Kinetics of the Reaction of HI with Ethyl Iodide and the Heat of Formation of the Ethyl Radical

Abstract

The kinetics of the reaction between EtI and HI have been studied in the temperature range 263° to 303°C using a spectrophotometric technique for the determination of I2. The mechanism obeyed is the following: I2⇌2I, KI2I+EtI⇌ lim 21Et+I2,Et+HI⇌ lim 43EtH+I,,which has the rate law (neglecting Step 4): d(I2)dt=k1k3KI212(EtI)(HI)(I2)12k2(I2)+k3(HI).. Decomposition of EtI by unimolecular elimination of HI is negligible. A differential method was employed to determine the rate constants k1 and the ratio k2/k3. The Arrhenius parameters of these constants were evaluated experimentally and combined with known thermodynamic data to give the parameters of the individual constants k2, k3, k4. As with other I2 molecule systems, the A factors of Reactions (1) and (4) are high, being approximately equal to collision frequencies. With the assumption E2=0.2 kcal and the experimental activation energy E1, the C–I bond-dissociation energy in EtI is found to be 52.9±1.0 kcal at 25°C. The heat of formation of the ethyl radical as determined here [ΔHf°(Et)=25.3±1.2 kcal] indicates the C–H bond energy in ethane to be 97.7±1.2 kcal. These values agree well with independent estimates.