Aliphatic hydrocarbons are compounds of carbon and hydrogen. They may be saturated or unsaturated open chain, branched or unbranched molecules, the nomenclature being as follows:
- paraffins (or alkanes)—saturated hydrocarbons
- olefins (or alkenes)—unsaturated hydrocarbons with one or more double bond linkages
- acetylenes (or alkynes)—unsaturated hydrocarbons with one or more triple bond linkages
The general formulae are CnH2n+2 for paraffins, CnH2n for olefins, and CnH2n-2 for acetylenes.
The smaller molecules are gases at room temperature (C1 to C4). As the molecule increases in size and structural complexity it becomes a liquid with increasing viscosity (C5 to C16), and finally the higher molecular weight hydrocarbons are solids at room temperature (above C16).
The aliphatic hydrocarbons of industrial importance are derived mainly from petroleum, which is a complex mixture of hydrocarbons. They are produced by the cracking, distillation and fractionation of crude oil.
Methane, the lowest member of the series, comprises 85% of natural gas, which may be tapped directly from pockets or reservoirs in the vicinity of petroleum deposits. Large amounts of pentane are produced by fractional condensation of natural gas.
Uses
The saturated hydrocarbons are used in industry as fuels, lubricants and solvents. After undergoing processes of alkylation, isomerization and dehydrogenation, they also act as starting materials for the synthesis of paints, protective coatings, plastics, synthetic rubber, resins, pesticides, synthetic detergents and a wide variety of petrochemicals.
The fuels, lubricants and solvents are mixtures which may contain many different hydrocarbons. Natural gas has long been distributed in the gaseous form for use as a town gas. It is now liquefied in large quantities, shipped under refrigeration and stored as a refrigerated liquid until it is introduced unchanged or reformed into a town gas distribution system. Liquefied petroleum gases (LPGs), consisting mainly of propane and butane, are transported and stored under pressure or as refrigerated liquids, and are also used to augment town gas supply. They are used directly as fuels, often in high-grade metallurgical work in which a sulphur-free fuel is essential, in oxypropane welding and cutting, and in circumstances where a heavy industrial demand for gaseous fuels would strain public supply. Storage installations for these purposes vary in size from about 2 tons to several thousands of tons. Liquefied petroleum gases are also used as propellants for many types of aerosols, and the higher members of the series, from heptane upwards, are used as motor fuels and solvents. Isobutane is used to control the volatility of gasoline and is a component of instrument calibration fluid. Isooctane is the standard reference fuel for octane rating of fuels, and octane is used in antiknock engine fuels. In addition to being a component of gasoline, nonane is a component of biodegradable detergent.
The principal use of hexane is as a solvent in glues, cements and adhesives for the production of footwear, whether from hide or from plastics. It has been used as a solvent for glue in the assembling of furniture, in adhesives for wallpaper, as a solvent for glue in the production of handbags and suitcases from hide and artificial hide, in the manufacture of raincoats, in the retreading of car tyres and in the extraction of vegetable oils. In many uses, hexane has been replaced by heptane because of the toxicity of n-hexane.
It is not possible to list all the occasions when hexane may be present in the working environment. It may be advanced as a general rule that its presence is to be suspected in volatile solvents and grease removers based on hydrocarbons derived from petroleum. Hexane is also used as a cleaning agent in the textile, furniture and leather industries.
Aliphatic hydrocarbons used as starting materials of intermediates for synthesis may be individual compounds of high purity or relatively simple mixtures.
Hazards
Fire and explosion
The development of large storage installations first for gaseous methane and later for LPGs has been associated with explosions of great magnitude and catastrophic effect, which have emphasized the danger when a massive leakage of these substances occurs. The flammable mixture of gas and air may extend far beyond the distances that are regarded as adequate for normal safety purposes, with the result that the flammable mixture may become ignited by a household fire or automobile engine well outside the specified danger zone. Vapour may thus be set alight over a very large area, and flame propagation through the mixture may reach explosive violence. Many smaller—but still serious—fires and explosions have occurred during the use of these gaseous hydrocarbons.
