What Is Glycerin?
Glycerin, also known as glycerol, is a trivalent alcohol with the chemical formula C3H8O3 and a CAS number of 56-81-5. It has a molecular weight of 92.09, a melting point of 17.8°C, and a boiling point of 290°C. Glycerin is a clear, colorless, odorless liquid at room temperature, highly viscous with a sweet taste. Its density is 1.261 g/cm3, and it is soluble in water and ethanol, but extremely difficult to mix with diethyl ether.
Glycerin is used in various industries, including pharmaceuticals. It is also present in living organisms as the backbone of triglycerides, phospholipids, glycolipids, and other substances.
Uses of Glycerin
Glycerin has diverse applications, including as a food additive, in pharmaceuticals, and cosmetics. As a food additive, it serves as a sweetener, preservative, humectant, and thickening stabilizer, and does not cause tooth decay. In pharmaceuticals and cosmetics, it is used as a moisturizer and lubricant, and in the preparation of enema solutions, cough syrups, gargles, toothpaste, soaps, and lotions.
In the machinery industry, glycerin is sometimes used in antifreeze solutions. It also serves as a raw material in synthetic chemistry for products like nitroglycerin and allyl iodide as well as in the manufacture of surfactants and polyurethane resins.
Properties of Glycerin
1. Synthesis of Glycerin
Glycerin can be obtained from natural sources like soybean oil and tallow through hydrolysis of triacylglycerols (triglycerides), or synthetically from propylene. In chemical synthesis, it is produced by hydrolysis under basic conditions via epichlorohydrin. Glycerin is also a byproduct in the synthesis of fatty acid methyl esters for biodiesel fuel, often incinerated due to impurities.
2. Physical Properties of Glycerin
Glycerin is soluble in water and exhibits strong hygroscopicity. Its aqueous solution has a freezing point depression, making it useful as an antifreeze. It has a melting point of 17.8°C but can supercool to a glassy state at around -100°C. Crystals can be obtained by further cooling and slow reheating.
3. Chemical Reactions of Glycerin
Glycerin, a trivalent alcohol, reacts with various substances. It yields allyl alcohol when heated with formic acid, and acrolein when heated with potassium hydrogen sulfate. Glycerin can undergo dehydration with acetone to yield a protected derivative. One of its notable reactions is the Skraup quinoline synthesis, where acrolein reacts with aniline derivatives under oxidative conditions to form quinoline.
Types of Glycerin
Glycerin is available in various forms, including pharmaceuticals, food additives, industrial chemicals, cosmetic raw materials, and research reagents. In pharmaceuticals, it is commonly used in enemas, as well as in ointments and lubricating agents. For industrial use, it is sold in large quantities like 22 kg cans and 250 kg drums, while smaller capacities are available for research and development purposes.