Reproducibility of kinetic results

Several additions have been kinetically studied by more than one research group, but most of them were studied in solution and different workers used different solvents, so that the published data cannot be compared for reproducibility, Such a difficulty does not arise for reactions in the gas and in the pure liquid phases. Two popular Diels-Alder reactions, the dimerisation of cyclopentadiene to endo-dicyclopentadiene and that of butadiene to 4-vinyl-cyclohexene are suitable for a comparison of experimental results. Rate coefficients from 8 different sources for the former reaction in the pure liquid phase at various temperatures (in the range where comparison is possible) are given in Table 2. The coefficients are all extrapolated to zero time, as both secondary reactions and variation of the environment during the reaction cause a drift in the observed values of k. Rate coefficients for butadiene dimerisation in the gas phase, from different sources, are collected in Table 3. In this case also the temperature range has been limited to that where comparison is possible. In both Tables 2 and 3 activation parameters, as given by the authors (unless otherwise indicated) are also listed. 

EXPERI MENTA L RATE COEFFICI ENTS (EXTRAPOLATED TO / = 0)( 10 it IN L.MOLE sec ) FOR CYCLOPENTADIENEDIMERISATION IN THE PURE LIQUID phaseintherance-2°to +50°C, taken from different sources 

Values in parentheses were calculated from experimental values at the nearest temperatures, by means of the Arrhenius equation, to facilitate comparison. 

Two types of comparison are possible, between coefficients and between activation parameters. Rate coefficients at the same temperature agree in the worst case within a factor of 1.7, more often within a factor of 1.5. Taking into account the difficulty, that more or less exists in the whole field of chemical kinetics, of reproducing data by different experimenters in various laboratories, the agreement between rate coefficients is satisfactory. The same cannot be said of the activation energies and the frequency factors. Values of E in Table 2 vary from 16.2 to 18.3 kcal.mole . In Table 3, E values between 23.7 and 26.8 kcal.mole are reported for the same reaction a value of 28.0 kcal.mole has also been obtained from experiments in the range 400-700°C . 

Similar variations are found for the dimerisation of gaseous cyclopentadiene. Two sources give rate coefficients having a ratio of 1.7 (at 150°C), but activation energies as different as 14.9 and 16.7 kcal.mole , and log/1 equal to 4.93 and 6.1, respectively. 

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