OCR Text |
Show Then at any given temperature (from the isobutene concentration) we find kdisp,Cl =[C6H6]/[(CH3)2C6H4] * kdisp, CH3 where the brackets refer to the measured concentration. For the other chlorinated compounds results were derived in an analogous fashion. We suspect that the best basis for comparison are the rate constants at a particular temperature. These can also be found in Table II. We have also included the rate constants for displacement for a variety of other compounds. Our results demonstrate that the range of rate constants for displacement of chlorine by hydrogen is not particularly large. Nevertheless the small changes are worthy of comment. Note that the rate of displacement per chlorine atom is virtually constant for the three systems chlorobenzene, orthodichlorobenzene and paradichlorobenzene. However when one goes to 1,2,3- trichlorobenzene rate constants for displacement are increased by a factor of 1.8 for the displacement of the outer two chlorines and a factor of 2.4 for the inner chlorine. In the case of methyl substitution it is clear that displacement is enhanced and the rate constants on a per chlorine atom basis is close to that for the trichloro system. It is also interesting that unlike the situation for the dichlorobenzene, methyl substitution in the ortho position increases the· rate constant for displacement. The differences in the rate constants for the chlorinated benzenes are worthy of comment. At first glance it is surprising that there should be a difference between the rate constant for the displacement of a chlorine in o-dichlorobenzene and the 1 and 3 chlorine in 1,2,3 trichlorobenzene. We do note however that the chlorine in a compound such as hexachlorobenzene are bent 120 out of the ring plane21 • It may be that in the 1,2,3 trichlorobenzene there is already some manifestation of this effect. The 13 |