Mechanism of sulfonation

The reaction mechanism is complex because of the large number of different electrophilic species that may be present depending on the nature of the sulfonating reagent and the experimental conditions. The mechanism of sulfonation by sulfuric acid has been extensively reviewed, and it has been observed that the rate of sulfonation speeds up as the concentration of the sulfuric acid used increases. The rate increase is very pronoimced in the region of 100 wt% acid this implies that the mechanism of sulfonation in sulfuric acid containing 

Kinetic studies supported the idea that, under some conditions, the precise nature of the electrophile remains in doubt because several kinetically equivalent species are possible. When the concentration of sulfuric acid is 85%, the predominant electrophilic species is H3S04 and the sulfonation mechanism is shown by Equations 13-15. 

In 90-96 wt% acid, the electrophile becomes sulfuric acid solvated with sulfru trioxide (pyrosulfiiric acid, H2S2O7) and the mechanism is modified so that the first step is as shown in Equation 16. 

In 97-100 wt% acid, the observed isotope effect indicated that removal of the proton from the intermediate is rate decisive, consequently the reaction of the electrophile with the aromatic species (Equation 16) is now fast while Equation 14 is slow. In fuming sulfuric acid (oleum), the electrophile is probably free sulfur trioxide which is more reactive than the solvated species and reacts rapidly with the aromatic moiety (Equation 17). 

This step is subsequently followed by the reactions shown in Equations 14 and 15, one of which must be a relatively slow stage. When the sulfonation is effected by sulfur trioxide in aprotic solvents, the mechanism varies with the solvent because complexes are formed in nitromethane and nitrobenzene, but not in halogenated solvents. Hinshelwood and co-workers showed that in nitrobenzene and nitromethane sulfonation was second order in sulfur trioxide which is in agreement with one molecule of sulfur trioxide assisting the polarization of the other. On the other hand, Cerfontain proposed the mechanism depicted in Equations 18 and 19. 

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Among the variety of electrophilic species present m concentrated sulfuric acid sulfur tnoxide (Figure 12 4) is probably the actual electrophile m aromatic sulfonation We can represent the mechanism of sulfonation of benzene by sulfur tnoxide by the sequence of steps shown m Figure 12 5... 

Scheme 5 Reaction Mechanism of Sulfonation of the Indole Group 821...

The plant soluble STs have around 25 to 30% amino acid identity with mammalian soluble STs, and are of a similar size. Comparisons between F. chloraefolia F3ST and F4 ST, combined with mutational analysis and data from the crystal structure of mouse estrogen ST, have defined amino acid residues important for PAPS binding, substrate binding and catalysis, and the mechanism of sulfonate transfer. " Sequence relatedness has been used to divide the STs into families and subfamilies in a similar manner as for P450s. ... 

Human neutrophil elastase (HNE) is a serine containing enzyme which is one of the most destructive of proteolytic enzymes, being able to catalyze the hydrolysis of the components of connective tissue. It has been implicated in the development of diseases such as emphysema, cystic fibrosis, and rheumatoid arthrisis. /3-Sultams are excellent candidates for exploring the mechanism of sulfonation and possible inhibition of serine protease enzymes. A-Benzoyl... 

According to the principle of microscopic reversibility (p. 170), the mechanism of desulfonation must be the exact reverse of the mechanism of sulfonation. 

FIGURE 12.3 The mechanism of sulfonation of benzene. An electrostatic potential map of sulfur trioxide can be viewed on Learning By Modeling. 

Although the mechanism of sulfonation is conceived as generally similar to those of nitration and halogenation, there is an important difference. The intermediate compound (A) above is electrically neutral and, therefore, more stable than the analogous intermediates involved in nitration and halogenation, which bear positive charges. Consequently, step 2 of sulfonation is much dower thaU d he analogous second steps in the other substitution reactions and, in fact, is involved in rate determination, as has been shown by isotopic studies. This unusual stability of intermediate (A) is also consistent with the characteristically reversible nature of the sulfonation reaction. 

The very mechanisms of sulfonation of high polymers have been reviewed by Kucera and Janc. The sulfonation reaction proceeds easily in the presence of groups, such as —Cl, —NH2, —OH, —SH, etc. In fact, the active agent in the sulfonation reaction is the SO3 cation. The sulfonation of aromatic compounds is a reversible reaction. Sulfonating agents can be classified into three groups, namely... 

Perrot C, Gonon L, Marestin C, Morin A, Gebel G. Aging mechanism of sulfonated poly(aryl ether ketone) (sPAEK) in an hydroperoxide solution and in fuel cell. J Power Sources 2010 195(2) 493-502. 

The very mechanisms of sulfonation of high polymers have been reviewed by Kucera and Jan [39]. The sulfonation reaction proceeds easily in the... 

Figure 2.32 Proposed thermal decomposition mechanism of sulfonated polyaniline and mercaptopropanesulfonic acid substituted polyaniline. (Reprinted with permission from Macromolecules, 36, 7908. Copyright (2003) American Chemical Society.)...

Okumura et al.. Mechanism of Sulfonation of Fatty Acid Esters with Sulfur Trioxide and Properties of Alpha-Sulfo Fatty Acid Esters, Proceedings of the 7th CID in Moscow, 1976. 

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