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Wednesday, 03 August 2011 00:58

Esters, Acetates

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Acetates are derived from a reaction (called esterification) between acetic acid or an anhydrous compound containing an acetate group and the corresponding alcohol, with the elimination of water. Thus methyl acetate is obtained by the esterification of methyl alcohol with acetic acid in the presence of sulphuric acid as a catalyst. The reaction is reversible and must therefore be conducted with heat, eliminating water formed by the reaction. Ethyl acetate is obtained by the direct esterification of ethyl alcohol with acetic acid, a process which involves mixing acetic acid with an excess of ethyl alcohol and adding a small amount of sulphuric acid. The ester is separated and purified by distillation. Ethyl acetate is easily hydrolyzed in water, giving a slightly acid reaction. In another process the molecules of anhydrous acetaldehyde interact in the presence of aluminium ethoxide to produce the ester, which is purified by distillation. Propyl acetate and isopropyl acetate esters are produced by the reaction of acetic acid with the corresponding propyl alcohol in the presence of a catalyst.

Both butyl acetate and amyl acetate consist of mixtures of isomers. Thus butyl acetate comprises n-butyl acetate, sec-butyl acetate and isobutyl acetate. It is made by the esterification of n-butanol with acetic acid in the presence of sulphuric acid. n-Butanol is obtained by the fermentation of starch with Clostridium acetobutylicum. Amyl acetate is primarily a mixture of n-amyl acetate and isoamyl acetate. Its composition and characteristics depend on its grade. The flashpoints of the various grades vary from 17 to 35 °C.

Uses

The acetates are solvents for nitrocellulose, lacquers, leather finishes, paints and plastics. They are also used as flavouring agents and preservatives in the food industry, and fragrances and solvents in the perfume and cosmetics industries. Methyl acetate, generally mixed with acetone and methyl alcohol, is used in the plastics and artificial leather industries, and in the production of perfumes, colouring agents and lacquers. Ethyl acetate is a good solvent for nitrocellulose, fats, varnishes, lacquers, inks and airplane dopes; it is used in the production of smokeless powder, artificial leather, perfumes, photographic films and plates, and artificial silk. It is also a cleaning agent in the textile industry, and a flavouring agent for pharmaceuticals and food.

n-Propyl acetate and isopropyl acetate are solvents for plastics, inks and nitrocellulose in the production of lacquers. They are utilized in the manufacture of perfumes and insecticides, and in organic synthesis. Butyl acetate is a commonly used solvent in the production of nitrocellulose lacquers. It is also used in the manufacture of vinyl resins, artificial leather, photographic film, perfumes, and in the preserving of foodstuffs.

In its commercial form amyl acetate, a mixture of isomers, is used as a solvent for nitrocellulose in the manufacture of lacquers, and, because of its banana-like smell, it is used as a fragrance. Amyl acetate is useful in the manufacture of artificial leather, photographic film, artificial glass, celluloid, artificial silk, and furniture polish. Isoamyl acetate is used for dyeing and finishing textiles, perfuming shoe polish, and manufacturing artificial silk, leather, pearls, photographic films, celluloid cements, waterproof varnish and metallic paints. It is also used in artificial glass manufacturing and in the straw hat industry as a constituent of lacquers and stiffening solutions. Sodium acetate in used in tanning, photography, electroplating and preserving meat, as well as in the manufacture of soaps and pharmaceuticals.

Vinyl acetate primarily functions as an intermediate for the production of polyvinyl alcohol and polyvinyl acetals. It is also used in hair sprays and in the production of emulsion paint substances, finishing and impregnation materials, and glue. 2-Pentyl acetate has many of the same functions as the other acetates and serves as a solvent for chlorinated rubber, metallic paints, cements, linoleum, washable wallpaper, pearls, and coatings on artificial pearls.

Hazards

Methyl acetate is flammable, and its vapour forms explosive mixtures with air at normal temperatures. High concentrations of vapour can cause irritation to the eyes and mucous membranes. Exposure to the vapours can also cause headache, drowsiness, dizziness, burning and tearing of the eyes, heart palpitations, as well as a constricted feeling in the chest and shortness of breath. Blindness arising from eye contact has also been reported.

Ethyl acetate is a flammable liquid and produces a vapour that forms explosive mixtures with air at normal temperatures. Ethyl acetate is an irritant of the conjunctive and mucous membrane of the respiratory tract. Animal experiments have shown that, at very high concentrations, the ester has narcotic and lethal effects; at concentrations of 20,000 to 43,000 ppm, there may be pulmonary oedema with haemorrhages, symptoms of central nervous system depression, secondary anaemia and damage of the liver. Lower concentrations in humans have caused irritation of the nose and pharynx; cases have also been known of irritation of the conjunctiva with temporary opacity of the cornea. In rare cases exposure may cause sensitization of the mucous membrane and eruptions of the skin.

The irritant effect of ethyl acetate is less strong than that of propyl acetate or butyl acetate. These two propyl acetate isomers are flammable, and their vapours form explosive mixtures with air at normal temperatures. Concentrations of 200 ppm can cause irritation of the eyes, and greater concentrations give rise to irritation of the nose and larynx. Amongst workers occupationally exposed to these esters, there have been cases of conjunctival irritation and reports of a feeling of constriction of the chest, and coughing; however, no cases of permanent or systemic effects have been found in exposed workers. Repeated contact of the liquid with the skin may lead to defatting and cracking.

Amyl acetate. All the isomers and grades of amyl acetate are flammable and evolve flammable mixtures of vapour in air. High concentrations (10,000 ppm for 5 h) can be lethal to guinea-pigs. The principal symptoms in cases of occupational exposure are headaches and irritation of the mucous membranes of the nose and of the conjunctiva. Other symptoms mentioned include vertigo, palpitations, gastrointestinal disorders, anaemia, cutaneous lesions, dermatitis and adverse effects on the liver. Amyl acetate is also a defatting agent, and prolonged exposure may produce dermatitis. Butyl acetate is significantly more irritating than ethyl acetate. In addition, it can exert behavioural symptoms similar to amyl acetate.

Hexyl acetate and benzyl acetate are used industrially and are flammable, but their vapour pressures are low and, unless they are heated, they are unlikely to produce flammable concentrations of vapour. Animal experiments indicate that the toxic properties of these acetates are greater than those of amyl acetate; however, in practice, due to their low volatility, their effect on workers is limited to local irritation. There are few data upon which to evaluate hazards.

Cyclohexyl acetate can exert extreme narcotic effects in animals and appears to be a stronger irritant experimentally that is amyl acetate; however, there are insufficient data on human exposure to evaluate. The chemical does not tend to accumulate in the body, and many effects appear to be reversible.

Vinyl acetate is transformed metabolically into acetaldehyde, which raises a question of carcinogenicity. Based on this and on the positive results of animal assays, the International Agency for Research on Cancer (IARC) has classified vinyl acetate as a Group 2B carcinogen, possibly carcinogenic to humans. In addition, the chemical can be irritating to the upper respiratory tract and eyes. It is defatting to the skin.

Acetates 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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More in this category: « Epoxy Compounds Esters, Acrylates »

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