Substitution reactions of aromatic hydrocarbons

Anodic substitution reactions of aromatic hydrocarbons have been known since around 1900 [29, 30]. The course of these processes was established primarily by a study of the reaction between naphthalene and acetate ions. Oxidation of naphthalene in the presence of acetate gives 1-acetoxynaphthalene and this was at first taken to indicate trapping of the acetyl radical formed during Kolbe electrolysis of... 

As an example, it has been suggested 37-) that many anodic substitution reactions of aromatic hydrocarbons are ECEC processes, as shown below for anodic aromatic acetoxylation Eqs. (2-5) ) ... 

FIGURE 5.34. Plot of log (PRF) for methyl (O) and trifluoromethyl ( ) substitution reactions of aromatic hydrocarbon. Data from Ref. 12. 

Figure 16.3 gives the structural formulas of some of the more important aryl halides. These compounds are made by the substitution chlorination of aromatic hydrocarbons, as shown, for example, by the reaction below for the synthesis of a polychlorinated biphenyl ... 

In the early 1960 s it was described 20,24,55-5 ) salt-like compounds of aromatic hydrocarbons are o-complexes, i.e. their cations AH possess the structure of arenium ions. This conclusion was first based on indirect arguments ensuing from the analysis of the AH -cation electronic absorption spectra (in particular, from the similarity of the spectra of anthracene and 1,1-diphenylethylene solutions in cone. HjSO j. It also results from the linear dependence of the logarithms of the. relative stability constants of A HF BF3 complexes on tho% of the rate constants of electrophilic substitution reaction of the hydrocarbons A Direct proof of this point of view was obtained from studies into the A HY mMY complexes and the solutions of aromatic hydrocarbons or their derivatives in various acids (HF, HF + BFj, HSO3F and others) by the nuclear magnetic resonance nKasurements of Dutch investigators... 

Heterocycles are aromatic compounds, and they undergo aromatic substitution reactions similar to reactions of aromatic hydrocarbons (see Chapter 21, Section 21.3). In some cases, electrophilic aromatic substitution reactions are faster than benzene due to the presence of the heteroatom, but in other cases the reaction is slower. In other words, the nature of the heteroatom and the size of the ring have a profound influence on the rate of reaction as well as the site of reaction. The basic principles of reactivity and regioselectivity in these cases are governed by the same fundamental principles discussed for benzene derivatives in Chapter 21. For electrophilic aromatic substitution reactions of heterocycles, a cationic intermediate is formed however, the presence of the electron-rich heteroatom must be taken into account. The major site of substitution in this reaction is the one that gives the more stable intermediate. 

An important property of aromatic hydrocarbons is that they are much more stable and less reactive than other unsaturated compounds Ben zene for example does not react with many of the reagents that react rapidly with alkenes When reaction does take place substitution rather than addition is observed The Kekule formulas for benzene seem mcon sistent with its low reactivity and with the fact that all of the C—C bonds m benzene are the same length (140 pm)... 

ALCOHOL represents a convenient method of converting allyl alcohol to 2-substituted 1-propanols, while a one-pot reaction sequence of alkylation (alkyl lithium) and reduction (lithium—liquid ammonia) provides excellent yields of AROMATIC HYDROCARBONS FROM AROMATIC KETONES AND ALDEHYDES. 

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

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... 

Representative couplings of aromatic hydrocarbons are summarized in Table 10. Alkyl-substituted aromatic hydrocarbons can be coupled to diphenyls and/or diphenylmethanes depending on their substitution pattern (Table 10, numbers 1-6). Reactions occur according to Scheme 9, paths (a) and (c). 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

Reactions between aromatic hydrocarbon radicabcations and cyanide ions, with few exceptions, give low yields of nuclear substitution products [76], In some cases, better results have been obtained by anodic oxidation of the aromatic compound in an emulsion of aqueous sodium cyanide and dichloromethane with tetra-butylammonium hydrogen sulphate as a phase transfer agent [77, 78]. Methoxy-benzenes give exceptionally good yields from reactions in acetonitrile containing tetraethylammonium cyanide, sometimes with displacement of methoxide [79, 80]... 

Stilbenes, photocyclization of, 30, 1 StiUe reaction, 50, 1 Stobbe condensation, 6, 1 Substitution reactions using organocopper reagents, 22, 2 41, 2 Sugars, synthesis by glycosylation with sulfoxides and sulfinates, 64, 2 Sulfide reduction of nitroarenes, 20, 4 Sulfonation of aromatic hydrocarbons and aryl halides, 3, 4 Swem oxidation, 39, 3 53, 1... 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

Summary of Electrophilic Substitution Reactions of Polynuclear Aromatic Hydrocarbons... 

Figure 13.3 An example of a substitution reaction of an aromatic hydrocarbon compound (biphenyl) to produce an organochlorine product (2,3,5,2, 3 -pentachlorobiphenyl, a PCB compound). The product is 1 of 210 possible congeners of PCBs, widespread and persistent pollutants found in the fat tissue of most humans and of considerable environmental and toxicological concern.

The found parallels between donor activity in a series of aromatic hydrocarbons, fullerene solubility in these hydrocarbons and their reactivity relative to electrophilic attack (series 2) will become regular if the process of C6o dissolution in aromatic hydrocarbons is considered as an intermediate stage for the reaction of electrophilic substitution in an aromatic series. 

The parallels between Cgo solubility in alkyl derivatives of benzene and reactivity of these derivatives to the reactions of electrophilic substitution have been established. The parallels allow the Cgo dissolution to be considered as a reaction of electrophilic substitution of aromatic hydrocarbons. 

It is well known that the reaction of electrophilic substitution of aromatic hydrocarbons is a two-stage process to form 7t-complexes at an intermediate... 

One of the most studied and used reactions of tellurium tetrachloride 16 is the electrophilic aromatic substitution with activated aromatic hydrocarbons.5,9,11,12 Traditionally, the reaction is performed in carbon tetrachloride or chloroform under reflux.50-54 The aryltellurium trichlorides 5 are insoluble in these solvents and precipitate as crystalline solids. Usually, the reaction gives high yields. The /> ra-substituted aryltellurium trichlorides are formed. The reaction can take several hours under reflux to reach completion.5 It can be performed also in the absence of... 

The ability of azoles to electrophilic substitution reactions is determined by the activity of reagents, the basicity of substrates, and the acidity of media. This caused some uncertainty in the interpretation of results and complicated a comparison of the reactivity of various azoles. The situation has changed after Katritzky and Johnson [1] have reported the criteria allowing, with a sufficient degree of reliance, the establishment in what form (base or conjugative acid) the compound reacts. The information on the mechanism of nitration of azoles was basically borrowed from the extensive literature on the nitration of aromatic hydrocarbons [2-8] therefore, we have found expedient to discuss briefly some works in this field. 

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