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Wednesday, 03 August 2011 05:52

Hydrocarbons, Aromatic

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Aromatic hydrocarbons are those hydrocarbons that possess the special properties associated with the benzene nucleus or ring, in which six carbon-hydrogen groups are arranged at the corners of a hexagon. The bonds joining the six groups in the ring exhibit characteristics intermediate in behaviour between single and double bonds. Thus, although benzene can react to form addition products such as cyclohexane, the characteristic reaction of benzene is not an addition reaction but a substitution reaction in which a hydrogen is replaced by a substituent, univalent element or group.

Aromatic hydrocarbons and their derivatives are compounds whose molecules are composed of one or more stable ring structures of the type described and can be considered as derivatives of benzene according to three basic processes:

  1. by replacement of hydrogen atoms with aliphatic hydrocarbon radicals
  2. by linking of two or more benzene rings, either directly or by intermediate aliphatic chains or other radicals, or by intermediate aliphatic chains or other radicals
  3. by condensation of benzene rings.

 

Each of the ring structures can form the basis of homologous series of hydrocarbons in which a succession of alkyl groups, saturated or non-saturated, replaces one or more of the hydrogen atoms of the carbon-hydrogen groups.

The main sources of the aromatic hydrocarbons are the distillation of coal and a number of petrochemical operations—in particular, catalytic reforming, distillation of crude oil, and alkylation of lower aromatic hydrocarbons. Essential oils, containing terpenes and p-cymene, can also be obtained from pines, eucalyptus and aromatic plants, and are a by-product in the papermaking industry using the pulp of pines. Polycyclic hydrocarbons occur in the smoke of urban atmospheres.

Uses

The economic importance of the aromatic hydrocarbons has been significant since coal tar naphtha was used as a rubber solvent early in the nineteenth century. The current uses of the aromatic compounds as pure products include the chemical synthesis of plastics, synthetic rubber, paints, dyes, explosives, pesticides, detergents, perfumes and drugs. These compounds are used mainly as mixtures in solvents and constitute a variable fraction of gasoline.

Cumene is used as a high-octane blending component in aviation fuel, as a thinner for cellulose paints and lacquers, as an important starting material for the synthesis of phenol and acetone, and for the production of styrene by cracking. It serves as a constituent of many commercial petroleum solvents in the boiling range of 150 to 160 °C. It is a good solvent for fats and resins and has, therefore, been used as a replacement for benzene in many of its industrial uses. p-Cymene occurs in several essential oils and can be made from monocyclic terpenes by hydrogenation. It is a by-product in the manufacture of sulphite paper pulp and is used chiefly with other solvents and aromatic hydrocarbons as a thinner for lacquers and varnishes.

Coumarin is used as a deodorizing and odour-enhancing agent in soaps, tobacco, rubber products and perfumes. It is also used in pharmaceutical preparations.

Benzene has been banned as an ingredient in products intended for use in the home, and its uses as a solvent and component of dry-cleaning liquid have been discontinued in many countries.

Benzene has been used extensively in the manufacture of styrene, phenols, maleic anhydride and a number of detergents, explosives, pharmaceuticals and dye-stuffs. It has been used as a fuel, chemical reagent and extracting agent for seeds and nuts. The mono-, di- and trialkyl derivatives of benzene are used primarily as solvents and thinners in and in the manufacture of perfumes and dye-stuff intermediates. These substances are present in certain petroleums and in distillates of coal tar. Pseudocumene is used in the manufacture of perfumes, and 1,3,5-trimethylbenzene and pseudocumene are used also as dye-stuffs intermediates, but the chief industrial use of these substances is as solvents and paint thinners.

Toluene is a solvent for oils, resins, natural rubber (mixed with cyclohexane) and synthetic rubber, coal tar, asphalt, pitch and acetyl celluloses (hot-mixed with ethyl alcohol). It is also a solvent and diluent for cellulose paint and varnishes, and a diluent for photogravure inks. When mixed with water, it forms azeotropic mixtures that have a depolishing effect. Toluene is found in mixtures that are used as cleaning products in a number of industries and in handicrafts. It is used in the manufacture of detergent and artificial leather, and as an important raw material for organic syntheses, especially those of benzoyl and benzilidene chlorides, saccharine, chloramine T, trinitrotoluene and many dye-stuffs. Toluene is a constituent of aviation fuel and automobile gasoline. This substance was to be withdrawn from these uses in the European Union as a result of EC Council Regulation 594/91.

