Polynuclear Hydrocarbons and Naphthalene Synthesis

• Polynuclear hydrocarbons (PNAs), also known as polycyclic aromatic hydrocarbons (PAHs), are organic compounds containing two or more fused aromatic rings
• These compounds can be classified based on the arrangement of the rings:
  • Linearly fused (e.g., anthracene)
  • Angularly fused (e.g., phenanthrene)
  • Clustered (e.g., pyrene)

Synthesis of Naphthalene

• Naphthalene consists of two fused benzene rings and is the simplest PNA
• Haworth Synthesis:
  • Reaction of benzene with succinic anhydride under Friedel-Crafts acylation conditions to yield β-benzoylpropionic acid
  • Reduction of the keto group using Clemmensen reduction (Zn-Hg, HCl) to form γ-phenylbutyric acid
  • Intramolecular cyclization of γ-phenylbutyric acid using heat and acid catalyst to give α-tetralone
  • Reduction of the carbonyl group to a methylene group using Clemmensen reduction to yield tetralin
  • Dehydrogenation of tetralin using catalysts like Pd/C or Se at high temperatures to yield naphthalene

Reactions of Naphthalene

• Electrophilic Aromatic Substitution:
  • Naphthalene undergoes electrophilic aromatic substitution reactions more readily than benzene
  • Substitution primarily occurs at the α-position (C-1) due to greater stability of the Wheland intermediate
  • Examples include nitration, sulfonation, halogenation, and Friedel-Crafts alkylation/acylation
  • Sulfonation of naphthalene is temperature dependent:
    • At low temperatures, α-naphthalenesulfonic acid is the major product due to kinetic control
    • At high temperatures, β-naphthalenesulfonic acid predominates due to thermodynamic control

Synthesis of Anthracene

• Anthracene consists of three linearly fused benzene rings
• Elbs Reaction:
  • Pyrolysis of diaryl ketones at high temperatures to form PNAs with the elimination of water
  • For example, o-toluylphenylketone yields anthracene upon heating

Reactions of Anthracene

• Anthracene is reactive at the 9 and 10 positions
• Diels-Alder Reaction:
  • Anthracene acts as a diene in Diels-Alder reactions due to the reactivity of the 9 and 10 positions
  • Reacts with dienophiles like maleic anhydride to form adducts

Synthesis of Phenanthrene

• Phenanthrene consists of three angularly fused benzene rings
• Pschorr Cyclization:
  • Deamination of an o-aminobiaryl compound followed by intramolecular cyclization
  • For example, o-aminobiphenyl can be converted to phenanthrene via diazotization and subsequent decomposition

Reactions of Phenanthrene

• Less reactive than anthracene, but still undergoes electrophilic aromatic substitution
• Oxidation:
  • Oxidation of phenanthrene yields phenanthraquinone, with oxidation occurring at the 9 and 10 positions

Reactivity Considerations

• PNAs generally undergo electrophilic aromatic substitution reactions, with the position of substitution depending on the specific PNA and reaction conditions
• The resonance stabilization energy decreases as the number of fused rings increases, leading to enhanced reactivity compared to benzene
• PNAs can undergo oxidation and reduction reactions, with the ease of oxidation increasing with the number of rings

Environmental Significance

• Many PNAs are carcinogenic and are formed during incomplete combustion of organic materials
• Sources include burning of fossil fuels, tobacco smoke, and industrial processes
• Monitoring and remediation of PNAs in the environment are important due to their potential health hazards