Dipole moments and molecular conformation in aliphatic diazoketones

The dipole moments of diazoacetone RCOCHn₂ and its three chloro derivatives substituted at the alkyl group have been determined. The experimental values are consistent with a rigid planar structure of the COCHN₂, where the CO and CN₂ groups may be either in cis or trans configuration In the parent molecule, an equilibrium is operative between the two and in the halogenated derivatives the cis configuration only is present. As to a possible free or hindered rotation of the halogenated alkyl group R around the R—Co bond, a comparison between found and calculated dipole moment values, and a qualitative consideration of internal steric and electrostatic factors, suggest that a single steric configuration is preferred in the monochloro and dichloro derivatives. The angle of 150° and 40° between the molecular plane containing the COCHN₂ grouping, and the plane defined by the C(carbonyl)—C(alkyl)—X(X = Cl in the former, H in the latter molecule) was found. A similar rigid structure is also expected in the parent molecule and, to a greater extent, in its trichloro derivative.