A glycol is an aliphatic organic compound in which two hydroxyl (OH) groups are present. The most important glycols are those in which the hydroxyl groups are attached to adjacent carbonatoms, and the term glycol is often interpreted as applying only to such compounds. The latter are also called vicinal diols, or 1,2-diols. Compounds in which two hydroxyl groups are attached to the same carbon atom (geminal diols) normally cannot be isolated.
The most useful glycol is ethylene glycol (IUPAC name: 1,2-ethanediol). Other industrially important glycols include propylene glycol (IUPAC name: 1,2-propanediol), diethylene glycol (IUPAC name: 3-oxa-1,5-pentanediol) and tetramethylene glycol (IUPAC name: 1,4-butanediol), see Figure 1.
The common glycols are colorless liquids with specific gravities greater than that of water. The presence of two hydroxyl groups permits the formation of hydrogen with water, thereby favoring miscibility with the latter. Each of the glycols shown above is completely miscible with water. Intermolecular hydrogen bonding between glycol molecules gives these compounds boiling points which are higher than might otherwise have been expected; for example, ethylene glycol has a boiling point of 388.5°F (198°C).
The most convenient and inexpensive method of preparing a glycol in the laboratory is to react an alkene with cold dilute potassium permanganate, KMnO4 (Figure 2).
Yields from this reaction are often poor; better yields are obtained using osmium tetroxide, OsO4. However, this reagent is both expensive and toxic.
In the industrial preparation of ethylene glycol, ethylene (IUPAC name: ethene) is oxidized to ethylene oxide (IUPAC name: oxirane) using oxygen
and a silver catalyst. Ethylene oxide is then reacted with water at high temperature or in the presence of an acid catalyst to produce ethylene glycol. Diethylene glycol is a useful by-product of this process (Figure 3).
Alternative methods of preparing ethylene glycol that avoid the use of toxic ethylene oxide are currently being investigated.
Much of this ethylene glycol is used as antifreeze in automobile radiators. Adding ethylene glycol to water causes the freezing point of the latter to decrease, thus the damage that would be caused by the water freezing in a radiator can be avoided by using a mixture of water and ethylene glycol as the coolant. An added advantage of using such a mixture is that its boiling point is higher than that of water, which reduces the possibility of boilover during summer driving. In addition to ethylene glycol, commercial antifreeze contains several additives, including a dye to reduce the likelihood of the highly toxic ethylene glycol being accidentally ingested. Concern over the toxicity of ethylene glycol—the lethal dose of ethylene glycol for humans is 1.4 ml/kg—resulted in the introduction of antifreeze based on nontoxic propylene glycol in 1993.
The second major use of ethylene glycol is in the production of poly(ethylene terephthalate) or PET.
Diol —An aliphatic organic compound containing two hydroxyl (OH-) groups.
Intermolecular hydrogen bonding —The attractive force between a hydrogen atom in one molecule and a strongly electronegative atom, such as oxygen, in a second molecule.
IUPAC —International Union of Pure and Applied Chemistry, the world oranization known for its efforts to standardize chemical names and symbols.
Polyester —A polymer in which the identical repeating units are linked by ester groups.
Poly(ethylene terephthalate) —A polymer formed by the reaction of ethylene glycol and terephthalic acid (or its dimethyl ester).
Polymer —A compound of high molecular weight whose molecules are made up of a number of identical repeating units.
Polyurethane —A polymer formed through the reaction of a glycol with a diisocyanate.
Unsaturated polyester resin —A product in which long-chain polyester molecules containing carbon-carbon double bonds have been joined (cross-linked) to other identical molecules.
This polymer, a polyester, is obtained by reacting ethylene glycol with terephthalic acid (IUPAC name: 1,4-benzenedicarboxylic acid) or its dimethyl ester (Figure 4).
Polyethylene terephthalate) is used to produce textiles, large soft-drink containers, photographic film, and overhead transparencies. It is marketed under various trademarks including Dacron®, Terylene®, Fortrel®, and Mylar®. Textiles containing this polyester are resistant to wrinkling, and can withstand frequent laundering. Polyethylene terephthalate) has been utilized in the manufacture of clothing, bed linen, carpeting, and drapes.
Other glycols are also used in polymer production, including tetramethylene glycol to produce polyesters, and diethylene glycol in the manufacture of polyurethane and unsaturated polyester resins. Propylene glycol is used to make polyurethane foam used in car seats and furniture. It is also one of the raw materials required to produce the unsaturated polyester resins used to make car bodies and playground equipment.
Bailey, James E. Ullmann’s Encyclopedia of Industrial Chemistry. New York: VCH, 2003.
Budavari, Susan, ed. The Merck Index. 11th ed. Rahway, NJ: Merck, 1989.
Loudon,G. Mark. Organic Chemistry. Oxford: Oxford University Press, 2002.
Szmant, H. Harry. Organic Building Blocks of the Chemical Industry. New York: Wiley, 1989.
Agency for Toxic Substances and Disease Registry. “Toxic FAQs for Ethylene Glycol and Propylene Glycol” <http://www.atsdr.cdc.gov/tfacts96.html> (accessed November 25, 2006).
Mallinckrodt Baker. “Material Safety Data Sheet: Ethylene Glycol” <http://www.jtbaker.com/msds/englishhtml/e5125.htm> (accessed November 25, 2006).
Arthur M. Last
"Glycol." The Gale Encyclopedia of Science. . Encyclopedia.com. (August 14, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/glycol
"Glycol." The Gale Encyclopedia of Science. . Retrieved August 14, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/glycol