OCR Text |
Show aromatics during the past two years. These include not only the four ' chlorinated benzenes which is the main focus of present concern but also the chlorotoluenes, phenol, aniline and toluene. In the last three case there are also reaction channels involving abstraction of the hydrogen on the side chain. Unlike the ring hydrogens, these are accessible due to the weakening of the C-H bond by resonance effects. In the following we briefly summarize the experimental procedure. The experiment apparatus is a single pulse shock tube. A schematic is shown in Figure II. For the present purposes, it is sufficient to consider the shock tube as simply a pulse heater with a heating time of about 500 microseconds. Subsequent to the experiment about 10% of the shocked gas is withdrawn for gas chromatographic analysis with flame ionization detection. The sensitivity of the latter permits us to work across wide concentration ranges. Together with the short reaction times, and the gas dynamic heating we can now work under conditions which are mechanistically clean and thus obtain results of high accuracy. More complete details can be found in an earlier publicationl5 • The basic features of our experiments are as follows. We begin with trace quantities of a source of hydrogen atoms in the form of a highly unstable hydrocarbon, hexamethylethane in large quantities of two target aromatic compounds. One of these two compounds is the chlorinated benzene of interest while the other is either toluene or 1,3,5 trimethylbenzene. The rate constants as a function of temperature for the displacement of the methyl group from these compounds have been determined in previous experiments. The use of the two standards was made necessary for gas chromatographic purposes. In the course of an experiment, hexamethylethane is decomposed via 11 |