STRUCTURAL SIMILARITIES AND DIFFERENCES BETWEEN N-PHENYLUREAS AND N-PHENYLTHIOUREAS

Crystal structures of N-phenyl-N'-propylthiourea and the complex of N, N-bis(4,4'-difluoro-2-biphenyl)urea with N,N,N',K-tetramethylurea have been determined by single-crystal X-ray diffraction. Both structures are stabilized by formation of intra- and intermolecular hydrogen bond s. The first compound adopts an anti-syn conformation whereas the seco nd one has a syn-syn conformation. In both structures the nearest envi ronment for the N atoms is planar. Structural similarities and differe nces between N-phenylureas and N-phenylthioureas are discussed togethe r with their average geometry and relationships between changes of the structural parameters of these fragments. Hydrogen bonding and mesome rism involving ionic structures seem to be the dominant effects in cha nging the geometrical parameters of ureas and thioureas. The differenc es in the electronegativities of S and O atoms are responsible for a d ifferent mobility of pi-electrons in the examined fragments and in con sequence for the differences in the variation of structural parameters .