A Catalyst for the Synthesis of Soap Several organic laboratory manuals ( 1 4 )and some freshman chemistry laboratory books (5-7). especially those for nonscience students, include an exercise on the synthesis of soap. As the animal fat (or vegetable oil) from which the soap is made is not soluble in water, simple synthesis involves a two-phase system in which reaction can occur only at the interface. The usual experimental procedure (I-5,7) calla for the use of ethanol to effect mutual solubility and thus speed up the reaction. After the reaction is comolete. the soav is salted out bv the addition of a saturated sodium chloride solution f35.7) or the alcohol is removed by distklation (2).w e have found that use of alcohol and the subsequent steps of salting out or distillation can he avoided and the reaction time shortened by the use of a phase-transfer catalyst. In a typical reaction, 110 g of lard or other fat is weighed out into a 800-ml beaker and heated gently, if necessary, to melt the fat. Then, sodium hydroxide solution (15 g NaOH in 50 ml of water) is added in 10-ml portions, with stirring. A few drops of catalyst, Adogenm464 or Aliquae 336, are added with the first portion of sodium hydroxide solution. The solution is heated gently with a small flame, steam bath, or hot plate. The end of the reaction is indicated by the disappearance of oily globules. The soapean be washed with 25mlof distilled water (by breaking it upand stirring witbaglass stirring rod), then filtered on a Buchner funnel. The soap on the funnel is rinsed with two additional 10-ml portions of ice-cold water. Finally, the soap is cured for a week or so before it is used. We have monitored the reaction with a pH meter. As compared to a control, the pH drops more rapidly with the catalyst present. The pH drop, as expected, corresponds to the thickening of the reaction mixture as soap is formed. We have found that the catalyst not only enhances the reaction rate a t elevated temperatures, but it enables us to make soap at room temperature. We simply mix the fat, sodium hydroxide solution, and catalyst and let stand overnight. Reaction is complete as evidenced by the disappearance of oily globules. A control showed considerable unreacted oil. T o our knowledge, phase-transfer catalysts are not used by the soapmaking industry. Their use could result in considerable energy savings.
Literature Cited
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Adogene is a trademark of Ashland Chemical Company. AliquatQ is a trademark of General Mills Chemicals. The catalvst is essentiallv a methvltrialkvlammonium chloride. where the alkvls are CrCln. John W: nii; Shenle J. Solberg. Cynthia S. H~II University of Wisconsin River Falls, WI 54022
788
Journal of Chemical Education
