Sulfonation, of aromatics

Sulfonation of aromatic hydrocarbons with sulfuric acid is cataly2ed by hydrogen fluoride or, at lower temperatures, by boron trifluoride (144). The products obtained are more uniform and considerably less sulfuric acid is needed, probably because BF forms complexes with the water formed ia the reaction, and thus prevents dilution of the sulfuric acid. 

M low cost, easily handable reactions not stoichio-metric generally hydrotrope sulfonation of aromatics using ... 

Nitration and sulfonation of aromatic compounds probably occur via the formation of the nitryl and sulfonyl cations ... 

Except for these studies of their protonation behavior, almost the only other aspect of the chemistry of sulfonic acids that has been investigated to any extent from a mechanistic point of view is the desulfonation of aromatic sulfonic acids or sulfonates. Since this subject has been well reviewed by Cerfontain (1968), and since the reaction is really more of interest as a type of electrophilic aromatic substitution than as sulfur chemistry, we shall not deal with it here. One should note that the mechanism of formation of aromatic sulfonic acids by sulfonation of aromatic hydrocarbons has also been intensively investigated, particularly by Cerfontain and his associates, and several... 

Stilbenes, photocyclization of, 30, 1 StiUe reaction, 50, 1 Stobbe condensation, 6, 1 Substitution reactions using organocopper reagents, 22, 2 41, 2 Sugars, synthesis by glycosylation with sulfoxides and sulfinates, 64, 2 Sulfide reduction of nitroarenes, 20, 4 Sulfonation of aromatic hydrocarbons and aryl halides, 3, 4 Swem oxidation, 39, 3 53, 1... 

Earle and Katdare reported a process for the sulfonation of an aromatic compound in the presence of an IL. The method for the sulfonation of aromatic compounds in water-stable ILs offered advantages over conventional sulfonation reactions (1) no by-products were formed (2) the IL was not consumed, and (3) the sulfonating agent (e.g., SO3) was relatively inexpensive. 

Sulfonation of aromatic compounds is generally carried out with sulfuric acid, halosulfuric acids, or sulfur trioxide as reagent with or without solvent.458,459 Friedel-Crafts catalysts such as aluminum chloride and boron trifluoride are effective catalysts in certain sulfonations with sulfuric acid and chlorosulfuric acid. 

Fig. 5.17. Effective electrophiles in sulfonations of aromatic compounds with sulfuric acids of different concentrations.

C. M. Suter and A. W. Weston, Direct Sulfonation of Aromatic Hydrocarbons and Their Halogen Derivatives, Org. React. 1946, 3, 141-197. 

Direct sulfonation of aromatic amines is even possible. This is very surprising because in sulfuric acid essentially all the amine will be protonated. The protonated amine would react in the meta position just like Ph—NMelj but in these reactions the para-sulfonic acid is formed. 

Important reactions have included sulfur as nucleophile and leaving group in the Sn2 reaction (illustrated here see also Chapter 17), sulfonation of aromatic rings (Chapter 22), formation and reduction of thioacetals (Chapter 24) and La wesson s reagent for converting carbonyl groups to thiocarbonyl groups (Chapter 44). 

The sulfonation of aromatic amines such as aniline can give a mixture of products that must be separated prior to dye synthesis. 

Sulfones are often produced as by-products in the sulfonation of aromatic hydrocarbons (method 540). Aromatic hydrocarbons react with sulfonic acids less readily than with sulfuric acid. The success of the reaction depends upon the removal of the water as it is formed. An automatic water separator is used in the conversion of a refluxing mixture of benzene and sulfuric acid to diphenyl sulfone (80%). A similar technique has been employed in the preparation of unsymmetrical sulfones. 

Sulfonation of aromatic compounds is very important in the manufacture of dyes. As sulfonic acids are generally water-soluble, the sulfonation of aromatic compounds provides a means to make aromatic compounds water-soluble. Also, the sulfonic acid group is easily displaced by a variety of other groups. Therefore, aryl-sulfonic acids are useful synthetic intermediates. 

Nitration and sulfonation of aromatics, Diels-Alder condensation, polyethylene, molecular complexes, CO-paraffin reaction, detergents, etc. 

Trapping of the Beckmann intermediates with enol silyl ethers affords facile entry to a variety of en-amino ketones. This condensation takes place with retention of regiochemical integrity in both oxime sulfonates and enol silyl ethers. Reaction of 6-methyl-l-(trimethylsiloxy)-l-cyclohexene (41) or 1-methyl-2-(trimethylsiloxy)-l-cyclohexene (42) with cyclohexanone oxime mesylate furnishes (43) or (44), respectively, as the sole isolable products (equation 25). Another striking feature of the reaction is the high chemospecificity. The condensation of the enol silyl ether (45), derived from p-acetoxyaceto-phenone, occurs in a chemospecific fashion with cyclododecanone oxime mesylate, the acetoxy moiety remaining intact (equation 26). Oxime sulfonates of aromatic ketones and cyclopentanones are not employable since complex reaction mixtures are formed. 

Concentrated or fuming sulfuric acid (oleum) is widely used for the direct sulfonation of aromatic compounds (see Chapter 7, p. 97).5,6 The active sulfonating agent in sulfuric acid is the electrophile sulfur trioxide, and the sulfonating power of sulfuric acid is proportional to the concentration of S03. Consequently, fuming sulfuric acid, which contains excess sulfur trioxide, is a more powerful sulfonating agent than concentrated sulfuric acid. The sulfonation of an aromatic hydrocarbon is depicted in Scheme 26. 

In the sulfonation of aromatic substrates containing a free amino group with hot concentrated sulfuric acid, the initially formed amine sulfate may rearrange to yield the required sulfonic acid. Thus, when aniline (31) is heated with concentrated sulfuric acid at 200°C (the baking process ), the amine sulfate (32) is probably converted into phenylsulfamic acid (33) which subsequently rearranges by migration of the sulfonic acid group into orthanilic acid (34) and finally to the most thermodynamically stable sulfanilic acid (35) (Scheme 18). 

The rearrangement of arylaminosulfuric acids (arylsulfamic acids) 220 to the corresponding ring-sulfonated anilines 221 and 222 was of great interest from both mechanistic and synthetic viewpoints, because these above-named acids have been postulated as intermediates in the sulfonation of aromatic amines with sulfuric acid305 (equation 85). 

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