The isomerisation of some ortho-substituted nitrobenzenes: a structure correlation analysis

Abstract

A search of the Cambridge Crystal Structure file has revealed a number of benzenoid molecules containing an NO₂ group ortho to an XYZ unit. X, Y, and Z are second row atoms, the X atom is bonded to the benzene ring, and the CXYZ unit is essentially planar. A scatter plot of the dihedral angles between the benzene ring and the NO₂ and XYZ groups, respectively, leads via the principle of structure correlation analysis to the conclusion that these molecules isomerise by disrotatory torsions of the two substituents, such that the sum of the dihedral angles is always 90°. A degree of resonance stabilisation is thus retained throughout the isomerisation, while non-bonded interactions are minimised. Alternative mechanisms do not satisfy one or other of these conditions.