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Preparation of Soap

By Walter Scharf and Charles Malerich
Natural Sciences/Chemistry
Baruch College
New York, NY 10010


Soap, from a chemical standpoint, is a salt (or a mixture of salts) of fatty acids. As with all salts, soap contains a positive ion, usually Na+ or K+, and a negative ion, usually the anions of long-chained carboxylic acids obtained by the hydrolysis of animal or vegetable fats. These fatty acids, of which there are about 20 naturally occurring members, are carboxylic acids containing 14, 16, or 18 carbon atoms in an unbranched chain. The even numbered chains result from the fact that fats are synthesized in cells by the polymerization of a 2-carbon acetate unit.

In addition to the normal (straight chain) saturated acids, there occur several with hydroxyl groups, and/or one or more double bonds in the carbon chain. the presence of unsaturation (double bonds) in molecules of fatty acids, fats, or soup tend to lower the melting point of these compounds and to cause them to be in the liquid state at room temperature. Thus, vegetable fats are relatively unsaturated and liquid under the ordinary conditions, while animal fats, being relatively more saturated, are solid, or semi-solid, at the same temperature. For this reason, vegetable fats are commonly refereed to as vegetable oils. (We say relatively saturated, or unsaturated because both vegetables oils and animal fats contain saturated and unsaturated chains). The reason why double bonds lower the melting point of a fatty acid chain, is that the sections of the chain attached to the double bond are attached cis-wise to each other (probably because the double bond cis configuration produces a bent chain which does not easily adhere to a neighboring chain by a Van der Waals attraction hence a lower temperature (lower mobility) is required for these molecules to adhere in order to form a solid crystal lattice. It is interesting to note, that straight chained carboxylic acids having an odd number of carbon atoms melt lower than even chained acids of comparable molecular weights. [pic]

The most commonly encountered fatty acids in natural fats are given below:

Saturated Acids

Lauric Acid (Dodecanoic Acid)


Myristic Acid (tetraderanoic Acids)


Palmitic acid (hexadecanoic acid)


Stearic acid (octadecanoic acid)


Unsaturated acids:

Oleic acid (octadec-9-enoic acid)

CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH- CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH Linoleic acid (octadec-9, 12-dienoic acid)


All fats are esters (though, not all esters are necessarily, fat!) of the trihydroxy alcohol, glycerin, and fatty acids. Since each molecule of fat contains three ester groups, fats are often referred to as triglycerides to distinguish them from other materials having a fatty or greasy texture, such as waxes (mono esters of long chain alcohols are carboxylic acids) mineral oil, and petroleum jelly (long-chain hydrocarbons).

If R-COOH represents a generalized fatty acid (R is a hydrocarbon chain of 13, 15, or 17 carbons) then, the general formula for a fat is

Note that the fatty-acid residues in a typical fat molecule are usually different! Fats from different sources have different percentages of the common fatty acids, and can be distinguished from one another by a quantitative analysis of these constituents.

Soap is prepared by hydrolyzing a fat under alkaline (basic) conditions. The reaction is called saponification, and produces one molecule of glycerin and three molecules of soap, for each molecule of fat taken.

The fats and...
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