Wednesday, 03 August 2011 05:26

Heterocyclic Compounds

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The heterocyclic compounds are used as chemical intermediates and solvents in the pharmaceutical, chemical, textile, dye-stuffs, petroleum and photography industries. Several compounds also function as vulcanization accelerators in the rubber industry.

Acridine and benzanthrone are used as starting materials and intermediates in the manufacture of dyes. Benzanthrone is also used in the pyrotechnics industry. Propyleneimine is used in flocculants in petroleum refining and as a modifier for rocket propellant fuels. It has been used in oil additives as a modifier for viscosity control, for high-pressure performance, and for oxidation resistance. 3-Methylpyridine and 4-methylpyridine serve as waterproofing agents in the textile industry. 4-Methylpyridine is a solvent in the synthesis of pharmaceuticals, resins, dye-stuffs, rubber accelerators, pesticides and waterproofing agents. 2-Pyrrolidone is also used in pharmaceutical preparations and functions as a high-boiling solvent in petroleum processing. It is found in specialty printing inks and in certain floor polishes. 4,4'-Dithiodimorpholine is used in the rubber industry as a staining protector and a vulcanizing agent. In the rubber industry, 2-vinylpyridine is made into a terpolymer that is used in adhesives for bonding tire cord to rubber.

Several heterocyclic compounds—morpholine, mercaptobenzothiazole, piperazine, 1,2,3-benzotriazole and quinoline—function as corrosion inhibitors for copper and industrial water treatment. Mercaptobenzothiazole is also a corrosion inhibitor in cutting oils and petroleum products, and an extreme-pressure additive in greases. Morpholine is a solvent for resins, waxes, casein and dyes, and a defoaming agent in the paper and paperboard industries. In addition, it is found in insecticides, fungicides, herbicides, local anaesthetics, and antiseptics. 1,2,3-Benzotriazole is a restrainer, developer and antifogging agent in photographic emulsions, a component of military aircraft de-icing fluid, and a stabilizing agent in the plastics industry.

Pyridine is utilized by numerous industries as both a chemical intermediate and a solvent. It is used in the manufacture of vitamins, sulpha drugs, disinfectants, dye-stuffs and explosives, and as a dyeing assistant in the textile industry. Pyridine is also useful in the rubber and paint industries, oil and gas well drilling, and in the food and non-alcoholic beverage industries as a flavouring agent. The vinylpyridines are utilized for the production of polymers. Sulpholane, a solvent and a plasticizer, is used for the extraction of aromatic hydrocarbons from oil refinery streams, for textile finishing, and as a component of hydraulic fluid. Tetrahydrothiophene is a solvent and a fuel gas odorant used in fire safety stench warning systems in underground mines. Piperidine is used in the manufacture of pharmaceuticals, wetting agents and germicides. It is a hardening agent for epoxy resins and a trace constituent of fuel oil.


Acridine is a powerful irritant which, in contact with the skin or mucous membrane, causes itching, burning, sneezing, lacrimation and irritation of the conjunctiva. Workers exposed to acridine crystal dust in concentrations of 0.02 to 0.6 mg/m3 complained of headache, disturbed sleep, irritability and photosensitization, and presented oedema of the eyelids, conjunctivitis, skin rashes, leucocytosis and increased red cells sedimentation rates. These symptoms did not appear at an acridine airborne concentration of 1.01 mg/m3. When heated, acridine emits toxic fumes. Acridine, and a large number of its derivatives have been shown to possess mutagenic properties and to inhibit DNA repair and cell growth in several species.

In animals, near-lethal doses of aminopyridines produce increased excitability to sound and touch, and cause tremor, clonic convulsions and tetany. They also cause contraction of skeletal muscle and smooth muscle, producing vasconstriction and increased blood pressure. It has been reported that aminopyridines and some alkyl pyridines exert inotropic and chronotropic action on the heart. Vinyl pyridines cause less dramatic convulsions. Acute poisoning can occur either from inhalation of the dust or vapour at relatively low concentrations, or by skin absorption.

A common hazard of benzanthrone is skin sensitization due to exposure to benzanthrone dust. Sensitivity varies from person to person, but after exposure of between a few months and several years, sensitive persons, especially those who are blond or red-headed, develop an eczema which may be intense in its course and the acute phase of which may leave a hazel or slate-grey pigmentation, especially around the eyes. Microscopically, atrophy of the skin has been found. Skin disorders due to benzanthrone are more frequent in the warm season and are significantly aggravated by heat and light.

Morpholine is a moderately toxic compound by ingestion and by cutaneous application; undiluted morpholine is a strong skin irritant and a potent eye irritant. It does not appear to have chronic toxic effects. It is a moderate fire hazard when exposed to heat, and thermal decomposition results in the release of fumes containing nitrogen oxides.

