THE HIGH-TEMPERATURE CHLORINATION OF OLEFIN

[CONTRIBUTION FROM SHELL DEVELOPMENT COMPANY]

THE HIGH-TEMPERATURE CHLORINATION OF OLEFIN HYDROCARBONS' FREDERICK F. RUST

AND

WILLIAM E. VAUGHAN

Received April 99, 1940

Although the reactions of olefins with chlorine in the liquid phase [see, for example, (2, 7, B)] and in the gas phase at comparatively low temperatures (3, 11) have been extensively studied, the investigations have not until recently been extended to higher temperatures. The recent work of Groll and Hearne (10) has shown that straight chain olefins will readily react with chlorine at elevated temperatures to form allylic chlorides. The present study, undertaken in an attempt t o provide a better understanding of the mechanisms of reactions above 200°, falls into three distinct divisions, namely, I. Olefin-chlorine reactions-a study of the mechanisms and conditions determining substitution and addition. 11. Catalysis of chlorine substitution reactions by oxygen. 111. Olefin inhibition of chlorination. The companion paper (20), to which frequent reference will be made, dealt with the high-temperature chlorination of the lower paraffin hydrocarbons. MATERIALS AND TECHNIQUE

The ethylene and vinyl chloride were commercial samples, which analysis showed to be 99.5% and 99-100% pure, respectively. The propylene, 2-butene and isobutylene were refinery products; their compositions were as follows: Propylene.. .................... 98.(4)% CaHs; 1.(6) C4Ha; (100% olefin). 97.(5)% 2-CdHs; (98+% olefin). 2-Butene.. ..................... Isobutylene.. . . . . . . . . . . . . . . . . . . . 99.(4)% iso-C4Hs. The chlorine was the specially prepared material of very low oxygen content (20). The same technique and apparatus were employed in this research as in the preceding (20). All materials which are normally gaseous, except of course the chlorine, were freed of oxygen by means of chromous chloride or sulfate solutions. The compounds normally liquid were vaporized at a fixed temperature from saturating 1 This paper was presented a t the 99th Meeting of the American Chemical Society, Cincinnati, Ohio, April 8-12, 1940. 472

CHLORINATION OF OLEFIN HYDROCARBONS

473

devices by means of a flow of oxygen-free nitrogen or carbon dioxide; after a period of sweeping t o remove dissolved oxygen, the flows were diverted to the reactor. The reactor, 45 cm. long X 1.35 cm. diameter, was equipped with two preheating coils above the mixing jet and both tube and coils were housed in a massive aluminum core in a vertical furnace. Three separate heating units permitted manual regulation of the furnace temperature to a constancy of '1 over the entire length. The uniformity of temperature within the reactor itself is illustrated by the following test. The position of a thermocouple in a thin-walled glass well in the center of the reactor was varied from the extreme top, next to the mixing jet, to the bottom, near the delivery capillary. Although 65% of the chlorine in a flow containing 50 cc. per minute of halogen was reacting, a maximum temperature variation of only '1 was noted. Of course, under certain conditions, inflammation does occur with consequent severe temperature gradients. This is always accompanied by carbon and t a r formation and the appearance of smoke. When such uncontrolled reaction was observed, an experience somewhat more common with olefins than with paraffins, the experiment was discontinued and the reactor cleaned with hot sulfuric-nitric acid mixture. The analysis of the products of olefin chlorination is a serious problem, especially when a considerable amount of unreacted halogen remains in the effluent gases. There is the possibility of a catalyzed reaction between chlorine and hydrocarbon occurring in the potassium iodide or potassium hydroxide solution used to absorb the unreacted halogen and the acid produced by substitution. If unsatisfactory absorbing solutions are employed, the error thus introduced may amount t o as much as 30% of the chlorine. This problem has been discussed a t length in a previous paper (11). In the following experiments, titration analyses for the amounts of addition and substitution were made in conformity with earlier experience, and were often supplemented by collection and distillation of the products. I. MECHANISMS OF THE OLEFIN-CHLORINE REACTIONS

Ethylene. a) Reactions Inhibited by Oxygen-Chain Reactions. It is considered well established that many chlorination reactions occur via a chain mechanism. As is often characteristic of such processes, exact control is sometimes very d s c u l t , especially when high concentrations of reactants are present. This has been noted by many workers. Similar observations have been made in the present case, and it has been found that only by diluting the reactants with an inert gas can a systematic study by dynamic methods be made. For example, with rising temperature, undiluted 1:1 ethylene-chlorine mixtures inflamed almost with the inception of reaction at approximately 215'. With small amounts of nitrogen, the temperature of the onset of uncontrolled reaction was raised somewhat. With larger quantities of diluent, control can be maintained to relatively high temperatures. This is shown by Figures 1, 2, and 3 which are temperature profiles for the following mixtures, for which the concentration of the diluent, nitro-

474

F. F. RUST AND W. E. VAUGHAN

gen, was varied while holding constant the ratio of the reactants and the total throughput: Flows i n cc./min. GI2

C2H4

Diluent

Totsl

35 50 75

35 50 75

230 200

300 300 300

150

Below ca. 235' no change occurs. The addition-reaction then sets in prior to the substitution and rises gradually with the temperature. However, due to the onset of substitutive processes and their higher temperature coefficient, the amount of addition reaches the maximum and then falls

TEMPERATURE, F.

FIG.1. CHLORINATION OF ETHYLENE. TEMPERATURE PROFILEB Flow (cc./min.) : 35 C1,; 35 CSHI;230 Nz

towards zero. At higher temperatures the amount of hydrogen chloride produced is in excess of the stoichiometric quantity expected from substitution. At 390' in Figure 2, at the point denoted by the arrow, smoking and charring occurred. In Table I are given some pertinent data correlating the several mixtures. At a given temperature (275' in Table I),the fraction of the reacted halogen which goes to addition progressively decreases as the concentration of the reactants increases. This means that while both reactions are highly dependent upon the partial pressures of olefin and chlorine, the substitution is of a higher order than the addition and rapidly becomes an important process of change. Curves practically identical with those of Figure 3 were obtained when nitrogen or helium rather than carbon dioxide was used as the diluent; the differences are probably not significant. In order that the significance of the curves might be more clearly understood, the actual composition of the product at two different temperatures


475

has been determined. The data in Table 11, which clearly shows the effect of temperature on the distribution of products, correspond to points on the curves of Figure 2. The agreement of the distillation analyses with those by titration is very satisfactory. However, it is generally better to have both bits of complementary information. At the lower temperature,

I

TOTAL REACTION ADDITION

A

SUBSTITUTION

0

TEMPERATURE, %.

FIG. 2. CHLORINATION OF ETHYLENE.TEMPERATURE PROFILES Flow (cc./min.) : 50 C11; 50 CzHd; 200 Nz

TEIPERATURE, *C.

FIG. 3. CHLORINATION OF ETHYLENE. TEMPERATURE PROFILES Flow (cc./min.): 75 Clz; 75 C2H&;150 CO1

308O, the total amount of addition is much greater than that of substitution; conversely, at the higher, 346', the substitutive steps are the dominant ones. The mole percentages of trichlorides and tetrachlorides are relatively constant, and the principal variations are in the amounts of unsaturates and the simple addition-product. The formation of higher chlorides from vinyl chloride has an important bearing on the understanding of the reaction; this point will be discussed later.

476


In conjunction with these product distributions, it is interesting to note that at higher temperatures (say at 485", as denoted by the arrow in Figure l), where extensive decomposition ("excess HC1") occurs, acetylene is TABLE I CHLORINATION OF ETHYLENE

CL _

_

CIH~ NrorCOs _ ~

Total

Total % CL

35 50 100 75

300 300 300" 300

22 33 57 58

% of resoted Clr added

rescted

TEMP. OF MAHYUY AMOUNT OF ADDITION

'C.

35 50 50 75 E

230 200 150 150

20

91 88 86 74

29 49 43

315 310 288 275

Data for this mixture taken from Figure 13 of (20). TABLE I1 CHLORINATION OF ETHYLENE. ANALYSES OF PRODUCT Flow (cc./min.) : 50 C12; 50 CtHc; 200 NI Mom % OF CHLOBINATID PBODUCT 808'

Vinyl chloride. .................................. 1,l- and 1,2-Dichloroethylene. . . . . . . . . . . . . . . . . . . 1,2-Dichloroethane. ............................. l11,2-Trichloroethane............................ Tetrachloroethanes (est.) ........................

20.7 4.6 64.0 10.3 0.4

52.7 14.8 20.2 9.9 2.4

% of total Clt reacted (see Figure 2). . . . . . . . . . . . .

57

77

By distillation analysis: % total Clz added.. . . . . . . . . . . . . . . . . . . . . . . . . . . . % total Clz substituted. . . . . . . . . . . . . . . . . . . . . . . .

37 20

25 57

By titration analysis: % total Clr added.. . . . . . . . . . . . . . . . . . . . . . . . . . . . % total Clt substituted. . . . . . . . . . . . . . . . . . . . . . . .

37 20

23 54

found in the gases; this undoubtedly results from a splitting of hydrogen chloride from vinyl chloride. In the attempt t o clarify the mechanism of this gas-phase additionreaction, the dependency of the rate on the concentrations of the reactants was studied. The same procedure described previously (20) was used.


477

Although the experiments were conducted at lower temperatures where substitution is relatively unimportant, the data obtained lacked satisfactory precision. This is not surprising, since the addition as determined is a difference involving three experimental values (chlorine input, amount unreacted, and amount substituted). Variation of the halogen input at 268' indicated a rate proportional to the first power of the chlorine concentration. At the same temperature the velocity appeared to vary as the square root of the mole-fraction of olefin. While this function gave an approximate representation of the Sndings and more or less definitely ruled out a linear dependency, the precision of the data leaves something to be desired. I

A GLASS

TU8ES;SURFACE 800 FR!

x GLASS WOOL.

TEMPERATURE, 'C

Fro. 4. CHLORINATION OF ETHYLENE.EFFECTOF SURFACE Flow (cc./min.) : 50 Cls; 100 C2H,;150 COS

Norrish (15, 16) and Williams (22) found that the addition of bromine to ethylene a t room temperature in the absence of a liquid film of product was highly dependent on the amount and character of the surface and that the rate was of difficultly reproducible order. They indicate that a dependence on the first powers of both reactant concentrations is probably as good a decision as can be made. Williams ascribes the trouble to surface variation, which would probably be an even more serious factor at the lower temperature at which he worked than in the present study. In this connection, Figure 4 shows clearly the pronounced influence of surface, in the light of which a study of exact rate dependences seems rather futile. At low temperature, where only addition occurs, increased surface causes an increase in the amount of reaction, probably as a result of catalyzed bimolecular association as well as initiation of chains. Glass wool is particularly effective. A t higher temperatures, surface suppresses

478


reaction, presumably as a consequence of termination of chains initiated in the gas phase. The chains involve both addition and substitution at these temperatures. It is seen that there is a rough parallelism for all three curves for packed systems. Glass wool packing, even at the higher temperatures, accelerates the amount of reaction a t the surface [compare Figure 4 with Figure 7 of (20)]. The superiority of quartz over glass as a material for the reactor is noteworthy, as with the former the formation of tar and carbon occurs to a smaller extent, although slightly higher temperatures are necessary to obtain an equivalent amount of reaction. Hearne and La France (12) have made similar observations for the chlorination of propylene. The powerful inhibiting effect of oxygen on numerous halogenations has been considered strong evidence that the reactions proceed by chain mechanisms involving radicals. In this work when even a very concenTABLE I11 CHLORINATION OF ETHYLENE. CATALYSIS BY TETRAETHYL LEAD Flow (cc./min.) : 50 Chlorine; 150 Nitrogen; 100 Ethylene through Tetraethyl Lead at 0' PER CENT OF CHLORINl REACTINQ

I

loo"

Without PbEt4. . , . , . . . . . , . . . . . With PbEt4. . . . . . . . . . . . . . . , . . . .

132'

Substitution

Addition

Substitution

Addition

0 0

2.6 16.4

0 0

9.1 23.3

trated mixture (137.5 cc. Clz/min.; 137.5 CzHd; 25 0,) was flowed through the reactor, no reaction occurred to as high a temperature as 288' (contrast with 227" for 8% Nz). At this temperature the mixture ignited, as much as 1.67 moles of hydrogen chloride being formed for every mole of chlorine used. If at any temperature below 288" the oxygen flow was stopped, ignition immediately occurred. When 3% oxygen was used (145 Clz; 145 C Z H ~10; OZ),the ignition temperature was 240"; the observations were otherwise the same. The fact that controlled inhibition by oxygen does not persist to as high temperature with olehs as with paraffins (20), will be explained under oxygen catalysis in Section 11. The chain character of the gas-phase reactions of addition and substitution into olefins under certain conditions is further confirmed by experiments wherein catalysis of the processes was obtained by use of tetraethyl lead. The great efficiency of this material in promoting the chlorination of saturates has been discussed at some length (20). The effect is attributed to the formation of radicals by the interaction of chlorine and the

479


lead alkyl, the radicals initiating a chain reaction involving the halogen and hydrocarbon. It has now been found that in like manner chlorine and olefins can be made to react at temperatures considerably below those normally needed. Table 111, which is self-explanatory, shows how the lead alkyl promotes the addition-reaction for ethylene. (The small amounts of reaction in the absence of tetraethyl lead vapor may be a consequence of reaction catalyzed by the lead chloride deposited on the walls.) More striking are the results for the chlorination of propylene given in Table IV. Although these data are also subject to the criticism of the possibility of surface-catalyzed reaction due to lead chloride (as with ethylene), nevertheless the enhancement of reaction due to tetraethyl lead is very definite. The increased substitution becomes even more significant on the basis of a distillation analysis of product made at 186O, TABLE IV CHLORINATION OF PROPYLENE. CATALYSIS BY TETRAETHYL LEAD Flow (cc./min.): 50 Chlorine; 100 Propylene; 50 Nitrogen; 100 Carbon Dioxide through Tetraethyl Lead at 0" % CHLORINE BEACTINQ TEMPERATUBE

In prmenoe of lead alkyl

I

In absence of lead alkyl

Substitution

Addition

Substitution

Addition

25.3 28.2 36.1

51 .O 51.8 47.9

2.7 3.2 6.0

34.0 34.8 37.8

OC.

