Elimination kinetics of β-hydroxynitriles in the gas phase

Abstract

The gas-phase elimination kinetics of primary, secondary and tertiary β-hydroxynitriles were examined in static seasoned vessels over the temperature range 360–450 °C and pressure range 47–167 Torr (1 Torr = 133.3 Pa). These reactions are homogeneous, unimolecular and follow a first-order rate law. The rate coefficients are given by the Arrhenius equation: for 3-hydroxypropionitrile log k1 = (14.29 ± 0.47) − (234.9 ± 6.3) kJ mol−1 (2.303 RT)−1; for 3-hydroxybutyronitrile log k1 = (13.76 ± 0.10) − (222.6 ± 0.7) kJ mol−1 (2.303RT)−1; and for 3-hydroxy-3-methylbutyronitrile log k1 (s−1) = (13.68 ± 0.68) − (212.5 ± 8.7) kJ mol−1 (2.303RT)−1. The decomposition rates of the β-hydroxynitriles increase from primary to tertiary carbon containing the OH group. The rates for the β-hydroxynitriles are found to be slower than those for the corresponding β-hydroxyacetylene analogs. The value of log A from 13.7 to 14.4 and the small positive ΔS ≠ indicate a mechanism different from a six-centered cyclic transition state. These data appear to indicate that a four-membered cyclic transition state or a quasi-heterolytic mechanism is conceivable. Copyright © 1999 John Wiley & Sons, Ltd.