Phenothiazine has harmful irritant properties, and industrial exposure may produce skin lesions and photosensitization, including photosensitized keratitis. As far as systemic effects are concerned, severe intoxication in therapeutic use has been reported to be characterized by haemolitic anaemia and toxic hepatitis. Because of its low solubility, the rate of its absorption from the gastrointestinal tract is dependent on particle size. A micronized form of the drug is absorbed rapidly. The toxicity of the substance varies a great deal from animal to animal, the oral LD50 in rats being 5 g/kg.

Although phenothiazine oxidizes fairly easily when it is exposed to air, the risk of fire is not high. However, if involved in a fire, phenothiazine produces highly toxic sulphur and nitrogen oxides, which are dangerous lung irritants.

Piperidine is absorbed by inhalation and through the digestive tract and the skin; it produces a toxic response in animals similar to that obtained with the aminopyridines. Large doses block ganglionic conduction. Small doses cause both parasympathetic and sympathetic stimulation due to action on the ganglia. Increased blood pressure and heart rate, nausea, vomiting, salivation, laboured breathing, muscular weakness, paralysis and convulsions are signs of intoxication. This substance is highly flammable and evolves explosive concentrations of vapour at normal room temperatures. The precautions recommended for pyridine should be adopted.

Pyridine and homologues. Some information on pyridine is available from clinical reports of human exposure, primarily through medical treatments or through exposure to the vapour. Pyridine is absorbed through the gastrointestinal tract, through the skin and by inhalation. Clinical symptoms and signs of intoxication include gastrointestinal disturbance with diarrhoea, abdominal pain and nausea, weakness, headache, insomnia and nervousness. Exposures less than those required to produce overt clinical signs may cause varying degrees of liver damage with central lobular fatty degeneration, congestion and cellular infiltration; repeated low-level exposures cause cirrhosis. The kidney appears to be less sensitive to pyridine-induced damage than is the liver. In general, pyridine and its derivatives cause local irritation on contact with the skin, mucous membranes and cornea. The effects on the liver may occur at levels that are too low to elicit a response from the nervous system, and so no warning signs may be available to a potentially exposed worker. Further, although the odour of pyridine is easily detectable at vapour concentrations of less than 1 ppm, odour detection cannot be relied upon because olfactory fatigue occurs quickly.

Pyridine in both the liquid and vapour phase may constitute a severe fire and explosion hazard when exposed to flame; it may also react violently with oxidizing substances. When pyridine is heated to decomposition, cyanide fumes are released.

Pyrrole and pyrrolidine. Pyrrole is a flammable liquid and, when burning, gives off dangerous nitrogen oxides. It has a depressant action on the central nervous system and, in severe intoxication, is injurious to the liver. Few data are available about the degree of occupational risk that this substance presents. Fire protection and prevention measures should be adopted and means of extinguishing fire should be provided. Respiratory protective equipment should be available for persons fighting a fire involving pyrrole.

The human experience with pyrrolidine is not well documented. Prolonged administration in rats caused reduction of diuresis, inhibition of spermatogenesis, decreased haemoglobin content in blood, and nervous excitation. As with many nitrates, the acidity of the stomach can convert pyrrolidine into N-nitrosopyrrolidine, a compound which has been found to be carcinogenic in laboratory animals. Some workers may develop headaches and vomiting from exposure.

The liquid is capable of evolving flammable concentrations of vapour at ordinary working temperatures; consequently, open lights and other agencies liable to ignite the vapour should be excluded from areas in which it is used. When burning, pyrrolidine gives off dangerous nitrogen oxides, and persons exposed to these combustion products should be supplied with suitable respiratory protection. Bunding and sills should be provided to prevent the spread of liquid accidentally escaping from storage and process vessels.

Quinoline is absorbed through the skin (percutaneously). The clinical signs of toxicity include lethargy, respiratory distress, and prostration leading to coma. This substance is irritating to the skin and may cause pronounced permanent corneal damage. It is a carcinogen in several animal species but there are inadequate data available on the human cancer risk. It is moderately flammable but does not evolve a flammable concentration of vapour at a temperature below 99 °C.

Vinylpyridine. Brief exposure to the vapour has caused eye, nose and throat irritation and transient headache, nausea, nervousness and anorexia. Skin contact causes burning pain followed by severe skin burns. Sensitization may develop. The fire hazard is moderate, and decomposition by heat is accompanied by the release of dangerous cyanide fumes.

Safety and Health Measures

The normal safety precautions are required for handling the dusts and vapours of the chemicals in this grouping. Since skin sensitization is associated with a number of them, it is particularly important that adequate sanitary and washing facilities be provided. Care should be taken to assure that workers have access to clean eating areas.

Heterocyclic compounds 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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