The largest fires involving liquid hydrocarbons have occurred when large quantities of liquid have escaped and flowed towards a part of the factory where ignition could take place, or have spread over a large surface and evaporated quickly. The notorious Flixborough (United Kingdom) explosion is attributed to a leak of cyclohexane.
Health hazards
The first two members of the series, methane and ethane, are pharmacologically “inert”, belonging to a group of gases called “simple asphyxiants”. These gases can be tolerated in high concentrations in inspired air without producing systemic effects. If the concentration is high enough to dilute or exclude the oxygen normally present in the air, the effects produced will be due to oxygen deprivation or asphyxia. Methane has no warning odour. Because of its low density, methane may accumulate in poorly ventilated areas to produce an asphyxiating atmosphere. Ethane in concentrations below 50,000 ppm (5%) in the atmosphere produces no systemic effects on the person breathing it.
Pharmacologically, the hydrocarbons above ethane can be grouped with the general anaesthetics in the large class known as the central nervous system depressants. The vapours of these hydrocarbons are mildly irritating to mucous membranes. The irritation potency increases from pentane to octane. In general, alkane toxicity tends to increase as the carbon number of alkanes increases. In addition, straight-chain alkanes are more toxic than the branched isomers.
The liquid paraffin hydrocarbons are fat solvents and primary skin irritants. Repeated or prolonged skin contact will dry and defat the skin, resulting in irritation and dermatitis. Direct contact of liquid hydrocarbons with lung tissue (aspiration) will result in chemical pneumonitis, pulmonary oedema, and haemorrhage. Chronic intoxication by n-hexane or mixtures containing n-hexane may involve polyneuropathy.
Propane causes no symptoms in humans during brief exposures to concentrations of 10,000 ppm (1%). A concentration of 100,000 ppm (10%) is not noticeably irritating to the eyes, nose or respiratory tract, but it will produce slight dizziness in a few minutes. Butane gas causes drowsiness, but no systemic effects during a 10-minute exposure to 10,000 ppm (1%).
Pentane is the lowest member of the series that is liquid at room temperature and pressure. In human studies a 10-min exposure to 5,000 ppm (0.5%) did not cause mucous membrane irritation or other symptoms.
Heptane caused slight vertigo in men exposed for 6 min to 1,000 ppm (0.1%) and for 4 min to 2,000 ppm (0.2%). A 4-min exposure to 5,000 ppm (0.5%) heptane caused marked vertigo, inability to walk a straight line, hilarity and incoordination. These systemic effects were produced in the absence of complaints of mucous membrane irritation. A 15-min exposure to heptane at this concentration produced a state of intoxication characterized by uncontrolled hilarity in some individuals, and in others it produced a stupor lasting for 30 min after the exposure. These symptoms were frequently intensified or first noticed at the moment of entry into an uncontaminated atmosphere. These individuals also complained of loss of appetite, slight nausea, and a taste resembling gasoline for several hours after exposure to heptane.
Octane in concentrations of 6,600 to 13,700 ppm (0.66 to 1.37%) caused narcosis in mice within 30 to 90 min. No deaths or convulsions resulted from these exposures to concentrations below 13,700 ppm (1.37%).
Because it is likely that in an alkane mixture the components have additive toxic effects, the US National Institute for Occupational Safety and Health (NIOSH) has recommended keeping a threshold limit value for total alkanes (C5 to C8) of 350 mg/m3 as a time-weighted average, with a 15-min ceiling value of 1,800 mg/m3. n-Hexane is considered separately because of its neurotoxicity.
n-Hexane
n-Hexane is a saturated, straight-chain aliphatic hydrocarbon (or alkane) with the general formula CnH2n+2 and one of a series of hydrocarbons with low boiling points (between 40 and
90 °C) obtainable from petroleum by various processes (cracking, reforming). These hydrocarbons are a mixture of alkanes and cycloalkanes with five to seven carbon atoms
(n-pentane, n-hexane, n-heptane, isopentane, cyclopentane, 2-methylpentane,
3-methylpentane, cyclohexane, methylcyclopentane). Their fractional distillation produces single hydrocarbons that may be of varying degrees of purity.
