Elimination kinetics of β-hydroxynitriles in the gas phase

The gas-phase elimination kinetics of primary, secondary and tertiary beta-hydroxynitriles were examined in static seasoned vessels over the temperature range 36-450 degrees 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 k(1) = (14.29 +/- 0.47) - (234.9 +/- 6.3) kJ mol(-1) (2.303 RT) for 3-hydroxybutyronitrile log k(1) = (13.76 +/- 0.10) - (222.6 +/- 0.7) kJ mol(-1) (2.303RT)(-1); and for 3-hydroxy-3-methylbutyronitrile log k(1) (s(-1))= (13.68 +/- 0.68) - (212.5 +/- 8.7) kJ mol(-1) (2.303RT)(-1). The decomposition rates of the beta-hydroxynitriles increase from primary to tertiary carbon containing the OH group. The rates for the beta-hydroxynitriles are found to be slower than those for the corresponding beta-hydroxyacetylene analogs. The value of log A from 13.7 to 14.4 and the small positive Delta S* 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 (C) 1999 John Wiley & Sons, Ltd.