132-134 156-159 196

which shows 25 mole-per cent allyl chloride and 75 mole-per cent dichloride. Even though the titration analysis indicated a somewhat larger yield of allyl chloride, it is noteworthy that we have produced the compound at much lower temperatures than has been heretofore considered possible (10). b) Reactions Unaffected by Oxygen. The ability of oxygen to suppress chains involving radicals has been used to detect other reaction processes. These are as follows: Association a t the surface. Figure 5 illustrates the effect of oxygen on the reacting system in the presence of glass wool. For such flows (50 cc. Clz/min.; 100 CzHr; 150 COZ) in an unpacked reactor, 5 cc. of oxygen per minute completely inhibited all reaction to at least 284". Glass wool, however, strongly promotes the addition even in the presence of oxygen. The suppression, which is apparently independent of the amount of inhibitor, at least in the six-fold range investigated, presumably corresponds to nearly the total amount of true gas-phase radical chain reactions, both

480


addition and substitution. The magnitude of the persisting reaction is indicative of the surface catalyzed addition. Gas-phase bimolecular association. A t higher temperatures (above 300"), another addition-reaction which is unaffected by oxygen occurs.

x 2Scc Os PER MIN.

TEMPERATURE, *a

FIG. 5. CHLORINATION OF ETHYLENE OVER GLASS WOOL. EFFECT OF OXYQEN Flow (cc./min.): 50 ClZ;100 C a d ; 150 (GOz 02)

+

I6

+

FIG.6. CHLORINATION OF ETHYLENE OVER GLASS RODPACKING.UNSUPPRESSIBLBI ADDITION Flow (cc./min.): 25 C11; 50 CzHa; 200 Nz; 25 On

Its temperature profile is shown by Figure 6. The negative temperature coefficient is not due to onset of a dominant substitutive reaction; under the conditions of the experiments, even at the highest temperature, substitution never amounted to more than 10% of the total chlorine. In


481

this range of temperature, packing has relatively little effect (Figure 20). The major amount of the addition occurring can logically be interpreted as a straightforward, gas-phase, bimolecular association, a type of reaction which can occur when the intermediate complex is sufficiently large to have a long enough life to permit stabilization by collision with a third body. Gas-phase bimolecular metathesis. The existence of a gas-phase bimolecular substitution of chlorine into olefins, unaffected by oxygen, can only be postulated by analogy with the findings on the chlorination of paraffins (20). Sherman, Quimby, and Sutherland (18) have made a rather exhaustive theoretical treatment of the possible reactions between ethylene and the halogens. They have calculated by the Eyring method the activation energies of the several bimolecular reactions. They have also assumed chain mechanisms which would lead to the same products, and estimated the energies involved. In almost all cases they find that free radical chain mechanisms are slightly more favorable than the corresponding bimolecular process. They also show that addition is likely to be dominant over substitution. In the addition of chlorine to ethylene, they find the exception that the molecular process is the more probable. The chain assumed involves the steps: 1. Cl2 Tf 2C1; 2.

C&"4

+ C1+

C2HaC1; 3. GH&1

+ Cl2 + GHL% + C1,

which do not lead to the present finding of rate dependence. The marked effect of surface found experimentally indicates that the course of the production of the l,2-dichloroethane is not the result of a single, simple process, but the facts given herein do show that chain addition is dominant. The substitution chain given by Sherman, Quimby, and Sutherland contains the straightforward chain-carrying steps:

1. C&"a

+ C1+

CzHs

+ HC1;

2. GHs

+ Cl2 + CzHaCl + C1.

From this they deduce that the chain process is very slightly the more favorable, which is in accord with experiment. At higher temperatures, however, molecular mechanisms undoubtedly are operative for both addition and substitution. Other Olefins. The temperature profiles for propylene, 2-butene, and isobutylene also have been determined; these are given in Figures 7, 8, and 9. The trend of the curves is the same as for ethylene and from this it may reasonably be inferred that the reactions are occurring in the manner outlined above. Control of the reactions BO as to eliminate igni-

482


tion was much more difficult, and for this reason the curves were not extended to higher temperatures. With propylene, the products are principally the addition-compound and allyl chloride (10). Isobutylene at higher temperatures also reacts by

F 30..

TEMPERATURE, .C.

FIa. 7. CHLORINATION OF PROPYLENE.TEMPERATURE PROFILES Flow (cc./min.): 50 C11; 100 CsHo; 150 Nz

1

70

0

I

TOTAL REACTION ADDITION

/

A SUBSTITUTION

TEMPERATURE, F.

FIG. 8.

CHLORINATION OF BUTENE-8. TEMPERATURE PROFILES Flow (cc./min.) : 50 C11; 100 C I H ~150 ; N,

both addition and substitution (Figure 9). Below 240°, above whic, the reaction became violent, 5% of oxygen completely suppressed all activity, indicating that both reactions occur by radical chain mechanisms. This is quite contrary to the behavior of isobutylene in liquid phase, where only substitution unaffected by oxygen occurs. It has been found (2) that light accelerates the vapor-phase addition. Admixture of oxygen to such


483

an illuminated system suppresses the photo process which is further evidence of the presence of radicals.

90

-

o TOTAL REACTION I

ADDITION

A SUBSTITUTION

70 60

8 I 2 4

200

220

240

26C

TEMPERATURE, 'C.

FIG. 9. CHLORINATION OF ISOBUTYLENE. TEMPERATURE PROFILES Flow (cc./min.): 50 Clz; 100 CdHs; 150 NI

FIG.10. CHLORINATION OF VINYLCHLORIDE.TEMPERATURE PROFILES Flow (cc./min.) : 50 (211; 100 CZHsC1; 150 NZ

As mentioned earlier (Table II), appreciable quantities of isomeric dichloroethylenes are found in the product from the high-temperature chlorination of ethylene. These secondary products have also been ob-

484


tained by chlorination of vinyl chloride. Figure 10 illustrates the temperature profiles for the addition- and substitutionreactions of this latter compound. Addition to give 1,1,2-trichloroethane and substitution to give the dichloroethylenes occur simultaneously. Thus, it seems unlikely that the latter compounds are formed by pyrolytic decomposition of the former. 11. OXYQEN-CATALYZED CHLORINE SUBSTITUTION-REACTIONS

Chlorination of Olejin-Oxygen Mixtures. The inhibiting effect of oxygen on gas-phase chlorination-reactions has long been known (l),and workers in the field have usually taken particular care to avoid possible complicaTABLE V CHLORINATION OF ETHYLENE.CATALYSIS BY OXYQEN Flow (cc./min.): 50 Clg; 100 TDMP.

I

'C.

245 245

+ 3%

0 2

35.2 0.0

2.0 0.0

33.2 0.0

264 264

+ 3%

66.6 93.4

11.5 93.4

55.1

On

272 272 272 272

0 2

67.6 95.2 69.2 95.5

12.0 95.2

55.6

4- 3%

17.1

52.1

+ 3%

0 2

95.5

?a

?a

?a

4 Approximately 20% more HC1 evolved than corresponds to the amount of substitution on the baais of the chlorine reacted.

tions from this source. Consequently, the discovery that, under certain conditions, oxygen strongly catalyzes substitution of chlorine into olefins was quite unexpected. So powerful is the effect that, for example, in one case with ethylene, as little as 0.5% of oxygen in the gas flow increased the amount of substitution from 10% to 93% of the chlorine. The catalysis for ethylene and propylene is clearly shown in Tables V and VI, and even more strikingly by Figures 11-14, where the results with more dilute reaction-mixtures and varying oxygen inputs are presented graphically. Only small amounts of oxygen induce catalysis and in fact larger amounts cause the expected inhibition.2 Experimental conditions,

* In this connection, i t may be mentioned that Willard and Daniels (21) observed acceleration of addition followed by inhibition by larger amounts of oxygen in the liquid-phase photobromination of tetrachloroethylene.

485


especially temperature, are very important in the definition of the magnitude of the effect. In general, it may be said that the catalysis seems to be much more pronounced, although more critically dependent on the concentration of the catalyst, with ethylene. For example, in Figure 11, TABLE VI CHLORINATION OF PROPYLENE.CATALYSIS BY OXYQEN Flow (cc./min.): 50 Clr; 100 CsHe; 150 NI TIDIIP.

oc. 261 263

274 272 286 286

+ 3%

0 2

-t3% 0%

+ 3% 0%

% Clt BmACTBD

% CL B W B .

% c h ADDmD

62.1 0.0

31 .O 0.0

33.1 0.0

90.8 76.4

90.8 45.2

0.0 31.2

89.6 83.1

88.6 55.0

1 .o 28.1

0

TOTAL REACTION

=

ADDITION

0

SUBSTITUTION

c c . 0 /MIN. ~

FIG. 11. CHLORINATION OF ETHYLENE AT 324°C. EFFECTOF OXYGEN Flow (cc./min.): 25 C11; 50 CIHI; 225(N, 02)

+

2 cc. of oxygen per minute in the flow causes an increase in the amount

of substitution from 13% of the total chlorine to 87%, while addition drops from 25% to 4%, presumably because of the faster rate of the competing substitution. Increase in the oxygen input to 4 cc. per minute causes a drop in the latter reaction to only 7% of the total chlorine and

486

F. F. RUST AND W . E. VAUGHAN

addition undergoes a slight rise to ca. 15%. Further oxygen results in an asymptotic leveling-off of addition at 4% and substitution falls toward zero. With propylene chlorination, the positive catalysis is less, amount-

o TOTAL REACTION

*

ADDITION

b

SUBSTITUTION

3 L

ffi&lMIN.

FIG. 12. CHLORINATION OF ETHYLENE AT 334°C. EFFECT OF OXYGEN Flow (cc./min.): 25 Cla; 50 CEH,;225 (N1 01)

+

d

: L L x

IO

ing at the maximum t o only 11% more than the 36% found for substitution in the absence of oxygen (Figure 13). Further oxygen causes substitution to decrease regularly t o IS%, and addition to 8%. The chlorination of %butene is also subject to positive catalysis by oxygen. Below 260°, reaction was strongly inhibited by 3% oxygen (in


487

50 cc. Cl*/min.; 100 2-CdHs; 150 N1), but at this temperature and above, chlorination was catalyzed. Without oxygen, 76.4% of the chlorine reacted, 56.4% by substitution and 20.0% by addition. With 3% oxygen,

0

TOTAL REACTION

I ADDITION

60

A SOBSTlTUTlON

-

a 2 4030 -

20

cc.OdMIN.

FIG. 14. CHLORINATION OF PROPYLENE AT 315°C. EFFECTOF OXYGEN Flow (cc./min.) : 25 Clz; 50 CSHS;225 (Na 0 2 )

+

TABLE VI1 OXYGEN-CATALYZED CHLORINATION OF ETHYLENE AT 324" Flow (cc./min.): 25 Clz; 50 C2Hh; 225 NI; 1.5 01 d e psr

csnt

Vinyl chloride.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ethylene dichloride. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Unsaturated dichlorides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61 26 13

OXYGEN-CATALYZED CHLORINATION OF PROPYLENE AT 300" Flow (cc./min.): 50 Cla; 100 CsHs; 150 Nz; 1.5 0 2 d

e

w 7 63 17 13

cent

Mixed allyl and vinyl type chlorides., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Allyl chloride. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Propylene dichloride. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Unsaturated dichlorides, and polychlorides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

97.2% of the chlorine was consumed, so violently that attendant decomposition made impossible the determination of the relative amounts of the two reactions. Product distributions for the oxygen-catalyzed chlorinations of ethylene and propylene are given in Table VII. With the ethylene mixture in the

488


absence of oxygen, addition, according to titration analysis, amounted to 25% and substitution to 13%; with oxygen the corresponding figures were 4% and 88% of the halogen. Oxygen in the propylene reaction-mixture increased substitution from 55 to 7370, and apparently reduced addition from 25% to 15% of the chlorine. The significant finding is that of the increased yields of unsaturated chlorides, especially with ethylene. This catalysis of substitution by oxygen is confined to olefins. Despite extensive tests over a considerable range of operating conditions, no similar effect for paraffins was found; only inhibition of chlorination was observed. Chlorination of OleJin-Oxygen-Parafin Mixtures. Ethane is a powerful inhibitor of oxygen-catalyzed chlorine substitution into olefins. This paraffin in a mole concentration of ca. 15Y0suppressed substitution in one case from 95% to 13%. The effect of larger concentrations of ethane is TABLE VI11 CHLORINATION OF ETHANE-ETHYLENE IN THE PRESENCE OF OXYGEN AT 327". EFFECT OF ETHANE CONCENTRATION Flow (cc./min.): 25 CIS; 50 C3H4; 220 (NI GHs); 7.5 air

+

cc. CiE6/Um.

