Collect. Czech. Chem. Commun. 1985, 50, 1565-1572

Evidence in favour of Exner isokinetic method to determine activation parameters

Pascual Segura

University of California, Santa Cruz, California 95064, U.S.A.


Exner method to ascertain the isokinetic relationship (IKR) and obtain isokinetic activation parameters in a reaction series, although well known, is relatively little applied; A great many chemists do not appear to be convinced of the superiority of isokinetic activation parameters over conventional ones, and the IKR itself is still a controversial matter. From our kinetic results on the decarboxylation of a series of 4-substituted-2,6-dinitrobenzoate ions in aqueous solution, activation parameters were calculated, both by Exner method and by a conventional one. The obtained isokinetic activation enthalpies correlate much better (r = 0.998, 5 points) than the conventional ones (r = 0.97, 5 points), with the energies (calculated at the STO-3G ab initio level) for the formation of substituted aryl anions, the latter being a process expected to be energetically related to the decarboxylation reaction. This is the first example of non-statistical evidence in favour of Exner method to determine activation parameters, a method that we strongly advocate.