Preparation of Esters & Soap

An interesting application of the esterification reaction is the preparation of soap. The soap making process is relatively simple: a source of fat and a strong base are heated together to produce a fatty acid salt. In primitive times, people made their own soap using a form of animal fat. The strong base, commonly lye, was prepared from fire ash, which could be dissolved in water to form sodium hydroxide (NaOH). Unfortunately, sodium hydroxide is quite harsh on the skin, causing drying and peeling at high concentrations. Lye soap, as it was called, was not particularly likable because of this. After you have made your own soap, you will attempt to purify it by removing as much of the remaining NaOH as possible to create more pleasant soap.

How does soap work? The cleaning action of soap works by removing dirt from skin or clothes. “Dirt” is a general term for any complex mixture of soil and debris. Usually this mixture is composed of both polar and nonpolar parts. Thinking back to our in-class discussion of polarity, we know that “like dissolves like.” Therefore, water (which is polar) can dissolve and wash away the polar parts, but it is necessary to remove an oil surface layer with a nonpolar material first. Thus, a good soap has to have both polar and nonpolar parts in order to effectively remove dirt. Recall that soap is the salt of a long-chain fatty acid. The long hydrocarbon “tail” is nonpolar, while the “head” is polar. These fatty acid salts, therefore, make effective soaps because of their polar AND nonpolar parts.