SULPHONIC ACIDS, organic acids containing sulphur and having the general formula, where R is either an alkyl or an aryl radical. (See CHEMISTRY, ORGANIC.) Aromatic Suiphonic Acids.—The acids of this more important group are obtained generally by digesting aromatic compounds with concentrated sulphuric acid, the process being called sul phonation. Such operations are frequently conducted on a manu facturing scale as in the preparation of the sulphonic acids of benzene (q.v.), naphthalene (q.v.) and anthraquinone (q.v.).
The sodium salt of benzenesulphonic acid is of importance since on fusion with caustic soda it yields phenol. During the World War large quantities of synthetic phenol were made by this process in order to eke out the supply of coal tar phenol, which was then insufficient for the wholesale production of the ex plosive, picric acid (q.v.). Benzenedisulphonic acid, made from oleum (fuming sulphuric acid) and benzene, yields by alkali fu sion the important colour intermediate, resorcinol.
The sulphonic acids of naphthalene are manufactured on an extensive scale, for by alkali fusion naphthalene-a-sulphonic acid, made by low-temperature sulphonation, yields a-naphthol, whereas naphthalene-j3-sulphonic acid, prepared at 140° C, when similarly treated, furnishes j3-naphthol.
Anthraquinone, the oxidation product of anthracene (q.v.), when sulphonated with oleum at high temperatures yields anthra quinone-0-sulphonic acid, the sodium salt of which ("silver salt") is an important intermediate used in the manufacture of alizarin (q.v.) and the indanthrene vat colours. (See DYES, SYNTHETIC.) Sulphonation of anthraquinone is facilitated by a mercuric sul phate catalyst, and in this way anthraquinone-I :5- and -1 :8 disulphonic acids are produced. These disulphonic acids are also used as intermediates in the dye industry. Sulphonation may be
effected by chlorosulphonic acid whereby sulphonic chlorides are formed. (See SACCHARIN.) In addition to direct sulphonation, the sulphonic group may be introduced into the aromatic nucleus through the diazo-reaction (see DIAZO-COMPOUNDS).
Sulphonic acids of aromatic hydrocarbons are very soluble in water and when obtained crystalline are extremely hygroscopic. With phosphorus pentachloride they yield sulphonic chlorides, which are reducible to thiophenols. On distillation with sodium cyanide or ferrocyanide the sodium arylsulphonates give rise to nitriles.
Sulphanilic acid, a sulphonic acid of aniline, is sparingly soluble in cold water and is to be regarded as an internal salt, This compound and the similar naphthylamine sulphonic acids are important colour intermediates.
Aliphatic Sulphonic Acids are much less important than those of the aromatic series. They are obtained generally (i.) by the oxidation of mercaptans, alkyl disulphides, alkyl thiocyanates or alkyl sulphinic acids; (ii.) by the interaction of alkyl iodides and metallic sulphites; (iii.) by direct action of sulphur trioxide on hexane and its homologues (j. Worstall, 1898). They are either viscous liquids or crystalline solids, converted into sulphonic chlorides by phosphorus pentachloride and into alcohols and alkali sulphites by alkali fusion.
Hydroarornatic Sulphonic Acids.—Camphor yields a character istic sulphonic acid on sulphonation with sulphuric acid in acetic anhydride. The optically active camphor sulphonic acids from dextro- and laevo-modifications of camphor have been very use ful in the resolution of racemoid bases. (See STEREOCHEMISTRY.) (G. T. M.)