Phenanthrene electrophilic aromatic substitution

Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures. 

Polynuclear aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene undergo electrophilic aromatic substitution reactions in the same manner as benzene. A significant difference is that there are more carbon atoms, more potential sites for substitution, and more resonance structures to consider. In naphthalene, it is important to recognize that there are only two different positions Cl and C2 (see 122). This means that Cl, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at Cl, C4, C5, and C8 as labeled in 122, only one product is formed 1-chloronaphthalene (121), which is the actual product isolated from the chlorination reaction. Chlorination of naphthalene at Cl leads to the five resonance structures shown for arenium ion intermediate 127. 

In anthracene (123), there are three different positions (Cl, C2, and C9) and there are five different positions (Cl, C2, C4, C5, and C9) in phenanthrene (124). Electrophilic aromatic substitution of anthracene leads to substitution primarily at C9 because that gives an intermediate with the most resonance forms and the most intact benzene rings. A comparison of attack at Cl and at C2 in anthracene will show that there are more resonance forms for attack at Cl and more fully aromatic rings. Attack at C9 leads to an intermediate with even more resonance, and electrophilic substitution of anthracene leads to C9 and Cl products, with little reaction at C2. 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives 56, 57, 104, 105, 106, 107, 108, 109. Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures 59, 60, 61, 62, 63, 64, 65, 85, 104, 106, 107, 108,109,110,113,114,118. 

Butler and Crossley (102) studied the reactivity of PAH, naturally present on soot particles, generated in a flame from an eth-ylene-air burner, in a reaction chamber with ambient air containing 5-10 ppm of sulfur dioxide or a mixture of nitrogen oxides. While exposure of up to three months to SO did not yield any significant loss of PAH, degradation by NO occurred readily and resulted in half lives varying between 7 and 30 days (for benzo(a)-pyrene and phenanthrene respectively),in correspondence with the order of reactivity for electrophilic substitution of aromatic systems. 

Electrophilic substitution of phenanthrene is more complicated because there is less difference in the stabihty of the intermediate cations formed from each substitution position. Nitration of phenanthrene (124) leads to five products. Phenanthrene is a very interesting compound for a different reason. The middle ring is not stabilized by aromaticity to the same extent as the others (as mentioned earlier), making it susceptible to reactions not usually observed with aromatic systems. For example, phenanthrene reacts with diatomic bromine in the absence of a Lewis acid, much hke a simple alkene, to give 129. This reaction does not occur with benzene or naphthalene the distortion of 7i-electrons in 124 is responsible for it. 

Although the attack on (or by) the aromatic ring that leads to substitution can be brought about by free radical, electron-rich (nucleophilic) or electron-poor (electrophilic) reagents, the surfeit of electrons associated with aromaticity provides a richness that dictates that the latter should be most common. Nonetheless, aU three substitution pathways will be considered. Indeed, the three that will be examined are benzene (CeHe) and the polycyclic arenes (naphthalene [CioH ], anthracene [C14H10], and phenanthrene [CmHjo]). 

Substituted benzene compounds belong to a class of conjugated compounds called arenes. Examples include benzene, naphthalene, anthracene, and phenanthrene. The common structural feature of arenes is a monocyclic or polycyclic system of k electrons that results in a special stability called aromaticity. As a result, aromatic compounds are much less reactive in electrophilic addition reactions than we would expect based on the reactivity of polyenes. 

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