1911 Encyclopædia Britannica/Alcohols
ALCOHOLS, in organic chemistry, a class of compounds which may be considered as derived from hydrocarbons by the replacement of one or more hydrogen atoms by hydroxyl groups. It is convenient to restrict the term to compounds in which the hydroxyl group is attached to an aliphatic residue; this excludes such compounds as the hydroxy-benzenes, naphthalenes, &c., which exhibit many differences from the compounds derived from the aliphatic alkyls.
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. Monatomic or monohydric alcohols contain only one hydroxyl group; diatomic, two, known as glycols (q.v.); triatomic, three, known as glycerols (q.v.); and so on.
The second principle leads to alcohols of three distinct types, known as primary, secondary and tertiary. The genesis and formulation of these types may be readily understood by considering the relation which exists between the alcohols and the parent hydrocarbon. In methane, CH4, the hydrogen atoms are of equal value, and hence only one alcohol, viz. CH3OH, can be derived from it. This compound, methyl alcohol, is the simplest primary alcohol, and it is characterized by the grouping ·CH2OH. Ethane, C2H6, in a similar manner, can only give rise to one alcohol, namely ethyl alcohol, CH3CH2OH, which is also primary. Propane, CH3CH2CH3, can give rise to two alcohols—a primary alcohol, CH3CH2CH2OH (normal propyl alcohol), formed by replacing a hydrogen atom attached to a terminal carbon atom, and a secondary alcohol, CH3·CH(OH)·CH3 (isopropyl alcohol), 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, C4H10, exists in the two isomeric forms—normal butane, CH3·CH2·CH2·CH3, and iso-butane, CH(CH3)3. Each of these hydro-carbons gives rise to two alcohols: n-butane gives a primary and a secondary; and iso-butane 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 alcohols 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.
In 1860 Hermann Kolbe predicted the existence of secondary and tertiary alcohols from theoretical considerations. Regarding methyl alcohol, for which he proposed the name carbinol, as the simplest alcohol, he showed that by replacing one hydrogen atom of the methyl group by an alkyl residue, compounds of the general formula R.CH2.OH would result. These are the primary alcohols. By replacing two of the hydrogen atoms, either by the same or different alkyls, compounds of the formula (R·R1)CH·OH (i.e. secondary alcohols) would result; while the replacement of the three hydrogen atoms would generate alcohols of the general formula (R·R1·R2)C·OH, i.e. tertiary alcohols. Furthermore, he exhibited a comparison between these three types of alcohols and the amines. Thus:—
To distinguish Primary, Secondary and Tertiary Alcohols.—Many reactions serve to distinguish these three types of alcohols. 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 phosphorus 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 treatment a primary nitro-alkyl yields a nitrolic acid, the potassium salt of which forms an intense red solution; a secondary nitro-alkyl forms a pseudo nitrol, which gives an intense blue solution, while the tertiary compound does not act with nitrous acid. The reactions outlined above may be thus represented:—
R·CH2OH→ R·CH2I→ R·CH2·NO2→ R·C⪕NOH
Primary alcohol. Nitrolic acid.
Secondary alcohol. Pseudo nitrol.
(R1R2R3)C·OH → (R1R2R3)C·I → (R1R2R3)C·NO2
By heating to the boiling point of naphthalene (218°) tertiary alcohols are decomposed, while heating to the boiling point of anthracene (360°) suffices to decompose secondary alcohols, the primary remaining unaffected. These changes can be followed out by determinations of the vapour density, and so provide a method for characterizing alcohols (see Compt. Rend. 1904, 138, p. 984).
Alcohols may be readily prepared from the corresponding alkyl haloid by the action of moist silver oxide (which behaves as silver hydroxide): by the saponification of their esters; or by the reduction of polyhydric alcohols with hydriodic acid, and the subsequent conversion of the resulting alkyl iodide into the alcohol by moist silver oxide. Preparation. Primary alcohols are obtained by decomposing their sulphuric acid esters (from sulphuric acid and the olefines) with boiling water; by the action of nitrous acid on primary amines; or by the reduction of aldehydes, acid chlorides or acid anhydrides. Secondary alcohols result from the reduction of ketones; and from the reaction of zinc alkyls on aldehydes or formic acid esters.
CH3CHO → CH3·CH＜C2H5
OZnC2H5 → CH3·CH＜C2H5
Acetaldehyde.Methyl ethyl carbinol.
|HC⪕||O||→ HC⪪||OZnCH3||→ HC⪪||OZnCH3||→ HC⪪||OH|
Formic ester. Isopropyl alcohol.
Tertiary alcohols may be synthesized by a method devised by A. Butlerow in 1864, who thus discovered the tertiary alcohols. By reacting with a zinc alkyl (methyl or ethyl) on an acid chloride, an addition compound is first formed, which decomposes with water to give a ketone. If, however, a second molecule of a zinc alkyl be allowed to react, a compound is formed which gives a tertiary alcohol when decomposed with water.
|R·C⪕||O||→ R·C⪪||CH3||→ R·C⪪||CH3||→ R·C⪪||CH3|
Acid chloride.Tertiary alcohol.
It is interesting to note that, whereas zinc methyl and ethyl give tertiary alcohols, zinc propyl only gives secondary alcohols. During recent years (1900 onwards) many brilliant syntheses have been effected by the aid of magnesium-alkyl-haloids.
The alcohols 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 crystallization. Properties. Sodium or potassium dissolves in them with the formation of alcoholates, the hydrogen of the hydroxyl group being replaced by the metal. With strong acids water is split off and esters are formed. The haloid esters of the paraffin alcohols formed by heating the alcohols with the halogen acids are the monohaloid derivatives of the paraffins, and are more conveniently prepared by the action of the phosphorous haloid on the alcohol. Energetic dehydration gives the olefine 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 1 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 10° for each CH2 increment; this is characteristic of all homologous series of organic compounds. Of the primary, secondary and tertiary alcohols having the same empirical formula, the primary have the highest, and the tertiary the lowest boiling point; this is in accordance with the fairly general rule that a gain in symmetry is attended by a fall in the boiling point.