Hexane is sold commercially as a mixture of isomers with six carbon atoms, boiling at 60 to
70 °C. The isomers most commonly accompanying it are 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane and 2,2-dimethylbutane. The term technical hexane in commercial use denotes a mixture in which are to be found not only n-hexane and its isomers but also other aliphatic hydrocarbons with five to seven carbon atoms (pentane, heptane and their isomers).
Hydrocarbons with six carbon atoms, including n-hexane, are contained in the following petroleum derivatives: petroleum ether, petrol (gasoline), naphtha and ligroin, and fuels for jet aircraft.
Exposure to n-hexane may result from occupational or non-occupational causes. In the occupational field it may occur through the use of solvents for glues, cements, adhesives or grease-removing fluids. The n-hexane content of these solvents varies. In glues for footwear and rubber cement, it may be as high as 40 to 50% of the solvent by weight. The uses referred to here are those that have caused occupational disease in the past, and in some instances hexane has been replace with heptane. Occupational exposure to n-hexane may occur also through the inhalation of petrol fumes in fuel depots or workshops for the repair of motor vehicles. The danger of this form of occupational exposure, however, is very slight, because the concentration of n-hexane in petrol for motor vehicles is maintained below 10% owing to the need for a high octane number.
Non-occupational exposure is found mainly among children or drug addicts who practise the sniffing of glue or petrol. Here the n-hexane content varies from the occupational value in glue to 10% or less in petrol.
Hazards
n-Hexane may penetrate the body in either of two ways: by inhalation or through the skin. Absorption is slow by either way. In fact measurements of the concentration of n-hexane in the breath exhaled in conditions of equilibrium have shown the passage from the lungs to the blood of a fraction of the n-hexane inhaled of from 5.6 to 15%. Absorption through the skin is extremely slow.
n-Hexane has the same skin effects previously described for other liquid aliphatic hydrocarbons. Hexane tends to vaporize when swallowed or aspirated into tracheobronchial tree. The result can be rapid dilution of alveolar air and a marked fall in its oxygen content, with asphyxia and consequent brain damage or cardiac arrest. The irritative pulmonary lesions occurring after the aspiration of higher homologues (e.g., octane, nonane, decane and so on) and of mixtures thereof (e.g., kerosene) do not appear to be a problem with hexane. Acute or chronic effects are almost always due to inhalation. Hexane is three times more acutely toxic than pentane. Acute effects occur during exposure to high concentrations of n-hexane vapours and range from dizziness or vertigo after brief exposure to concentrations of about 5,000 ppm, to convulsions and narcosis, observed in animals at concentrations of about 30,000 ppm. In humans, 2,000 ppm (0.2%) produces no symptoms in a 10-min exposure. An exposure of 880 ppm for 15 min can cause eye and upper respiratory tract irritation in humans.
Chronic effects occur after prolonged exposure to doses that do not produce obvious acute symptoms and tend to disappear slowly when the exposure ends. In the late 1960s and early 1970s, attention was drawn to outbreaks of sensorimotor and sensory polyneuropathy among workers exposed to mixtures of solvents containing n-hexane in concentrations mainly ranging between 500 and 1,000 ppm with higher peaks, although concentrations as low as 50 ppm could cause symptoms in some instances. In some cases, muscular atrophy and cranial nerve involvements such as visual disorders and facial numbness were observed. About 50% showed denervation and regeneration of the nerves, Tingling, numbness and weakness of distal extremities were complained of, mainly in the legs. Stumbling was often observed. Achilles tendon reflexes disappeared; touch and heat sensation were diminished. Conduction time was decreased in the motor and sensory nerves of the arms and legs.
The course of the disease is generally very slow. After the appearance of the first symptoms, a deterioration of the clinical picture is often observed through an aggravation of the motor deficiency of the regions originally affected and their extension to those which have hitherto been sound. This deterioration can occur for some months after exposure has ceased. The extension generally takes place from the lower to the upper limbs. In very serious cases ascending motor paralysis appears with a functional deficiency of the respiratory muscles. Recovery may take as long as 1 to 2 years. Recovery is generally complete, but a diminution of the tendon reflexes, particularly that of the Achilles tendon, may persist in conditions of apparent full well-being.
