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acyclic compounds containing two hydroxyl groups. The simplest glycol (ethylene glycol, HOCH2— CH2OH) and other lower glycols are viscous, colorless liquids, readily soluble in water and alcohol, and sweet to the taste. The higher glycols are solids that are readily soluble in alcohol and ether and less readily soluble in water. Depending on the relative arrangement of the OH groups in the molecule, a distinction is made among the 1,2-glycols, such as the 1,3-glycols, such as ; and so on. Glycols containing two OH groups at the same carbon atom—for example, chloral hydrate, CCl3CH(OH)2—are stable only in the presence of strong electronegative groups.
In their chemical properties, glycols resemble the monohydric alcohols in many respects (they form alkoxides, ethers, esters, and other compounds). Depending on the relative arrangement of the OH group in the molecule and on the dehydration conditions, dehydration of glycols may yield carbonyl compounds or cyclic ethers. Thus, ethylene glycol yields acetaldehyde or 1,4-dioxane, whereas 1,4-butane diol yields tetrahydrofuran.
A general synthetic method for glycols is the hydrolysis of the corresponding dihalides (I) or chlorohydrins (II); 1,2-glycols are also prepared by the hydration of the corresponding epoxides (III) or by the oxidation of olefins (IV):
The 1,2-glycols and their derivatives have the greatest practical importance. The esters of higher glycols are used as plasticizers.