On the Mechanism of Sulfonation of the Aromatic Nucleus and Sulfone Formation

Abstract

Although sulfonations of aromatic hydrocarbons date back to near the beginning of modern organic chemistry, little had been published on the mechanisms of these reactions prior to the publication on this subject by Michael and Weiner in 1936 (1). The advent of stabilized sulfur trioxide on a commercial scale has increased the importance of the reactions of this material (2-4).In a recent article (5), Leierson, Host, and LeBaron presented their findings on the sulfur trioxide sulfonation of benzene in liquid sulfur dioxide to yield benzenesulfonic acid as the main product and lesser amounts of diphenylsulfone.These workers reported that the amount of diphenylsulfone formed, when the reaction was carried out at -8°, near the boiling point of liquid sulfur dioxide, was dependent upon one or more of three factors:(a) The order of addition of benzene and sulfur trioxide to liquid sulfur dioxide, (b) the warming of the reaction mixture after mixing of the reagents from -8°to room temperature with accompanying expulsion of sulfur dioxide, and (c) the purity of benzene.We have re-examined the reaction of benzene with sulfur trioxide in liquid sulfur dioxide and have found that the extent of concomitant sulfone formation is not dependent upon the purity of benzene to a significant degree as claimed by Leierson, Bost, and LeBaron. Instead, the percentages of sulfone in the reaction product were found to be dependent chiefly on the temperature of reaction and, especially, on the mole ratio of benzene to sulfur trioxide at various stages of the synthesis, when the hydrocarbon is added to the sulfur dioxide solution of sulfur trioxide.Furthermore, it was found that carboxylic acids, present in the sulfonation medium even in small amounts, exercise a profound effect on sulfone formation; that is, their presence remarkably lowers the percentages of sulfone in the product as compared with the results obtained in their absence. Suter (6) states that the presence of carboxylic groups influences the sulfonation in the aromatic series because the sulfonating agent is more soluble in carboxylic acids and because the formation of a mixed anhydride or an addition compound may occur as the first step in the reaction. Our results confirm these ideas.These combined findings led to the conclusion that pyrosulfonic acids are, in all probability, formed in the reaction of the aromatic hydrocarbon with sulfur trioxide and that these pyrosulfonic acids, through further reaction with benzene, are precursors to sulfones. In principle, at least, this concept agrees with that of Michael and Weiner (1). In our opinion, carboxylic acids react readily 455 SUMMARY The sulfur trioxide sulfonation of simple aromatic hydrocarbons produces sulfonic acids in high yield along with lesser amounts of sulfones. The amount of sulfone formed depends upon the temperature of reaction, the mode of addition of the reagents and, especially, upon the presence of small amounts of carboxylic acids, intentionally added to the reaction mixture.These factors lead to the co...

Recent Advances in the Synthesis of Sulfones

“…27 Sulfones are frequently encountered side-products in the sulfonation of arenes with sulfuric acid. 28 They are formed by the reaction of the intermediate sulfonic acid with an excess of the arene. Removal of water during the reaction furnishes the symmetrical diaryl sulfone as the major product (Scheme 5).…”

The Reactions of Sulfur Trioxide, and Its Adducts, with Organic Compounds.

“…Use of chloroform as reaction solvent reduces the sulfone to about 2% (130,306). Liquid S02 has been studied most extensively as the solvent for this reaction, because of its favorable boiling point and cost, and because it dissolves both reagents and products (117,118,219,306,399). One of these studies (399) has led to several conclusions regarding the probable course of the reaction at 1-to-l molar ratio, as well as the extent and possible mechanism of sulfone formation.…”

“…Liquid S02 has been studied most extensively as the solvent for this reaction, because of its favorable boiling point and cost, and because it dissolves both reagents and products (117,118,219,306,399). One of these studies (399) has led to several conclusions regarding the probable course of the reaction at 1-to-l molar ratio, as well as the extent and possible mechanism of sulfone formation. The reaction is seen as occurring in two steps I CsH6 + 2SOs -C6H5S020S03H (Benzenepyrosulfonic acid) (A II C8H6S020S03H + C"hJ^C 8HsS02C8H8 + H2S04 l-2C"H6S03H…”

Experimental and molecular modelling studies on aromatic sulfonation

“…1 The mechanism of the process is complex, reversible in many cases, and depends mainly on (1) the nature of the electrophilic species, which may be derived from sulfuric acid, pyrosulfuric acid, chlorosulfonic acid, acetylsulfonic acid or sulfur trioxide; and (2) the reactivity of the aromatic system under attack. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] At low temperatures, the reverse reaction is very slow, and the forward reaction is essentially irreversible. Several transient intermediate species are thought to be involved but few have been properly characterized.…”

“…Molecular energies of the reactants, intermediates and products(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) involved in the sulfonation of toluene calculated at the ab initio 4-31G/S* level…”