Naphthalene is used as the starting product in the organic synthesis of a wide range of chemicals, as a pesticide in mothballs, and in wood preservatives. It is also employed in the manufacture of indigo and is applied externally on livestock or poultry to control lice.

Styrene is used in the manufacture of a wide range of polymers (e.g., polystyrene) and copolymer elastomers, such as butadiene-styrene rubber or acrylonitrile-butadiene-styrene (ABS), that are obtained by the copolymerization of styrene with 1,3-butadiene and acrylonitrile. Styrene is widely used in the production of transparent plastics. Ethylbenzene is an intermediate in organic synthesis, particularly in the production of styrene and synthetic rubber. It is employed as a solvent or diluent, a component of automative and aviation fuels, and in the manufacture of cellulose acetate.

There are three isomers of xylene: ortho- (o-), para- (p-) and meta- (m-). The commercial product is a blend of the isomers, the largest proportion consisting of the meta- compound (up to 60 to 70%) and the smallest percentage of the para- compound (up to 5%). Xylene is used commercially as a thinner for paints, for varnishes, in pharmaceuticals, as a high-octane additive to aviation fuels, in the synthesis of dyes and for the production of phthalic acids. Since xylene is a good solvent for paraffin, Canada balsam and polystyrene, it is used in histology.

Terphenyls are used as chemical intermediates in the manufacture of non-spreading lubricants and as nuclear reactor coolants. Terphenyls and biphenyls are used as heat transfer agents, in organic synthesis and in perfume manufacture. Diphenylmethane, for instance, is used as a perfume in the soap industry and as a solvent for cellulose lacquers. It also has some applications as a pesticide.

Hazards

Absorption takes place by inhalation, ingestion and in small quantities through the intact skin. In general the monoalkyl derivatives of benzene are more toxic than the dialkyl derivatives, and the derivatives with branched chains are more toxic than those with straight chains. Aromatic hydrocarbons are metabolized through the bio-oxidation of the ring; if there are side chains, preferably of the methyl group, these are oxidized and the ring is left unchanged. They are, in large part, converted into water-soluble compounds, then conjugated with glycine, glucuronic or sulphuric acid, and eliminated in the urine.

Aromatic hydrocarbons are capable of causing acute and chronic central nervous system effects. Acutely, they can cause headaches, nausea, dizziness, disorientation, confusion and listlessness. High acute doses can even result in loss of consciousness and respiratory depression. Respiratory irritation (cough and sore throat) is a well-known acute effect. Cardiovascular symptoms can include palpitations and light-headedness. Neurological symptoms of chronic exposure can include behavioural changes, depression, mood alterations, and changes in personality and intellectual function. Chronic exposure has also been known to cause or contribute to distal neuropathy in some patients. Toluene has also been associated with a persistent syndrome of cerebellar ataxia. Chronic effects can also include dry, irritated, cracked skin, and dermatitis. Hepatotoxicity has also been associated with exposure, in particular to the chlorinated group. Benzene is a confirmed carcinogen in humans, having been known to cause all types of leukaemia but primarily acute nonlymphocytic leukaemia. It can also cause aplastic anaemia and (reversible) pancytopenia.

Aromatic hydrocarbons as a group pose a significant flammability hazard. The US National Fire Prevention Association (NFPA) has classified most compounds in this group with a flammability code of 3 (where 4 is severe hazard). Measures must be in place to prevent accumulation of vapours in the work environment and to deal with leakages and spills promptly. Extremes of heat must be avoided in the presence of vapours.

Benzene

Benzene is often referred to as “benzol” in its commercial form (which is a mixture of benzene and its homologues) and should not be confused with benzine, a commercial solvent which consists of a mixture of aliphatic hydrocarbons.

Mechanism. Absorption of benzene usually occurs through the lungs and gastrointestinal tract. It tends not to be well absorbed through the skin unless exceptionally high exposures occur. A small amount of benzene is exhaled unchanged. Benzene is widely distributed throughout the body and is metabolized mainly to phenol, which is excreted in the urine after conjugation. After exposure ceases, body tissue levels decline quickly.

