Fused Aromatic Rings 1 Naphthalene

This section is concerned with the hydrogenation of naphthalene and of tetralin (tetrahydro-naphthalene) and their alkyl derivatives, and the reactions with deuterium. The focus of interest lies in the stereochemistry of the reactions, in the analysis of the products (sometimes very complicated), and their significance for the understanding of reaction mechanisms. 

There is much evidence from early work summarised by H.A. Smith, and amply confirmed by later careful work, that naphthalene can be hydrogenated to tetralin, i.e. one of the rings could be saturated, with a high degree of selectivity on nickel and the noble metals (Table 10.5), although in some cases it declined somewhat at higher temperatures. In further hydrogenation, yields of octalin were 

TABLE 10.5. Selectivities in the Hydrogenation of Naphthalene to Tetralin (5r), Octalins (5o) and Decalin ( d) on Metals of Groups 8 to 10  

The presence of a single alkyl substituent in the 1-position has a marked effect on which ring is reduced first with Pd/C as catalyst, the proportions of reduction of the ring carrying the substituent increase in the sequence methyl (34%) ethyl (45%) isopropyl (68%) fert-butyl (97%). With platinum, however, the unsubstituted ring is the more reactive. The size and position of the substituent also influences its rate of reduction. There are no kinetic studies to differentiate between effects on the adsorption coefficients and on the rates of reaction of the molecules once they have been adsorbed, of the kind conducted with benzene and toluene a combination of steric and electronic factors may be at work. 

The consequential studies of the hydrogenation of the octalins have been reviewed in Section 7.5.3. To cut a very long story short, the suggested route to fi-decalin has been validated, or at least has been found to be satisfactory of course if we believe Karl Popper there may be an even better explanation around the corner. 

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Aromatic compounds with two or more fused aromatic rings. Naphthalene is a polynuclear aromatic hydrocarbon (PAH or PNA). Indole is a polynuclear aromatic heterocycle, (p. 735)... 

Simple aliphatic olefins are practically nonelectroactive unless activated by conjugation. Fused aromatic rings (naphthalene, biphenyl, anthracene) are also electroactive. 

The structures of some PAHs of environmental interest are given in Figure 9.1. Naphthalene is a widely distributed compound consisting of only two fused benzene rings. It is produced commercially for incorporation into mothballs. Many of the compounds with marked genotoxicity contain 3-7 fused aromatic rings. Benzo[a] pyrene is the most closely studied of them, and will be used as an example in the following account. 

In general, the triarylmethane leuco skeleton can be represented by structures 1-4. Traditional leuco di- and triphenylmethane dyes frequently include compounds of type 1 and 3. The closely related compounds 2 and 4 are derived from 1 and 3. Another closely related type is the lactone or phthalide 5 (see Chapter 4). In all of these leuco dyes, one or more of the phenyl rings can be replaced by a hetaryl ring or by a fused aromatic ring such as a naphthalene. 

Most of the substituent increments presented in Table 4.82 can be derived from 3C shifts of benzenoid carbons in monosubstituted benzenes as listed in Table 4.53. Additional substituent increments are available for fused aromatic rings such as naphthalene and... 

The carbon-to-hydrogen ratios are benzene = 1 naphthalene = 1.25 anthracene = 1.40 pyrene = 1.60. The percentage of carbon in a structure increases with the number of fused aromatic rings. 

Compounds containing fused aromatic rings are called polycyclic aromatic hydrocarbons. Some common examples (shown in Fignre 20.5 ) include naphthalene, the snbstance that composes mothballs, and pyrene, a carcinogen fonnd in cigarette smoke. 

The accepted configuration of naphthalene, ie, two fused benzene rings sharing two common carbon atoms in the ortho position, was estabUshed in 1869 and was based on its oxidation product, phthaUc acid (1). Based on its fused-ring configuration, naphthalene is the first member in a class of aromatic compounds with condensed nuclei. Naphthalene is a resonance hybrid ... 

Substituents on benzene or benzenoid rings in fused pyridazines, i.e. in cinnolines and phthalazines, usually exhibit reactivity which is similar to that found in the correspondingly substituted fused aromatic compounds, such as naphthalene, and is therefore not discussed here. 

The polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenan-threne undergo electrophilic aromatic substitution and are generally more reactive than benzene. One reason is that the activation energy for formation of the c-complex is lower than for benzene because more of the initial resonance stabilization is retained in intermediates that have a fused benzene ring. 

Another class of compounds is called condensed-ring or fused-ring systems. These structures contain two or more aromatic rings that share a pair of carbon atoms. Examples include naphthalene, anthracene, and phenanthrene, the latter two being isomeric structures. 

More recently, charge-transfer emission was anticipated when solutions of hydrocarbon anion radical salts in dimethoxyethane were mixed with Wurster s blue perchlorate.15 Emission was seen in every instance however, with eight anion radicals derived from 3 to 5 ring-fused aromatic hydrocarbons, the emission was derived from the hydrocarbon rather than the complex. Preliminary studies with smaller hydrocarbons, biphenyl and naphthalene, did show emission in the region (18 kK) where charge transfer was expected. The question as to what pairs of ion radicals will be emissive under what conditions has only begun to be considered. Much opportunity for further experimentation exists in this area. 

Azonia substitution at a naphthalene bridgehead position gives the quinolizinium ion (16). Oxonia substitution, elsewhere, forms the 1- and 2-benzopyrylium ions (17) and (18). The two most well-known monoaza systems with three aromatic fused rings are acridine (19), derived structurally from anthracene, and phenanthridine (20), an azaphenanthrene. The better-known diaza systems include phenazine (21) and 1,10-phenanthroline (22), while systems with three linearly fused pyridine rings are called anthyridines, e.g. the 1,9,10-isomer (23). 

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