EFFECT OF SUBSTITUENTS ON THE PHENYL COUPLING REACTION ON CU(111)

By using selectively substituted iodobenzenes, we have been able to st udy the kinetics of the Ullmann phenyl coupling reaction on the Cu(111 ) surface. The substituted iodobenzenes used in these experiments were (iodobenzene (C6H5I), 3-iodotoluene (m-CH3C6H4I), 1-fluoro-2-iodobenz ene (o-FC6H4I), 1-fluoro-3-iodobenzene (m-FC6H4I) and 1-fluoro-4-iodob enzene (p-FC6H4I)). Variable heating rate temperature programmed react ion (TPR) experiments were used to determine the energy barriers and p reexponential factors in the rate constant for phenyl coupling. These studies have allowed us to probe the nature of charge separation in th e transition state. Fluorination of iodobenzene in the meta or para po sition decreased the activation barrier, indicating that the transitio n state is electron rich with respect to the initial state. Reaction r ate constants were calculated from the kinetic parameters and linear f ree energy relationships (LFER) or Hammett plots were constructed usin g tabulated values of the substituent constants. The reaction constant , rho, was determined to be 6.0 +/- 3.3 from the Hammett plot. The sig n of the reaction constant indicates the electron rich character of th e transition state relative to the initial state.