From the biological point of view, it seems that the bone marrow and blood disorders found in chronic benzene poisoning can be attributed to the conversion of benzene to benzene epoxide. It has been suggested that benzene might be oxidized to epoxide directly in bone marrow cells, such as erythroblasts. As far as the toxic mechanism is concerned, benzene metabolites seem to interfere with nucleic acids. Increased rates of chromosome aberrations have been observed both in humans and in animals exposed to benzene. Any condition likely to inhibit further metabolism of benzene epoxide and conjugation reactions, especially hepatic disorders, tends to potentiate the toxic action of benzene. These factors are of importance when considering differences in individual susceptibility to this toxic agent. Benzene is discussed in more detail elsewhere in this Encyclopaedia.

Fire and explosion. Benzene is a flammable liquid, the vapour of which forms flammable or explosive mixtures in air over a large range of concentrations; the liquid will evolve vapour concentrations in this range at temperatures as low as -11 °C. In the absence of precautions, therefore, at all normal working temperatures flammable concentrations are liable to be present where the liquid is being stored, handled or used. The risk becomes more pronounced when accidental spillage or escape of liquid occurs.

Toluene and derivatives

Metabolism. Toluene is absorbed into the body mainly through the respiratory tract and, to a lesser extent, through the skin. It penetrates the alveolar barrier, the blood/air mixture being in the proportion of 11.2 to 15.6 at 37 °C, and then spreads through the different tissues in amounts depending upon their perfusion and solubility characteristics respectively.

The tissue-to-blood proportion is 1:3 except in the case of those tissues rich in fat, which have a coefficient of 80:100. The toluene then becomes oxidized to its lateral chain in the liver microsomes (microsomal mono-oxygenation). The most important product of this transformation, which represents about 68% of the absorbed toluene, is hippuric acid (AH), which appears in the urine through renal excretion mainly by being excreted in the proximal tubules. Small quantities of o-cresol (0.1%) and p-cresol (1%), which are the result of oxidation in the aromatic nucleus, can also be detected in the urine, as discussed in the Biological monitoring chapter of this Encyclopaedia.

The biological half-life of AH is very short, being of the order of 1 to 2 hours. The level of toluene in the expired air at rest is of the order of 18 ppm during an exposure rate of 100 ppm, and this drops very rapidly after exposure has terminated. The amount of toluene retained in the body is a function of the percentage of fat present. Obese subjects will retain more toluene in their body.

In the liver the same enzymatic system oxidizes toluene, styrene and benzene. These three substances therefore tend to inhibit each other competitively. Thus, if rats are heavily dosed with toluene and benzene, a reduction in the concentration of benzene metabolites will be seen in the tissue and in the urine, and similarly an increase of benzene in the expired air. In the case of trichloroethylene, the inhibition is not competitive since the two substances are not oxidized by the same enzymatic system. Simultaneous exposure will result in a reduction of AH and the appearance of trichlor compounds in the urine. There will be higher absorption of toluene under effort than at rest. With an output of 50 watts, the values detected in the arterial blood and in the alveolar air are doubled in comparison with those obtained at rest.

Acute and chronic health hazards. Toluene has an acute toxicity somewhat more intense than that of benzene. At a concentration of about 200 or 240 ppm, it gives rise after 3 to 7 h to vertigo, dizziness, difficulty in maintaining equilibrium, and headache. Stronger concentrations may result in a narcotic coma.

The symptoms of chronic toxicity are those habitually encountered with exposure to the commonly used solvents, and include: irritation of the mucous membrane, euphoria, headaches, vertigo, nausea, loss of appetite, and alcohol intolerance. These symptoms generally appear at the end of the day, are more severe at the end of the week, and become less or disappear during the weekend or on holiday.

Toluene has no action on the bone marrow. Those cases that have been reported relate either to an exposure to toluene together with benzene or are not clear on this subject. In theory it is possible that toluene can give rise to a hepatotoxic attack, but this has never been proved. Certain authors have suggested the possibility of its causing an autoimmune illness similar to the Goodpasture syndrome (autoimmune glomerulonephritis).

Several cases of sudden death are to be noted, especially in the case of children or adolescents given to glue sniffing (inhaling fumes from adhesives containing toluene among other solvents), resulting from cardiac arrest due to ventricular fibrillation with loss of catecholamines. Animal studies have shown toluene to be teratogenic only at high doses.

Fire and explosion. At all normal working temperatures, toluene evolves dangerously flammable vapours. Open lights or other agencies liable to ignite the vapour should be excluded from areas where the liquid is liable to be exposed in use or by accident. Appropriate facilities for storage and shipment are required.

