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We report a new method for determination of rate constants of processes in solution using electrospray ionization mass spectrometry (ESI-MS). The investigated reaction is C-H activation of acetanilides by palladium(II)trifluoroacetate leading to stable organopalladium complexes. The rate constants can be determined from an experiment with a couple of differently substituted acetanilides being in competition activated by the palladium salt. The formed organopalladium complexes can be detected by ESI-MS. The time dependence is achieved by adding one of the acetanilides to the reaction mixture with a time delay. The kinetics can be then evaluated from the evolution of the ratio of the ESI-MS signals of differently substituted complexes as a function of the time delay. The Hammett analysis of the rate constants obtained for a series of meta-and para-substituted acetanilides provides the ρ value of-1.5, which is in agreement with values reported for similar C-H activations. We have investigated the very same reaction also with UV-Vis spectroscopy that gave us about three times smaller rate constants, but the same trend with the ρ value of-1.6. The rate constants determined by ESI-MS are directly linked to the occurrence of organopalladium complexes, whereas the UV-Vis data are associated with an absorption spectra change that could involve more reaction steps. DFT calculations support the interpretation of the reaction mechanism as cyclopalladation and provide the ρ value in the same range. The rate determining step corresponds to the agostic C-H transition structure.

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