OCR Text |
Show undoubtedly very important, is the abstraction of a hydrogen atom by a fluorine atom. b: Kinetics: At the single step thennal reaction level, with the molecules of interest here, only first and second order reactions need to be considered. Unimolecular reactions are processes that occur when a molecule receives sufficient energy from the bath so that its internal energy exceeds a certain threshold value. The result is the creation of one or two new species. Table 2 contains rate expressions for representative processes. Also included in Table 2 so as to provide a basis for comparison are the temperature needed to obtain 99.99% destruction assuming that the sole destruction mechanism is by unimolecular decomposition and that the residence time is 1 sec. This is based on the equations given earlier. Under the general category of unimolecular reactions are a variety of bimolecular processes. An example is radical combinat~9n. It is related to the reverse unimolecular reaction through the equilibrium constant Rate constants for the combination of two radicals are relatively unaffected by fluorine substitution and these process usually proceed with a large or near collisional rate constant. Combination of CF2 radicals are however an exception. They have a small activation energy of about 8 kJ/mol and an A-factor close to an order of magnitude smaller than radical combination. Another class of bimolecular processes that are covered within the unimolecular framework are those resulting from the combination of two reactive species which may then fragment into two new compounds as a result of the failure to remove the high energy content of the molecule through collisional energy transfer. The rate constants of these processes at various temperatures and pressures are calculable using well established theories13 • Aside from the breaking of a single bond, fluorinated compounds can also decompose through the 1,2 and 1,1 elimination of hydrogen fluoride. These can occur thermally or through the type of reaction described above; combination of two radicals followed by elimination of HF from the hot molecule. Such a decomposition channel does not exist for ordinary alkanes. On the other hand, this is a much more facile decomposition channel for chloroalkanes in comparison to the fluorinated compounds. Also included in Table 2 are data on the decomposition of radicals. They are all much 9 |