Other monoalkyl derivatives of benzene. Propylbenzene is a depressant of the central nervous system with slow but prolonged effects. Sodium dodecylbenzene sulphonate is produced by catalytic reaction of tetrapropylene with benzene, acidification with sulphuric acid, and treatment with caustic soda. Repeated contact with the skin may cause dermatitis; in prolonged exposure it might act as a bland irritant of mucous membranes.

p-tert-Butyltoluene. The presence of the vapour is detectable by odour at 5 ppm. Slight conjunctival irritation occurs after exposure to 5 to 8 ppm. Exposure to the vapour gives rise to headaches, nausea, malaise and signs of neurovegetative dystonia. The metabolism of this substance is probably similar to that of toluene. The same fire and health precautions should be taken in the use of p-tert-butyltoluene as those described for toluene.

Xylene

Like benzene, xylene is a narcotic, prolonged exposure to which results in impairment of the haemopoietic organs and disturbances of the nervous system. The clinical picture of acute poisoning is similar to that of benzene poisoning. The symptoms are fatigue, dizziness, drunkenness, shivering, dyspnoea and sometimes nausea and vomiting; in more serious cases there may be unconsciousness. Irritation of the mucous membranes of the eyes, the upper airways and the kidneys are also observed.

Chronic exposure results in complaints about general weakness, excessive fatigue, dizziness, headache, irritability, sleeplessness, loss of memory, and ringing noises in the ear. Typical symptoms are cardiovascular disorders, sweetish taste in the mouth, nausea, sometimes vomiting, loss of appetite, strong thirst, burning in the eyes, and bleeding from the nose. Functional disorders of the central nervous system associated with pronouned neurological effects (e.g., dystonia), impairment of protein-forming function and reduced immunobiological reactivity may be observed in certain cases.

Women are liable to suffer from menstrual disorders (menorrhagia, metrorrhagia). It has been reported that female workers exposed to toluene and xylene in concentrations which periodically exceeded the exposure limits were also affected by pathological pregnancy conditions (toxicosis, danger of miscarriage, haemorrhage during childbirth) and infertility.

The blood changes manifest themselves as anaemia, poikilocytosis, anisocytosis, leukopenia (sometimes leukocytosis) with relative lymphocytosis, and in certain cases strongly pronounced thrombocytopenia. There are data on differences in individual susceptibility to xylene. No chronic intoxication has been observed in certain workers exposed for a few decades to xylene, whereas a third of the personnel working under the same conditions of exposure presented symptoms of chronic xylene poisoning and were disabled. Prolonged exposure to xylene may reduce the resistance of the organism and render it more susceptible to various kinds of pathogenic factors. Urinalysis reveals proteins, blood, urobilin and urobilinogen in the urine.

Fatal cases of chronic poisoning are known, in particular among workers of the intaglio printing industry but also in other branches. Cases of serious and fatal poisoning among pregnant women with haemophilia and bone-marrow aplasia have been reported. Xylene also causes skin changes, in particular eczema.

Chronic poisoning is associated with the presence of xylene traces in all organs, especially the suprarenal glands, bone marrow, spleen and nerve tissue. Xylene oxidizes in the organism to form toluic acids (o-, m-, p-methylbenzoic acids), which later react with glycine and glucuronic acid.

During the production or use of xylene there may be high concentrations in the workplace air if the equipment is not tight and open processes are used, sometimes involving large surfaces of evaporation. Large amounts are also released into the air during repair work and when cleaning the equipment.

Contact with xylene, which may have contaminated the surfaces of premises and equipment or also protective clothing, may result in its absorption through the skin. The rate of skin absorption in humans is 4 to 10 mg/cm2 per hour.

Levels of 100 ppm for up to 30 minutes have been associated with mild upper respiratory tract irritation. At 300 ppm, balance, vision and reaction times are affected. Exposure to 700 ppm for 60 minutes can result in headache, dizziness and nausea.

Other dialkyl benzene derivatives. Fire risks are associated with the use of p-cymene, which is also a primary skin irritant. Contact with the liquid can cause dryness, defatting and erythema. There is no conclusive evidence that it can affect the blood marrow. Acute exposure to p-tert-butyltoluene in concentrations of 20 ppm and above may cause nausea, metallic taste, eye irritation and giddiness. Repeated exposure has been found to be responsible for decreased blood pressure, increased pulse rate, anxiety and tremor, slight anaemia with leukopenia and eosinophilia. In repeated exposure it is also a mild skin irritant because of fat removal. Animal toxicity studies show effects on the central nervous sytem (CNS), with lesions in the corpus callosum and spinal cord.

