Stearic Acid

STEARIC ACID, CH₃.(CH₂)₁₆.CO.OH, is an organic acid belonging to the series of monobasic, saturated fatty acids. It is widely distributed in nature in the form of glycerol esters, alongside other higher fatty acids (palmitic, oleic, etc.), forming the main constituent of animal and vegetable fats. Generally, the higher the melting point of a fat, the greater its stearic acid content. Stearic acid is particularly abundant in mutton, beef, and pork tallow. To obtain stearic acid, tallow is subjected to saponification, the resulting soap is decomposed with hydrochloric acid, and the resulting mixture of solid and liquid acids is pressed. The solid residue, used for making candles under the name stearin, contains palmitic acid in addition to stearic acid; the latter can be isolated by separating it from an alcohol solution as a magnesium salt. Stearic acid can also be obtained from oleic acid by the addition of hydrogen to the latter. Stearic acid also occurs as a cetyl ester in spermaceti; by forming esters with glycerophosphoric acid, it is a component of lecithins and cephalins. Stearic acid is a fat-like, white, crystalline mass with a melting point of 69.3° (de Visser), 71–71.5° (Zaitsev), and boils at 291° (at 100 mm pressure). Specific gravity D₄^69.2 = 0.8428. It is almost insoluble in water, but dissolves well in hot alcohol and ether. Sodium salts of stearic acid, mixed with salts of other higher fatty acids, form soap; potassium salts of stearic acid are found in liquid soap (green medicinal soap); lead salts are used in the preparation of plasters. Esters of stearic acid with guaiacol and cresol were proposed as medicinal agents, but did not find wide application.—Tristearin is a fat composed of glycerol residues and three molecules of stearic acid (CH₂O.CO.C₁₇H₃₅.CHO.CO.C₁₇H₃₅.CH₂O.CO.C₁₇H₃₅). It melts at 71.5°, distills without decomposition in a vacuum, D₄⁸⁰ = 0.8621, is soluble in benzene, chloroform, and hot alcohol, less so in hot ether, and almost insoluble in cold alcohol and petroleum ether. It is found in many fats, especially in solid animal fats. It is the main constituent of mutton tallow.