planta piloto para la producción de cloruro de alilo groll hearne

HALOGENATION OF HYDROCARBONS Substitution of Chlorine and Bromine into Straight-Chain Olefins

H. P. A. GROLLl AND G. HEARNE Shell Development Company, Emeryville, Calif.

The reaction of halogens with olefins containing a double bond in an unbranched carbon chain is changed from addition to substitution by operating at elevated temperatures. This substitdtion, unlike the induced reaction described in the first paper ( P ) , occurs exclusively into the olefin with the formation principally of allyl-type unsaturated monohalides. The optimum temperature ranges from 300' to 600' C., depending on the nature of the olefin and halogen.

Operating conditions have been examined in some detail so that the work can serve as a basis which will make this class of products easily available.

LTHOUGH it is known that the reaction between chlorine and olefins containing a tertiary carbon atom A a t the double bond leads predominantly to chlorine

substitution in the allyl position, the only observation of formation of allyl-type chlorides from straight-chain olefins is that of Stewart and Weidenbaum (9). These investiga- tors found pentenyl chlorides in the chlorination product of 2-pentene, but only in small amounts. In general, the main chlorination product of straight-chain olefin is the dichloro paraffin.

The investigations described in the two preceding papers in this series (2, 4) failed to disclose any clue which could be utilized for increasing the amount of substitution into straight- chain olefins. The first paper (4) showed that the induced substitution is observed whenever an olefin is chlorinated in a liquid medium in the presence of a saturated compound capable of being substituted by chlorine. This induced substitution does not take place to any extent into olefin mole- cules, a t least not into those which possess paraffin chain radicals of only reasonably short lengths. (No vinyl chloride was obtained from the liquid-phase chlorination of ethylene, and the small amount of unsaturated monochlorides formed in the liquid-phase chlorination of butane-butene was not reduced by the presence of oxygen.) Any attempt to explain the behavior of isobutene by this induction phe- nomenon encounters the fact that this substitution reaction is not inhibited by the presence of oxygen, while the typical induced chlorination is easily inhibited in this manner. Therefore chlorine substitution of isobutene and its homo- logs containing a tertiary unsaturated carbon atom is still unique and unexplained.

This chlorine substitution of isobutene was shown in the second paper ( 2 ) to occur particularly well in the liquid phase,

1 Present address, Rhenania-Ossag Mineralolwerke A.-G., Hamburg, Germany.

but to be catalyzed also on solid surfaces. However, isobutene adds chlorine under the influence of light in the vapor phase. These observations indicate that substitution and addition are independent reactions and can be accelerated independently, at least in a few isolated instances. Therefore it was decided to try any available accelerating factors in the hope of finding one that would accelerate substitution in preference to addition in straight-chain olefins. In general, four means are available for accelerating any chlorination reaction-namely, radiation by actinic light, presence of a catalyst, induction by simultaneous chlorine addition, and heat. Light and such catalysts as have been tried merely increase the reaction velocity but have no significant effect on the course of chlorination of straight-chain olefins. Induction may be responsible for a limited amount of substitution into straight-chain olefins as has been observed recently (9). However, it appears that the yield of allyl-type chlorides obtainable by this means is very small and cannot be increased.

The application of heat to the chlorination of olefins has apparently never been tried, possibly because of several circumstances which a t first sight discourage such an attempt:

According to Williams the reaction velocity of halogenation of unsaturated hydrocarbons decreases with increasing temperature (11). However, this was demonstrated in the preceding papers (2 ,4 ) to be due to the disappearance of the liquid hase in a certain temperature range. Therefore this peculiar e&ct exists only in this same narrow zone.

There are considerable mechanical difficulties in chlorinating olefins at really high temperatures. The addition of chlorine to these compounds is by no means slow even at room temperature. Therefore if the gases are mixed cold, they will robably react by addition before they can be heated to the desire$ point, especially as the reaction is accelerated by solid surfaces which must be a plied for heating. On the other hand, if the gases are mixed at egvated temperatures, the danger of flame formation and ex-

1530

DECEMBER, 1939 INDUSTRIAL AND ENGINEERING CHEMISTRY 1531

cessive carbon deposition is even greater than in the thermal chlorination of paraffin hydrocarbons. This danger is greatest in those regions in which a favorable effect of the heat becomes noticeable.

