Fine Chemicals: Soaps and Detergents

James Richard Fromm


The fine chemicals branch of chemical industry is a somewhat arbitrary grouping of several specific industries, all of which are characterized by the synthesis of specific desired compounds from the raw materials provided by the petrochemical industry or occasionally from other sources. These products are often of high value and the quantities produced are relatively small. The fine chemicals industry contains as its major product groupings pharmaceuticals, dyes and pigments, pesticides, and soaps; minor product groupings include analytical reagents and photographic chemicals.

Soaps

The term soap is a class name for the sodium and potassium salts of stearic acid, C17H35COOH, and other fatty acids. Fatty acids are found in animal fats and in plant oils such as coconut oil, palm oil, castor oil, olive oil, or cottonseed oil. Soaps are, and have been for centuries, made by the addition of hot concentrated aqueous sodium hydroxide solutions to fats such as glyceryl stearate:

3NaOH + (C17H35>COO)3C3H5 rarrow.gif (63 bytes) 3C17H35COONa + C3H5(OH)3.

The two products are sodium stearate, a soap, and glycerol (glycerin). An alternative and more modern process is the hydrolysis of the fat followed by treatment of the fatty acids with caustic at about 48 atm and

250oC. The reaction is:

3H2O + (C17H35COO)3C3H5 rarrow.gif (63 bytes) 3C17H35COOH + C3H5(OH)3,

followed by

C17H35COOH + NaOH rarrow.gif (63 bytes) C17H35COONa + H2O.

Although most glycerol is obtained as a by-product of soap manufacture, it can also be synthesized from petrochemicals, the usual starting material being propene. Glycerol is widely used in some plastics, as an additive to food, drugs, cosmetics, and tobacco, and for the production of nitroglycerine-based explosives.

Synthetic Detergents

Synthetic detergents first came on the market about 1940 and held about half the North American market for cleaning materials by 1950. About 85% of the North American market is now held by synthetic detergents, and at least 50% of the world market as well. Several types of synthetic detergents are now manufactured, of which the largest quantity are alkylaryl sulfonates such as dodecylbenzene sulfonate. The feedstocks are benzene and propene. Propene reacts in the presence of phosphoric acid as a catalyst to form a tetramer, which is dodecane:

4CH2=CHCH3 rarrow.gif (63 bytes) H-[CH2-CH2-CH3]4-H

This tetramer, together with benzene, yields dodecylbenzene on treatment with aluminum chloride as a catalyst. The reaction is:

H-[CH2-CH2-CH2]4-H + C6H6 rarrow.gif (63 bytes) C6H5(CH2)12-H

Treatment with strong sulfuric acid yields the sulfonic acid, which is then neutralized to give the desired sodium salt, C6H5(CH2)12-SO3Na.

Various materials are added to synthetic detergents to improve the cleansing ability. Among these are the common "builders" sodium sulfate, Na2SiO3, and Na4P2O7. The latter compound is being dropped for environmental reasons. Phosphate-containing effluents resulting from detergents have caused undesirable growth of algae in streams and lakes. Minor constituents such as dyes or enzymes may also be added to soaps and detergents.


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Copyright 1997 James R. Fromm