Styrene and ethylbenzene. Styrene and ethylbenzene poisoning are very similar and are consequently dealt with together here. Styrene may enter the body by both vapour inhalation and, being lipid soluble, by absorption through intact skin. It rapidly saturates the body (in 30 to 40 min), is distributed throughout the organs and is rapidly eliminated (85% in 24 h) either in the urine (71% in the form of oxidation products of the vinyl group—hippuric and mandelic acids) or in the expired air (10%). As regards ethylbenzene, 70% of it is eliminated with the urine in the form of various metabolites—phenylacetic acid, α-phenylethyl alcohol, mandelic acid and benzoic acid.

The presence of the double bond in the side chain of styrene significantly increases the irritant properties of the benzene ring; however, the general toxic action of styrene is less pronounced than that of ethylbenzene. Liquid styrene has a local effect on the skin. Animal experiments have shown that liquid styrene irritates the skin and causes blistering and tissue necrosis. Exposure to styrene vapours may also give rise to skin irritation.

Vapours of ethylbenzene and styrene in concentrations of over 2 mg/ml may cause acute poisoning in laboratory animals; the initial symptoms are irritation of the mucous membranes of the upper respiratory tract, the eyes and mouth. These symptoms are followed by narcosis, cramps and death due to respiratory-centre paralysis. The main pathological findings are oedema of the brain and lungs, epithelial necrosis of the renal tubules, and hepatic dystrophy.

Ethylbenzene is more volatile than styrene, and its production is associated with a greater hazard of acute poisoning; both substances are toxic by ingestion. Animal experiments have shown that digestive absorption of styrene causes symptoms of poisoning similar to those resulting from inhalation. Lethal doses are as follows: 8 g/kg body weight for styrene and 6 g/kg for ethylbenzene; lethal inhalation concentrations are between 45 and 55 mg/l.

In industry acute styrene or ethylbenzene poisoning may occur as the result of a breakdown or faulty plant operation. A polymerization reaction that gets out of control is accompanied by a rapid release of heat and necessitates prompt purging the reaction vessel. Engineering controls that avoid a sudden rise of the styrene and ethylbenzene concentrations in the workplace atmosphere are essential or workers involved can be exposed to the dangerous levels with sequelae such as encephalopathy and toxic hepatitis unless they are protected by suitable respirators.

Chronic toxicity. Both styrene and ethylbenzene may also cause chronic poisoning. Prolonged exposure to styrene or ethylbenzene vapours in concentrations above permitted levels may result in functional disorders of the nervous system, irritation of the upper airways, haematological changes (in particular leukopenia and lymphocytosis) and also in hepatic and biliary tract conditions. Medical examination of workers employed for more than 5 years in polystyrene and synthetic rubber plants in which the atmospheric styrene and ethylbenzene concentrations were around 50 mg/m3 revealed cases of toxic hepatitis. Prolonged exposure to styrene concentrations of less than 50 mg/m3 caused disorders of certain liver functions (protein, pigment, glycogen). Polystyrene production workers have also been found to suffer from asthenia and nasal mucosa disorders; ovulation and menstruation disorders have also been observed.

Experimental research in rats has revealed that styrene exerts embryotoxic effects at a concentration of 1.5 mg/m3; its metabolite styrene oxide is mutagenic and reacts with microsomes, proteins and the nucleic acid of the liver cells. Styrene oxide is chemically active and several times more toxic for rats than styrene itself. Styrene oxide is classified as a Group 2A probable carcinogen by IARC. Styrene itself is considered a Group 2B possible human carcinogen.

Animal experiments on the chronic toxicity of ethylbenzene have shown that high concentrations (1,000 and 100 mg/m3) may be harmful and cause functional and organic disturbances (nervous system disorders, toxic hepatitis and upper respiratory tract complaints). Concentrations as low as 10 mg/m3 may lead to catarrhal inflammation of the upper respiratory tract mucosae. Concentrations of 1 mg/m3 give rise to disorders of liver function.

Trialkyl derivatives of benzene. In the trimethylbenzenes three hydrogen atoms in the benzene nucleus have been replaced by three methyl groups to form a further group of aromatic hydrocarbons.The risk of injury to health and a fire risk are associated with the use of these liquids. All three isomers are flammable. The flashpoint of pseudocumene is 45.5 °C, but the liquids are commonly used industrially as constituents of coal tar solvent naphtha, which may have a flashpoint anywhere in a range from 32 °C to below 23 °C. In the absence of precautions, a flammable concentration of vapour may be present where the liquids are used in solvent and thinner operations.

