In 1990, it was discovered by the USA soft drinks industry that benzene could be produced at low levels in certain soft drinks containing a benzoate preservative and ascorbic acid. Since benzene is a known human carcinogen, its presence in food and beverages is clearly undesirable.
Biochem/physiol Actions
Environmental carcinogen; hematoxin that is linked to increased incidence of leukemia in humans.
Carcinogenicity
Benzene is known to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in humans.
Environmental Fate
Benzene is released to air primarily by vaporization and combustion emissions associated with its use in gasoline. Other sources are vapors from its production and use in manufacturing other chemicals. In addition, benzene may be in industrial effluents discharged into water and accidental releases from gas and oil production, refining and distribution industries. Benzene released to soil will either evaporate very quickly or leach to groundwater. It can be biodegraded by soil and groundwater microbes. Benzene released to surface water should mostly evaporate within a few hours to a few days, depending on quantity, temperature, water turbulence, etc. Although benzene does not degrade by hydrolysis, it may be biodegraded by microbes.
storage
work with benzene
should be conducted in a fume hood to prevent exposure by inhalation, and splash
goggles and impermeable gloves should be worn at all times to prevent eye and skin
contact. Benzene should be used only in areas free of ignition sources.
Purification Methods
For most purposes, *benzene can be purified sufficiently by shaking with conc H2SO4 until free from thiophene, then with H2O, dilute NaOH and water, followed by drying (with P2O5, sodium, LiAlH4, CaH2, 4X Linde molecular sieve, or CaSO4, or by passage through a column of silica gel, and for a preliminary drying, CaCl2 is suitable), and distillation. A further purification step to remove thiophene, acetic acid and propionic acid, is crystallisation by partial freezing. The usual contaminants in dry thiophene-free *benzene are non-benzenoid hydrocarbons such as cyclohexane, methylcyclohexane, and heptanes, together with naphthenic hydrocarbons and traces of toluene. Carbonyl-containing impurities can be removed by percolation through a Celite column impregnated with 2,4-dinitrophenylhydrazine, phosphoric acid and H2O. (Prepared by dissolving 0.5g DNPH in 6mL of 85% H3PO4 by grinding together, then adding and mixing 4mL of distilled H2O and 10g Celite.) [Schwartz & Parker Anal Chem 33 1396 1961.] *Benzene has been freed from thiophene by refluxing with 10% (w/v) of Raney nickel for 15minutes, after which the nickel is removed by filtration or centrifugation. Dry *benzene is obtained by doubly distilling high purity *benzene from a solution containing the blue ketyl formed by the reaction of sodium-potassium alloy with a small amount of benzophenone. Thiophene has been removed from *benzene (absence of bluish-green coloration when 3mL of *benzene is shaken with a solution of 10mg of isatin in 10mL of conc H2SO4) by refluxing the *benzene (1.25L) for several hours with 40g HgO (freshly precipitated) dissolved in 40mL glacial acetic acid and 300mL of water. The precipitate is filtered off, the aqueous phase is removed and the *benzene is washed twice with H2O, dried and distilled. Alternatively, *benzene dried with CaCl2 has been shaken vigorously for 0.5hour with anhydrous AlCl3 (12g/L) at 25-35o, then decanted, washed with 10% NaOH, and water, dried and distilled. The process is repeated, giving thiophene-free *benzene. [Holmes & Beeman Ind Eng Chem 26 172 1934.] After shaking successively for about an hour with conc H2SO4, distilled water (twice), 6M NaOH, and distilled water (twice), *benzene is distilled through a 3-ft glass column to remove most of the water. Absolute EtOH is added and the *benzene-alcohol azeotrope is distilled. (This low-boiling distillation leaves any non-azeotrope-forming impurities behind.) The middle fraction is shaken with distilled water to remove EtOH, and again redistilled. Final slow and very careful fractional distillation from sodium, then LiAlH4 under N2, removed traces of water and peroxides. [Peebles et al. J Am Chem Soc 82 2780 1960.] *Benzene liquid and vapour are very TOXIC and HIGHLY FLAMMABLE, and all operations should be carried out in an efficient fume cupboard and in the absence of naked flames in the vicinity. [Beilstein 5 H 175, 5 I 95, 5 II 119, 5 III 469.] Rapid purification: To dry benzene, alumina, CaH2 or 4A molecular sieves (3% w/v) may be used (dry for 6hours). Then benzene is distilled, discarding the first 5% of distillate, and stored over molecular sieves (3A, 4A) or Na wire.
Toxicity evaluation
Benzene enters the air, water, and soil as a result of industrial
processes, emissions from burning coal and oil, tobacco
smoke, gasoline exhaust, and gasoline leaks, and from natural
sources, including volcanoes and forest fires. Benzene in the
atmosphere chemically degrades in only a few days. Benzene
released to soil or waterways is subject to volatilization,
photooxidation, and biodegradation. Benzene has a short halflife
in surface water because it is so volatile.
Toxics Screening Level
The acute initial threshold screening level (ITSL) for benzene (CAS # 71-43-2) is 30 μg/m3 based on a 24-hour averaging time.
Regulations
Current USA and EU legislation does not set maximum limits for benzene in soft drinks. However, the FDA has adopted the Environmental Protection Agency (EPA) maximum contaminant level (MCL) for drinking water of 5 ppb as a quality standard for bottled water. ThisMCL has been used to evaluate the significance of benzene contamination in the soft drinks tested in surveys. The FSA has used the World Health Organization (WHO) guideline level for benzene in water of 10 mg kg-1 as a point of reference for its own survey results.
