OCR Text |
Show possibility of being used for a multiplicity of applications is a particular advantage of this type of chemical kinetic data base. In the following, we briefly summarize some of the new thermodynamic and kinetic properties contained in the data base. This describes in a very satisfactory manner the chemistry brought about by the introduction of fluorine into the hydrocarbon framework. a. Thermochenlistry Fundamental to the thermal and chemical stability of any moiecule are the energies of the bonds that hold a molecule together. Table 1 contains a number of relevant bond energies. Also included are those for comparable hydrocarbon and chlorohydrocarbon molecules. These are thennodynamic quantities or more specifically, the enthalpy of reaction at 298 K. For the present application, which is mostly kinetics in nature, they set limits for rate processes. In the framework of thennochemical kineticsll they are also the basis of estimation. The important features to be noted are the conspicuously higher values for C-F and H-F bonds in comparison to their hydrogen and chlorine analogs. F-OH, F-O and F-F bonds are all however much weaker than that for the H-OH, H-O and H-H compounds. It would appear that except at the highest temperatures (in the flame front) once H-F bonds are fonned, abstraction of the hydrogen radical by OH, Hand 0, being highly endothermic, are unlikely. This is in contrast to the situation with hydrogen chloride, where the abstraction reactions are close to thermal neutral and can therefore release Cl back into the system. Abstraction of fluorine bonded to carbon by OH and 0 are endothermic. Thus rate constants will be smaller than those involving abstraction of Hand Cl. Abstraction of fluorine by hydrogen is exothermic. However, as will be seen subsequently, there appears to be a significant activation barrier. There are some changes in C-C bond energies upon the substitution of fluorine for hydrogen. The most conspicuous difference is the weakness of the double bond in CF2=CF2• This is tied to the stability of the CF2 radical. This is in contrast to the situation with ethylene, where the double bond is so strong so that C-H is the weakest bond. Also included are bond strengths for a number of radicals. In all cases, the introduction of a free radical site leads to a drastic lowering of bond energies beta to the radical site. The presence of fluorine, particularly in large amounts, introduces some new elements. 7 |