These difficulties were overcome by developing suitable apparatus and operating technique. Thus it could be shown that when straight-chain olefins are chlorinated at high temperatures, they form predominantly unsaturated monochlorides of the allyl type (3). The yield increases with increasing temperature up to a point where the chloride formed is pyrolyzed before it can be withdrawn from the hot zone. The temperatures actually applied, with the optimum ranging from 300" to 600" C. according to the nature of the olefin, are surprisingly high for the production of such un- stable compounds as allyl chloride and crotyl chloride. Even at 650" C. propylene gives yields similar to those obtained a t 600" C.; but there is a large drop a t 700" C., and decidedly poorer yields are obtained a t still higher temperatures. The optimum temperature is dependent upon the nature of the olefins as well as of the halogen.

In view of the difficulties encountered with flame formation when reaction of separately preheated gases was tried, most of the preliminary work was carried out in a reactor system in which the chlorine was mixed with the hydrocarbon a t room temperature and the mixture passed into hot tubes. Finally, the difficulties connected with mixing the hot gases were overcome by suitable design of the mixing nozzles and employing the most favorable conditions of flow. However, since many factors had been investigated by the old system of mixing the gases at low temperature and it appeared unnecessary to reinvestigate these in the improved apparatus, both types of experiments are described here.

High-Temperature Chlorination of Propylene

The efforts to substitute chlorine into straight-chain olefins have been concentrated chiefly on propylene because of the value of allyl chloride as a chemical intermediate. In a previous communication from these laboratories (10) it was shown that this chloride occupies a key position in the synthesis of glycerol from petroleum. Therefore, as soon as the feasibility of preparing allyl chloride had been demon-

Cb F/ow Mefers Fee

sulfide, and steam was also determined. Tubes ranging in size from 0.25 to 1 inch (0.635 to 2.54 cm.) in diameter and 10 to 20 inches (25.4 to 50.8 cm.) in length were tested between temperatures from 400" to 700" C. The results were not always completely reproducible because various factors a t times caused a marked change in the reaction velocity and yield of products. By following the general trend of the results, however, it was possible to arrive a t approximately the optimum conditions without investigating each tube throughout the entire temperature range. The experiments which show the influence of the various factors on the reaction are listed in Table I. The results may be summarized as follows:

The best yield of allyl chloride was obtained when the temperature of the wall of the tube was from 600" to 650" C. (Table I, A ) . At lower temperatures there is an increased tendency to form the addition rather than the substitution product, and at higher temperatures the de- composition of the reaction products becomes excessive. It should be noted that the temperatures given here do not represent reaction temperatures but wall temperatures of a tube which serves the purpose of a preheater as well as a reactor. The influence of reaction temperatures is better shown in the section on "High-Temperature Bromination of Propyl- ene".

Most of the experiments were with a mole ratio of propylene to'chlorine of 2 to 1. Increasing the excess of propylene caused a small increase in the yield of allyl chloride with this type of reactor, but it was accompanied by a considerable decrease in the amount of chlorine that could be reacted (Table I, B). The advantage of using a large excess of propylene was not so marked in this type of reactor as in the arrangement in which the gases were preheated before mixing.

Optimum results were obtained when the flows of chlorine and propylene were adjusted so that not more than a trace of halogen remained unreacted a t the end

Temperature.

Mole Ratio of Propylene to Chlorine.

Throughput.

XC/ Scrubber

strated, the problem of as- sembling sufficient data to put the process on a larger scale was attacked. Under the direction of W. Engs and S. Wik, a plant capable of producing close to 2000 pounds of allyl chloride per day has been de- signed and operated for

has been employed for the experimental production of glycerol.

GASES MIXED AT Low TEMPERATURE AND REACTED AT of the reaction tube. No marked decrease in the yield of ELEVATED TEMPERATURES. The apparatus in which propyl- allyl chloride was observed by reducing the flows slightly, ene and chlorine were mixed at room temperature and passed but it is obvious that if the flows were greatly decreased, the through a heated tube is shown in Figure 1. The influences reaction products would be decomposed by the long contact of temperature, ratio of chlorine to propylene, throughput, time at elevated temperatures (Table I, C ) . and tube dimensions were studied. The effect of small Tube Dimensions. By adjusting the throughput and amounts of gases such as oxygen, sulfur dioxide, hydrogen temperature, it was possible to prepare allyl chloride in a

over a year. Part of this Cb/amnafed PmducT Producf FIGURE 1. APPARATUS FOR HIGH-TEMPERATURE CHLORINATION OF OLEFINS

1532 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 31, NO. 12

TABLE I.