% Clr BBPACTID

0 50 100 160

95.2 14.8 16.8 18.6

% c h SVB-D

95.2 12.8 14.8 15.0

also shown by Table VIII. It is to be noted that beyond a certain value, reaction is almost independent of the amount of paraffin. With a reaction-mixture corresponding to the following flow (cc./min.) : 50 Cla; 100 C&L; 100 GHs; 7.5air (1.50,);45 COZ,it was necessary to go to 336' to obtain an amount of reaction equivalent to that obtainable at ca. 265' in the absence of paraffin. By titration analysis, substitution amounted to 98.4%. The product distribution, Table IX, is striking in that it indicates that the ethylene was participating only slightly in the reaction. Yet if the olefin is replaced with nitrogen, the oxygen inhibits the reaction of chlorine and ethane to reduce substitution below 10% (see Figure 4 of Reference 20). This interesting result is further demonstrated by the following data for a mixture corresponding to flows of 50 cc./min. chlorine, 100 ethylene, 100 ethane, and 50 oxygen. With this greatly increased amount of oxygen the reaction was remarkably easy to control, and substitution rose from 10.5% at 343' to 72.0% at 404'. Illustrative of the effect of the olefin is

CHLORINATION OF OLEPIN #YDROCARBONS

489

an experiment made at 374" with the above mixture in which 60% of the halogen reacted; when the olefin was replaced by carbon dioxide, reaction dropped to only 4%. Despite the fact that chlorination under these conditions is dependent upon the presence of olefin, analysis of the product made at 414" shows that ethyl chloride was the principal constituent. There was very little increase in the proportion of vinyl chloride formed when the oxygen input was increased from 1.5 cc. per minute (see Table TABLE IX CHLORINATION OF ETHANE-ETHYLENE I N PRESENCE OF OXYGEN AT 336" Flow (cc./min.): 50 C12; 100 CZ"; 100 C2Hs;7.5 air; 45 COI mal4

w

Vinyl chloride.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ethyl chloride.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,l-Dichloroethane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . l,l,l-Trichloroethane and 1,PDichloroethane. ...............................

csnl

12

60 16 12

I

/ / /

3 L

MOLE FRACTION 01s

FIG. 15. OXYGEN-CATALYZED CHLORINATION OF ETHANE-ETHYLENE AT 340°C. OF CHLORINE INPUT VARIATION ; air (1.5 On); (180-130) N2 Flow (cc./min.): (12.5-62.5) Clr; 50 C2H4;50 C ~ H S7.5

IX) to the 50 cc. per minute used in this experiment. Of course, the two sets of data are not strictly comparable, as the former experiment was carried out a t 336" and the latter a t 414",the higher temperature being necessary to overcome the inhibiting effect of the excessive amount of oxygen. The inhibition by p a r a 5 s of oxygen-catalyzed substitution of chlorine into olefins permits the determination of the rate dependencies for the

490


over-all formation of hydrogen chloride, as the amount of reaction can be controlled so as to show variation with concentrations. Without the paraffin, the chlorine is almost completely consumed. Although in such mixtures the paraffin is the principal reactant, Table VI11 shows that

I

70

(MOLE % C;Hd*

FIG. 16. OXYGEN-CATALYZED CHLORINATION OF ETHANE-ETHYLENE AT 327°C. OF ETHYLENE INPUTWITH [C2H41 > [Cl~l VARIATION Flow (cc./min.): 12.5 Clz; (100-230)C~Ha;50 CZHe; 7.5 air (1.5 0 2 ) ; (130-O)Nz

MOLE

FRACTION C*Ha

FIG. 17. OXYOEN-CATALYZED CHLORINATION OF ETHANE-ETHYLENE AT 327°C. VARIATION OF ETHYLENE INPUTWITH [CZHJ5 [ C l ~ l (195-95)Nz Flow (cc./min.): 50 Clz; (O-lOO)CzHd;50 CPHe; 7.5 air (1.5 02);

variation in its concentration has relatively little effect. The catalysis is dependent upon the presence of both oxygen and olefin, and the following functions bear some relation to the interaction of these compounds, as this apparently initiates the over-all reaction. Figure 15 indicates that


491

the amount of substitution is directly proportional to the halogen concentration; the curve deviates from linearity at the higher chlorine inputs, probably because of onset of thermal chains. When the olefin is present in greater amount than the chlorine, the rate is proportional to the square of the mole-fraction of ethylene (see Figure 16). When chlorine exceeds the amount of olefin, the rate varies linearly with the concentration of the latter (see Figure 17). Figure 18 shows that for very low concentrations of oxygen the amount of substitution in a C12:CzH4:N2 mixture is directly proportional to the mole-fraction of oxygen; with greater increases in the oxygen input the catalysis is less marked, and still greater amounts sharply reduce reaction (see Figures 11 and 12).

I

80

A SUESTiTUTlON

aoi

aoz

0.03

no*

0.05

MOLE % O I

FIG. 18. CHLORINATION OF E T H Y L ~ N ATE315°C. EFFECT OF OXYGEN Flow (cc./min.): 25 Clp; 50 C2H4; 225 (Nz air)

+

Discussion of Oxygen-Catalyzed Chlorine Substitution. An explanation of the oxygen-catalysis of substitution of chlorine into olefins must take into consideration several facts, namely: (A) Chlorine reacts with olefins under conditions heretofore considered unfavorable for such processes, i.e., in the presence of oxygen. (B) Chlorine reacts principally by substitution under conditions of temperature, some of which, a t least, in the absence of oxygen favor addition. (C) Paraffins (e.g., ethane) inhibit the reaction with the olefin and react with the chlorine themselves. Yet, no oxygen-catalysisof paraffin chlorination is known. (D) Higher concentrations of oxygen strongly inhibit substitution.

492


(E) The rate of production of hydrogen chloride by substitction seems to vary linearly, (a) with the square of the ethylene concentration (b) with the chlorine concentration (c) with the oxygen concentration (for very small amounts of oxygen). These points can be formally correlated by postulating that the over-all rate-determining step is the interaction of an oxygen molecule with ethylene (one or two molecules). This process apparently gives rise to extremely reactive centers which can initiate a chain reaction involving substitution. The formation of a highly reactive intermediate complex which itself reacts substitutively with chlorine might explain the absence of addition by “protection” of the double bond, but it seems inconceivable that the small amounts of oxygen used, which show such pronounced catalysis, could so influence the large number of olefin molecules present. Further, it has been shown that the amount of vinyl chloride produced from an ethane-ethylene mixture was not increased by an increase in the oxygen input from 1.5 cc. to 50 cc. per minute; this finding is contrary to expectations based on the premise of formation of an intermediate complex, since the concentration of the latter should be increased by an increase in the oxygen mole-fraction. Likewise, the astonishingly sudden onset of catalysis with rise of temperature rules out the possibility that reaction originates in the interaction of a complex with chlorine, as the onset would necessitate an anomalous temperature coefficient for the production of the intermediate. Lenher (13), in his studies of the oxygen-ethylene reaction, postulated that one of the primary steps was the formation of an association complex which could react with another ethylene molecule to give two molecules of ethylene oxide, or with oxygen to give water and carbon monoxide. At higher temperatures he also found polymerization products, butenes, propylene, and amylenes. The formation of the last two led to the suggestion that the association complex was capable of rapid dissociation into oxygen and methylene radicals; the latter could react with ethylene or its dimer. He discussed the energetics of the process of disruption. Lenher (14) also came to similar conclusions regarding higher olefins formed at high temperatures from oxygen-propylene mixtures. In the present instance, a similar development of radicals must be occurring, giving rise to centers which initiate chlorination chains. We have shown that very small amounts of tetraethyl lead will cause reaction of chlorine with saturates (20) and olefins under conditions of temperature where ordinarily thermal reaction does not occur. This catalysis was attributed t o the production of centers for chain propagation by interaction of the


493

alkyl with chlorine. Thus, both addition and substitution are promoted in the case of olefins. The oxygen catalyRis leads to a generalization regarding the relative reactivities of various radicals in their vulnerability to oxygen. In short, radicals containing an ethylenic linkage seem to be more stable toward oxygen than those which have only single C-C bonds. Ethane markedly inhibits the oxygen-catalyzed reactions of chlorine with ethylene, propylene, and %butene. What reaction does take place involves principally (though not exclusively) the paraffin. In effect, two competing chains for substitution can occur, and the one involving the para& is the predominant one. [Ethane can be made almost the sole reactant in an ethane-ethylene mixture (20).] The radicals $reduced by interaction of olefin and oxygen initiate chains which immediately involve the paraffin. However, these chains are short because the ethyl radicals are more susceptible to elimination by oxygen than are the ethylenic ones. In the absence of the paraffi, the “catalytic” radicals initiate a chain of the type :

---

+ Cl2 + CzH4 C2Hs + C12

R C1

+

RC1 C1 CsHa HC1

---4

+

CzHaCl

+ C1

wherein R is the “catalytic” radical. Free vinyl is apparently less susceptible to removal by oxygen (possibly as a consequence of stabilization by resonance) and the chain can be of considerable length. Similarly, allyl (from propylene) and crotyl (from %butene) can persist even in the presence of oxygen. If the oxygen-catalyzed substitution is actually a consequence of chains initiated by radicals, it is to be supposed that chain addition should also be promoted. However, under the most favorable circumstances for catalysis, such addition is almost non-existent. Chain addition, as has H H been shown, involves a radical of the type Cl-C-C. According to the

H H above postulate, this radical should be highly susceptible to attack by oxygen, and therefore chains involving it alone should be of very short length. Figure 19 bears on this point. In the chlorination of propylene in the presence of a high concentration of oxygen, it may be assumed that radical chains are considerably shortened. It is seen that all reaction is eliminated by the large amount of oxygen until a temperature of about 300” is reached, where both addition and substitution set in. However, with a slight further rise in the temperature, the unsuppressed addition is the same as that found in the absence of oxygen. Thus it would seem that

494


chain addition is almost nonexistent, that is, a t this temperature, 330°, a t which the oxygen-catalysis first appears for lower concentrations of H H H oxygen, oxygen-vulnerable links in the chain, such as HC-C-CC1, are H H 8

SUBSTITUTI0N;NO 02

+ ADDlTlON~WITH01 0

SUBSTITUTION,WITH OI

50

TEMPERATURE, 'C.

FIG.19. CHLORINATION OF PROPYLENE IN THE PRESENCE OF OXYGEN. TEMPERATUR~ PROFILES; PACKED REACTOR Flow (cc./min.) : 25 Clz; 50 CaH6;225 Nz; or 25 CL; 50 CsHa;200 N2;25 O2

8-

I

,;*,, I

4-

,$" ,'. ' '

.v

' PACKED

REACTOR

.j:

practically eliminated. However, ethylenic radicals, such as allyl, persist as chain carriers for substitution. The nature of the addition occurring in the presence of the large amount


495

of oxygen is probably a straightfoxward, gas-phase, bimolecular association, rather than a surface reaction. Figure 20 shows that at 348", both the unsuppressible addition and substitution with ethylene are little affected by a 2.8-fold increase in surface. The addition, it is worth noting, is actually the dominant reaction, whereas in the absence of oxygen under these conditions, addition is subordinate to chain substitution. The amounts of both processes are nearly proportional to the olefin concentration. The foregoing is offered as an explanation of the oxygen catalysis of substitutive chlorination, and the evidence would indicate that the mechanism is one of chain initiation by radicals produced by interaction of olefin and oxygen, rather than reaction of an association complex itself with chlorine. 111. INHIBITION OF HYDROCARBON CHLORINATION BY OLEFINS

It was observed in the course of the study of the chlorination of npropyl chloride (see Figure 9 of ref. 20) that, whereas at lower temperatures the rate was very similar to that of propane, above 260" the rate of reaction was markedly less. This was attributed to inhibition of the chain substitution by propylene produced by induced decomposition of the chloride. Further, the substitutive chlorination of propylene itself (Figure 7) has a lower temperature coefficient than the reactions with ethane or propane (Figure 9 of ref. 20). That the inhibition is well defined is shown by Figures 21 and 22, which show the effects of propylene and 2-butene on the thermal chlorination of ethane. For example, replacement of 20 cc. of nitrogen per minute in a flow of 50 Clz; 100 C2H6; 150 Nz by an equal amount of propylene reduced the amount of reacted chlorine at 261" from 75% to 65%, and at 247" from 96% to 83%. The corresponding values for suppression by 2-butene are practically the same. The decreases, it should be noted, take place despite an increase in the actual amount of hydrocarbon present and the introduction of a simultaneous addition-reaction which consumes ca. 8% of the chlorine. In order to investigate more fully the factors influencing the inhibition, an apparatus was designed which permitted study of photochlorinations to temperatures as high as 200" under easily controllable conditions. Approximately equal amounts of substitution for the "standard" reaction could be readily obtained simply by control of the light intensity, which compensatesfor other variables. The apparatus consisted essentially of a glass reactor of the usual size (45 X 1.3 cm.) housed within a metal jacket through which paraffin oil was circulated at constant temperature from a thermostated reservoir. The reactor was illuminated through a slit in the jacket.

496

F. F. RUST AND W. E. VAUQHAN

The relative efficiencies of ethylene, propylene, %butene, and isobutylene as inhibitors of ethane chlorination were investigated at 75”, 1 1 5 O , 1 3 4 O , and 1 7 5 O , and the data were compared by means of an arbitrary

TEMPERATURE,%.

FIG.21. CHLORINATION OF ETHANE.INHIBITION OF PBOPYLENE

90

-

(LO.

70

-

a 60..

t soG

*

40.

30.

eo

-

01

‘

230

FIG.

22.

