trans‐Diphenyldiazene oxide

The crystal structure of the title compound (common name: trans‐azoxybenzene, t‐AXB), C12H10N2O, was determined at 299 ± 0.5 K, i.e. 6 K below its melting point. The azoxy group has the expected geometry, and the O atom is affected by libration normal to the N=N—O plane. The O atom is disordered over two positions; the site‐occupancy ratio is ∼3:2. Phenyl rings are out of the NNO plane, with dihedral angles ranging from 20 (1) to 34 (1)°. The dihedral angle between the two phenyl rings is 46.19 (11)°. This geometry contrasts strongly with the gas‐phase conformation of t‐AXB, previously determined by electron diffraction combined with ab initio calculations, which showed that the stable conformation is planar. This discrepancy is, however, consistent with low rotational barriers for phenyl rings. These features could be of interest in relation to the actual reaction mechanism of the Wallach rearrangement (acid‐catalyzed isomerization of t‐AXB into 4‐hydroxyazobenzene), which remains a controversial matter.