A dipole moment study of hindered benzophenones

Dipole moment analysis of 4-X-2,6-dimethylbenzophenones, 4′-X-2,6-dimethyl- and 4′-X-2,4,6-trimethylbenzophenones, with X = OMe and X = NMe2, showed that the (X⋯CO) interaction moments are much lower in the first family of compounds. 2,6-Dimethyl- and 2,4,6-trimethylbenzophenone exhibit an asymmetric gauche model with φ = 55° and φ′ = 0°. 2,6-Dimethyl-3′-chloro- and 2,4,6-trimethyl-3′-chlorobenzophenones occur as equimolecular mixtures of cis-(Cl,O) and trans-(Cl,O) conformers. Preferred conformations are proposed for p-chloro, p-methoxy- and p-dimethylaminobenzophenone, and the delicate problem posed by the actual conformation of benzophenone in solution is discussed. In the present work, 25 benzophenones were examined, and their dipole moments were analysed according to a coherent pattern.