Hydrogen bonding in certain oximes and their acid salts

Abstract

Diethyl α-hydroximinomalonate (pK_a 5·48) and ethyl α-hydroximinocyanoacetate (pK_a 4·66) are very acidic for oximes and their n.m.r. and i.r. spectra are presented. Diethyl α-hydroximinomalonate is shown to adopt an intramolecular hydrogen-bonded conformation in its liquid state and in non-polar solvents. An open conformation, in which the oximino-group functions as a hydrogen-bond donor to a basic site is preferred in dipolar, aprotic solvents. Ethyl α-hydroximinocyanoacetate adopts a similar conformation in dipolar, aprotic solvents; but in chloroform, an alternative conformation, in which the oximino-group functions as a hydrogen-bond acceptor is preferred. In concentrated solution and in the solid state one mole of both these conformations take part in the formation of a dimer by OH ⋯ O–N intermolecular hydrogen bonding. This evidence of intermolecular opposed to intramolecular association shows that the oximino-group must be anti- with respect to the ethoxycarbonyl group. These studies of conformational equilibria have provided a new and systematic approach to the assignment of the ν(CN) vibration frequencies to the various modes of hydrogen bonding shown by oximes.

Acid salts are formed by both these oximes and it is suggested that i.r. evidence for the localisation of negative charge on the oxygen atoms of the oximino-groups may be related to the very strong intermolecular hydrogen bonds in these salts.