Kinetics of the Gas‐Phase Addition of HI to C2H4 and the Pyrolysis of Ethyl Iodide

The addition of HI to C2H4 yields C2H5I at low temperatures and C2H6+I2 at higher temperatures. From 289° to 331°C the rate law is mixed—second order with C2H6+I2 as products. The rate is independent of added I2 and has a specific rate constant k2 (units, liter/mole‐sec): logk2=8.52−(28900/4.575T). The pyrolysis of C2H5I from 330° to 392°C goes by a first‐order process to C2H4+C2H6+I2. The first‐order rate constant is given by (units, sec—1): logku=13.66−(50000/4.575T).If it is assumed that both processes have the same rate determining step, namely the reversible dissociation of C2H5I, they yield an equilibrium constant in good agreement with independently calculated values of Keq for this dissociation. Under these conditions k1, the specific rate constant for the unimolecular split of C2H5I into C2H4+HI is given by ku/2. Arguments are presented to show that this common path involves the four‐center molecular transition state rather than a radical disproportionation. The latter, however, may account for as much as 20% of the pyrolysis at 392°C.