,--- Reaction Conditions 7

HIGH-TEMPERATURE CHLORINATION OF PROPYLENE WITH GASES MIXED AT Low TEMPERATURE AND REACTED AT ELEVATED TEMPERATURES

M 01 e % ratio, Yield HCI

Reactor Distance Feed propyl- -Material Balance of Chlorine Applied-- Mono- Forma- Space Dimensions from en- of ene Unsatd. Di- Tri- Total chloride tion, Mole .Veloc-

Di- Block Wall trance t o chlo- chlo- Un- mono- chlo- chlo- accounted on Clz % Clz Ity per Length ameter temp. temp. "hot spot" rlne rine HC1 reacted chloride ride Basis Applied Sec. In. (cm.) In. (cm.) C.

20 (50.8) 1 (2,54) 384 20 1 470 20 1 510 20 1 550 20 1 600

20 1 650

20 1 708

20 1/z (1.27) 525 525 528

20 1/2 525 20 '/a 20 1 / 2

20 1 600

20 1 600

20 1 650

20 1 650

20 1 706

20 1 708

20 */a (0.95) 500 20 '/a (1.27) 528 20 8/,(1.91 475 20 l(2.541 510 10(25.4) 8/4(1.91) 500

20 (50.8) 1/z (1.27) 500 20 '/a 500

500 :$: 500 20 20 20 1/2 500

c.

405 486 525 556 Not

detd. Not detd. Not

detd.

555 546 530 518

Not detd. Not

detd. Not detd. Not detd. Not

detd. Not

detd.

522 530 470 525 492

In. (cm.) G./min. A. INFLUENCE OF TEMPERATURm

11 (27.9) 15.6 1.76 42.3 0.1 26.5 22.8 9 (22.9) 16.7 1.89 46.4 . . . 29.8 17.6 7.5 (19.1) 19.4 1.87 48.4 0.1 29.7 16.7 7 (17.8) 19.7 2.10 48.4


was reduced to less than 2 per cent by preheating the gases to about 400" C. In some cases a larger amount of saturated dichloride was found even when the gases were mixed a t high temperatures, but this could be explained by the fact that the flows employed in these instances were so fast that part of the reaction occurred at a lower temperature in the cooling coil at the end of the reaction chamber or in the aqueous solution employed for removing the hydrogen chloride. Apparently the chlorination is not complete a t the point of maximum temperature, so that the reactor should extend beyond this point for a sufficient distance to permit the reaction to come to completion.

The yield of unsaturated monochlorides obtained with a 1 to 2 ratio of chlorine to propylene was only slightly better than that obtained by mixing the gases cold and reacting them in a heated tube. However, the process is obviously superior for development on a larger scale because the problem of heating the mixed gases rapidly from room temperature to 600" C. without overheating in certain zones becomes increasingly difficult as the diameter of the reaction tube is increased. I n the semiscale plant a much larger temperature rise from the point of mixing was observed than in the laboratory; therefore i t was unnecessary to preheat to such high temperatures. The difference is due to larger heat losses in the laboratory apparatus.

I n order to determine the influence of temperature on the addition and substitution reaction of chlorine and propylene, a series of experiments was started in which the reaction temperature was controlled by employing a large excess of propylene. The results are listed in Table 11, B. The most noticeable change in the nature of the reaction products occurred between 200" and 300" C. At the lower temperature the product was predominantly dichloropropane formed by addition of chlorine to propylene, but at the higher temperature the substitution reaction predominated. The nature of the reaction product was not greatly altered by increasing the temperature from 300" to 600" C., but the rate of reaction was markedly increased. The chlorination was still slower a t

Preheafcr -

FIGURE 2. APPARATUS FOR HIGH-

OUT TUBE TEMPERATURE CONTROL TEMPERATURE CHLORINATION WITH-

200" C., and a space velocity of 0.02 per second was necessary to complete the reaction, whereas at 600" C. a space velocity of about 5 per second could be employed.