Symptoms in the central nervous system (defects of the visual function or the memory) have been observed in serious cases of intoxication by n-hexane and have been related to degeneration of the visual nuclei and the tracts of hypothalamic structures. These may be permanent.
With regard to laboratory tests, the most usual haematological and haemato-chemical tests do not show characteristic changes. This is also true of urine tests, which show increased creatinuria only in serious cases of paralysis with muscular hypotrophy.
The examination of the spinal fluid does not lead to characteristic findings, either manometric or qualitative, except for rare cases of increased protein content. It appears that only the nervous system shows characteristic changes. The electroencephalograph readings (EEG) are usually normal. In serious cases of disease, however, it is possible to detect dysrhythmias, widespread or subcortical discomfort and irritation. The most useful test is electromyography (EMG). The findings indicate myelinic and axonal lesions of the distal nerves. The motor conduction velocity (MCV) and the sensitive conduction velocity (SCV) are reduced, the distal latency (LD) is modified and the sensory potential (SPA) is diminished.
Differential diagnosis with respect to the other peripheral polyneuropathies is based on the symmetry of the paralysis, on the extreme rareness of sensory loss, on the absence of changes in the cerebrospinal fluid, and, above all, on the knowledge that there has been exposure to solvents containing n-hexane and the occurrence of more than one case with similar symptoms from the same workplace.
Experimentally, technical grade n-hexane has produced peripheral nerve disturbances in mice at 250 ppm and higher concentrations after 1 year of exposure. Metabolic investigations have indicated that in guinea-pigs n-hexane and methyl butyl ketone (MBK) are metabolized to the same neurotoxic compounds (2-hexanediol and 2,5-hexanedione).
The anatomical modifications of the nerves underlying the clinical manifestations described above have been observed, whether in laboratory animals or in sick human beings, through muscular biopsy. The first convincing n-hexane polyneuritis reproduced experimentally is due to Schaumberg and Spencer in 1976. The anatomical modifications of the nerves are represented by axonal degeneration. This axonal degeneration and the resulting demyelination of the fibre start at the periphery, particularly in the longer fibres, and tend to develop towards the centre, though the neuron does not show signs of degeneration. The anatomical picture is not specific to the pathology of n-hexane, for it is common to a series of nervous diseases due to poisons in both industrial and non-industrial use.
A very interesting aspect of n-hexane toxicology lies in the identification of the active metabolites of the substance and its relations with the toxicology of other hydrocarbons. In the first place it seems to be established that the nervous pathology is caused only by n-hexane and not by its isomers referred to above or by pure n-pentane or n-heptane.
Figure 1 shows the metabolic pathway of n-hexane and methyl n-butyl ketone in human beings. It can be seen that the two compounds have a common metabolic pathway and that MBK can be formed from n-hexane. The nervous pathology has been reproduced with 2-hexanol, 2,5-hexanediol and 2,5-hexanedione. It is obvious, as has been shown, moreover, by clinical experience and animal experiment, that MBK is also neurotoxic. The most toxic of the n-hexane metabolites in question is 2,5-hexanedione. Another important aspect of the connection between n-hexane metabolism and toxicity is the synergistic effect that methyl ethyl ketone (MEK) has been shown to have in the neurotoxicity of n-hexane and MBK. MEK is not by itself neurotoxic either for animals or for humans, but it has led to lesions of the peripheral nervous systems in animals treated with n-hexane or MBK that arise more quickly than similar lesions caused by those substances alone. The explanation is most likely to be found in a metabolic interference activity of MEK in the pathway which leads from n-hexane and MBK to the neurotoxic metabolites referred to above.
Figure 1. The metabolic pathway of n-hexane and methyl-n-butyl ketone
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Safety and Health Measures
It is clear from what has been observed above that the association of n-hexane with MBK or MEK in solvents for industrial use is to be avoided. Whenever possible, substitute heptane for hexane.
