Reactivity of Polycyclic Aromatic Compounds

The polycyclic aromatic hydrocarbons, particularly naphthalene, anthracene and phenanthrene, as well as simple substituted derivatives, generally undergo the 

Phenanthrene and anthracene are both much more reactive than benzene, and there is a preference for substitution to occur in the center ring. That this behavior would be expected is evident even from simple resonance considerations. The o--complexes that result from substitution in the center ring have two intact benzene rings. The total resonance stabilization of this intermediate is larger than that of the naphthalene system that results if substitution occurs in one of the terminal rings. 

Both phenanthrene and anthracene have a tendency to undergo addition reactions under the conditions involved in certain electrophilic substitutions. Halogenation and nitration may proceed in part via addition intermediates.For example, in the nitration of anthracene in the presence of hydrochloric acid an intermediate addition product is isolated. 

Cycloaddition reactions are also favorable for anthracene since the resonance stabilization of two benzene rings is comparable to that of the anthracene ring. 

Friedel-Crafts Chemistry, Wiley Interscience, New York (1973). 

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Chemical Reactivity of Polycyclic Aromatic Compounds Adsorbed on Coal Stack Ash... 

That these effects may rationalize the reactivity of polycyclic aromatic compounds... 

Dewar and his co-workers, as mentioned above, investigated the reactivities of a number of polycyclic aromatic compounds because such compounds could provide data especially suitable for comparison with theoretical predictions ( 7.2.3). This work was extended to include some compounds related to biphenyl. The results were obtained by successively compounding pairs of results from competitive nitrations to obtain a scale of reactivities relative to that of benzene. Because the compounds studied were very reactive, the concentrations of nitric acid used were relatively small, being o-i8 mol 1 in the comparison of benzene with naphthalene, 5 x io mol 1 when naphthalene and anthanthrene were compared, and 3 x io mol 1 in the experiments with diphenylamine and carbazole. The observed partial rate factors are collected in table 5.3. Use of the competitive method in these experiments makes them of little value as sources of information about the mechanisms of the substitutions which occurred this shortcoming is important because in the experiments fuming nitric acid was used, rather than nitric acid free of nitrous acid, and with the most reactive compounds this leads to a... 

Dihydrovinylphenanthrenes are more reactive than the corresponding vinyl phenanthrenes and undergo Diels-Alder reactions easily. They have been used in the synthesis of polycyclic aromatic compounds and helicenes. Examples of cycloaddition reactions of the 3,4-dihydro-1-vinylphenanthrene (70), [61] 3,4-dihydro-2-vinylphenanthrene (71) [68] and l,2-dihydro-4-vinylphenanthrene (72) [69] are reported in Equation 2.22 and Schemes 2.27 and 2.28. 

Highly selective formation of phenyl acetate was observed in the oxidation of benzene with palladium promoted by heteropoly acids.694 Lead tatraacetate, in contrast, usually produces acetoxylated aromatics in low yields due to side reac-tions. Electrochemical acetoxylation of benzene and its derivatives and alkoxylation of polycyclic aromatics789 790 are also possible. Thermal or photochemical decomposition of diacyl peroxides, when carried out in the presence of polycyclic aromatic compounds, results in ring acyloxylation.688 The less reactive... 

In 2001, ab initio, density-functional and semiempirical calculations on the reactivity of polycyclic aromatic hydrocarbon episulfides 10-22 were reported. Episulfides are predicted to open more easily than the corresponding 0-protonated derivatives, epoxides and diol epoxides <2001HCA3588>. Calculation results for the episulfide ring opening of the j -protonated compounds are shown in Table 1. 

Similar relative reactivity of polycyclic aromatics is found with noble metal catalysts.1112 The development of noble metal catalysts more resistant to sulfur and nitrogen compounds has led to their more prevalent use, particularly by those refiners who wish to have low aromatic base stocks. Nickel catalysts can also be used for this purpose, but do not appear to have achieved wide acceptance except historically in white oil applications. 

Simple HMO calculations have had marginal success in correlating the reactivity of series of polycyclic aromatic compounds, as we shall see shortly. Little success was achieved with substituted benzenes, however, since HMO calculations are not adequate for describing effects that originate in the interaction of the ring with heteroatomic substituents. The more complete semiempirical methods that have been available since the mid-1960 s show more promise. 

