Evidence for Intramolecular Attraction between Hydroxyl and Carbonyl Oxygen Atoms

Abstract

THE accurate crystal structure analyses of α-oxalic acid¹ and oxalic acid dihydrate² show that the length of the central C—C bond is indistinguishable from the single bond-length of 1.5445 A. in diamond within a probable limit of ± 0.025 A. It is inferred that there is no appreciable degree of π-conjugation across the central bond, presumably because of polarization of the carbon 2p electrons by the oxygen atoms; some other explanation must therefore be sought for the fact that the oxalic acid molecule is perfectly flat in both α and β anhydrous forms and in the dihydrate. It seems unlikely that in all three crystal structures the intermolecular hydrogen-bonding system should be specifically that of lowest energy only when the molecules are planar. In both the α anhydrous form and the dihydrate, the C—O bonds are distinguishable in length, one tending towards a double bond, the other towards a single bond. The centre of symmetry of the molecule requires a trans relationship between the bonds as in (I).