Mechanism aromatic sulfonation

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... 

Imidazolides of aromatic sulfonic acids react much more slowly in alcoholysis reactions than the carboxylic acid imidazolides. Although the reaction with phenols is quantitative when a melt is heated to 100 °C for several hours, with alcohols under these conditions only very slight alcoholysis is observed. In the presence of 0.05 equivalents (catalytic amount) of sodium ethoxide, imidazole sodium, of NaNH2, however, imidazolides of sulfonic acids react with alcohols almost quantitatively and exothermically at room temperature in a very short time to form sulfonic acid esters (sulfonates). (If the ratio of sulfonic acid imidazolide to alcoholate is 1 2, ethers are formed see Chapter 17). The mechanism of catalysis by base corresponds to that operative in the synthesis of carboxylic esters by the imidazolide method. Because of the more pronounced nucleophilic character of alkoxide ions, sulfonates can also be prepared in good yield by alcoholysis of their imidazolides in the presence of hydroxide ions i.e., with alcoholic sodium hydroxide. 45 Examples of syntheses of sulfonates are presented below. 

Excess acidity correlations have been used to show that some aromatic sulfonic acid desulfonations have an A-SE2 mechanism.188,189 This mechanism (alternative terminologies are Ad-E2 and A(E) +A(N))190 has also been found to apply to the hydration of acetylene itself,191 to ynamines192 and to many other alkynes,193-195 as well as to many different alkenes196-199 and vinyl ethers.200-203 The excess acidity method has been used to evaluate aA values for several alkene hydrations.204 205... 

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... 

Allhough rtol shown, the nitrogen atom of the sulfonic acid is probably protonated, or even sLilfonated. under the reaction conditions, exaggerating the steric problems experienced by the substituent at C-8. The mechanism of the desuifonation ol aromatic sulfonic acids occurs via the reverse of the sulfonation process. 

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. 

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... 

Maintaining an eye for ciarity With every edition we improve the presentation of topics, reactions, and diagrams where the opportunity arises. In this edition some examples include improved discussion and diagrams regarding endo and exo Diels-Alder transition states, the effect of diene stereochemistry in Diels-Alder reactions (Section 13.1 OB), and improved mechanism depictions for aromatic sulfonation and thionyl chloride substitution. 

The highly intractable chemical stmcture vMch. inq)arts the outstanding mechanical properties also makes the PATs very difficult to process (4, 5). In the ftilly imidized form PAI is not processable hence a poly(amic acid) (PAA) precursor is the usual form in which they are supplied and bricated. The precursors themselves have very hi viscosities in the melt state and hence the flow characteristics tend to be very poor. Semicrystalline and amorphous polyamides (6) and aromatic sulfone polymers such as poly(phenylene sulfide), poly(ether sulfone) and polysulfone (7) have been blended with the precursor to PAI, to obtain better flow characteristics. 

Recall the mechanism of aromatic sulfonation from your introductory organic chemistry course, or took at Chapter 10. Now draw two reaction coordinate diagrams on the same plot that show the relative energies of organic intermediates and products. Place the reactant at the center and the products at the left and right. Explain why the ratio is different at different temperatures, why one transition state is more stable than another, and why one product is more stable than another. 

Poly(ether ether ketone) (PEEK) is an aromatic, high performance, semicrystalline polymer with extremely good thermal stability, chemical resistance, and electrical and mechanical properties. This polymer shows little solubility in organic solvents due to the sanicrystalline nature of certain poly(ether ketone)s. The most common way to modify aromatic polymers for application as a PEM is to employ electrophilic aromatic sulfonation. By introdncing sulfonic acid groups to the backbone, the crystallinity decreased and solnbility increased [19,20]. 

Several carbon-hydrogen bond substitutions are involved in the palladium-catalysed reaction of arylsulfonic acids with arenes to yield aromatic sulfones. A plausible mechanism, shown in Scheme 9, involves the initial formation of the palladacycle (99) which, after eoupling with the arene, yields the biphenyl derivative (100). Further coordination and carbon-hydrogen activation gives the seven-membered palladacycle (101) that affords the sulfone, (102), after reductive elimination. ... 

Os(bipy)3] he found that SO can act as an aromatic sulfonating agent with phenanthroline ligands. Margerum has used pulsed accelerated flow spectrophotometry to study the oxidation of I2 and by SO 3 and HSOf. For the former reductant the mechanism is shown in Eqs. (47)-(49). The ISO species contains an I—S bond and not an I—O bond... 

Complexation of bromine with iron(III) bromide makes bromine more elec trophilic and it attacks benzene to give a cyclohexadienyl intermediate as shown m step 1 of the mechanism (Figure 12 6) In step 2 as m nitration and sulfonation loss of a proton from the cyclohexadienyl cation is rapid and gives the product of electrophilic aromatic substitution... 

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