C_6H_5SO_2.Cl + K_2S = C_6H_5SO_2.SK + KCl
Sulphonic chlorides, dissolved in ether, yield sulphinic acids on reduction with zinc dust or metallic sodium—
C_6H_5SO_2.Cl + H_2 = C_6H_5SO_2.H + HCl
In the sulphonic acid compounds it is assumed that the sulphur is hexavalent, and it is hence possible to consider the sulphones to be esters of sulphinic acid.
==O R—S==O —H
The sulphones are mostly solid bodies, which soften prior to melting when heated. They are very stable towards chemical reagents; for instance, saponification of a mono-sulphone very rarely yields sulphinic acid.
If a hydroxyl is substituted for a hydrogen atom in the aromatic hydrocarbons, the action of sulphuric acid is greatly facilitated; thus, by merely mixing phenol with sulphuric acid, the sulphonic acid is at once formed, whereby, in the cold, o-phenolsulphonic acid prevails which on heating for some time to 100°-110° C. is completely converted into p-phenolsulphonic acid. In the absence of free sulphuric acid the conversion of o- into p-phenolsulphonic acid is brought about by heating the aqueous solution. Phenol-2,4-disulphonic acid is prepared from o- or p-phenolsulphonic acid, whereas phenol-2,4,6-trisulphonic acid is prepared directly from phenol by heating with concentrated sulphuric acid in presence of phosphorus pentoxide. Phenolsulphonic acids are also obtained by fusing benzenedisulphonic acid with alkali.
Cresol is not so easily sulphonated as is phenol; o-cresol when heated eight to ten hours at 90° C. with one and one-half times its weight of concentrated sulphuric acid, yields o-cresol-p-sulphonic acid.
The phenolsulphonic acids are strong, rather stable acids; their alcoholic hydroxyl-hydrogen atom may, similarly to that of the phenols, be substituted by a metal or an alkyl radical.
From [Greek: a]- and [Greek: b]-naphthol a number of sulphonic acids may easily be prepared; viz., mono-, di-, and trisulphonic acids. Nearly all these acids are important as basic materials in the dyestuff industry, especially 2,6-[Greek: b]-naphtholmonosulphonic acid (S-acid), 2,3,6-[Greek: b]-naphtholdisulphonic acid (R-acid) and 2,6,8-[Greek: b]-naphtholdisulphonic acid (G-acid).