Health hazards. The main information as to the toxic effects of the trimethylbenzenes 1,3,5-trimethylbenzene and pseudocumene, both on animals and also on human beings, has been derived from studies of a solvent and paint thinner which contains 80% of these substances as constituents. They act as depressants of the central nervous system and can affect the blood coagulation. Bronchitis of an asthmatic type, headache, fatigue and drowsiness were also complained of by 70% of the workers exposed to high concentrations. A large proportion of 1,3,5-trimethylbenzene is oxidized in the body into mesitylenic acid, conjugated with glycine and excreted in the urine. Pseudocumene is oxidized into p-xylic acid, then excreted as well in the urine.

Cumene. Regard must be paid to certain health and fire hazards when cumene is used in an industrial process. Cumene is a skin irritant and can be slowly absorbed through the skin. It also has a potent narcotic effect in animals, and the narcosis develops more slowly and lasts longer than with benzene or toluene. It also has a tendency to cause injury to the lungs, liver and kidneys, but no such injuries have been recorded in human beings.

Liquid cumene does not evolve vapours in flammable concentrations until its temperature reaches 43.9 °C. Thus flammable mixtures of vapour and air will be formed only in the course of uncontrolled operations that involve hotter temperatures. If solutions or coatings containing cumene are heated in the course of a process (in a drying oven, for instance), fire and, under certain conditions, explosion readily occur.

Health and Safety Measures

Given that the major route of entry is the lungs, it becomes important to prevent these agents from entering the breathing zone. Effective exhaust ventilation systems to prevent accumulation of toxins is one of the most important methods of preventing excessive inhalation. Open containers should be kept covered or closed when not in use. The above precautions to ensure that a harmful concentration of vapour is not present in the working atmosphere are fully adequate to avoid flammable mixtures in the air in normal circumstances. To cover the risk of accidental leakage or overflow of liquid from storage or process vessels, additional precautions are needed such as mounds round storage tanks, sills at doorways or specially designed floors to limit the spread of escaping liquid. Open flames and other sources of ignition should be excluded where these agents are stored or used. Efficient means of dealing with leakage and spills must be available.

Respirators, while effective, should be used only as backup (or in emergencies) and are entirely user-dependent. Protection from the second major route of exposure, the skin, can be provided by protective clothing such as gloves, facial protectors/shields, and gowns. Furthermore, protective eyeware should be given to workers at risk of splashing these substances in their eyes. Workers should avoid wearing contact lenses when working in areas where exposure (especially to the face and eyes) is a possibility; contact lenses can potentiate the harmful effect of these substances and often make eyewashes less effective unless the lenses are removed immediately.

If skin contact with these substances occurs, wash the skin immediately with soap and water. If clothing has been contaminated, remove it promptly. Aromatic hydrocarbons in the eyes should be removed by irrigating with water for at least 15 minutes. Burns from splashes of liquefied compounds require prompt medical attention. In case of severe exposure, the patient should be taken into the fresh air for rest until the arrival of a physician. Give oxygen if the patient appears to have difficulty in breathing. The majority of persons quickly recover in fresh air, and symptomatic therapy is rarely required.

Substitution for benzene. It is now recognized that the use of benzene should be abandoned for any industrial or commercial purpose where an effective, less harmful substitute is available, although often a substitute may be unavailable when the benzene is being used as a reactant in a chemical synthesis. On the other hand it has proved possible to adopt substitutes in almost all the very numerous operations where benzene has been used as a solvent. The substitute is not always as good a solvent as benzene, but it may still prove the preferable solvent because less onerous precautions are required. Such substitutes include benzene
homologues (especially toluene and xylene), cyclohexane, aliphatic hydrocarbons (either pure, as is the case with hexane, or as mixtures as is the case with the wide range of petroleum solvents), solvent naphthas (which are relatively complex mixtures of variable composition obtained from coal) or certain petroleum products. They contain virtually no benzene and very little toluene; the main constituents are homologues of these two hydrocarbons in proportions that vary depending on the origin of the mixture. Various other solvents may be chosen to suit the material to be dissolved and the relevant industrial processes. They include alcohols, ketones, esters and chlorinated derivatives of ethylene.

Aromatic hydrocarbons tables

Table 1 - Chemical information.

Table 2 - Health hazards.

Table 3 - Physical and chemical hazards.

Table 4 - Physical and chemical properties.

 

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Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides