138
INDUSTRIAL A N D ENGINEERING CHEMISTRY
acts also to terminate chains. The termination of a chain m y occur in several ways: the collision of a radical or atom with a mlid surface such as the wall, collision of a radical or atom with oxygen, and the combination of radicals and atom. Rewntly (6t) it wag shown that very smsll amounts of tetraethyllead, which presumably yields ethyl radicala when heated, accelerate gas and liquid-phase chlorinations enormously. Also, hexsphenylethane, which yields tripheuyhethyl readily, catalyzes liquid-phase chlorination, and momethane acta as a catalyst in the vapor phase. These d t a constitute additional evidence in support of the chain mechunism. The formation of olefins and chloro-olehs during the chlorination of many saturated compounda at elevated temperatures is a fact well establiabed. Such side reactions cannot be explained as simple pyrolysis of chlorinated produ& because with comparable experiments much higher temperatum are usudly required to obtain the m e degree of ole& formation in the absence of free chlorine. It seems likely that at least some of this secondary reaction occnra, as by ‘‘iiduced” decomposition (63). A newly formed energyrich alkyl chloride may lose energy by losing hydrogen chloride or by collision with another alkyl chloride which, by mamu of ita acquired energy, undergoes disruption. a t Bath chl&tiea.
One of the intemting recent developments in the field of ohlorination is the technique of using a molten salt bath in direct contact with the reagents. Low-melting mixtuh 6f substancea such as scdium, calcium, and aluminum chlorides
Vol. 33, No. 2
are maintained at a suitable temperature, and the chlorine and the material to he chlorinated are bubbled through. The agitation thus produced in the bath fseilitates good control of the temperature which, in these extremely exothermic reactions, is higbly desirable. Since metallic halides frequently serve as catalysts for the pyrolysis of organic chlw rides, salt bath cblorinatiom may be controlled to yield olefins or chlorwle6na as well as saturated substitution products. The fission of carbon-carbon bonds to yield products of fewer carbon atoms than the starting material has also been repohd to occur under these conditions. Grebe,M y , and Miley (16)patanted a prows for preparing carbon chlorides which consists ementially in passing a mixture of chlorine and a saturated aliphatic hydmosrbon or its p&y chlorinated derivative into molten metal chlorides maintained at a temperature above 250‘ C. The following equations were given to illustratS the reaction: ca‘cl, CaCL C,“I
+ 3cI*-C*CL + 4HCl + BCIS-GCL + CCL + 6HC1 + 8CL C.CL + CCL + 8HCI
-
(a) (4) (6)
More recently, this work was extended to chlorinations in which the rewtion products contsin hydrogen in addition to carbon and chloriqe. Reilly disclosed the preparation of l,l,%ttichloroethane (61) and the chlorination of acetylene (SS), ethylene chloride (60), ethane (@), and ben5ene (68). In the preparation of l,l,%trichlomethane, ethylene chloride is chlorinafed in a molten salt bath at 300‘ to 426O C. If a higher bath temperature is employed, the principal prcduota are di- and trichloroethylene. Acetylene is mixed with a nonflammable chlbrinated hydrocarbon such 88 emhn
tetraahloride and chlorinated under the surface of a molten metsl chloride at 176-250’ C. to yield tetracblorcethylene 88 the principal product. For the production of ethyl chloride and vinyl chloride, etbane and chlorine are passed into a mdten bath at 250-460’ C. Essentially the eame technique h used to prepare chlorobenzene from benzene. Chlorinationof Olefull
The chlorination of oMm haa been the subject of much study. GmU, Hearne, Rust, and Vaughan (MYextended the work of DeansleJr (0) on the chlorination of olefin-pr&n &urea. “Induced” suWtution is ohserved when ah oleh is chlorinated in the liquid p6aee. This Muced substitution w m on the dichloride addition product or COpKeent Bstnrated hydrooarbons. In the chlorination of ethylene in the liquid p b ” , no vinyl chloride w a ~obtained. Also, only rnnall amounts of unsaturated mondoridea rem& in the liquid-phase chlorination of propene and butene. Hence, it appesrs that if substitution into ole6na ocuua by induction, the &ect is not very pronounced. It haa been shown that addition and concurrent substitution ocmr in the dark kt tempersturee ranging from -10 to +1W C., provided a liquid p b ” is p-t, but no gakphase thermal renction o c m over & m e tempratwe range. Approximstely 50 per mt of the totalchlorine reacted with a pmpane-pmpene mixture at 22. C. by induced suhetitution, either with or without illumination. One per mt of oxygen mtly reduced the amount of chlorine reacting by sub ntitution. Tbeae re6uIta may be interpreted by that induced substitution is a chain reaction mitirrted by irradiation or the addition reaction of chlorine and olefins and that the l a t h occurs most readily in a liquid medium. This phenomenon has been considered to rasult from apeajsc moleah transfer of the lelstively high heat of formation of dichlorides (67, 68). Such a tran6fer of energy may ( ~ 1 8 8 the dimciation of a chlorine molecule and thus initiate a ohain reaction wbich yields substitution products. There is considerable evidence to indicate that both addition and substitution reactions of olefins with chlorine OCC~V, at leaat in part, particularly at high t e m p e r a h by obain meohanisma. Therefore,it is partimhrly interesting to note that the substitutive chlorination of olemns is cataly5ed by low concentrations of oxygen (64). This has been explaid by amuming tbat radio& me produced by the interaction of ole6n and oxygen and that the ethylenic radicals W6r from alkyl radi& in being less readily*aliminated by oxygem. In the cow% of their careful work on the mechanism of the chlorination of olefin hydmosrbons, Rust and Vmgbm found that olemns inhibit bigh-temperature chlorination m actione. Such inbibition of reactiom by oleha in which ohsinllwhamm * m believed to occur have been noted before,but the true maahslusm ’ of what OWUI‘B is not kuown. Burgin, Engs, GroU, and Hearne (4) studied the influenoe of many factors on the chlorination of olefins contsining an unesturated tartimy carbu atom. The foUowing reactionsocrmr nonnally during the chlorination of isobutene:
a
”I
