[CONTRIBUTION FROM
THE
LABORATORIES O F SHELL DEVELOPMENT COMPANY]
T H E OLEFIN-OXYGEN-HYDROGEN BROMIDE PHOTO-REACTION’ FREDERICK F. RUST
AND
WILLIAM E. VAUGHAN
Received July 1 , 1942
Oxygen has been shown by Kharasch, Mayo, and their co-workers ( 5 ) to be an effective catalyst for the ‘Labnormal”or chain addition of hydrogen bromide t o olefins. However, oxygen is also known to retard chlorination and bromination chains (10) which, in common with “abnormal” hydrobromination, involve halogen atoms and alkyl radicals. The recent demonstration (11) that this latter reaction can occur in the vapor phase permits the investigation of the effect of larger concentrations of oxygen than has heretofore been possible. In agreement with expectations, we have found that high concentrations of oxygen do inhibit photo-hydrobromination. Of particular interest is the ultimate fate of the oxygen. Urushibara and Simamura (9) and Simamura (8), who have investigated the liquid-phase interaction of oxygen, hydrogen bromide, and allyl bromide, state that water, 1,3dibromopropane, 1,2,3-tribromopropane, and an unidentified peroxide are products of the reaction. This last compound they assume to be the peroxide obtained by Bockemuller and Pfeuffer (2) from the bromination of allyl bromide in an atmosphere of oxygen, namely bis-1 ,3-dibromoisopropyl peroxide. Offhand, it seems rather unexpected that such peroxidic constituents would persist for long in an atmosphere of hydrogen bromide. In actuality, bromoacetone, which we have identified in the C3Hs-HBr-02reaction product, simulates peroxides in its ability to oxidize acidified potassium iodide, ferrous ion, and numerous other reagents ordinarily used for peroxide identification. No positive evidence of peroxidic compounds was obtained. I n agreement with earlier work, bromine addition products were found, and another product, bromohydrin, is a major constituent when oxygen is in excess. The complex reaction mixture is probably best explained by assuming an initial photo-oxidation of hydrogen bromide analogous to that suggested by Cook and Bates (3) for hydrogen iodide. Bromine atoms produced by photolysis lead to some “abnormal” hydrobromination. This is accompanied by the formation of bromohydrin and dibromide from olefin and the products of the hydrogen bromide oxidation-bromine and water. In the case of propylene, subsequent oxidation of the bromohydrin by bromine is most likely responsible for the bromo ketone. It has been found that this latter compound is an unusually effective catalyst for the ‘Labnormal”addition of hydrogen bromide to olefins. K 4 T E R I A L S AND TECHNIQUE
The materials, apparatus, and technique were the same as described previously (11). I n brief, hydrogen bromide was prepared by combination of the elements and purified by 1 Presented before the Division of Organic Chemistry of the American Chemical Society a t its 103rd meeting, Memphis, Tennessee, April 20-23, 1942. 491
492
F. F. RUST AND W. E. VAUGHAN
repeated distillation a t low temperatures and high vacuum. Ethylene and propylene were similarly purified. Oxygen was passed through a liquid air-cooled trap and used without further treatment. A three-liter Pyrex flask, fitted with a quartz window (5 cm. dia.) served as the reactor, and the pressure changes during illumination with the General Electric Company Laboratory Uviarc were followed by means of a quartz spiral manometer used as a null point instrument. The bulk of the product condensed out and collected in a small appendix on the bottom of the reactor. The vapor-phase product and unconsumed oxygen were passed through a liquid air-cooled trap and the condensate added to the material in the appendix. Analysis was difficult because of the small amounts with which one had to work. RESULTS AND DISCUSSION
EthyEene. I n Figure 1 is shown the course of the pressure change in an illuminated mixture of ca. 200 mm. each of hydrogen bromide, oxygen, and ethylene. It is seen that reaction is much slower than that occurring in the vapor phase in the absence of oxygen (compare with Figure 3 of Reference ll),reaching a point of essentially constant pressure only after 3.5 hours compared t o 10 minutes. Approximately 8.5 cc. of a two-phase product containing water was collected from five such runs. The water-extractable fraction amounted to ca. 4.0 cc. and an insoluble portion to ca. 4.5 cc. The water extract was distilled and the resulting constant-boiling mixture (99.0-99.5') saturated with sodium sulfate, yielding a heavy oil. A second distillation removed dissolved water from the oil. The 3 ,li-dinitrobenzoate derivative was then prepared and recrystallized from an alcohol-water solution. Its melting point was 81-82'. The same derivative of ethylene bromohydrin (Eastman Kodak, redistilled) and a mixture of the known and unknown esters also melted in the same range. Ethylene bromohydrin is therefore one product of the reaction. The water-insoluble material was distilled in microcolumn. A few drops of low-boiling liquid were obtained which had the index ni'1.4240. Ethyl bromide has the index ni'1.4239 (4). The major fraction (ca. 4 cc.) boiled a t 130.4' and had the index ni'1.5375. The literature (4) gives for the boiling point of ethylene dibromide 131.6' and nk'1.5379. A test for aldehyde was easily obtained with Schiff's reagent, although indications were that the amount was very small. It might possibly have formed by decomposition of the bromohydrin. ,4cidified potassium iodide solution and ferrous sulfate-ammonium isothiocyanate solution were both very slowly oxidized by the product. Peroxide, if actually present, must have been in very small amount. Test with still another peroxide reagent, vanadic acid, was also negative. In this connection it should be mentioned that we found vanadic acid reagent (6) to be the only peroxide indicator which is not affected by bromo ketone.2
* The Japanese workers (8, 9) based their conclusion of the presence of peroxide in the hydrogen bromide-oxygen-allyl bromide reaction product solely on the liberation of iodine from acidic potassium iodide solution. With Dr. B. Barnett we have attempted to duplicate the liquid-phase experiments of these workers. Ketonic material (presumably sym-dihromoacetone and its hydrolysis product) has been positively identified. While this material is capable of reaction with potassium iodide, vanadic acid reagent is unaffected by it.
493
PHOTO-REACTIONS WITH HYDROGEN BROMIDE
Propylene. When propylene was substituted for ethylene in the olefinoxygen-hydrogen bromide mixture, the photo-reaction was complete in fifty minutes as compared with three and one-half hours in the case of ethylene (see Figure 1). However, oxygen does inhibit the “abnormal” addition reaction which in the absence of oxygen occurs in approximately ten minutes. The water-containing, two-phase product was extremely lachrymal. This material, which presumably consisted in part of CH3COCH2Br, gave a brominecontaining precipitate with 2,4-dinitrophenylhydrazine, and its hydrolysis proI
I
I
I
I
I
1
I
I
600
560
520
LEGEND
o---O
480
x--3(
199.0 MM.C2Hq-204.9MH.02- 198.4 MM. HBr 209.6 MM. C3 He 2070 MM. 02 * 199.1 199. I MM. Her
-
i
I w
440
LL
3 v) rA
400
a
360
320
280
240
I
I
I
I
20
40
60
80
I
I
100 I20 TIME, MINUTES
I
I
I
140
160
180
200
FIG. 1. PHOTO-REACTION OF OLEFIN-OXYGEN-HYDROGEN BROMIDE (Full quartz mercury arc radiation)
duct (presumably CH&OCH20H) reduced both ammoniacal silver nitrate and Fehling’s solution in the cold. There was no positive test for aldehyde. Both acidified potassium iodide and ferrous sulfate-ammonium isothiocyanate mixture were oxidized by the product. However, vanadic acid solution, which readily gives a red color with ether peroxides, gave no test. The behavior of a sample of prepared bromoacetone toward these peroxide reagents was in agreement with the responses of the product. A portion of the reaction mixture was extracted with water, and the aqueous solution, to which was added hydroxylamine to remove bromoacetone, was
494
F. F. RUST AND W. E. VAUGHAN
distilled. A sweet smelling oil which separated when the distillate was saturated with sodium sulfate, after drying contained 57.0% bromine (theory for propylenebromohydrin: 57.5%). The water-insoluble portion of the product was distilled in a microcolumn, and a first fraction obtained which had the boiling point 69.7" [n-propyl bromide b.p. 70.9" (4)]. The higher-boiling material contained not only propylene dibromide but also high molecular weight compounds which seemed to be condensation products of bromoacetone. This fraction was washed three times with 10% sodium hydroxide solution, then with water, dried and distilled. The material so obtained contained 77.9% bromine and had the boiling point 142.0" [theory for propylene dibromide: 79.2% Br and b.p. 141.6" (4)]. There is a possibility that the bromohydrin and the bromoacetone might have a common origin in an unstable perokide formed by interaction of the l-bromoisopropyl radical and oxygen in accordance with a chain mechanism such as the following : HBr
Br
+ RCH=CHz
I
RCHCHzBr
+ HBr
hv
H
-
+ Br
1."
