[CONTRIBUTION FROM S H E L L
DEVELOPMENT COMPANY]
THE HIGH-TEMPERATURE CHLORINATION OF PARAFFIN HYDROCARBONS WILLIAM E. VAUGHAN
AND
FREDERICK F. RUST
Received April 29, 1940
Since the discovery of the photochemical chlorination of methane in 1840 by Dumas ( 6 ) , a vast amount of study has been given the subject of the halogenation of hydrocarbons. Extensive reviews2 make unnecessary any broad recapitulation at the present time. The following study3 is an extension of the very considerable amount of research done previously in these laboratories on halogenation processes and was initiated with the object of better correlating and explaining existing data. A number of articles bearing importantly on this and its companion paper have recently appeared and reference to these will be made at the appropriate points. Unusually interesting has been the finding of Groll and Hearne that, contrary to the implications of “classical” organic chemistry, under certain conditions propylene can be readily chlorinated substitutively rather than additively (10, 11, 12). Thus, at temperatures ranging from 400’ t o 600°, by an apparently homogeneous reaction, the halogen in a 1:7 mixture with the olefin is utilized almost entirely by substitution, and allyl chloride is the principal product. This can be employed as a useful intermediate in syntheses, for example, that of glycerol (27), and consequently the process has far-reaching implications. This seemingly paradoxical reaction, wherein chlorine reacts with a saturated group in direct preference to an ethylenic linkage, was deemed to merit further study. While an absolute delimitation cannot be made because of the interrelationships and interdependencies of the reactions involved, in this paper an attempt will be made to deal primarily with the gas-phase chlorination of paraffins (and of the chlorides derived therefrom), while the succeeding one will treat the high-temperature reactions of chlorine with olefins. This paper was presented a t the 99th Meeting of the American Chemical Society, Cincinnati, Ohio, April 8-12, 1940. 2 See, for example, (7), (81,and (9). * A number of points revealed during the work are now subjects of several patent applications. 449
450
W. E. VAUGHAN AND F. F. RUST
The subject of the chlorination reactions of both saturates and unsaturates in liquid phase and in the presence of a liquid film,especially in regard t o substitution, has been discussed at length by previous workers (see, for example, 3, 13, 22, 23). In passing brief consideration of these researches, it may be noted that the extremely fast liquid-phase comPROPOSED MECHANISM O F HIQH-TEMPERATURE PARAFFIN HYDROCARBON CHLORINATION Cn&n+l
+ Clr
1
Initiation of reaction chains on the surface. (presumably are unaffected by oxygen). Gas hase radical chains. (are susceptibfe to interruption by oxygen). Excited molecules of monochloride result from the chains. These are capable of several modes of reaction.
Excited molecules are deactivated by collision with the wall, inert gases, and hydrocarbons.
1
Excited molecules plus alkyl halide give induced decomposition of the halide.
fC1
Monochlorides can enter into radical chain processes with C1; suppressible by oxygen. Polychloride formation by this manner secondary to formation from excited molecules, a t least for ethane.
Excited molecules plus chlorine give polychlorides.
Excited molecules initiate bimolecular thermal chains resulting in monochloride formation by supplying activation energy to the hydrocarbon. Not affected by oxygen.
bination of isobutene with chlorine to give methallyl chloride and hydrogen chloride is not inhibited by oxygen (3), while the relatively slow substitution into paraffins or other saturates is markedly suppressed. Addition, which in solution could readily occur as an association process, and presumably does to a considerable extent, is likewise seemingly little affected. However, so-called “induced substitution” into paraffins or
CHLORINATION OF PARAFFIN HYDROCARBONS
45 1
saturated chlorides concurrently present with an olefin participating in addition reaction, is effectively diminished by oxygen, indicating that the process involves free radicals or atoms initiated in some manner by the interaction of chlorine with the ethylenic linkage. These observations indicate that chlorination reactions take place in a diversity of ways, such as by chain mechanisms involving atoms and radicals, bimolecular metatheses, and association processes involving a third body. The same complexity is found in the vapor-phase experiments to be described. Attention has thus far been focused on only the simplest compounds, to reduce analytical difficulties to a reasonable degree. Even with this simplification, it has been possible to gain an increased insight into the mechanisms of the several reactions. MATERIALS AND TECHNIQUE
A flow system has been utilized, as i t permits greater flexibility in variation of conditions and in obtaining samples of product for analysis. Against these advantages, however, is the objection that the rate dependencies are somewhat less certain, especially when the amount of reaction is high. Despite this difficulty some kinetic functions may be given with certainty. The ethane, ethylene, and propane were the same samples used previously (13). The ethyl chloride was a commercial product, which analysis showed to be better than 99.8% pure. Eaatman n-propyl and isopropyl chlorides and ethyl bromide were carefully refractionated t o give pure materials. Only chlorine purified by the distillation process described in the earlier paper was employed; i t is felt that this material contained at most only a very low percentage of oxygen (
