Proton transfer equilibria in halobenzene systems: Entropy changes and relative proton affinities

Abstract

The free energy changes associated with proton transfer equilibria in fluorinated benzene systems, AH + + B ⇌ BH + + A, have been determined by measuring the equilibrium constants for reactions in a number of systems. Results of equilibria involving benzene, and chlorobenzene are also included. For the proton transfer equilibria in benzene-fluorobenzene, benzene-chlorobenzene, fluorobenzene-chlorobenzene, and m-difluorobenzene-fluorobenzene systems, entropy changes were experimentally determined. These entropy changes were in good agreement with those predicted from statistical mechanical considerations, and again verified the existence of the “intermolecular” entropy change, which has recently been pointed out. From measured free energy changes in various overlapping equilibria, it was possible to construct a scale of relative gas phase basicities at 373 K, and from this, to calculate a scale of relative proton affinities. The positions in this scale of most of the fluorine-substituted benzenes can be rationalized by considering the number of non-fluorinated reaction sites available, and the positions of those sites ortho to or para to a fluorine substituent.