Racemic 1,2,4-Trimethylurazole

The structural and stereochemical parameters of 1,2,4-trimethyl-1,2,4-triazolidine-3,5-dione, C₅H₉N₃O₂, (1), were examined in view of the contrast between its physical properties and those of urazoles having one or more unsubstituted NH groups. The X-ray structure shows that (1) is devoid of intermolecular hydrogen bonding but retains the structural features of strongly hydrogen-bonded urazoles. Its trans H₃C-N-N-CH₃ torsion angle, 72.3 (5)°, is slightly larger than the corresponding angle in urazoles having an unsubstituted or monomethylated N-N function, but smaller than that in urazoles monosubstituted with bulkier groups. Like other crystalline urazoles, (1) is racemic by virtue of N1 and N2 chirality. In its packing pattern of parallel molecular sheets, two pass through the unit cell; within a sheet, each molecule is surrounded by two of its own chirality and four of opposite chirality. While physical properties of crystalline urazoles can be associated with their degree of intermolecular hydrogen bonding, this study shows that their chirality is an intrinsic molecular phenomena.

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