# 1911 Encyclopædia Britannica/Sulphonic Acids

See also Sulfonic acid on Wikipedia; and our 1911 Encyclopædia Britannica disclaimer.
​SULPHONIC ACIDS, in organic chemistry, a group of compounds of the type R·SO$_3$H, where R is an alkyl or an aryl group.
Aliphatic Sulphonic Acids.—The members of this class may be prepared by the direct sulphonation of some paraffins (I. Worstall, Amer. Chem. Journ., 1898, 20, p. 664), by the oxidation of mercaptans with concentrated nitric acid (H. Kopp, Arm., 1840, 35, p. 346); in the form of their salts from the alkyl halides and alkaline sulphates, and as esters from the alkyl halides and silver sulphite. They are colourless oils or crystalline solids which are extremely hygroscopic, very soluble in water and have a strongly -acid reaction. They are unaffected by heating with aqueous alkalis or acids and are stable towards concentrated nitric acid. Phosphorus pentachloride converts them into the corresponding acid chlorides, R·SO$_2$Cl, which are decomposed slowly by water. These chlorides, on reduction by zinc and sulphuric acid, pass readily into the mercaptans, whilst if zinc dust and alcohol be used they are converted into the sulphinic acids, R·SO$_2$H.
Methyl sulphonic acid, CH$_3$SO$_3$H, was obtained by H. Kolbe (Ann., 1845, 54, p. 174) by reducing trichloromethyl sulphonic chloride (formed from ch orine and carbon bisul hide in the presence of water:CS$_2$+5Cl2+2H$_2$O＝CCl3·SO$_2$Cl+4HCl+SCl$_2$) with sodium amalgam. It is a colourless syrup which decomposes when heated above 130° C. The corresponding acid chloride is an extremely stable solid which melts at 135° C. It is formed by the action of carbon bi sulphide on potassium bichromate in the presence of nitric and hydrochloric acids (Loew, Zeit. f. Chem., 1869, p. 82). When heated under pressure it decomposes with the final production of carbonyl and thionyl chlorides: CCl$_3$·SO$_2$Cl＝CCl$_4$+SO$_2$＝COCl$_2$+SOCl$_2$. Ethyl sulphonic acid, C$_2$H$_5$·SO$_3$H, is a crystalline deliquescent solid formed by oxidizing ethyl mercaptan or by reducing vinyl sulphonic acid, CH2:CH·SO;H (Kohler, Amer. Chem. Journ., 1898, 20, p. 687).
Thiosulphonic acids of the type R-S02-SH are formed by the action of the sulphochlorides on a concentrated solution of potassium sulphide: R·SO$_2$Cl+K$_2$S＝R·SO$_2$K+S+KCl＝KCl+R·SO$_2$·SK; or by the action of the salt of a sulphinic acid on an alkaline sulphide in the presence of iodine (Otto, Ber., 1891, 24, p. 144).
Aromatic Sulphonic Acids.—The acids of this group are very similar to the corresponding aliphatic sulphonic acids and are usually obtained by the direct heating of an aromatic hydrocarbon with concentrated sulphuric acid, fuming sulphuric acid or sulphur chlorhydrin. After the action is completed they may frequently be “salted out” by adding common salt to the acid solution until no more dissolves, when the sodium salt of the acid separates (L. Gattermann, Ber., 1891, 24, p. 2121). They are also formed by oxidizing thiophenols or by decomposing diazonium salts with sulphurous acid. The free acids are usually hygroscopic, crystalline solids which are readily soluble in water. When heated under pressure with concentrated hydrochloric acid to about 150° C. they yield hydrocarbons and sulphuric acid. The salts usually crystallize well, and those of the alkali metals are employed in the preparation of phenols, into which they pass when fused with the caustic alkalis. When distilled with potassium cyanide they yield the aromatic nitriles. The sulphonic acids with phosphorus pentachloride are converted into sulphochlorides which are stable to cold water, but with ammonia they yield sulphonamides, R·SO$_2$NH$_2$, and with alcohols esters of the sulphonic acids.
Benzene sulphonic acid, C$_6$H$_5$·SO$_3$H,11/2H$_2$O, crystallizes in small plates and is very deliquescent. Benzene sulphochloride, C$_6$H$_5$·SO$_2$Cl, is a colourless fuming liquid which boils at 120° C. (10 mm.). The amino benzene sulphonic acids, particularly the meta and para compounds, are of importance owing to their employment in the colour industry. The direct sulphonation of aniline yields the para acid, sulphanilic acid, C$_6$H$_4$(NH$_2$)(SO$_3$H), which crystallizes in small plates and is sparingly soluble in cold water. When fused with caustic potash it yields aniline, whilst oxidation with chromic acid yields benzoquinone. In constitution it is probably to be regarded as a cyclic ammonium salt, C$_6$H$_4$NH$_3$ SO$_3$. When diazotized in acid solution and coupled with dimethyl aniline it yields helianthine; ​the sodium salt of which is used as an indicator (q.v.). Metanilic acid C$_6$H$_4$(NH$_2$) (SO$_3$H) [1.3], which crystallizes in prisms, is formed by the reduction of meta-nitrobenzene sulphonic acid and is used in the preparation of various azo dyes.
Sulphinic acids, R·SO$_2$H, are formed by reducing sulphochlorides with zinc dust; by the action of sulphur dioxide on the zinc alkyls (Hobson, Ann, 1857, 102, p. 72; 1858, IO6, p. 287); by the action of sulphochlorides on mercaptans in alkaline solution; and by the action of the Grignard reagent on sulphur dioxide or thionyl chloride (Rosenheim, Ber., 1904, 37, p. 2152; Oddo, R. Accad. Lin., 1905 (5), 14 (i.), p. 169). The free acids are unstable. They are readily oxidized to sulphonic acids and reduced to mercaptans., Their alkali salts on treatment with the alkyl halides yield sulphones, R$_2$SO$_2$. Ethyl sulphinic acid, C$_6$H$_5$·SO$_2$H, is a colourless syrup. Benzene sulphinic acid, C$_6$H$_5$·SO$_2$H, crystallizes in large prisms and acts as a reducing agent. It decomposes when heated with water under pressure: 3C$_6$H$_5$·SO$_2$H＝C$_5$H$_5$·SO$_2$H+C$_5$H$_5$·SO$_2$·S·C$_6$H$_5$+H$_2$O. The potassium salt when fused with caustic potash yields benzene and potassium sulphite.