Glycerol is the common name of the organic compound whose chemical structure is HOCH2-CHOHCH2OH. Propane-1,2,3-triol or glycerin (USP), as it is also called, consists of a chain of three carbon atoms with each of the end carbon atoms bonded to two hydrogen atoms (C-H) and a hydroxyl group (-OH) and the central carbon atom is bonded to a hydrogen atom (C-H) and a hydroxyl group (-OH). Glycerol is a trihydric alcohol because it contains three hydroxyl or alcohol groups. Glycerin is a thick liquid with a sweet taste that is found in fats and oils and is the primary triglyceride found in coconut and olive oil. It was discovered in 1779, when the Swedish chemist Carl Wilhelm Scheele (1742-1786) washed glycerol out of a heated a mixture of lead oxide (PbO) and olive oil. Today, it is obtained as a byproduct from the manufacture of soaps.
One important property of glycerol or glycerin is that is not poisonous to humans. Therefore it is used in foods, syrups, ointments, medicines, and cosmetics. Glycerol also has special chemical properties that allow it to be used where oil would fail. Glycerol is a thick syrup that is used as the "body" to many syrups, for example, cough medicines and lotions used to treat ear infections. It is also an additive in vanilla extracts and other food flavorings. Glycerin is added to ice cream to improve the texture, and its sweet taste decreases the amount of sugar needed. The base used in making toothpaste contains glycerin to maintain smoothness and shine. The cosmetic industry employs glycerin in skin conditioning lotions to replace lost skin moisture, relieve chapping, and keep skin soft. It is also added in hair shampoos to make them flow easily when poured from the bottle. The raisins found in cereals remain soft because they have been soaked in glycerol. Meat casings and food wrapping papers use glycerin to give them flexibility without brittleness. Similarly, tobacco is treated with this thick chemical to prevent the leaves from becoming brittle and crumbling during drying. It also adds sweetness to chewing tobacco. Glycerol is added during the manufacture of soaps in order to prepare shiny transparent bars. The trihydric alcohol structure of glycerin makes it a useful chemical in the manufacture of various hard foams, like those that are placed under siding in buildings and around dish washers and refrigerators for insulation and sound proofing. Analogously, the chemical structure of glycerol makes it an excellent catalyst in the microbiological production of vinegar from alcohol.
In the manufacture of foods, drugs, and cosmetics, oil cannot be employed as a lubricant because it might come in contact with the products and contaminate them. Therefore, the nontoxic glycerol is used to reduce friction in pumps and bearings. Gasoline and other hydrocarbon chemicals dissolve oil-based greases, so glycerin is used in pumps for transferring these fluids. Glycerol is also applied to cork gaskets to keep them flexible and tough when exposed to oils and greases as in automobile engines. Glycerin is used as a lubricant in various operations in the textile industry, and can be mixed with sugar to make a nondrying oil. Glycerol does not turn into a solid until it is cooled to a very low temperature. This property is utilized to increase the storage life of blood. When small amounts of glycerin are added to red blood cells, they can be frozen and maintained for up to three years.
Chemical derivatives of glycerol or propane-1,2,3-triol are important in a wide range of applications. Nitroglycerin is the trinitrate derivative of glycerol. One application of this chemical is as the key ingredient in the manufacture of dynamite explosives. Nitroglycerin can also be used in conjunction with gun cotton or nitrocellulose as a propellant in military applications. In the pharmaceutical industry, nitroglycerin is considered a drug to relieve chest pains and in the treatment of various heart ailments. Another derivative, guaiacol glyceryl ether, is an ingredient in cough medicines, and glycerol methacrylate is used in the manufacture of soft contact lenses to make them permeable to air. Glycerol esters are utilized in cakes, breads, and other bakery products as lubricants and softening agents. They also have similar applications in the making of candies, butter, and whipped toppings. A specially designed glycerol ester called caprenin can be used as a low calorie replacement for cocoa butter.
The acetins are derivatives of glycerol that are prepared by heating glycerol with acetic acid. Monoacetin is used in the manufacture of dynamite, in tanning leather, and as a solvent for various dyes. Diacetin, another derivative of glycerol, is used as a solvent and a softening agent. Triacetin, the most useful of the acetins, is used in the manufacture of cigarette filters and as a component in solid rocket fuels. It is also used as a solvent in the production of photographic films, and has some utility in the perfume industry. Triacetin is added to dried egg whites so that they can be whipped into meringues.
See also Fat.
Carey, Francis A. Organic Chemistry. New York: McGraw-Hill, 2002.
Newman, A.A. Glycerol. Cleveland: C.R.C. Press, 1968.