Glycols
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 carbon atoms, 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).
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.
Yields from this reaction are often poor and better yields are obtained using osmium tetroxide, OsO4. While OsO4 is both expensive and toxic, powerful synthetic methods have been developed (notably by Barry Sharpless) that use catalytic quantities of osmium oxidants, often with excellent stereochemical control with respect to the 1,2-diol product (the Sharpless asymmetric dihydroxylation reaction). For this reason, osmatebased oxidants have nearly completely displaced permanganate oxidants, such as KMnO4, for the laboratory preparation of glycols.
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.
Alternative methods of preparing ethylene glycol that avoid the use of toxic ethylene oxide are currently being investigated.
In 1995, the production of ethylene glycol totaled 6.2 billion lb (2.8 x 109 kg), with a steady annual growth rate of 5-6%. Much of this ethylene glycol is used as antifreeze in automobile radiators. The addition of 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 boil-over 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, in 1993, of antifreeze based on the less toxic propylene glycol.
The second major use of ethylene glycol is in the production of poly(ethylene terephthalate), or PET. This polymer, a polyester, is obtained by reacting ethylene glycol with terephthalic acid (IUPAC name: 1,4- benzenedicarboxylic acid) or its dimethyl ester.
Poly(ethylene 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. Poly(ethylene terephthalate) has been utilized in the manufacture of clothing, bed linen, carpeting, and drapes.
Other glycols are also used in polymer production; for example, tetramethylene glycol is used to produce polyesters, and diethylene glycol is used in the manufacture of polyurethane and unsaturated polyester resins. Propylene glycol is used in the manufacture of the 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.
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