ALCOHOLS, in organic chemistry, a class of compounds of which ethyl alcohol (see ALCOHOL) is perhaps the most common member derived from paraffinoid and non-aromatic hydrocarbons by the replacement of one or more hydrogen atoms by hydroxyl groups. This definition excludes such compounds as the phenol (hydroxy derivatives of benzene, naphthalene and other aromatic hydrocarbons) which are more acidic than the true alcohols. Alcohols are classified on two distinct principles, one depending upon the number of hydroxyl groups present, the other on the nature of the remaining groups attached to the carbon atom which carries the hydroxyl group. Monohydric alcohols contain only one hydroxyl group; dihydric, two known as glycols; trihy dric, three known as glycerols ; and so on (see CHEMISTRY: Organic).
The second principle leads to alcohols of three distinct types, known as primary, secondary and tertiary (H. Kolbe, 186o). The genesis of these types may be readily understood by consid ering the relation which exists between the alcohols and the parent hydrocarbon. In methane, the hydrogen atoms are of equal value, and hence only one monohydric alcohol, viz., can be derived from it. This compound, methyl alcohol (carbinol), is the simplest primary alcohol, and it is characterized by the grouping Ethane, in a similar manner, can only give rise to one alcohol, namely ethyl alcohol or methylcarbinol, which is also primary. Propane, can give rise to two alcohols—a primary alcohol, (normal propyl alcohol or ethylcarbinol), formed by replacing a hydrogen atom attached to a terminal carbon atom, and a sec ondary alcohol, (isopropyl alcohol or di methylcarbinol), when the substitution is effected on the middle carbon atom. The grouping CH.OH characterizes the secondary alcohols ; isopropyl alcohol is the simplest member of this class. Butane, exists in the two isomeric forms—normal butane, and isobutane, Each of these hy drocarbons gives rise to two alcohols: n-butane gives a primary and a secondary; and isobutane a primary, when the substitution takes place in one of the methyl groups, and a tertiary, when the hydrogen atom of the ;CH group is substituted. Tertiary alco hols are thus seen to be characterized by the group : C•OH, in which the residual valencies of the carbon atom are attached to alkyl groups. The three types of alcohol may be formulated as follows (Primary) ; CH.OH (Secondary) ; (Tertiary).
Properties.—Thealcohols are neutral in reaction, and the lower members possess the property of entering into combination with salts, in which the alcohol plays the role of water of crystal lization. Sodium or potassium dissolves in the alcohols with the formation of alcoholates (alkyloxides), the hydrogen of the hy droxyl group being replaced by the metal. With strong acids water is split off and esters are formed The halide esters of the paraffin alcohols, formed by heating the alcohols with the halogen acids, are more conveniently prepared by the action of the phos phorous halide on the alcohol. Energetic dehydration gives rise to olefinic hydrocarbons, but under certain conditions ethers (see ETHER) are obtained.
The physical properties of the alcohols exhibit a gradation with the increase of molecular weight. The lower members are colourless mobile liquids, readily soluble in water and exhibiting a characteristic odour and taste. The solubility decreases as the carbon content rises. The normal alcohols containing i to 16 carbon atoms are liquids at the ordinary temperatures ; the higher members are crystalline, odourless and tasteless solids, closely resembling the fats in appearance. The boiling points of the normal alcohols increase regularly about 19° for each CH2 increment; this is characteristic of all homologous series of or ganic compounds. Of the primary, secondary and tertiary alco hols having the same empirical formula, the primary have the highest, and the tertiary the lowest boiling point ; this is in ac cordance with the fairly general rule that a gain in symmetry is attended by a fall in the boiling point.
Distinctive Tests for Primary, Secondary and Tertiary Alcohols.—Manyreactions serve to distinguish these three types of alcohol. Of chief importance is their behaviour on oxidation. The primary alcohols are first oxidized to aldehydes (q.v.), which, on further oxidation, yield acids containing the same number of carbon atoms as in the original alcohol. Secondary alcohols yield ketones (q.v.), which are subsequently oxidized to a mixture of two acids. Tertiary alcohols yield neither aldehydes nor ketones, but a mixture of two or more acids. Another method is based upon the different behaviour of the corresponding nitro alkyl with nitrous acid. The alcohol is first acted upon with phos phorus and iodine, and the resulting alkyl iodide is treated with silver nitrite, which gives the corresponding nitro-alkyl. The nitro-alkyl is then treated with potassium nitrite dissolved in concentrated potash, and sulphuric acid is added. By this treat ment the primary nitro-alkyl yields a nitrolic acid, the potassium salt of which forms an intense red solution ; the secondary nitro alkyl forms a pseudo-nitrole which gives an intense blue solution, while the tertiary compound does not react with nitrous acid. The reactions outlined above may bet hus represented:— ride ; an addition compound was first formed, which decomposed with water to give a ketone. When, however, a second molecule of a zinc alkyl was allowed to react, a compound was formed which gave a tertiary alcohol when decomposed with water.
It is interesting to note that, whereas zinc methyl and ethyl give tertiary alcohols, zinc propyl only gives secondary alcohols. The Grignard reaction (q.v.) affords a means of synthesizing the three classes of monohydric alcohols, the process being based on the addition of a Grignard reagent to a carbonyl group. When added to formaldehyde, a primary alcohol is obtained. With any other aldehyde a secondary alcohol is produced, whereas with a ketone the product is a tertiary alcohol.
The following monohydric alcohols receive special treatment under their own headings :—ALCOHOL (ETHYL), ALLYL ALCOHOL,