SOME OBSERVATIONS ON THE PI-COMPLEX OF CL ATOMS WITH BENZENE

An estimate of the entropy change in the attachment of a Cl atom to benzene together with the reported equilibrium constant led to a DELTAH-degrees for the reaction and a DELTA(f)H-degrees (Ph-H:Cl) which coincides with that estimated for the chlorocyclohexadienyl radical. Kinetic considerations rule out the latter as a direct participant in chlorination. Instead its role appears to be that of a carrier for reversibly bound Cl atoms. It is in rapid cage equilibrium with the much more weakly bound pi-complex which probably has never been seen spectroscopically but which still remains the most probable agent responsible for the increased selectivity of chlorination. Pyridine which has been observed32 to be some 4-fold more selective than benzene in chlorination is shown to form a stronger complex (by 2.6 kcal/mol). However, this >N-Cl bonded complex is also not the selective chlorinating agent but rather again the more loosely bound pi-complex in equilibrium with it. Effective cage concentrations of substrate RH and benzene around nascent and free Cl atoms are different as well as being different from bulk solution concentrations. An outline is given of an empirical method to estimate the thermochemistry of Cl and HCl and other gases in various solvents as a function of concentration.