Phenanthrene

Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings--as the above formula shows. The name Phenanthrene is a composite of phenyl and anthracene. It provides the framework for the Steroids. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitising skin to light.

The compound with a phenanthrene skeleton and nitrogens at the 4 and 5 position, so IUPAC name 4,5-diazaphenanthrene, is known as phenanthroline.

Chemistry
A classical phenanthrene synthesis is the Bardhan-Sengupta Phenanthrene Synthesis (1932). In the second step of this reaction 9,10-dihydrophenanthrene is oxidized with elemental selenium. Phenanthrene can also be obtained photochemically from certain diarylethenes.

Reactions of phenanthrene typically occur at the 9 and 10 positions. Some basic reactions:
 * Organic oxidation to Phenanthrenequinone with chromic acid
 * Organic reduction to 9,10-dihydrophenanthrene with hydrogen gas and raney nickel
 * Electrophilic halogenation to 9-bromophenanthrene with bromine
 * Aromatic sulfonation to 2 and 3-phenanthrenesulfonic acids with sulfuric acid
 * Ozonolysis to diphenylaldehyde

Canonical forms of Phenanthrene
Phenanthrene has five resonance structures, given below:



The third and fourth resonance structures appear to be the same; they are in fact just 180° rotations of each other, as the molecule is planar. However, they do deserve to be treated as separate structures, as what matters are the bonds on the individual carbon atoms, which remain fixed even when we rotate our frame of reference.

Phenanthrene is more stable than its linear isomer anthracene. A classic and well established explanation is based on Clar's rule. A novel theory invokes so-called stabilizing hydrogen-hydrogen bonds between the C4 and C5 hydrogen atoms.

More information on phenanthrene is available.