Sulfo Compounds

SULFO COMPOUNDS, organic compounds containing a sulfuric acid residue, the so-called sulfoxyl or sulfo group, -S-OH; sulfones containing the group -S- linked to two radicals are also formed. The formation of sulfonic acids proceeds especially easily in aromatic compounds by the direct action of concentrated sulfuric acid on aromatic hydrocarbons according to the equation C₆H₆ + HO.SO₂.OH -> H₂O + C₆H₅.SO₂.OH. Free sulfonic acids of the aromatic series are crystalline bodies, usually readily soluble in water, sometimes deliquescent in air. Due to the ease of obtaining sulfo compounds of this series and the possibility of replacing the sulfo group with other groups, sulfo compounds have found wide application as intermediate compounds in a number of chemical industries. Thus, for example, phenols can be obtained from sulfo compounds by fusing sulfo compounds with alkalis: C₆H₅-SO₃Na + NaOH -> C₆H₅OH + Na₂SO₃; by the action of phosphorus pentachloride on sulfonic acids, sulfonic acid halogen anhydrides are obtained, from which sulfonic acid amides or sulfamides are obtained by the action of ammonia. Thiophenols are obtained by the reduction of sulfo compounds. The substitution of the sulfo group by a cyan group makes it possible to convert sulfonic acids into carboxylic acids. The introduction of the sulfo group increases the water solubility of aromatic compounds and decreases their ability to be oxidized, which is observed upon the introduction of the sulfo group into aromatic amines and phenols. Among individual sulfo compounds, the following should be noted: sulfobenzoic acid C₆H₅SO₃H; sulfotoluene CH₃.C₆H₄SO₃H; sulfanilic acid, HSO₃.C₆H₄.NH₂, serves as an initial product for obtaining azo dyes—methyl orange, tropaeolin, and others, and is used for the detection of nitrous acid in sanitary studies (Griess reaction); the red dye Congo, used for the detection of hydrochloric acid in gastric contents, belongs to the sulfonic acid derivatives of naphthylamine; o-sulfobenzoic acid C₆H₄(COOH)SO₃H, a derivative of which is saccharin (see); sulfosalicylic acid C₆H₃(OH)(CO.OH)SO₃H finds application as a reagent for protein; β-sulfo-α-aminopropionic acid, or cysteic acid HSO₃.CH₂.CH(NH₂).COOH, is obtained by the oxidation of cystine, and upon decarboxylation it is converted into the sulfamine taurine NH₂.CH₂.CH₂.SO₂H, which takes part in the formation of bile acids; thiocol is the potassium salt of sulfoguaiacol KSO₃.C₆H₃(OH)O.CH₃; sulfonal (CH₃)₂C(SO₂C₂H₅)₂ is a hypnotic medicinal preparation, as are trional (C₂H₅)(CH₃).C(SO_{2}.C₂H_{5})_{2} and tetronal (C₂H₅)₂C(SO_{2}.C₂H_{5})_{2}. Formaldehyde sulfoxylate HO.CH₂.SO₂Na is a strong reducing agent and is part of neosalvarsan and novoarsenol (see Salvarsan). The introduction of the sulfo group strongly decreases the toxic properties of a substance. Thus, for example, the introduction of a sulfo group into the ring of poisonous aniline leads to the formation of the weakly toxic sulfanilic acid; thiocol is significantly less toxic compared to guaiacol. The decrease in toxicity is closely connected with the decrease in the substance's ability to be oxidized after the introduction of the sulfo group.