T h e yield of monochlorides was increased considerably by operating a t constant tempera tures . A t 600" C. the yield was 78.6 per cent, which corresponds to about 75 per cent of allyl chloride. The amount of allyl chlor ide formed is actually higher than this, for, as will be shown below, the yield can be improved greatly by increasing t h e efficiency of the

recovery system. The most important observation is that the addition reaction between chlorine and propylene is almost completely eliminated. Less than 0.3 per cent of dichloropropane was formed, and the chlorine substitution as indicated by the amount of hydrogen chloride formed was almost quantitative. The improvement in yield of unsaturated monochlorides was also accompanied by a decrease in the amount of unsaturated dichlorides. These are probably

700

606

Y $ $ 5 500

k

900

0 I 2 3 4 5 6 7 8 9 1 0 1 1 I 2 1 3 1 4

DISTANC fROM POINT Of M/XJNG (INCM.7)

FIGURE 3. TEMPERATURE PROFILE OF PROPYLENF~ CHLORINATION

formed by chlorination of allyl chloride; therefore the extent of tYleir formation should be reduced by increasing the excess of propylene, since this decreases the concentration of allyl chloride in the chlorination mixture.

Since recovery of allyl chloride from propylene with a ratio of chlorine to propylene of 1 to 6 may offer a rather difficult recovery problem, the reaction was investigated with a somewhat smaller excess of propylene-i. e., 3.5 to 1 ratio of propylene to chlorine. When the gases were preheated to 500" C., a temperature rise of only 85" was observed. The yield of unsaturated monochlorides (74 per cent) was not much lower than that obtained with a 1 to 6 ratio. This indicated that it might be unnecessary to employ such a large excess of propylene, provided the temperature in the reactor is maintained nearly constant.

In order to maintain an even temperature with a mole ratio of chlorine to propylene of 1 to 2, however, it was necessary to apply external cooling to the zone of maximum reaction temperature because of the exothermic nature of the reaction. [The heat evolved in the reaction CaHe + Clz + CaH&l+ HC1 is probably about 26 kg. cal. per mole. No reliable data exist for the direct calculation of the heat evolved, but i t can be estimated from the heat evolved in the substitution of chlorine into methane (23.9 kg. cal. per mole), ethylene (24.1), and ethane (26.8), using the data of Bichowsky and Rossini, 1 .]

The apparatus for these experiments is shown in Figure 4. By using a long narrow reaction tube and fast flows, the normal high-temperature zone was removed a sufficient distance from the mixing jet so that cooling could be applied. In this apparatus two air jets were employed to cool the hot zone, and the latter part of the tube was insulated to


TABLE 11. HIGH-TEMPERATURE CHLORINATION OF PROPYLENE WITH GASES MIXED AND REACTED AT ELEVATED TEMPERATURES Reaction Conditions

Mole -- Material Balance of Chlorine Applied I % HC1 Distance ratio, Un- Yield Forma-

Reactor Pre- from Feed propyl- satd. Mono- tion, Space Dimensions heat- entrance of ene Unsatd. di- Satd. Tri- Total chloride Mole Veloc-

Di- ing Tube t o chlo- chlo- Un- mono- chlo- dichlo- accounted on Clr Yo Clz ity per Length ameter temp. temp. "hotspot" rine rine HC1 reacted chloride ride chloride ride for Basis Reacted Sec.

In, (cm.) I n . (en.) C. ' C. In. (cm.) G./min. A . EXPERIMENTS USING A 1:2 MOLE RATIO OF CHLORINE TO PROPYLENE (NO TEMPERATURE CONTROL)

14(35.6) 3/4(1.91) 400 5955 6 (15.2) 28.7 2.05 51.0 1 0 . 3 34.2 1 . 9 5 . 8 2 .4 95.3 68.5 102 4.87 460 675Q 2 (5.1) 28.7 2.03 56.5 None 31.6 8 . 4 1 . 9 98.4 63.3 113 4.84 425 6125 9 .5 (24.1) 48.1 2.08 44.8 10.1 29.7 14.7 5.2 94.4 59.5 89.6 8.38

14 3/4 460 6655 6 (15.2) 49.6 2.00 55.0 None 34.9 1.8 5.6 2 . 2 99.5 69.8 110 8.30

14 3/4 375 559a 5.5(14.0) 21.6 2.08 46.7 0.02 32.9 5.0 5 . 0 3 . 4 93.0 65.9 93.5 3.70 14 3/4 14 3/4

B. CONSTANT-TEMPERATURE EXPERIMENTS (TEMPERATURE CONTROLLED BY LARQE EXCESS OF PROPYLENE) 600 590b . . . 12.2 6.6 50.5


A / r Coo/er No i

React/on Tube

from one vesvel to another. The p r o p y l e n e r e - moved by distilla- t i o n w a s c o n - d e n s e d a n d analyzed for chlorine to determine whether any of the product was lost dur ing dis t i l la- tion.

