Substitution, electrophilic sulfonation

For electrophiles such as Me3SiCH2X strong ground-state destabilization has been observed for X = 4-nitrobenzoate[130]. For X= halide, on the other hand, this ground-state destabilization is significantly smaller, and it may therefore be advisable to choose carboxylates or sulfonates as leaving groups when alkylations with a-silyl-substituted electrophiles are to be performed. 

Hydroxypyridines undergo a variety of other electrophilic substitution reactions. Sulfonation of 2-pyridone with 10% oleum at 180° gave the 5-sulfonic acid.69 110 A-Methyl-2-pyridone is similarly sulfonated with chlorosulfonic acid. The action of fuming sulfuric acid gave a mixture of the 5-sulfonic acid and the 3,5-disulfonie acid. A nitro group at C-5 is said not to hinder the reaction, sulfonation at the... 

The chemical reactivity of (1) and (2) is a dichotomy. Benzenoid character is indicated in normal electrophilic substitution (nitration, sulfonation and catalyzed halogenation, Section 4.26.4.1) and in the lack of dienophile reactivity in the Diels-Alder reaction. On the other hand, typical dienoid character is exhibited in the facile ozonolysis of the benzene ring of (1) and in the easy, non-catalyzed tetra-addition of halogen (see Section 4.26.4.1). 

Selenophene undergoes various electrophilic substitutions nitration, sulfonation, halogenation, mercuration, chloromethylation, aminomethylation, acylaminomethylation, acylation, formylation, and hydrogen exchange. 

Direct electrophilic sulfonation of imidazole and 2-substituted imidazoles has been traditionally carried out using oleum at around 160°C. Yields of the 4-sulfonic acid can be above 80%, but if a three-fold excess of sulfur trioxidc in refluxing 1,2-dichloroethane is used, the acids arc obtained under much milder conditions. Care has to be taken, for if only a 1 1 or 1 2 ratio of S03 imidazole is used, a charge transfer complex is formed instead [6]. 

Unlike most other electrophilic substitution reactions, sulfonation shows a moderate isotope effect ordinary hydrogen (protium) is displaced from an aromatic ring about twice as fast as deuterium. Docs this mean that sulfonation takes place by a different mechanism than nitration, one involving a single step Almost certainly not. 

Unlike most other electrophilic substitution reactions, sulfonation is reversible, and this fact gives us our clue. Reversibility means that carbonium ion II can lose SO3 to form the hydrocarbon. Evidently here reaction (2) is not much... 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

In contrast to aromatic nitration and other electrophilic aromatic substitutions aromatic sulfonation is reversible. Sulfonation takes place in strong acidic conditions and desulfonation is the mode of action in a hot aqueous acid. 

Because sulfonation is reversible, a sulfonic acid group is sometimes introduced to occupy a position on the ring, temporarily protecting that position from attack by another electrophile. Sulfonation is also a useful way of directing further substitution to a specific position. 

Electrophilic sulfonation of isoxazole is of no preparative value the substitution of only the phenyl-substituent of 5-phenylisoxazole with chlorosulfonic acid makes the same point. Both isothiazole and pyrazole can be satisfactorily sulfonated at C-4. 

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. 

Electrophilic substitution is a straightforward way to functionalize polyaniline. Substitution of sulfonic acid groups on the backbone of polyaniline, as shown in Figure 2.2, was first introduced by Epstein et al. [39] in the very first report of self-doped water soluble polyaniline. Their synthetic method involved the sulfonation of polyaniline using fuming sulfuric acid. The emeraldine base form of polyaniline (0.5 g) was dissolved in 40 mL of fuming sulfuric acid with constant stirring. 

Electrophilic substitution of sulfonic acid on the polyaniline backbone in nonaqueous media has been carried out electrochemically by Sahin et al. [147]. They prepared self-doped sulfonated polyaniline electrochemically in acetonitrile containing anhydrous FSO3H. FSO3H was used as both sulfonation reagent for aniline and also as supporting electrolyte. In situ sulfonation occurred during polymerization under these conditions. The... 

Besides electrophilic sulfonation, sulfonated phosphines can be produced by nucleophilic substitution reactions using PH3, primary or secondary phosphines, and fluoroarenes in superbasic media. This method has been applied for the synthesis of pura-isomer of TPPTS (p-TPPTS) (Scheme 10) as well as dozens of other interesting ligands including chelating, chiral, and amphiphilic phosphines (a selection is in Scheme 11). ... 

Introduction to Electrophilic Aromatic Substitution Halogenation Sulfonation... 

A review deals with the reactivity of pyrrole rings in fficxo-tetraazaporphyrins and benzene rings in substituted analogues to electrophilic substitution. Bromination, sulfonation, chlorosulfonation, and chloromethylation reactions are covered. 

By electrophilic substitution the sulfonic acid and phosphonic acid groups were grafted on the backbone (Scheme 4.2 and Scheme 4.3). It is well known that the acidity of phosphonic acids is lower than that of sulfonic acids. 

All the halogenothiazoles, depending on the electron-withdrawing power of the halosubstituent, together with the electron-withdrawing power of the azasubstituent, are only slightly susceptible to electrophilic substitution reactions such as nitration, sulfonation, and so on, while the polyhalogenatjon reaction can take place. 

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... 

Electrophilic aromatic substitution (Sec tion 22 14) Arylamines are very reac tive toward electrophilic aromatic sub stitution It IS customary to protect arylamines as their N acyl derivatives before carrying out ring nitration chio rination bromination sulfonation or Friedel-Crafts reactions... 

Sulfonation (Section 12 4) Replacement of a hydrogen by an —SO3H group The term is usually used in connection with electrophilic aromatic substitution... 

PoIysuIfonyIa.tlon, The polysulfonylation route to aromatic sulfone polymers was developed independendy by Minnesota Mining and Manufacturing (3M) and by Imperial Chemical Industries (ICI) at about the same time (81). In the polymerisation step, sulfone links are formed by reaction of an aromatic sulfonyl chloride with a second aromatic ring. The reaction is similar to the Friedel-Crafts acylation reaction. The key to development of sulfonylation as a polymerisation process was the discovery that, unlike the acylation reaction which requires equimolar amounts of aluminum chloride or other strong Lewis acids, sulfonylation can be accompHshed with only catalytic amounts of certain haUdes, eg, FeCl, SbCl, and InCl. The reaction is a typical electrophilic substitution by an arylsulfonium cation (eq. 13). 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

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