Nitration and Sulfonation of Benzene

A quick calculation confirms that electrophilic bromination of benzene is exothermic. A phenyl-hydrogen bond (approximately 112 kcal mol Table 15-1) and a bromine molecule (46 kcal moF ) are lost in the process. Counterbalancing this loss is the formation of a phenyl-bromine bond DH° = 81 kcal moF ) and an H-Br bond (DH° = 87.5 kcal moF ). Thus, the overall reaction is exothermic by 158 — 168.5 = —10.5 kcal moF (43.9 kJ moF ). 

As in the radical halogenation of alkanes (Section 3-8), the exothermic nature of aromatic halogenation decreases down the periodic table. Huorination is so exothermic that direct reaction of fluorine with benzene is explosive. Chlorination, on the other hand, is controllable but requires the presence of an activating catalyst, such as aluminum chloride or ferric chloride. The mechanism of this reaction is identical with that of bromination. Finally, electrophilic iodination with iodine is endothermic and thus not normally possible. Much like the radical halogenation of alkanes, electrophilic chlorination and bromination of benzene (and substituted benzenes. Chapter 16) introduces functionality that can be utilized in further reactions, in particular C-C bond formations through organometallic reagents (see Problem 54, Section 13-9, and Real Life 13-1). 

When benzene is dissolved in D2SO4, its H NMR absorption at 8 = 7.27 ppm disappears and a new compound is formed having a molecular weight of 84. What is it Propose a mechanism for its formation. (Caution In all mechanisms of electrophilic aromatic substitution, always draw the H atom at the site of electrophilic attack.) 

Professor G. Olah and his colleagues exposed benzene to the especially strong acid system HF-SbFs in an NMR tube and observed a new H NMR spectrum with absorptions at 6 = 5.69 (2 H), 8.22 (2 H), 9.42 (1 H), and 9.58 (2 H) ppm. Propose a structure for this species. 

In Summary The halogenation of benzene becomes more exothermic as we proceed from I2 (endothermic) to F2 (exothermic and explosive). Chlorinations and brominations are achieved with the help of Lewis acid catalysts that polarize the X-X bond and activate the halogen by increasing its electrophilic power. 

---

Nitration and sulfonation of benzene introduce two different functional groups on an aromatic ring. Nitration is an especially useful reaction because a nitro group can then be reduced to an NH2 group, a common benzene substituent, in a reaction discussed in Section 18.14. 

The sequence of steps for the nitration and sulfonation of benzene is similar to that for chlorination and bromination. For nitration, the electrophile is the nitronium ion, NOg, generated by the reaction of nitric acid with sulfuric acid. In the following equations nitric acid is written HONOg to show more clearly the origin of the nitronium ion. 

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... 

Anthraquinone dyes are derived from several key compounds called dye intermediates, and the methods for preparing these key intermediates can be divided into two types (/) introduction of substituent(s) onto the anthraquinone nucleus, and (2) synthesis of an anthraquinone nucleus having the desired substituents, starting from benzene or naphthalene derivatives (nucleus synthesis). The principal reactions ate nitration and sulfonation, which are very important ia preparing a-substituted anthraquiaones by electrophilic substitution. Nucleus synthesis is important for the production of P-substituted anthraquiaones such as 2-methylanthraquiQone and 2-chloroanthraquiaone. Friedel-Crafts acylation usiag aluminum chloride is appHed for this purpose. Synthesis of quinizatia (1,4-dihydroxyanthraquiQone) is also important. 

The mode of attack of electrophilic reagents (E ) at ring carbon atoms is jS to the heteroatoms as shown, for example, in (11) and (12) the intermediates usually revert to type by proton loss. Halogenation takes place more readily than it does in benzene (Section 4.02.1.4.5). Nitration and sulfonation also occur however, in the strongly acidic environment required the compounds are present mainly as less reactive hydroxyazolium ions, e.g. (13). 

Aromatic aminosulfonic acids are synthesized by a sequence of important industrial processes, including sulfonation of benzene. This is followed, wherever necessary, by chlorination, nitration, and reduction, or by aniline sulfonation, possibly involving subsequent baking [7,8]. 

Biphenyl and terphenyls may be regarded as substituted benzenes that undergo acylation, alkylation, halogenation, nitration, sulfonation, and other reactions common to benzene. The points of initial attack on chlorination, miration, and sulfonation of biphenyl occur at the 2- and 4-positions the latter group predominates. 

These steps illustrate how to generate the electrophile E for nitration and sulfonation, the process that begins any mechanism for electrophilic aromatic substitution. To complete either of these mechanisms, you must replace the electrophile by either or S03H in the general mechanism (Mechanism 18.1). Thus, the two-step sequence that replaces H by E is the same r ardless of E. This is shown in Sample Problem 18.1 u.sing the reaction of benzene with the nitronium ion. 

Polymers with phenyl pendant groups such as those present in polystyrene undergo all of the characteristic reactions of benzene, such as alkylation, halogenation, nitration, and sulfonation. Thus, oil-soluble polymers (e.g., poly(vinyl cyclohexylbenzene) used as viscosity improvers in lubricating oils are obtained by the Friedel-Crafts reaction of polyst3frene... 

The characteristic frequencies of some common substituent groups, such as C=0, NH, S=0, and benzene ring, were tabulated in the earlier review. Carbohydrate nitrates and sulfonates have since been studied in detail and another useful correlation that enables the methoxyl group to be detected by means of a characteristic C—H stretching band at 2882... 

Some indication of how theory compares with observation is given in Table II which shows the relative rates at which methyl radicals attack some of the compounds of interest (2). Notice that all rates are ordered inverse to delocalization energies. Values of DE quoted in Table II and hereafter have been nondimensionalized by the Hiickel factor 2/2 this nondimensional DE is also known as the Dewar number. The pattern of Table II is observed for a variety of other aromatic reactions such as nitration and sulfonation (I). Among the molecules considered in this study (Table I), benzene and anthracene represent the extremes of reactivity. 

Representative Electrophilic Aromatic Substitution Reactions of Benzene 457 Mechanistic Principles of Electrophilic Aromatic Substitution 458 Nitration of Benzene 459 Sulfonation of Benzene 461 Halogenation of Benzene 462 Biosynthetic Halogenation 464 Friedel-Crafts Alkylation of Benzene Friedel-Crafts Acylation of Benzene Synthesis of Alkylbenzenes by Acylation-Reduction 469 Rate and Regioselectivity in Electrophilic Aromatic Substitution 470 Rate and Regioselectivity in the Nitration ofToluene 472... 

The electrophilic aromatic substitutions that we studied in Sections 15-9 and 15-10 can be stopped at the monosubstitution stage. Why do Friedel-Crafts alkylations have the problem of multiple electrophilic substimtion It is because the substituents differ in electronic structure (a subject discussed in more detail in Chapter 16). Bromination, nitration, and sulfonation introduce an electron-withdrawing group into the benzene ring, which renders the product less susceptible than the starting material to electrophilic attack. In contrast, an alkylated benzene is more electron rich than unsubstituted benzene and thus more susceptible to electrophilic attack. 

---