SECONDARY STERIC EFFECTS IN SNAR OF THIOPHENES - A COORDINATE KINETIC, THERMODYNAMIC, UV-VIS, CRYSTALLOGRAPHIC AND AB-INITIO STUDY

The reactivity of some dinitro-benzene and -thiophene derivatives with piperidine and sodium benzenethiolate has been examined giving eviden ce that benzenes show large and thiophenes show small kinetic secondar y steric effects, respectively, The acid dissociation reaction and the UV-VIS spectra of some nitrothiophenamines have also been studied, Th e crystal structure and the absolute conformation of 2-iodo-3,5-dinitr othiophene and of 2-iodo-4-methyl-3,5-dinitrothiophene have been deter mined, For different conformations of the analogous chlorodinitro deri vatives in the thiophene and benzene series, ab initio energy calculat ions have been performed, The results collected show that steric strai n among adjacent groups affects the benzene and thiophene compounds an d the kinetic, thermodynamic and spectroscopic properties of the molec ules examined differently, Differences in geometry of the two aromatic rings and then in rotation of nitro groups with respect to the rings themselves as well as differences along reaction coordinates (essentia lly depending on hybridation changes) are used to explain the above da ta.