I

c

240

250

CHLORINATION O F

,

e60

I

270 TEYPERATURE,‘C.

I

980

ETHANE. INHIBITION BY

BUTENE-2

“inhibition factor.” It is necessary to introduce this factor in order to compensate for the amount of chlorine reacting by addition with the olefins themselves; such a side-reaction, varying in extent from compound

TABLE X ETHANXI cHLORINATt6N BY OLBPINS Flows (cc./min.): 50 C ~ Z100 ; C Z H ~150 ; Nz;and 50 Clz; 100 C2Hs; 130 Nz; 2OC, €I2, 75 ~NRIBITION OF

-

+CiHa Is* C4HS

(1) % Cla substituted (9) % Clt added % C11 subs. X 100 (3) Clz input (100%) % C12 added ( 4 ) Inhibition factor 73.9 (8)

-

73.9 57.1 15.7 67.7

40.2 18.4 49.6

1 I

-

6.2

45.1 46.3 24.3 18.1 59.6 55.7

14.3 18.2

115" (1)

(8)

% Cla substituted % C11 added

% Clr Subs. X 100 Cl2 input (100%) - % Cla added (4) Inhibition factor (a) 72.7 - (3) (b) 79.0 - (3) (c) 76.3 - (8) ($1

(1)

% Cla substituted

(i?)

% Cl2 added

(3)

% C11 subs. X 100 C11 input (100%) - % C1, added

-

(C) (a) 72.7 (a) 36.8 (b) 79.0 (b) 41.7 (0) 76.3 62.3 I 13.0 (a) 14.4 (b) 16.7 71.6 (a) 43.0 (b) 50.1 (a) 29.7 (b) 28.9

(b)

(4 46.8

47.6 16.3

17.0

56.9 56.5 16.2 22.1

4.7

(4 (a) 74.5 61.5 (b) 73.5 (c) 73.2 (d) 72.3 11.8

69.7

(b)

(d) (a) 45.5

45.1 (c) 42.1

37.6 14.4 (a) 14.3 17.5 (c) 18.3 43.9 (a) 53.1 54.7 (c) 51.5

( 4 ) Inhibition factor (a) 74.5 (b) 73.5 (c) 73.2 (d) 72.3

(3) (3) (3) (3)

4.8

% Cln substituted

(a) 72.4 63.4 (b) 76.5 7.4 68.3

-

(1)

% C11 added (3) % C1, Subs. X 100 Clninput (100%) - % Clr added (4) Inhibition factor (g)

-

(a) 72.4 (3) (b) 76.6 - (3)

(a) 21.4 18.8 (c) 21.7

28.4

(a)

4.1

(a) 39.6

(a>

(b)

45.6

14.5 46.4

50.4 15.3 13.3 53.2 58.2

26.0

19.2 18.3

498


to compound, obviously alters the concentration of the chlorine available for the principal substitution process, that with ethane. The effective quantity of chlorine is obtained by deducting the amount of “chlorine added” from the “chlorine input.” The amount of substitution is expressed as a percentage of this amount of chlorine theoretically available for such reaction. No distinction is made between the substitution into para& and that into olefin, since our studies have indicated that the mechanism of substitution is, under these conditions, the same for both types of compound. Moreover, in the present cases, because the concenTABLE X I OF ETHANECHLORINATION BY CHLORO~LEFINS INHIBITION Flows (cc./min.): 50 Clz; 100 CzHa; 150Nz; and50 Clz; 100CZHs; 1 3 0 N ~20C,H2,-1C1 ; 134” C,Hs

CrHs

+ CIHICI CzHs -I-CaHsCl (a)

(1)

% Clz substituted

(a) 74.0 (b) 67.7

( 8 ) % Cla added % Clr substituted X 100 (8) C12 input (100%) - 5% C12 added . ( 4 ) Inhibition factor (a) 74.0 - (5) (b) 67.7 - (3)

(b)

50.5 13.6 58.5

35 .O 8.5 38.3

15.5 29.4

175”

% C11 substituted (8) % C12 added % C1, substituted X 100 (5) C ~ input Z (100%) - % C12 added ( 4 ) Inhibition factor 72.4 - (3) (1)

72.4

52.0 6.8 55.8

16.6

tration of olefin is small compared with that of the paraffin, substitution into the olefins may be considered of minor importance. The “inhibition factor,” which indicates the amount of suppression, is merely the numerical difference between the percentage of substitution, as determined above, and the percentage of chlorine reacting with ethane in the absence of unsaturates, but under otherwise strictly comparable conditions. The data on the olefins are given in Table X, and similar findings for vinyl and allyl chlorides as inhibitors are shown in Table XI. The duplicate values give a good idea of the reproducibility of the effect, despite considerable variation in the amount of reaction. It is also noteworthy

499


that temperature has very little effect on the magnitude of the inhibition. The exception to this, with %butene at 75”, is probably only apparent, and is due to the large amount of addition. Propylene and allyl chloride are the most effective inhibitors, and ethylene the least. Figure 23 illustrates the effect at 138” of variation of the concentration of the most powerful inhibitor, propylene. While substitution decreases regularly, the addition rises. It is logical to attribute the effect of the olefins in suppressing reaction to an interaction of these substances with radicals produced in the chains involving substitution. This association process would result in the formation of larger radicals which, by reason of orientation requirements for 80

-

0

0

TOTAL REACTION SUBSTITUTION

cc. C$H(/MIN.

FIG. 23.

ETHANEAT 138°C. INHIBITION AMOUNTSOF PROPYLENE

PHOTOCHLORINATION OF

BY VARYING

Flow (cc./min.) : 50 C12; 100 C2H8; (150-llO)N2; (0-40)CsH~

successful collision, react less readily with chlorine than do the smaller ones. The magnitude of the inhibition is most likely dependent upon two factors, (a) the reactivity of the olefinic bond, and (b) the sizes and configurations of both of the hydrocarbon reactants which form the complex. Thus the data of Tables X and XI probably represent the consequences of reaction of ethyl radicals with the various unsaturates. This interpretation is substantiated by the finding that propane chlorination is less affected by propylene than is ethane chlorination. Thus the “inhibition factor” for the former reaction is 26.0 (see Table XII) as compared to 29.0 for the equivalent ethane-propylene mixture. Another line of attack was also used. The suppression by the highly reactive olefin, propylene, of the chlorinations of a number of compounds

500


should afford a measure of the reactivities of the various radicals involved

H H in the chains. Methyl, ethyl, propyl, butyl, 6-chloroethyl (Cl-C-C), H H

H H

and a-chloroethyl (C-CH) have been produced by photochlorination of C1 H methane, ethane, propane, n-butane, ethylene, and ethyl chloride. The results are condensed in Table XII. To determine the various inhibition factors, the compounds were caused to react by adjusting the light intensity until 70-75% of the chlorine was consumed. Methane was found TABLE XI1 OF VARIOUS CHLORINATION REACTIONS BY PROPYLENE AT 135' INHIBITION Flows (cc./min.) : 50 Clz; 100 compound; 150 Nz; and 50 C1,; 100 compound; 130 N2; 20 CaHs COMPOUND CHLORINATED

CHP

C:He

CsHa

n-

CxHi

C:HsCl

CtHlO --- -

(1)

% C1, substituted when no CsHs 67.4 73.5 74.3 72.4 74.7b 72.6

(8)

