Substitution 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. 

Alkylbenzenes and other activated aromatics, such as polynuclear compounds, may react without a catalyst. Halogenation, in this case, occurs with the involve-ment of molecular halogens. Polarization of chlorine and bromine molecules is brought about by their interaction with the aromatic ring. For example, bromination in the presence of acetic acid is suggested to take place through the transition state 5, leading to the formation of the a complex  

The kinetic order in bromine, which is often higher than unity, indicates the involvement of a second molecule of bromine in the rate-determining step assisting ionization. 

Among the fluorinating agents of practical significance,44 only fluoroxytrifluoro-methane45 and iV-fluoroperfluoroalkylsulfonimides46 were shown to fluorinate aromatic hydrocarbons by an electrophilic mechanism. 

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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... 

First of all, substitution of aromatic hydrocarbons will be discussed. Two principal mechanisms have been proposed for these reactions.47 The first, mechanism 1, proposes a one-step 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... 

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 first paper of the frontier-electron theory pointed out that the electrophilic aromatic substitution in aromatic hydrocarbons should take place at the position of the greatest density of electrons in the highest occupied (HO) molecular orbital (MO). The second paper disclosed that the nucleophilic replacement should occur at the carbon atom where the lowest unoccupied (LU) MO exhibited the maximum density of extension. These particular MO s were called "frontier MO s . In homolytic replacements, both HO and LU.were shown to serve as the frontier MO s. In these papers the "partial" density of 2 pn electron, in the HO (or LU) MO, at a certain carbon atom was simply interpreted by the square of the atomic orbital (AO) coefficient in these particular MO s which were represented by a linear combination (LC) of 2 pn AO s in the frame of the Huckel approximation. These partial densities were named frontier-electron densities . 

Derivatives of aromatic hydrocarbons may have chains of carbon atoms substituted on the aromatic... 

Benkeser, R. A. etal., Tetrahedron Lett., 1984, 25, 2089-2092 The use of calcium in 1,2-diaminoethane as a safer substitute for sodium or lithium in liquid ammonia for the improved Birch reduction of aromatic hydrocarbons is described in detail. 

Means, J.C., Wood, S.G., Hassett, J.J., Banwart, W.L. (1982) Sorption of amino-and carboxy-substituted polynuclear aromatic hydrocarbons by sediments and soils. Environ. Sci. Technol. 16, 93-98. 

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... 

These data imply that aromatic hydrocarbons incorporated into sediments are not preferentially accumulated in relation to increased alkyl substitution, as shown with dietary and seawater exposures. Moreover, the apparent lack of accumulation of the fluorene and phenanthrene suggests that unsubstituted aromatic hydrocarbons having more than two benzenoid rings may not be readily sequestered by fish exposed to petroleum-impregnated sediment. These differences are presumably related, at least in part, to physico-chemical interactions of aromatic hydrocarbons with sediment matrices that regulate their bioavailability. 

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). 

Wise, S.A., Sander, L.C., Lapouyade, R., and Garrigues, P., The anomalous behavior of selected methyl-substituted polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography, J. Chromatogr., 514, 111, 1990. 

Keeley, D.F., Hoffpauir, M.A., and Meriwether, J.R. Solubility of aromatic hydrocarbons in water and sodium chloride solutions of different ionic strengths C2-substituted benzenes, / Chem. Eng. Data, 36(4) 456-459, 1991. 

There are many other aromatic hydrocarbons, i.e. compounds like benzene, which contain rings of six carbon atoms stabilised by electron delocalisation. For example, if one of the hydrogen atoms in benzene is replaced by a methyl group, then a hydrocarbon called methylbenzene (or toluene) is formed. It has the structural formulae shown. Methylbenzene can be regarded as a substituted alkane. One of the hydrogen atoms in methane has been substituted by a or —group, which is known as a phenyl group. So an alternative name for methylbenzene is phenylmethane. Other examples of aromatic hydrocarbons include naphthalene and anthracene. 

Main-chain manipulation offers an opportunity to dramatically change the electronic and physical properties of the PAEs. Popular approaches are the introduction of meta linkages into the polymers, the introduction of aromatic hydrocarbons other than benzene, the introduction of heterocycles, and the substitution of a fraction of the connecting alkyne groups by double bonds. The last strategy leads to polymers that are hybrids between PPEs and PPVs. 

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... 

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... 

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). 

Recently, the synthesis of symmetrically substituted dialkylpolynuclear aromatic hydrocarbons, such as 2,6-diisopropylnaphthalene and 4,4 -diisopropyl-biphenyl has been studied because they are superior candidates of components for advanced materials.4,7 Polynuclear aromatics require larger space for the transition state intermediate composed of reactants and acid sites inside the pores than do mononuclear reactants. For these reasons, twelve-membered ring zeolites, especially HM, are suitable for the formation of the smallest products although the selectivity varies with reactants and zeolites. In this paper, we review the shape-selective alkylation of polynuclear aromatics catalysed by zeolites. 

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. 

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