Alcohols

ALCOHOLS, or spirits, are a group of organic compounds representing hydrocarbons in which one of the hydrogen atoms is replaced by an OH-group (hydroxyl). Thus, all alcohols correspond to the general formula R.OH, where R is a monovalent hydrocarbon radical. In nature, alcohols are rarely found in a free state; the most important representative of alcohols—ethyl alcohol—has been detected in blood, in animal tissues, and also in certain parts of plants. Synthetically, alcohols are obtained by: 1) heating alkyl halides with water or alkalis under pressure: RCl + H₂O = R.OH + HCl; 2) reduction of aldehydes and ketones (see); 3) the action of nitrous acid on primary amines (see): R.CH₂.NH₂ + HNO₂ = R.CH₂.OH + N₂ + H₂O; 4) by organomagnesium synthesis (see Grignard reaction) from ketones, aldehydes, and carboxylic acid derivatives; 5) saponification of complex esters: R.COOR + KOH = R.COOK + R.OH. These methods have, mainly, laboratory significance: for various kinds of research and organic syntheses with the aim of obtaining alcohols of more or less complex structure or in a chemically pure form. All technically important alcohols, except for methyl alcohol, which is obtained by the dry distillation of wood, are obtained almost exclusively by the fermentation of carbohydrates (see Fermentation). The lower members of the homologous series of alcohols are mobile, colorless liquids with a characteristic odor, which are highly soluble in water; with an increase in molecular weight, the solubility of alcohols in water decreases; starting from the fifth member, they become less mobile, take on an oily consistency, and finally, at ordinary temperatures, turn into solid, colorless, water-insoluble bodies that crystallize well from ethyl alcohol. The chemical properties of alcohols are reduced to the reactive capacity of the OH-group: 1) upon the action of alkali and certain other metals on alcohols, salt-like compounds are obtained—alcoholates (see); 2) oxygen-containing mineral and organic acids react with alcohols to form complex esters; 3) halide compounds of phosphorus cause the replacement of the hydroxyl with a halide: R.OH + PCl₅ = R.Cl + HCl + POCl₃; 4) under the influence of catalysts that promote the splitting off of water, alcohols, depending on the conditions of the experiment and the nature of the catalyst, are converted either into simple ethers (R.OH + HO.R = R.O.R + H₂O) or into unsaturated hydrocarbons (C₂H₅.OH = C₂H₄ + H₂O). Some chemical properties of alcohols depend on the number of free hydrogen atoms located at the carbon atom bearing the OH-group. Therefore, three types of alcohols are distinguished: primary—R.CH₂.OH, secondary—R₂CH.OH, and tertiary—R₃C.OH. The difference in their properties is sharply manifested during oxidation: primary alcohols are converted into aldehydes, which, upon further oxidation, turn into carboxylic acids: R.CH₂.OH -> R.CHO -> R.COOH; secondary alcohols form ketones: R₂.CH.OH + O = R₂CO + H₂O; tertiary alcohols, however, either do not undergo oxidation at all or decompose, forming substances with a smaller number of carbon atoms. Besides monohydric alcohols, there exist dihydric, trihydric, etc.—polyhydric alcohols—with two, three, or more hydroxyls. One of the well-known trihydric alcohols is glycerin (see). Polyhydric alcohols are obtained by the reduction of the simplest sugars—monoses (see Carbohydrates). Regarding individual alcohols, see under the corresponding names of the alcohols, e.g., amyl alcohol, butyl alcohol, etc.