Elimination kinetics of beta-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 temperatu re 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 r ate 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-methylbutyronitri le 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 pr imary to tertiary carbon containing the OH group. The rates for the beta-hy droxynitriles 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 smal l positive Delta S* indicate a mechanism different from a six-centered cycl ic transition state. These data appear to indicate that a four-membered cyc lic transition state or a quasi-heterolytic mechanism is conceivable. Copyr ight (C) 1999 John Wiley & Sons, Ltd.