The Investigation of Electronic Interactions in some Polyhalogenoaromatic Compounds using Electric Dipole Moment and NMR Studies

Abstract

The electric dipole moment of several polyfluoro-, polyfluoronitro-and polychlorofluoro-benzenes have been determined at 25°C in dilute solution in cyclohexane. The dipole moments of a range of polybromo-and polybromonitro---benzenes have been determined in p---xylene at 25°C The departures from additivity in the moments are discussed in terms of variations in atom polarisation, changing mesomeric effects or steric distortion; no evidence is found for these explanations. The assignment of an empirically calculated value to the C-NO₂ and C-halogen bond moments in various environments gives excellent correlation between the observed and calculated moments. This is explained in terms of an induced moment in the pi-electron system caused by the primary group moment. The relative importance cf the mesomeric and pi-inductive effects of the nitro- and halogen---groups in conjugated systems are considered.

The influence of a substituent on the chemical shift of a magnetic nucleus is described. The substituent constant parameters are determined in terms of chemical shifts in ppm of the observed nuclei (1H, 13C, 19F) at the ortho-, meta- and para-positions to the substituent in monosubstituted benzenes. The extent of correlation between the observed and calculated chemical shifts in the above range of compounds have been examined. The latter shifts are the sum of the appropriate substituent constants. No considerable agreement has been observed between these chemical shifts and their corresponding calculated values. The deviations are larger for the polysubstituted benzenes. No single explanation could be given for the origin of the effects causing the non-additivity of these chemical shifts.

The F-H coupling constants of the polychlorofluorobenzenes have been determined for the three spin systems of ABX type. The H-H and C-H coupling constants in polybromo- and polybromonitro-benzenes have been determined for several first order spectra.

The products of the reaction of sodium borohydride as a reducing agent in dimethyl sulfoxide, as solvent, over the range of polybromo-benzenes have been examined. The mechanism of the reaction could be a nucleophilic attack of borohydride ion on the least electro-negative bromine atom of the ring.