Kinetics of the Gas-Phase Reaction of CH3I with HI

Abstract

By modification of a Beckmann DB spectrophotometer for use with high-temperature gas systems it has been possible to follow the kinetics of reaction of CH3I+HI which goes quantitatively to CH4+I2 in the temperature range, 260° to 316°C. The kinetics fit the mechanism proposed earlier by Benson and O'Neal very well. I2⇋2 I,(K),I+CH3I⇋ lim 21CH3+I2,CH3+HI⇋ lim 43CH4+I.The rate law for this system is (neglecting 4) d(I2)/dt=k1k3K12(CH3I)(HI)(I2)12/[k2(I2)+k3(HI)].Although this equation can be integrated exactly, the integrated forms are not sensitive to the data, and various expansions must be used. Values for k1 and k3/k2 are found to be in excellent agreement with earlier estimates. From the activation energy E1 and the assumption that E2≤0.5 kcal, the bond energy of CH3I is found to be about 55±1 kcal/mole, in excellent agreement with other estimates. Values are given, in addition, for the individual rate constants, k2, k3 and k4, as well as their Arrhenius parameters. The A factors for reactions 1 and 4 are shown to be in excess of collision frequencies, indicating steric factors slightly in excess of unity and thus the formation of very loose transition-state complexes. This appears to be typical of iodine-atom or I2-molecule reactions and thus far quite unique.