NMR studies and rotational isomerism in fluoroacetones

Abstract

The subject matter of this dissertation is an NMR study of α-haloacetones. In order to probe into the related structural questions of rotational isomerism, configurational equilibria, conformational analysis, etc., proton and fluorine chemical shifts and long-range H-F coupling constants were used. The fluorinaced acetones 1,1,1-trifluoroacetone, 1,1,1-trifluoro-3-chloroacetone, 1,1,1-trifluoro-3-bromoacetone, 1,1,1,3-tetrafluoroacetone, 1,1,1-trifluoro-3.3-dibromoacetone, 1-fluoro-3-chloroacetone, 1-fluoro-1-chloroacetone, 3-fluorobutan-2-one and α, α, α-trifluoroacetophenone were prepared and carefully purified. For NMR study, solutions of the above fluoroketones in different solvents were vacuum degassed and sealed. The chemical shifts and coupling constants were taken from the spectra obtained on Varian A-60, HA-60 and HA-100 spectrometers. Variable temperature studies were carried out in some representative solvents such as carbon tetrachloride and acetonitrile. The mole fractions of rotational isomers of CF₃COCH ₂X (X=Br, Cl, F) and CF₃COCHBR ₂ were estimated from variable temperature studies. The diamagnetic effect and π-complex formation in benzene solution and in the neat liquids are also discussed. From a study of the solvent dependence of the long-range H-F coupling in fluoroacetones, useful conclusions about the coupling signs and stereochemical dependences were drawn. The sign of the long-range H-F coupling was found to be positive in less halo-substituted acetones but it changes to negative for pentafluoroacetone. The through-bond and through-space coupling mechanisms were compared and discussed..