Hydrocarbons electrophilic substitution

The relative basicities of aromatic hydrocarbons, as represented by the equilibrium constants for their protonation in mixtures of hydrogen fluoride and boron trifluoride, have been measured. The effects of substituents upon these basicities resemble their effects upon the rates of electrophilic substitutions a linear relationship exists between the logarithms of the relative basicities and the logarithms of the relative rate constants for various substitutions, such as chlorination and... 

Given that many electrophiles form r-complexes with aromatic hydrocarbons, and that such complexes must be present in solutions in which electrophilic substitutions are occurring, the question arises... 

Accordingly, they do not easily add to reagents such as halogens and acids as do alkenes. Aromatic hydrocarbons are susceptible, however, to electrophilic substitution reactions in presence of a catalyst. 

Aromatic hydrocarbons, like paraffin hydrocarbons, react by substitution, but by a different reaction mechanism and under milder conditions. Aromatic compounds react by addition only under severe conditions. For example, electrophilic substitution of benzene using nitric acid produces nitrobenzene under normal conditions, while the addition of hydrogen to benzene occurs in presence of catalyst only under high pressure to... 

The introduction of the halogens onto aromatic rings by electrophilic substitution is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, but Lewis acid catalysts are normally needed to achieve desirable rates. Elemental fluorine reacts very exothermically and careful control of conditions is required. Molecular iodine can effect substitution only on very reactive aromatics, but a number of more reactive iodination reagents have been developed. 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

Like benzene, other aromatic hydrocarbons will undergo electrophilic substitution reactions. 

Oligospirocyclopropanated bicyclopropylidenes can be functionalized in the same manner as the parent compound 1 (see Scheme 8), but in the deprotonations and subsequent electrophilic substitutions on unsymmetrical hydrocarbons like 55 and 57, very little selectivity at best was observed [54]. 

The intermediate tricyclic ketones 495 and 496 have been transformed to the methoxy-substituted derivative 97284,285) latter ketone is subject to hydrogen-deuterium exchange only under basic conditions and appears to exist entirely in the keto form despite the ready formation of its anion and successful methylation on oxygen . In agreement with the aromatic nature of 490, the hydrocarbon undergoes electrophilic substitution reactions... 

Dewar, M. J. S., T. Mole, and E. W. T. Warford, Electrophilic Substitution. Part VI. The Nitration of Aromatic Hydrocarbons Partial Rate Factors and Their Interpretation, J. Chem. Soc., Part 111, 3581-3586 (1956). 

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. 

Since numerous monographs and review papers cover in detail nearly all aspects of electrophilic aromatic substitutions, only a brief overview of some important reactions resulting in heterosubstitution is given here. Electrophilic alkylation of aromatics, the most important electrophilic substitution with regard to hydrocarbon chemistry, including new C—C bond formation, in turn, is discussed in Section 5.1.4. 

The oxidation of hydrocarbons by cobalt(lll) acetate has been thoroughly investigated, due to its relevance to industrial homolytic oxidation processes.56 361 547 Radical intermediates are produced from one-electron oxidation of hydrocarbons according to an electron transfer or an electrophilic substitution mechanism previously described in equations (200)-(203). These oxidations are dramatically accelerated by the presence of strong acids or halide salts. 

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

After a brief discussion of the notion of molecular topology and the analogy principle as related to topology/reactivity relationships more recent developments in the field of reactivity indices for polynuclear benzenoid hydrocarbons are reviewed. Reaction mechanisms and correlations of reactivity indices with rates of electrophilic substitution and Diels-Alder reactions, thermally induced polymerization, and biochemical transformations of benzenoid hydrocarbons are discussed. 

In addition to the usual reactions of the catalyst with intermediate hydroperoxides, the second type of reaction undoubtedly involves direct reaction of the metal catalyst with the hydrocarbon substrate and/or with secondary autoxidation products. Two possible pathways can be visualized for the production of radicals via direct interaction of metal oxidants with hydrocarbon substrates, namely, electrophilic substitution and electron transfer. Both processes are depicted below for the reaction of a metal triacetate with a hydrocarbon. 

Keywords fiillerene, solubility, 71 -complex, electrophilic substitution, atomatic hydrocarbons. 

The parallels observed between CM) solubility and electrophilic substitution products are regular if C6o dissolution in aromatic hydrocarbons is considered as acid-base relationships. According to the theoretical research and experimental results, double bonds of aromatic hydrocarbons with mobile Tt-electrons are Lewis base. Consequently, they react with acids and Lewis acids to form complexes. It has been established that these complexes cannot be to a marked extent electrostatic. It has been found that they are often colored. Complexes with iodine (Lewis acid) give absorption bands at 300 nm in the UV region. These complexes are not true chemical compounds. According to Dewar, all the above facts are due to the formation of Tt-complexcs between an acid or Lewis acid and the entire Ji-electron system of an unsaturated compound which should be considered as Lewis base. Because in these complexes a double bond is an electron donor and Lewis acid is an electron acceptor, they are known as donor-acceptor complexes. The decrease in energy in complexing is conditioned by quantum-mechanical reasons. 

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... 

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