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 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 investigators 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 straightchain 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 molecules, 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 phenomenon 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 homologs 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,
A
1 Present address, Rhenania-Ossag Germany.
Mineralolwerke A.-G., Hamburg,
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 t o 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 t o 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 ex1530
DECEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
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 unstable 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
1531
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: Temperature. 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 decomposition 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 Propylene". Mole Ratio of Propylene to Chlorine. 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. Throughput. 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
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 XC/ Scrubber of glycerol from petroleum. Cb F/ow Mefers Therefore, as soon as the Fee feasibility of preparing allyl chloride had been demonstrated, the problem of assembling 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 designed and operated for over a year. Part of this Cb/amnafed PmducT Producf has been employed for the FIGURE 1. APPARATUSFOR HIGH-TEMPERATURE CHLORINATION OF OLEFINS 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 propylallyl 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
INDUSTRIAL AND ENGINEERING CHEMISTRY
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VOL. 31, NO. 12
TABLE I. HIGH-TEMPERATURE CHLORINATION OF PROPYLENE WITH GASES MIXEDAT Low TEMPERATURE AND REACTED AT ELEVATED TEMPERATURES ,---
Reaction Conditions
Reactor Dimensions DiBlock Length ameter temp. In. (cm.) In. (cm.) C.
Wall temp.
c.
Distance from entrance t o "hot spot"
In. (cm.)
7
Feed of chlorlne G./min.
M 01 e ratio, propylene chlorine A.
1 (2,54) 384 470 1 510 1 550
20 (50.8) 20 20 20 20
1 1
600
20
1
650
20
1
708
20 20 20 20
1/z
'/a 1/2 1/2
(1.27) 525 525 528 525
405 486 525 556 Not detd. Not detd. Not detd.
555 546 530 518
11 (27.9) 9 (22.9) 7.5(19.1) 7 (17.8)
1
600
20
1
600
20
1
650
20
1
650
20
1
706
20
1
708
*/a (0.95) 20 '/a (1.27) 20 20 8/,(1.91 20 l(2.541 10(25.4) 8/4(1.91)
500 528 475 510 500
20 (50.8) 1/z (1.27) 500 '/a 500 20 20 20
:$:
500 500
20
1/2
500
Not detd. Not detd. Not detd. Not detd. Not detd. Not detd.
HC1
1.76 1.89 1.87 2.10 1.98
42.3 46.4 48.4 48.4 47.3
Not detd.
42.0
2.07
Not detd.
49.3
2.01
Not detd.
16 12 12 12
Unreacted
Balance of Chlorine Unsatd. Dimonochlochloride ride
Applied-TriTotal chlo- accounted ride for
B. I N F L U E N C E O F (40.6) 18.4 1.3 (30.5) 10.1 1.7 6.5 2.1 3.0 3.0
0.1 0.1
