Sulfur trioxide, electrophilic aromatic

Among the variety of electrophilic species present in concentrated sulfuric acid, sulfur trioxide (Figure 12.4) is probably the actual electrophile in aromatic sulfonation. We can represent the mechanism of sulfonation of benzene by sulfur trioxide by the sequence of steps shown in Figure 12.5. 

The most common way to modify aromatic polymers for application as a PEM is to employ electrophilic aromatic sulfonation. Aromatic polymers are easily sulfonated using concentrated sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, or sulfur trioxide (or complexs thereof). Postmodification reactions are usually restricted due to their lack of precise control over the degree and location of functionalization, the possibility of side reactions, or degradation of the polymer backbone. Regardless, this area of PEM synthesis has received much attention and may be the source of emerging products such as sulfonated Victrex poly (ether ether ketone). 

Aromatic sulfonation occurs with fuming sulfuric acid, where the electrophile is sulfur trioxide. This... 

In this reaction, the aromatic ring is a nucleophile and the sulfur of sulfur trioxide is an electrophile. 

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. 

Aromatic sulfonation, like nitration, balogenation, alkylation, and acylation, is a typical electrophilic substitution reaction. Sulfonation, however, differs from these other reactions in two marked respects it is reversible, and reaction temperature can, in certain cases, have an important influence on the position of the entering group, as shown on p. 344. These characteristics have tended to complicate studies of the reaction mechanism and rate of sulfonation and to render difficult the drawing of general conclusions. Other factors having the same effect are the tendency of sulfur trioxide to form a complex with the sulfonic acids and the pronounced tendency of all Lubs, pp. 534ff. 

In aromatic sulfonation reactions (eq. 4.19), we use either concentrated or fuming sulfuric acid, and the electrophile may be sulfur trioxide, SO3, or protonated sulfur trioxide, SOjIT. The following resonance structures demonstrate that SO3 is a strong electrophile at sulfur. 

In the preceding section, benzene reacted with cations to form substituted benzene derivatives. The cations of interest include Br+, C1+, the nitronium ion, and sulfur trioxide or sulfuric acid, which react as electrophiles. In principle, benzene may react with any cation, including carbocations, once that cation is formed. Carbocations are generated by several different methods they react with nucleophiles, as described for reactions of alkenes with acids such as HX (Chapter 10, Section 10.2) and for S l reactions (Chapter 11, Section 11.4). If benzene reacts with a carbocation, a new carbon-carbon bond is formed, and electrophilic aromatic substitution will give an arene. The reaction of benzene and its derivatives with carbocations is generically called the Friedel-Crafts reaction, after the work of French chemist Charles Friedel (France 1832-1899) and his American protege, James M. Crafts (1839-1917). The reaction takes two fundamental forms Friedel-Crafts alkylation and Friedel-Crafts acylation. Both variations will be discussed, beginning with the alkylation reaction. 

Sulfonation (Section 22.1 B) The electrophile is either sulfur trioxide, SOj, or HSO3+ depending on experimental conditions. The mechanism involves reaction of SO3 as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the aiylsulfonic add product. 

Step 1 The active electrophile in the sulfonation of aromatic compounds is sulfur trioxide and reacts with benzene in the rate-determining step. 

Sulfonation with Sulfuric Acid and Sulfur Triozide. Various mechanisms for the reaction of aromatic hydrocarbons or aryl halides with sulfuric acid or with sulfur trioxide have been proposed. Since.the reaction is heterogeneous, it is not favorable for experimental study. Solvents that dissolve sulfuric acid or sulfur trioxide form addition compounds with the reagent hence any conclusion drawn from a homogene ous sulfonation might not be applicable to the ordinary sulfonation. One possibility is that an electrophilic reagent such as sulfur trioxide with its relatively positive sulfur atom or an ion such as HOaS" " in the case of sulfuric acid attacks the negative center of the polarized form of the hydrocarbon, as illustrated for benzene. 

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). 

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