OCR Text |
Show chlonnation processes for industnal processing. Pertinent data have been summarized in a previous publication[9]. For the present purpose it is worthwhile to consider their behavior in comparison to the appropriate hydrocarbons. From the viewpoint of thermal stability, chlorine for hydrogen substitution leads to a compound that is less stable from a unimolecular sense. This is brought about by the decrease in the strength of the C-Cl bond in companson to the C-H bond, a lowering of the strength of C-H bonds adjacent to chlonne atom and for larger organic compounds, the possibility of alternate lower lying decomposition channels leading directly to the formation of HC1. A key radical that is found in all chlorinated system is the chlorine atom. There is much data regarding its regarding is reactivity. In general they track that of the O H radical. Thus at higher temperatures they pnmanly abstract hydrogen atoms from other organic systems. The decomposition of such radicals then releases other radicals into the system. At lower temperatures they can add to unsaturated systems. However as the temperature is increased such reactions are much more easily reversed in comparison to the hydrocarbon analog. This is a direct consequence of the weaker C-Cl bonds. The consequence is that chlorination is basically a lower temperature process in companson to oxidation. The reactivity of chlorine atom to attack by hydrogen atom is only slightly less than that for the analogous hydrogen. W h e n chlorine is ipso to a site of unsaturation it can be readily displaced by a hydrogen atom. When the attacking species are O H and O, as in an oxidative system, the reaction endothermicity leads to a tremendous reduction in reactivity. Thus a chlorinated system is less susceptible to oxidation in comparison to the comparable hydrocarbon system as the degree of chlorination increases. This is contrary to the situation under pyrolysis where as noted earlier chlorinated compounds are less stable. Thus if one begins with an oxidative system containing chlorine at low temperatures chlorination is favored, however since combustion leads to high temperatures this leads ultimately to conditions where unimolecular decomposition kicks in and this then becomes the predominant channel for decomposition. Finally, it should be noted that due to the exponential nature of the rates of chemical reaction to temperature, destruction rates of a particular chemical are particularly sensitive to temperature. The physical situation is that as one proceed from low to high values there will be essentially no reaction until a particular temperature is reached, then a very small increase will lead to total destruction. This can also be the case for the formation of intermediates. Thus the consequence is that there may be very small windows where concentrations of intermediates are a maximum. The thermodynamic end state for chlorine is HC1. However the strength of the H-Cl bond is much less than the comparable H - O H bond. The consequence is that in an oxidative environment the reaction of O H with HC1 is exothermic. Thus in essence HC1 can burn and release more CI atoms into the system. Under such condition chlorine oxides and indeed the chlorine molecule can be formed. This is the well known Deacon process. |