Study of the Thermodynamics (Thermal and Cl Catalyzed) and Kinetics of the Cis and Trans Isomerizations of CF3CF═CHF, CF3CH═CHCF3, and CH3CH═CHCH3 in 100–950 Torr of N2 Diluent at 296–875 K: Effect of F and CF3 Substitution on the Isomerization Process Including the Fluorine “Cis Effect”

The equilibrium constants for the Z to E isomerizations of CF3CF═CHF (K1) and CF3CH═CHCF3 (K2) have been measured using GC/FID analysis over the temperature ranges 360–850 and 297–850 K, respectively. At lower temperature, K was determined using Cl atom catalysis. At higher temperature, K was measured without a catalyst. The temperature-dependent expressions are K1 (Z to E) = 1.45(±0.15)e(–[2845(±100)/RT]) and K2 (Z to E) = 1.9(±0.22)e(+[4330(±120)/RT]) (where the gas constant R ≡ 1.986 cal mol(–1) K(–1)). For isomerization 1, the Z (fluorine cis) isomer is 2.85 kcal mol(–1) lower in energy than the E (fluorine trans) isomer, providing another example of the fluorine "cis effect" in olefins. For isomerization 2, the E(trans) isomer is 4.3 kcal mol(–1) lower in energy than the corresponding Z(cis) isomer as is normal for olefins. The isomerization rate constant in a single direction was also measured for each fluorinated compound: k–1(E to Z) = 10(13.87±0.24)e(–59530(±887)/RT) s(–1); and k2(Z to E) = 10(13.89±0.23)e(–58845(±675)/RT) s(–1). To verify the experimental method, cis to trans (k3) and trans to cis (k–3) isomerization rate constants were also measured for cis- and trans-2-butene for comparison to several previous studies. The rate constants determined herein are k3 (cis to trans) = 10(13.95±0.23)e(–63245(±815)/RT) s(–1); and k–3 (trans to cis) = 10(14.32±0.28)e(–64993(±1132)/RT) s(–1). k3 agrees well with four previous measurements and represents the best available rate constant for 2-butene. All errors quoted here are 2σ. The typical total pressure for these experiments was 760 ± 100 Torr. Limited experiments performed at 100 Torr showed no pressure dependence for any of the compounds above 100 Torr. Thus, all isomerization rate constants represent high-pressure limits. The rates of the addition reactions of Cl to the double bonds of CF3CF═CHF (k4) and CF3CH═CHCF3 (k7) (used in Cl catalysis) were also measured in pure N2 and in pure O2. In O2, the rate constants expressions are k4 = 1.56 (±0.22) × 10(–11) e(+(643/RT)) cm3 molecule(–1) s(–1); and k7 = 1.05 × 10(–12) e(+(1874/RT)) cm(3) molecule(–1) s(–1). In N2, k4 and k7 decrease several orders of magnitude as the temperature increases because of the increasing reversibility of the Cl addition reaction, which produces the catalytic effect.