GLYCOLS, the generic name applied to dihydric alcohols on account of their sweet taste (see CHEMISTRY: Organic). The simplest member of the series, ethylene glycol, is a colourless, fragrant, oily liquid boiling at 197.4° C, melting at —17.4° C and having a specific gravity of 1.125 at o° C. It resembles glycerine (glycerol) in sweetness and in being com pletely miscible with water. Its importance lies mainly in the fact that it serves as a substitute for glycerine in the production of a high explosive, ethylene dinitrate, the successive processes of manufacture being as follows :—Ethy lene gas, produced by passing alcohol vapour over charcoal im pregnated with phosphoric acid at 400° C, is treated with chlorine water so that the ethylene is always in excess. In these circum stances ethylene chlorohydrin, is produced as a liquid miscible with water (boiling point 128-132° C) . When warmed with milk of lime the chlorohydrin is converted into ethylene glycol and this dihydric alcohol on treatment with nitric and sulphuric acids, as in the preparation of nitroglycerine (q.v.) from glycerine, is converted i:ito ethylene dinitrate, an oily explosive liquid (boiling point 114-116° C, specific gravity, 1.496/15° C), which has the advantage over nitroglycerine of not freezing in cold weather (see EXPLOSIVES). A more complex nitro explosive may be prepared by nitrating a solution of a carbo hydrate (q.v.) in ethylene glycol (Hibbert, U.S. Patent 1216367 1917).
Higher glycols, e.g., a-propylene glycol, and 2 :3-butylene glycol, (OH) •CH •OH, are also syrupy liquids of comparatively high boiling points. They are prepared from the corresponding propylene, : or symmetrical butylene, : through their chloro hydrins by treating these substances with milk of lime. Certain complex glycols, termed pinacones or more appropriately pinacols, are produced by the reduction of ketones (q.v.). For example, acetone when reduced either electrolytically or with sodium and water furnishes the simplest pinacol, 2 :3-butanediol, a crystalline solid melting at 38° C and boiling at 172° C. When distilled with dilute sulphuric acid this pinacol is transformed by loss of water into pinacoline, methyl tertiary butyl ketone, a liquid boiling at 1o6° C, insoluble in water and having an odour of peppermint.