This series of experiments with stepwise improvement in technique of recovering light e n d s s h o w s a steady increase of t h e over-all recovery from 94.4 to 96.4 per cent, and to 96.6 per cent, as well as of the yield of monochlorides from 79.8

FIQURE 4. APPARATUS FOR HIQH- to 81.8 per cent

TUBE TEMPERATURE CONTROL cent. Table 11, D, shows a more

detailed comparison of the best of these experiments with the best of the older series, with normal means of recovery. The improvement of total recovery of 2.4 per cent accounts for nearly 5 per cent improvement of the monochloride yield. That the latter was actually 7 per cent is a result of the special emphasis on the recovery of light ends. The discrepancy in the order of only 1 per cent in over-all material balance is well within the limit of error of such a complicated recovery system.

At any rate, we can draw the conclusion that the actual yields of allyl chloride were considerably higher than had been recorded throughout the investigation. Since no more saturated dichloride is formed in the present operation, we can visualize an ultimate high-temperature chlorination process producing, say, 85 to 90 per cent allyl chloride, 3 per cent methyl vinyl chlorides, and 4 to 5 per cent higher chlorides.

The composition of the unsaturated monochloride fraction from the high-temperature chlorination of propylene varies only slightly with the chlorination conditions. The distribution of the isomeric chlorides in this fraction is shown in Table 111, A , together with their physical properties. For comparison the composition of the unsaturated monochloride fraction from the pyrolysis of dichloropropane is also listed. The difference between the distribution of the isomeric monochlorides from the chlorination and from the pyrolysis is significant, for it shows definitely that dichloropropane is not an intermediate product in the formation of unsaturated monochlorides by high-temperature chlorination of olefins.

The composition of the dichloride fraction from the chlorination of propylene varies considerably with the reaction conditions. Dichloropropane from the addition of chlorine to the olefin is the principal product of the reaction at low temperature, but it can be almost avoided by operating entirely a t elevated temperatures. The unsaturated dichlorides which are present in this fraction are probably the result of substitution of chlorine into the unsaturated monochlorides formed as primary products. The extent of their formation is

Z uma/ scrubhnq condensmg 5y3fem

TEMPERATURE CHLORINATION WITH and to 85.5 per

NATURE OF CHLORINATED PRODUCT.

increased by decreasing the excess of olefin over chlorine. The unsaturated dichlorides are also formed in increasing amounts when the mixing of chlorine with propylene is not sufficiently rapid and efficient.

Since allyl chloride is the principal primary product from the chlorination of propylene, the principal dichlorides are the ones formed by its chlorination. The proportions in which these are formed are listed in Table 111, B, together with their physical properties. It should be noted that allylic rearrangement is possible between 1,l-dichloro-2- propene and 1,3-dichloro-l-propene; therefore it is impossible to state in what proportions these chlorides were formed in the chlorination. The rearrangement of l,l-dichloro-2-propene to 1,3-dichloro-l-propene by the action of hydrogen chloride was reported by van Romburgh (8).

Since the fractionation of the unsaturated dichlorides from dichloropropane (boiling point, 96.2" C.) is difficult, it was necessary to develop a special method for determining the relative amounts of saturated and unsaturated dichlorides present. Analysis by bromine absorption was unsuitable because unsaturated dichlorides do not give an accurate bromine number. A satisfactory but somewhat tedious method was developed, based on the fact that dichloropropane on treat- ment with alcoholic potassium hydroxide forms a quantitative mixture of 1- and 2-chloro-1-propene. The unsaturated dichlorides under similar conditions yield either unsaturated chloroethers or other products not boiling in the same range. The experimental procedure is as follows:

A weighed sample of about 25 cc. of the dichloride fraction is mixed with 250 cc. of 2 N alcoholic potassium hydroxide in the kettle of a still. The product is distilled slowly with good reflux until no more material boiling from 20' to 40' C . is obtained from the still head. From the weight of distillate (1- and 2- chloro-1-propene), the amount of dichloropropane in the starting material is calculated.

High-Temperature Bromination of Propylene The bromination of propylene was studied because it was

of interest to determine the influence of the nature of the halogen on the substitution and addition reactions with unbranched olefins. The results are listed in Table IV.

TABLE IV. BROMINATION OF PROPYLENE

Reaction conditions: Reactor dimensions, in.:

Preheating temp O C . Max. tube temp.:) O C . Distance from entrance to "hot Feed of bromine, g./min. Mole ratio, propylene/bromine

Length Diameter

spot",

Material balance of bromine applied: HBr Unreacted Unsatd. monobromides Dibromides Tribromides Total accoun%ed for

Yield on bromine basis, %: Monobromides Allyl bromide

in.

