tunneling between the two configurations. Aromaticity Aromaticity of the arsole manifests itself in delocalization and resonance of its ring electrons. It is closely related to planarity in that the more planar the molecule the stronger its aromaticity. Aromaticity of arsole and its derivatives has been debated for years both from experimental and theoretical point of view. A 2005 review combined with quantum chemical calculations concluded that arsole itself is "moderately" aromatic as its ring current is 40% that of pyrrole, which is known to be aromatic. However, comparable ring current was calculated for cyclopentadiene, which has long been regarded as non-aromatic. Other reports suggest that the aromaticity (and planarity) can vary between arsole derivatives. Chemical properties (arsole derivatives) Chemical properties of arsole derivatives have been studied experimentally; they are similar to those of phosphole and its derivatives. Substitution of all hydrogen atoms in arsole with phenyl groups yields yellow needles of crystalline pentaphenylarsole, which has a melting point of 215 °C. This complex can be prepared, at a yield of 50–93%, by reacting 1,4-diiodo-1,2,3,4-tetraphenylbutadiene or 1,4-dilithio-1,2,3,4-tetraphenylbutadiene with phenylarsenous dichloride (C6H5AsCl2) in ether. Substituting in this reaction arsenic trichloride for phenylarsenous dichloride yields 1-chloro-2,3,4,5-tetraphenylarsole, which also forms yellow needles but with a lower melting point of 182–184 °C. Pentaphenylarsole can further be oxidized with hydrogen peroxide resulting in yellow crystals with melting point of 252 °C. It can also be reacted with iron pentacarbonyl (Fe(CO)5) in isooctane at 150 °C to yield a solid organoarsenic compound with the formula C34H25As,Fe(CO)3. Reacting pentaphenylarsole with metallic lithium or potassium yields 1,2,3-triphenyl naphthalene. Reaction of phenylarsenous dichloride with linear diphenyls results in 1,2,5-triphenylarsole (see below), a solid with a melting point of about 170 °C. This compound forms various anions upon treatment with alkali metals. See also Pyrrole, a nitrogen analog. Furan, an oxygen analog. Thiophene, a sulfur analog. Simple aromatic rings Varsol, a petroleum distillate with a boiling range of 150–200 °C. References Arsenic heterocycles Five-membered rings