Kinetics of the Gas-Phase Addition of HI to Butene-2

The kinetics of the gas-phase reaction 2HI+butene-2→n-C4H10+I2 have been studied from 292° to 333°C using a spectrophotometer to follow I2 production. Except for the first few minutes of each run d[I2]/dt is a constant for up to 25% reaction. On the basis of two coupled sets of reactions: HI+butene−25⇋6sec−C4H9Isec−C4H9I+HI→n−C4H10+I2it is shown that the reaction path 5 consists of two separate steps, one a direct molecular addition of HI and the other an I-atom-catalyzed addition. Using estimates of the equilibrium constant K5·6 it is shown that the rate constants for molecular and I-atom-catalyzed paths are consistent with data of Ogg and Polanyi on the pyrolysis of sec-C4H9I. The Arrhenius parameters for the two addition paths are examined in terms of recent proposed theories of the transition states for four-center reactions and shown to be compatible with similar data in other systems. Some complexities of the present system are examined involving the butene-1, butene-2 isomerizations catalyzed by I2. The pyrolysis studies on the iodides are shown to be complicated by butadiene+HI production.