Without Temp.

Control

20 20

186 5i5 ; t o %o

8 7 . 5 14.7 2.03 2.14

16.5 None 13.6 58.3 6 . 6

94 .9

42.7 None 27.5 15.9 1 .9

88.0

Constant- Temp. Expts.

22 22 Is/c l S / 4 200 300 205 315

2:54 15:5 4.26 4.02

11.1 48.1 0.04 None

12.7 36.7 43.8 2 .5

5.6 4.0 73.2 91.3

27.2 55.0 2!.4 73.4 a 46.8 6 5 b

HBr mole 5% of bromine applied 33.0 85.4 22.2 96.2 Spac'e velocity per sec. 0.92 1.80 0.04 0.22 a Not determined. b At least.

It appears that the reaction of bromine with propylene is similar to that of chlorine. There is a pronounced shift in the nature of the reaction products caused by increasing the temperature from 200" to 300" C. At the lower temperature the principal product is dibromopropane formed by addition; but at the higher temperature the substitution


reaction predominates. In each case, however, the substitution reaction is more pronounced with bromine than with chlorine. This is the more interesting since there is no, or only little, substitution with tribromide formation a t low temperatures in the liquid phase. It is possible that the higher substitution by bromine a t elevated temperatures is caused by the difference in the thermal dissociation of bromine and chlorine. The dissociation of both halogens increases greatly with increasing temperature but is much greater for bromine than for chlorine at corresponding temperatures (Table V). At present it is impossible to state to what extent the dissociation of the halogen is responsible for the formation of unsaturated monohalides.

TABLE v. THERMAL DISSOCIATION O F CHLORINE AND BROMINE (7)

Temp., Degree of Dissooiation a c. Chlorine Bromine 25.1 2.16 x 10-19 4.02 X 10-16

127 5.46 X 10-3' 7.80 X 10-11 227 . . . . . . . . 2.15 x 10-9 327 1.08 x 10-8 1.24 X 10-8 427 . . . . . . . . 1.99 x 10-6 527 5.04 X 10-6 1.60 x 10-4 627 , . . . . . . . 8.10 x 10-4 727 2.07 x 10-4 3.00 X 10-8

Although the bromination was not investigated in so great detail as the chlorination, sufficient data were obtained to show that a good yield of allyl bromide is possible. The maximum yield of unsaturated monobromides obtained was 73 mole per cent, which represents a t least 65 per cent allyl bromide. The composition of the unsaturated monobromide fraction does not differ greatly from that of the unsaturated monochloride fraction from the high-temperature chlorination.

High-Temperature Chlorination of Other Olefins

In the preceding section it was shown that the type of reaction between oIefin and halogen is influenced by the nature of the halogen. Experiments with various olefins indicate that the nature of the olefin also has an important bearing on the course of the reaction. This was shown by comparison of the amount of chlorine substitution occurring at different temperatures with various olefins. The measurements were made a t almost constant temperatures in a reactor similar to that shown in Figure 2. The temperature throughout the reactor was controlled by using a large

excess of the olefin. In most cases the products were not fractionated, but the amount of chlorine reacting by substitution and by addition was determined by measurement of the hydrochloric acid formed. The results are listed in Table VI and Figure 5.

Ethylene requires a higher temperature to form substitution products than the other olefins. The product a t the elevated temperatures is predominantly vinyl chlo- r ide formed b y direct substitution of chlorine into the ethylene. (That the vinyl chloride might be formed by pyrolysis of an intermediate addition product is dis- proved by the fact that l12-dichloro- ethane is relatively stable a t the tempera tures a n d flows employed in the chlorination.) The high temperatures required for s u b s t i t u t i o n of chlorine into ethvl-

FIGURE 5. INFLUENCE OF TEMPERA- TURE ON THE CHLORINATION OF VARI-

OUS HYDROCARBONS

ene indicate that more rigorous conditions are required for substituting chlorine into the vinyl position than into the allyl position in an olefin. This is in line with the observation that the monochloride fraction from the high-temperature chlorination of both propylene and 2-butene contains about 96 per cent allyl-type and only 4 per cent vinyl-type unsaturated monochlorides.

