Missing Link in the Homologous Series of Lactams: The X-ray Structure of Valerolactam

Lactams are an important class of heterocyclic compounds and are widely used for industrial and pharmaceutical purposes. Even decades after initial lactam syntheses, research on their physical and chemical properties is still rewarding. It delivers valuable information on the reactivity of lactams and their conformational behavior. For the small- and medium-sized parent lactams, the X-ray structures have been well-known, except for the “missing link”, the six-membered valerolactam. An X-ray structure of valerolactam is described here for the first time, stimulating a comparative discussion of the homologous lactam series. The experimental solid state conformation of valerolactam differs significantly from the calculated and energy-minimized ones reported in the literature. The amide bond length in valerolactam is more or less equal to that in other lactams. A comparison with the structure of cyclohexene revealed striking similarities arising from the partial C–N double bond in valerolactam caused by amide...