On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

Dihydro-1-vinylnaphthalene (67) as well as 3,4-dihydro-2-vinylnaphtha-lene (68) are more reactive than the corresponding aromatic dienes. Therefore they may also undergo cycloaddition reactions with low reactive dienophiles, thus showing a wider range of applications in organic synthesis. The cycloadditions of dienes 67 and 68 and of the 6-methoxy-2,4-dihydro-1-vinylnaphthalene 69 have been used extensively in the synthesis of steroids, heterocyclic compounds and polycyclic aromatic compounds. Some of the reactions of dienes 67-69 are summarized in Schemes 2.24, 2.25 and 2.26. In order to synthesize indeno[c]phenanthrenones, the cycloaddition of diene 67 with 3-bromoindan-l-one, which is a precursor of inden-l-one, was studied. Bromoindanone was prepared by treating commercially available indanone with NBS [64]. 

When benzyne is generated in the absence of another reactive molecule it dimerizes to biphenylene.132 In the presence of dienes, benzyne is a very reactive dienophile and [4+2] cycloaddition products are formed. The adducts with furans can be converted to polycyclic aromatic compounds by elimination of water. Similarly, cyclopentadienones can give a new aromatic ring by loss of carbon monoxide. Pyrones give adducts that can aromatize by loss of C02, as illustrated by Entry 7 in Scheme 11.9. 

The NO + 03 chemiluminescent reaction [Reactions (1-3)] is utilized in two commercially available GC detectors, the TEA detector, manufactured by Thermal Electric Corporation (Saddle Brook, NJ), and two nitrogen-selective detectors, manufactured by Thermal Electric Corporation and Antek Instruments, respectively. The TEA detector provides a highly sensitive and selective means of analyzing samples for A-nitrosamines, many of which are known carcinogens. These compounds can be found in such diverse matrices as foods, cosmetics, tobacco products, and environmental samples of soil and water. The TEA detector can also be used to quantify nitroaromatics. This class of compounds includes many explosives and various reactive intermediates used in the chemical industry [121]. Several nitroaromatics are known carcinogens, and are found as environmental contaminants. They have been repeatedly identified in organic aerosol particles, formed from the reaction of polycyclic aromatic hydrocarbons with atmospheric nitric acid at the particle surface [122-124], The TEA detector is extremely selective, which aids analyses in complex matrices, but also severely limits the number of potential applications for the detector [125-127],... 

Before we examine the oxidation pathways available to aromatic systems, it is first instructive to review the most notorious role of these compounds in combustion chemistry their propensity to lead to soot formation. Soot is a byproduct of fuel-rich combustion, and soot particles can affect respiration and general health in humans." Soot production is a result of polycyclic aromatic hydrocarbon (PAH) formation in flames as reactive hydrocarbon radical intermediates combine to grow... 

Pistikopoulos, P., P. Masclet, and G. Mouvier, A Receptor Model Adapted to Reactive Species Polycyclic Aromatic Hydrocarbons Evaluation of Source Contributions in an Open Urban Site—I. Particle Compounds, Atmos. Environ., 24A, 1189-1197 (1990a). 

Fortunately, there is now a comprehensive body of knowledge on the metabolic reactions that produce reactive (toxic) intermediates, so the drug designer can be aware of what might occur, and take steps to circumvent the possibility. Nelson (1982) has reviewed the classes and structures of drugs whose toxicities have been linked to metabolic activation. Problem classes include aromatic and some heteroaromatic nitro compounds (which may be reduced to a reactive toxin), and aromatic amines and their N-acylated derivatives (which may be oxidized, before or after hydrolysis, to a toxic hydroxylamine or iminoquinone). These are the most common classes, but others are hydrazines and acyl-hydrazines, haloalkanes, thiols and thioureas, quinones, many alkenes and alkynes, benzenoid aromatics, fused polycyclic aromatic compounds, and electron-rich heteroaromatics such as furans, thiophenes and pyrroles. 

Polycyclic aromatic compounds also undergo electrophilic aromatic substitution reactions. Because the aromatic resonance energy that is lost in forming the arenium ion is lower, these compounds tend to be more reactive than benzene. For example, the brotni-nation of naphthalene, like that of other reactive aromatic compounds, does not require a Lewis acid catalyst ... 

Purpose. DNA adducts are nucleotide bases (i.e., purines and pyrimidines) that have been covalently modified by reactive electrophilic chemical intermediates or free radicals. The chemical structures of DNA adducts are diverse and vary from simple alkyl adducts induced by alkylating agents to complex bulky adducts such as those resulting from metabolic activation of polycyclic aromatic hydrocarbons, aromatic amines, and aflatoxins (Dipple 1995 Chiarelli and Jackson 1992 Rundle 2006 Xue and Warshawsky 2005). The purpose of measuring DNA adducts is to determine whether a DNA-reactive compound or a metabolically activated... 

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