The temperature required for substituting chlorine into the allyl position in unbranched olefins also varies with the nature of the olefin. This is evident from a comparison of the results obtained from 2-butene and 2-pentene with those from the chlorination of propylene. The temperature required for substituting chlorine into propylene is about 100 o C. higher than for the higher olefins. There is some indica- tion that 2-butene requires a slightly higher temperature than 2-pentene to form substitution products, but this difference could not be determined accurately with the apparatus available.

TABLE VI. INFLUENCE OF TEMPERATURE ON THE CHLORINATION OF VARIOUS HYDROCARBONS ' Mole Ratio, Reaotor Space Velocity Preheat Av. Temp. of 75 of Mole %

Hydrocarbon CL Rete Hydrooarbon/Clz Dimensions per Seo. Temp. Chlorination Unreacted Cln HC1 Formed O./min. Inches 0 C. 0 C.

0.38 7.7 18/4 X 22 0.02 200 198 19.0 0.0 21.6 255 254 0.0 0.02

300 314 0.0 47.3 0.38 7.7 Is/, X 22

8.9 l P / 4 X 22 0.06 0.04 300 326 1 . 3 78.2

0.88 361 0.7 90.3

1.26 4.1 18/4 x 22 0.05 350

400 436 0.0 103.5 1.30 4.6 Is/, X 22 4.30 6.6 l a / , x 22 0.21

Ethylene

3 .4 Is/, x 22 0.02 200 210 0 . 9 25.3 300 320 1 . 0 77.5

Propylene 0.70 6.0 Is/, X 22 0.08

4.59 6 .3 Is/, X 22 368 400 0 . 5 96.5 1.73

0.22 103.0 6 .3 a/d X 14 1.84 500 510


With this new evidence on the substitution reactivity of olefins, a theoretically attractive comparison of the behavior of all types of olefins can be made. The approximate critical temperature a t which chlorination of the olefins changes from addition to substitution with preservation of the double bond is as follows :

Olefin

Critioal Temp. Range, c.

Isobutene and other "tertiary olefins" Below -40 2-Pentene 125-200 2-Butene 150-225 Propylene 200-350 Ethylene 250-350a

a In the first paper (4) i t was shown that when ethylene and ohlorine were mixed a t room temperature and reacted over oaloium chloride, only a small amount of substitution ooourred when the temperature rose t o 337' C. Chlorination took plaee over a wide temperature range, and the results are not strictly comparable to the measurements a t oonstrtnt temperature.

This indicates that the nature of the carbon atoms joined by the double bond is the deciding factor for the substitution reactivity of the compound with chlorine. Ethylene containing only primary carbon atoms is least reactive. Propyl- ene which contains one secondary unsaturated carbon atom is easier to substitute. This tendency is somewhat further increased if both carbon atoms joined by the double bond be- come secondary as in 2-butene. The largest change in reactivity, however, takes place when one of the carbon atoms becomes tertiary. In this case the only reaction known is one which leads predominantly to the unsaturated substitution product. Whether isobutene rightfully belongs in the series listed above is not yet certain. One attempt to decide this question was the experiment a t -40" C. described in the second paper (2) of this series. Since the low temperature did not cause the expected shift from substitution to addition, the question as to whether there is a critical chlorination temperature for isobutene remains open.

TABLE VII. APPROXIMATE RATE OF CHLORINATION OF PROPYL- ENE BY SUBSTITUTION AND ADDITION (DATA FROM TABLE 11, B)

Mole c11 Av. Ratio, Reaot-

Reao- Pro- ing by C12 Reaoted per 100 Cc. Reactor Vol. tor pylene Substi- By addi- By substi-

Temp. Ch tutiona tion tution Total O c. % ----Grams p e r min.- 210 3.44 25.3 0.061 0,020 0,081 320 6.03 77.5 0.046 0.155 0.200 400 6.34 96 ,2 0.020 0.510 0.530 510 6.32 98.7 0.063 4.01 4 .06 590 6.6 99.7 0.033 10.9 10.9

(1 Estimated at 210-400 C., inclusive, by measurement of HC1 formed. Above 400' C. the amount of saturated dichloride formed was determined and the substitution oaloulated hu differenoe.

We would also expect some connection between the chlorination reactivity of these hydrocarbons and the known tendency of some of their derivatives to undergo various types of rearrangements ; examples are the allylic rearrangement and somewhat different rearrangements of compounds containing tertiary carbon atoms which have been investigated in these laboratories. The fact that the order in which the olefins are listed above coincides with the order of decreasing reac- tivities with acids is also worth noting. All these relations may prove of value in determining the mechanism for the substitution of halogens into olefins.

