Chemistry of 1,1,2,3,3,4,4-Heptafluorobut-1-ene Initiated by Chlorine Atoms

Abstract:

Unsaturated hydroflurocarbons (HFCs) are potential substitutes for long-lived saturated HFCs with high global warming potentials (GWP). Here we evaluate some properties of CF₂H-CF₂-CF=CF₂ (HFB) at room temperature. Photolysis of Cl₂ by a mercury lamp generated atomic Cl which reacted with a mixture of ethane and HFB diluted in about 1 bar of artificial air. Reactants and products were monitored via FT-IR spectroscopy and the rate coefficient was determined via relative rate methods. COF₂ was identified as a product and its yield was determined to be close to 1.

Plausible second products are CF₂H-CF₂-CFO, 2,2,3,3-tetrafluoropropanoyl fluoride (TPF), and CF₂Cl-CFO. The observed IR spectrum was matched to the first compound via density functional theory and anharmonic frequency calculations. These showed the importance of considering an equilibrium distribution of all conformers created by rotations around single bonds, in order to make a quantitative comparison. No evidence for C-Cl bonding was seen in the IR, nor the extra COF₂ that would be a co-product.

Absolute IR cross-sections for TPF were obtained which allow the assessment of its radiative efficiency (RE). Likewise, the first IR cross-sections and RE of HPF were obtained. In order to match them computationally, again an equilibrium mixture of rotational conformers was considered.

Possible reasons for the lack of formation of significant quantities of CF₂Cl-CFO are either kinetic control over which end of the π bond in HFB is preferentially attacked by atomic Cl, or variations in the bond strengths in the intermediates that determine which fragmentation is favored.