The activation of saturated hydrocarbons by transition-metal complexes in solution. Part III. Use of photoelectron spectra of halogen-substituted hydrocarbons and hydrocarbons containing quaternary carbon atoms to interpret the nature of platinum(II)-catalysed hydrogen–deuterium exchange

Abstract

Representative fluoro-, chloro-, bromo-, and iodo-substituted alkanes and aromatic compounds, and two branched-chain hydrocarbons that contain an ethyl group adjacent to a quaternary carbon atom, have been studied both by photoelectron spectroscopy and in hydrogen–deuterium exchange reactions catalysed by PtCl₄^2– in 50 mol % acetic [²H₁]acid–deuterium oxide. For the halogen-substituted compounds, H–D exchange occurs in those molecules for which the first ionisation potential in the photoelectron spectrum is associated with a molecular orbital centered on carbon–hydrogen bonds. For these compounds, an exponential relationship exists between the rate constant for exchange and this potential. When the first ionisation potential is associated with electrons of the halogen atom, then displacement of the halogen occurs. For the branched-chain hydrocarbons, the rate constant for exchange in the ethyl group correlates with the rate constants for the linear alkanes, provided an ionisation potential attributable to electrons of the ethyl group is used.