- The sulfur atom
- Analysis of organosulfur compounds
- Organic compounds of bivalent sulfur
- Organic compounds of polyvalent sulfur: sulfoxides and sulfones
- Other sulfinyl and sulfonyl compounds
In sulfoxides, R−S(=O)−R′, and sulfones, R−S(=O)2−R′, groups R and R′ both contain a carbon atom bonded to sulfur. A variety of other organosulfur compounds are known of types R−S(=O)−X, Y−S(=O)−X, R−S(=O)2−X, and Y−S(=O)2−X, in which X and Y are elements other than carbon—e.g., oxygen, nitrogen, or a halogen. Three types of organosulfur oxyacids are possible: sulfenic acids, RSOH; sulfinic acids, RS(O)OH; and sulfonic acids, RSO2OH. These compounds are named by attaching the name of the alkane, arene, and so on, to the name for the acid, as in trichloromethanesulfenic acid, ethanesulfinic acid, and p-toluenesulfonic acid. The sulfonic acids are very strong—comparable to hydrochloric acid and other mineral acids—and are the most common of the sulfur-containing acids. The colourless and odourless sulfonic acids and their salts are water soluble. Sulfonic acid salts, particularly those of linear alkylbenzenesulfonates, are very useful and widely used as detergents and synthetic surfactants. Sulfonic acid groups can greatly enhance the water solubility of compounds, as seen with the sulfonic acid derivative of triphenyl phosphine (TPPTS), P(C6H4-m-SO3Na)3. Metal complexes of this compound are used as homogeneous catalysts for the syntheses of organic compounds in two-phase systems (e.g., in a mixture of water and an organic solvent) in industry and the laboratory. A few sulfonic acids occur naturally—for example, the essential nutrient taurine (2-aminoethanesulfonic acid; NH2CH2CH2SO3H), the echinosulfonic acids A–C, and the sulfobacins and other sulfonolipids. Sulfinic acids are weaker (having a pKa of roughly 2.2) and less stable than sulfonic acids. Sulfenic acids and their salts are unstable compounds, are weaker (pKa ≈ 5) than sulfinic acids, and are rarely isolated. Sulfenic acids containing one or three carbon atoms are key intermediates formed when onion, garlic, and related plants are cut or crushed (see above Sulfoxides and sulfones: Reactions).
Aromatic sulfonic acids and sulfonyl chlorides can be prepared by sulfonation of benzene derivatives with fuming sulfuric acid and chlorosulfonic acid, ClSO3H, respectively, while aliphatic sulfonic acids are prepared by vigorous oxidation of thiols or by reaction of amine sulfur trioxide complexes (e.g., Me3NSO3) with organolithium compounds. Trifluoromethanesulfonic acid (triflic acid; CF3SO3H), one of the strongest known organic acids, is manufactured by electrochemical fluorination of methanesulfonyl chloride or fluoride and is used as a polymerization catalyst, in fuel cells, in gasoline production, and in synthesis of organic and organometallic compounds. Reaction of sulfonyl chlorides with amino compounds, R′NH2, gives sulfonamides and related compounds, RSO2NHR′, whereas reaction of alcohols in the presence of tertiary amines yields sulfonates, RSO2OR′.
Carbanions attack sulfonyl chlorides at chlorine rather than sulfur, forming carbon-chlorine rather than carbon-sulfur bonds. Attack at sulfur can be realized by substituting sulfonyl fluorides, RSO2F, for sulfonyl chlorides. Aromatic sulfonyl chlorides can be reduced to aromatic thiols such as thiophenol with zinc metal-hydrochloric acid (Zn/HCl). Sulfinyl chlorides can be made by treating disulfides with chlorine in the presence of acetic anhydride. Sulfinyl chlorides react with amines and alcohols to yield sulfinamides (RS(O)NR′2) and sulfinates (RS(O)OR′), respectively. As previously noted (see above Disulfides and polysulfides and their oxidized products: Reactions), sulfenyl chlorides can be prepared by reaction of disulfides with equimolar quantities of chlorine. Sulfenyl chlorides readily add to olefins to produce chlorine-containing sulfides and react with amines to form sulfenamides, RSNR′2.
Sulfonylureas, RSO2NHC(O)NRR′, which are widely used herbicides, inhibit acetolactic synthase, a key plant enzyme. Anticlotting medical plastics have been prepared containing sulfonated polymers that bind heparin, a natural polysulfate. Sulfonamides, RSO2NH2, played an important role in the development of certain medicines. Sulfanilamide, p-aminobenzenesulfonamide, a compound used in the manufacture of azo dyes, was found to inhibit the growth of bacteria. This discovery led to the development of sulfa drugs, which still find some use today in the treatment of infections, although they have been largely replaced by newer antibiotics, to which bacteria are less resistant. Other sulfonamides include sildenafil (Viagra), a popular drug for the treatment of erectile dysfunction; piroxicam (Feldene), a cyclic sulfonamide used to treat arthritis; and acetazolamide (Diamox), a diuretic used in the treatment of glaucoma.
Esters of sulfuric acid—such as dimethyl sulfate, MeOSO2OMe, and diethyl sulfate, EtOSO2OEt, made from the alcohols methanol and ethanol, respectively, as well as sulfur trioxide/sulfuric acid—are important industrial chemicals used to introduce methyl (Me) and ethyl (Et) groups into organic molecules. Both dimethyl and diethyl sulfate are highly toxic. Esters of sulfurous acid known as dialkyl sulfites—dimethyl sulfite, MeOS(O)OMe, for example—can be made from alcohols and thionyl chloride: 2MeOH + Cl2S=O → MeOS(=O)OMe. Cyclic sulfite esters, made in a similar manner from 1,2-diols (1,2-dialcohols), and their oxidation products, cyclic sulfate esters, find considerable use in organic synthesis.
Optically active unsymmetrically substituted cyclic sulfites are especially useful in the synthesis of other optically active compounds.