With regard to TLVs in force for n-hexane, modifications of the EMG pattern have been observed in workers exposed to concentrations of 144 mg/ml (40 ppm) that have not been present in workers not exposed to n-hexane. The medical monitoring of exposed workers is based both on acquaintance with the data concerning the concentration of n-hexane in the atmosphere and on clinical observation, particularly in the neurological field. Biological monitoring for 2,5-hexanedione in the urine is the most useful indicator of exposure, although MBK will be a confounder. If necessary, measurement of n-hexane in exhaled air at the end of shift can confirm exposure.
Cycloparaffins (Cycloalkanes)
The cycloparaffins are alicyclic hydrocarbons in which three or more of the carbon atoms in each molecule are united in a ring structure and each of these ring carbon atoms is joined to two hydrogen atoms, or alkyl groups. The members of this have the general formula CnH2n. Derivatives of these cycloparaffins include compounds such as methylcyclohexane (C6H11CH3). From the occupational safety and health point of view, the most important of these are cyclohexane, cyclopropane and methylcyclohexane.
Cyclohexane is used in paint and varnish removers; as a solvent for lacquers and resins, synthetic rubber, and fats and waxes in the perfume industry; as a chemical intermediate in the manufacture of adipic acid, benzene, cyclohexyl chloride, nitrocyclohexane, cyclohexanol and cyclohexanone; and for molecular weight determinations in analytical chemistry. Cyclopropane serves as a general anaesthetic.
Hazards
These cycloparaffins and their derivatives are flammable liquids, and their vapours will form explosive concentrations in air at normal room temperature.
They may produce toxic effects by inhalation and ingestion, and they have an irritant and defatting action on the skin. In general, the cycloparaffins are anaesthetics and central nervous system depressants, but their acute toxicity is low and, due to their almost complete elimination from the body, the danger of chronic poisoning is relatively slight.
Cyclohexane. The acute toxicity of cyclohexane is very low. In mice, exposure to 18,000 ppm (61.9 mg/l) cyclohexane vapour in air produced trembling in 5 min, disturbed equilibrium in 15 min, and complete recumbency in 25 min. In rabbits, trembling occurred in 6 min, disturbed equilibrium in 15 min, and complete recumbency in 30 min. No toxic changes were found in the tissues of rabbits after exposure for 50 periods of 6 h to concentrations of 1.46 mg/l (434 ppm). 300 ppm was detectable by odour and somewhat irritating to the eyes and mucous membranes. Cyclohexane vapour causes weak anaesthesia of brief duration but more potent than hexane.
Animal experimentation has shown that cyclohexane is far less harmful than benzene, its six-membered ring aromatic analogue, and, in particular, does not attack the haemopoietic system as does benzene. It is thought that the virtual absence of harmful effects in the blood-forming tissues is due, at least partially, to differences in the metabolism of cyclohexane and benzene. Two metabolites of cyclohexane have been determined—cyclohexanone and cyclohexanol—the former being partially oxidized to adipic acid; none of the phenol derivatives that are a feature of the toxicity of benzene have been found as metabolites in animals exposed to cyclohexane, and this has led to cyclohexane being proposed as a substitute solvent for benzene.
Methylcyclohexane has a toxicity similar to but lower than that of cyclohexane. No effects resulted from repeated exposures of rabbits at 1,160 ppm for 10 weeks, and only slight kidney and liver injury was observed at 3,330 ppm. Prolonged exposure at 370 ppm appeared to be harmless to monkeys. No toxic effects from industrial exposure or intoxication in humans by methylcyclohexane have been reported.
Animal studies show that the majority of this substance entering the bloodstream is conjugated with sulphuric and glucuronic acids and excreted in the urine as sulphates or glucuronides, and in particular the glucuronide of trans-4-methylcyclohexanol.
Saturated and alicyclic hydrocarbons tables
Table 1 - Chemical information.
Table 2 - Health hazards.
Table 3 - Physical and chemical hazards.
Table 4 - Physical and chemical properties.