Reactions of Polycyclic Aromatic Compounds

Many molecules are known where benzene rings are fused together along a bond. The simplest examples are naphthalene (the main ingredient in mothballs), anthracene, and phenanthrene (12.10-12.12), 

One of the most harmful of all the polycyclic aromatic compounds is benzo[a]pyrene, 12.13, which is a constituent of tobacco (and in greater quantities in marijuana) smoke, and was established to be the precarcinogenic compound in the cancers of the scrotum suffered by chimney sweeps in nineteenth-century Britain. In vivo, it is catalytically oxidized in the presence of various cytochromes and a hydrolase enzyme to 12.14, which is both carcinogenic and mutagenic (the epoxide group is susceptible to ring-opening reactions by nucleophiles such as the bases of DNA and proteins). 

Although naphthalene and indeed most polycyclic aromatic hydrocarbons are usually obtained from coal tar, naphthalene can be synthesized by a rather unusual Friedel-Crafts reaction 

Why is the 2-substitution preferred under thermodynamic conditions It is less sterically hindered because of the unfavorable / en-interaction as in 12.15. 

Plus other resonance forms in which there is no intact benzene ring 

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There are several reviews on various aspects of photooxygenation reactions. Bowen2 has reviewed type II direct photooxygenation reactions of polycyclic aromatic compounds in solution. 

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