The results from a few tests on the chlorination of propane are also of theoretical interest in this connection. Using space velocities comparable to those necessary to complete the chlorination of propylene a t the same temperature, no reaction was observed at 200" C., but the reaction was complete a t about 270" C. The temperature at which substitution into propane occurs is therefore slightly lower but not greatly different from that required for propylene.

The olefins, however, differ from the saturated hydrocarbons in that reaction is possible by addition as well as by substitution. Although substitution products are formed almost entirely by the chlorination of propylene at 600" C., a small amount (0.3 per cent) of dichloropropane is present in the reaction product. The influence of temperature on the rates of the substitutions and addition reactions of chlorine with propylene are shown in Table VII. These results show clearly that increasing the temperature greatly increases the rate of substitution of chlorine into propylene but does not appreci- ably influence the rate of the addition reaction. Although there are doubtless other factors, such m surface and ratio of chlorine to olefin, which may influence the rate of either reaction, the trend of the results does not appear to be af- fected. Within certain limits, the rate of addition of chlorine to olefins in the absence of liquid phase does not change with temperature. The increase in total rate of reaction caused by increasing the temperature is almost entirely the result of an increase in the rate of the substitution reaction.

Although at present the mechanism for the reaction of olefins with halogens a t elevated temperatures is not known, it is hoped that the data submitted here will prove useful as a basis for a study of the kinetics of the reaction.

Basis of Conclusions

.

The apparatus employed in this investigation was of the type shown in Figures 1, 2, and 4. T,he reaction conditions are shown in Tables I, 11, IV, and VI together with the yield of products. The only other experiments from which conclusions are drawn are those on the pyrolysis of dichloropropane and the chlorination of allyl chloride. The reaction conditions employed in these instances were as follows :

Dichloropropane at a rate of 20 cc. per minute was passed through a tube 18/18 inch in diameter and 22 inches long (2.06 X 55.9 cm.) inserted in a steel block heated to 700" C. The products recovered were 29.3 mole per cent of hydrogen chloride, 23.7 of monochlorides, and 67.6 of dichloropropane. The monochloride fraction contained 58 per cent allyl chloride, 40 per cent I-chloro-1-propene, and 2 per cent 2-chloropro ene The yield of allyl chloride on dichloride consumed was txerefore 42 mole per cent; of 1- and 2-chloro-1- ropene, 31 mole per cent.

In an apparatus similar to that sfown in Figure 1, 2.9 grams of chlorine per minute were mixed in a mole ratio of 1 to 2.1 with allyl chloride at 100" C. and passed through a tube 1/2 inch in diameter and 20 inches long (1.27 X 50.8 cm.) heated to 500" C. On distillation the dichloride fraction contained 11 per cent of l,l-dichloro-2-propene boiling at 84.4' C., 47 per cent of 1,3- dichloropropene boiling at 112.1 ' C., and 42 per cent of 1,3-dichloropropene boiling at 104.1 O C.

Acknowledgment The authors thank E. C. Williams for his active interest

in this work and are indebted to D. S. La France and M. L. Adams for skillful performance of experimental work.

Literature Cited (1) Bichowsky and Rossini, "Thermochemistry of Chemical Sub-

(2) Burgin, Engs, Groll, and Hearne, IND. ENG. CHEM., 31, 1413

(3) Groll, Hearne, Burgin, and La France, U. S. Patent 2,130,084

(4) Groll, Hearne, Rust, and Vaughan, IND. ENG. CHEM., 31, 1239

(5) Hubner and Geuther, Ann., 114, 36 (1860). (6) Kharasch, Engelmann, and Mayo, J. Org. Chem., 2, 301 (1937). (7) Landolt-Bornstein, Physikalisch-chemische Tabellen, 3rd Suppl.,

Vol. 111, pp. 2615-16, Berlin, Julius Springer, 1936. (8) Romburgh, van, BUZZ. SOC. chim., [2] 36, 551 (1881). (9) Stewart and Weidenbaum, J. Am. Chem. Soc., 58,98 (1936). (10) Williams, E. C., IND. ENG. CHEM., News Ed., 16, 630 (1938). (11) Williams, G., J. Chem. Soc., 1932, 1747. PRWENTED before the Division of Organic Chemistry at the 97th Meeting of the American Chemioal Society, Baltimore, Md.

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(Sept. 13, 1938).

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planta piloto para la producción de cloruro de alilo groll hearne

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