Effect of substituents on the hydrodechlorination reactivity of para-substituted chlorobenzenes

In order to determine the electronic factor affecting the hydrodechlorination reactivity of aromatic chlorinated compounds, chlorobenzene and para-substituted chlorobenzenes were hydrogenolyzed over a 5% Pd/C catalyst, at H-2 1 MPa and 573 K. The results are discussed from the standpoints of both the energy level and the orbital coefficient of the frontier orbital using a semi-empirical calculation method. When these chlorobenzenes were reacted, selective cleavage between the ipso-carbon and the chlorine atom took place and was accelerated by the presence of both electron-releasing and electron-withdrawing substituents. Between the logarithms of the reaction rate constant ln(k/k(0)) and the substitution constant sigma(p)(0), a good linear relationship for the two series was observed. The results of the PM3 calculation of the organics suggested a good correlation in chlorobenzene reactivities with a close energy gap between the HOMO (and the LUMO) of the organics and the unoccupied part (and occupied part) of the d-band of the catalyst. Furthermore, the magnitude of the relative values of the HOMO coefficient on the ipso-carbon atom to that on the chlorine atom, (C-HOMO,C-C)(2)/(C-HOMO,C-Cl)(2), and of the LUMO coefficient on the chlorine atom to that on the ipso-carbon atom, (C-LUMO,C-Cl)(2)/(C-LUMO,C-C)(2), resulted in the same orders of reactivity for the para-substituted chlorobenzenes with electron-releasing and electron-withdrawing substituents. (C) 2002 Elsevier Science B.V. All rights reserved.