% C1, substituted in the presence of 21.4 37.3 43.9 42.4 57.9') 22.1

present CsHa

(a) % Clz addition to CaHa % Cla subs. ( 8 ) X 100 (4) Clz input (100%) - % C11 added (6) Inhibition factor (I) - ( 4 )

39.0 16.3 9.3 6.8 16.8c 24.3 35.1 44.5 48.3 45.5 49.4d 29.2 32.3 29.0 26.0 26.9 25.3

43.4

* Experiments at 172". b% '.. Clz reacted. c % C12added to propylene (est.). d (% CI2reacted (8) - 16'8%) loo; 57.9 - 16.8 = % Clz added to CZHI. 100% - 16.8%

to be unusually di5cult to chlorinate-only two-thirds of the halogen substituted at 172" even under the highest intensity of light available. From the values of the inhibition factors, propylene seems to have the greatest effect upon methane and ethyl chloride chlorinations. Although at first sight, the latter with the very high inhibition factor of 43.4 would seem to be most affected, one must consider the fact that under the rigorous conditions required for methane chlorination, the small amount of admixed propylene becomes an important reactant. The inhibiting double bonds become saturated with chlorine and are thus rendered ineffective. The true order of reactivity, therefore, cannot be given at present. A comparison of the effect of location of the free valency in the radical

CHLORINATION OF OLEFIN BYDROCARBONS

501

is, however, worth noting. By chlorinating ethylene, the fi-chloroethyl radical would be expected. H K h C H ,-

+ Cl---+ .

H H Cl-C-C

H H

Ethylene itself has been shown to be of practically no consequence in s u p pressing reaction. The inhibition factor for the photochlorination of this olefin in the presence of propylene was 25.3. The a-chloroethyl radical is by comparison surprisingly reactive. At 135" its inhibition factor with propylene was 43.4,the highest factor yet obtained. Its mode of formation from ethyl chloride would be: H H H-C-C-C1 H H

+ C1-

H H H-C-C H C1

+ HC1

It has been shown [see Table I of (20)] that the a-position is the principal point of attack. When m e r e n t hydrocarbons are photochlorinated, comparisons of the reactivities of various radicals derived from them become much more complicated. In order to obtain comparableamounts of reaction, differing light intensities must be employed. This means that both rate of chain initiation and the chain lengths are variables. Furthermore, the rate of saturation of the inhibiting double bond, and therefore its average concentration, becomes variable. Allowance must be made for all of these complications and until more data are available to make these corrections, these present inhibition factors can only be arbitrary. The inhibition by olefins of high-temperature chlorination reactions apparently has not been observed previously. However, Groll and Burgin (9) noted that isobutylene inhibited the photochlorination of butane at temperatures below 100". Also, several workers have observed that propylene suppresses other reactions in which it is suspected that radical chain mechanisms play an important role. Thus, Echols and Pease (6) have found that the thermal decomposition of n-butane is inhibited by one of the products, propylene. The rate of dehydrogenation of ethane to form ethylene is decreased in proportion to the amount of added propylene up to concentrations of 14% (4). The pyrolyses of n-hexane and n-octane are retarded by the products of reaction, such as o l e h s (5). Snow and Frey (19)have found that the liquid-phase formation of resinous materials from olefins and sulfur dioxide is inhibited by isobutylene. To explain the inhibition by propylene of the thermal decompositions of paraffins and of some other compounds, Rice and Polly (17) postulate that the olefin destroys the radicals R of the chain by forming RII: and allyl; the latter

502


combines with itself to terminate the chain. On the basis of the present data, no distinction between this or any other mechanism can be offered. ACKNOWLEDGMENT

The authors wish to thank Dr. E. C. Williams, Vice-president in Charge of Research, for his stimulating interest in the work, and their colleagues for many valuable discussions. Mr. L. H. Bayley ably assisted in the analytical problems. SUMMARY

1. The high-temperature chlorination of olefins has been studied. It seems likely that under certain conditions, both addition- and substitutionreactions occur by radical chain mechanisms. However, reactions at surfaces and gas-phase bimolecular associations and metatheses also play a part. 2. It has been found that under carefully controlled conditions it is possible to catalyze strongly the substitutive chlorination of olefins by inclusion of oxygen in the input gases. The effect occurs with low concentrations of oxygen; larger amounts give the expected inhibition. The presence of paraffin hydrocarbons in an oleh-oxygen mixture greatly retards the rate of chlorination. The reaction which does occur is chiefly one of substitution of hydrogen atoms on saturated carbon atoms. 3. Olefins act as inhibitors of high-temperature chlorination reactions. Of those tested, propylene seems the most effective. EMERYVILLE, CALIF. REFERENCES

(1) (2) (3) (4)

BUNSSON AND ROSCOE, Pogg. Ann., 96, 373 (1855). BURGINS,ENGS,GROLL,AND HEARNE, Ind. Eng. Chem., 31, 1413 (1939). CONN,KISTIAKOWSKY, AND SMITH,J . A m . Chem. SOC.,60, 2764 (1938). DINTZES,KVYATKOVSKII, STEPUKHOVICH, AND FROST, J . Gen. Chem. (U. S. S. R.) 7, 1754 (1937); Chem. Abstr., 31, 8277. (5) DINTZESAND ZHERKO, J. Gen. Chem. (U.S. 8.E . ) , 6, 68 (1936); Chem. Abstr., 30, 4745. (6) ECHOLSAND PEASE,J . Ana. Chem. SOC.,61, 208 (1939). (7) ELLIS,“The Chemistry of PetroleumDerivatives,” Chem. Cat. Co., N. Y., 1934. (8) ELLIS,“The Chemistry of Petroleum Derivatives 11,” Reinhold, N. Y., 1937. (9) GROLLAND BURGIN,private communication. (10) GROLLAND HEARNE,Ind. Eng. Chem., 31, 1530 (1939). (11) GROLL,HEARNE,RUST,A N D VAUGHAN, Ind. Eng. Chem., 31, 1239 (1939). private communication. (12) HEARNEAND LAFRANCE, (13) LENHER,J . A m . Chem. SOC.,63, 3737, 3752 (1931). (14) LENHER,J. Am. Chem. SOC.,64, 1830 (1932).


(15) NORRISH, J . Chem. SOC.,123, 3006 (1923). (16) NORRISH AND JONES, J . Chem. SOC.,126, 55 (1926). J . Chem. Phys., 6, 273 (1938). (17) RICEAND POLLY, (18) SHERMAN, QUIMBY,A N D SUTHERLAND, J . Chem. Phys., 4 , 732 (1936). (19) SNOWAND FREY,Ind. Eng. Chem., 30, 176 (1938). (20) VAUGHAN AND RUST,J. Org. Chem., 6 , 449 (1940). (21) WILLARDAND DANIELS, J. Am. Chem. SOC.,67, 2240 (1935). (22) WILLIAMS,J . Chem. SOC.,1932, 1747, 1758.

503

THE HIGH-TEMPERATURE CHLORINATION OF OLEFIN

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