When sulphides or mercaptans in glacial acetic acid solution are heated with permanganate, the resulting sulphonic acid compounds exhibit great similarity to compounds containing free carboxyl groups. The sulphonic acid group may also be directly introduced either by concentrated, or by fuming sulphuric acid, or by elimination of halogen by the action of sodium or silver sulphite on the halogen derivatives of the aliphatic compounds. Saturated hydrocarbons do not react with sulphur trioxide, but unsaturated hydrocarbons are readily attacked by SO_3. Similarly, halogenated compounds and alcohols react with concentrated or fuming sulphuric acid forming sulphonic and hydrosulphonic acids respectively. The aromatic compounds form, as a rule, sulphonic acids with much greater facility. Benzene, for instance, is easily converted into the m-disulphonic acid by gently heating with fuming sulphuric acid; stronger heating converts the m- into the p-disulphonic acid, and at 190° C. the trisulphonic acid is formed. Toluene treated with fuming sulphuric acid first yields o- and p-sulphonic acids, finally o- and p-disulphonic acids, ethylbenzene at the boiling point p-ethylbenzene-sulphonic acid. Of the three isomeric xylenes o- and m-xylene dissolve in concentrated, p-xylene in fuming sulphuric acid only.
The action of sulphuric acid on naphthalene is stronger even than on benzene. Equal parts of naphthalene and sulphuric acid heated to 100° C. yield 80 per cent. [Greek: a] and 20 per cent. [Greek: b]-monosulphonic acid. At 160°-170°C. 25 per cent [Greek: a]- and 75 per cent. [Greek: b]-sulphonic acid is formed, and at higher temperatures [Greek: b]-monosulphonic acid only. If, on the other hand, 8 parts of naphthalene are heated with 3 parts of concentrated sulphuric acid to 180° C., two different naphthyldisulphonic acids are obtained.
Complete solution of the substance in sulphuric acid is, generally speaking, a criterion of complete sulphonation. A completely sulphonated compound should remain clear on dilution with water, or, in case precipitation occurs, the precipitate should be completely soluble in alkali or ammonia. It is necessary to submit the product to this test, since many organic substances are soluble in concentrated sulphuric acid without undergoing any alteration in composition.
Phosphoruspentoxide or potassium sulphate considerably increase the sulphonating property exhibited by fuming sulphuric acid.
The separation of the sulphonic acids from sulphuric acid is effected by salting out the former with common salt, or by removing the sulphuric acid with calcium, barium, or lead salts, provided that the sulphonic acid salts of these metals are soluble in water.
The sulphonic acid, in its chemically pure state, is best obtained from its crystalline barium salts, which are decomposed with the equivalent of sulphuric acid; another way is to decompose the calcium salts of the sulphonic acids with oxalic acid. The sulphonic acids are frequently hygroscopic and are easily soluble in water; the majority of their barium and lead salts are also soluble in water. The sulphonic acids are insoluble in ether. The halogens do not easily react with sulphonic acids, but when they do they usually replace the sulphonic acid group. In order to prepare the halogen substitution products, therefore, use is made of sulphonic chlorides. The latter are obtained by the action of chlorosulphonic acid on aromatic hydrocarbons; a simpler method, however, is to treat the dry alkali sulphonates with phosphorus pentachloride—
C_6H_5SO_3Na + PCl_5 = C_6H_5SO_2.Cl + NaCl + POCl_3
Derivatives of sulphonic chlorides are sulphonamides, which are easily prepared from the former by grinding with ammonium carbonate—
C_6H_5SO_2.Cl + (NH_4)_2CO_3 = C_6H_5.SO_2.NH_2 + NH_4Cl + CO_2 + H_2O
Sulphonic chlorides react with alkaline sulphides to form thiosulphonic acids—