Pyrolysis of 1,2‐dichloropropane

Abstract

The thermal decomposition of 1,2-dichloropropane at atmospheric pressure has been studied in the temperature range 227-59OoC, in a flow system. Above 45OoC, the reaction is homogenous and unimolecular with a rate constant:Below 45OoC, a low activation energy, probably heterogenous process competes with the gas phase reaction.The primary reaction products are HCl and the monochloropropene isomers; the relative amounts of each isomer depend on the temperature in the low but not in the high temperature region. The direction of the HCl elimination is discussed in terms of substituent effects at the a-and ,%carbon positions and compared with literature data on similar reactions.Secondary products are formed principally by further pyrolysis of ally1 chloride. The first-order rate constant of this reaction is given by: k = 10*.54*0.2 exp (-37,275 f 700/RT) sec-l.

The mechanism of thermal dehydrochlorination. Pyrolysis of 1‐Chloro‐1‐fluoroethane and 1‐chloro‐1, 1‐difluoroethane

“…Thus, when the inhibitor is a reaction product, as in the pyrolysis of 1,2-dichloropropane, small amounts of chlorine are unable to consume the macroscopic amounts of inhibitors and no effect is observed [2]. When the inhibitor is only a 6i radical, it can be consumed readily by the halogen and a chain process can occur, as in the cases of Anyway, one should keep in mind that an accelerating effect of chlorine is by no means a criterion of a free-radical process for the uninitiated dehydrochloririation and that such tests should be considered with great care.…”

“…The Arrhenius plot of the present results is compared in Figure 2 with previous literature data [3] and yields: (R in cal) (2) where the quoted error limits are twice the mean deviation. …”

“…This is very unlikely, since at the considered temperature (480°C) it can be calculated from recent results [16] that the stationary concentration ratio [C1']/[CzH3C12'] is at least 6 for a 1,2-C2H4Cl2 pressure of 200 torr, and the relative reactivities of the CzH3C12' radical and the C1' atom toward the C-H allylic bond of propene is expected to be by far lower than this value. Actually, from preliminary experiments on the addition-substitution competition during the thermal chlorination of propylene [22] and the rate constant of Cl' addition to propene [23], a rate constant k2' II This step, which is probably the source of allylic radicals, is faster by more than a power of ten than the hydrogen abstraction by a chlorine atom from chlorinated [ 181 or chlorofluorinated [ 131 ethanes at the pyrolyses temperatures. This suggests that even a few per cent propene should inhibit "completely" a free-radical dehydrochlorination as shown by a treatment analogous to that of Howlett [21], but assuming kz' as the source of &radicals.…”

“…Rate constants of unimolecular pyrolyses of chlorinated hydrocarbons have been recently reviewed and the influence of substituents on the reaction rate discussed [ 1,2]. Even though the unimolecular rate constants calculated assuming cumulative substituent effects are generally in reasonable agreement with measured values [Z], some literature Arrhenius parameters are definitely out of line.…”

“…The reactions were studied at 50O-60O0C, at atmospheric pressure, with residence times ranging from 0.5 to 25 sec, in a seasoned quartz reactor (S/V = 6 cm-l) described previously [2]. The reactor walls were cautiously seasoned with a carbonaceous film obtained by pyrolyzing an ethyl chloride flow during about two days at 60OoC.…”