Photolytic initiation
I
--+ RCHCH2Br
Abnormal?' addition
+ Br
RCHzCH2Br 0 2
I
RCHCHzBr
I + O2 --+RCHCHzBr OzH
0 2
I
RCHCHZBr
Possible chain for bromo ketone formation
I + HBr --+ RCHCH2Br + Br 0
OzH
I
II
RCHCHZBr -RCCH2Br
-
OzH
+ H20
OH
I + 2HBr RCHCH2Br + Br2 + H2O I RCHCHzBr + Br ---+ RCHBrCHZBr
RCHCHZBr
7.
Over-all reaction for 1-bromo-2-propanol
or Br RCH=CHs
+ Br --+M + Brz
i
Bra
__f
9. 10.
RCHBrCH2Br
or
I
RCHCH2Br
+ Brz
RCHBrCHZBr
+ Br
11.
Of course, the H atom could also act as a chain initiator,
' Reaction 4 possibly leads to chain termination; see text.
Olefin dibromide formation
PHOTO-REACTIOKS WITH HYDROGEX BROMIDE
495
If the bromohydrination of propylene were entirely analogous to liquid-phase chlorohydrination, one should find two isomeric products, 1-bromo-2-propanol and 2-bromo-1-propanol, while if the foregoing free radical chain were operative, only 1-bromo-2-propanol would be found (Reactions 6 and 7). Unfortunately, no distinction between these alternative mechanisms is possible, because only 1-bromo-2-propanol is formed during liquid-phase bromohydrination. Purified product from the addition of hydrogen bromide to propylene oxide and from the interaction of propylene and bromine water had almost identical refractive indices, nE’1.4768 and 1.4767 respectively. Certain facts, however, favor the belief that, with the exception of the small amount of “abnormal” addition (Reactions 2 and 3), the olefin reacts primarily with the products of the hydrogen bromide photo-oxidation, namely, bromine and water, yielding bromohydrin. The presence of bromoacetone is readily explained by the oxidation of the bromohydrin by bromine. This idea receives support from the fact that when propylene is bubbled into bromide water, some bromoacetone is formed. The foregoing evidence would indicate that the rate of formation of monobromides (ethyl bromide and n-propyl bromide), presumably formed via a chain mechanism, is greatly reduced by the presence of a large amount of oxygen. The inhibiting reaction is probably of the type R
+
0 2
+ ROr
wherein R represents a radical. The fact that under identical conditions the propylene-oxygen-hydrogen bromide reaction is faster than the corresponding change with ethylene, demonstrates the effectiveness of bromoacetone as a catalyst (see the following section). When ethylene is involved there is no opportunity for the generation of such a catalytic compound. Catalysis of “abnormal” h ydrobromination by bromoacetone. Since the generation of bromine atoms (for example, by interaction of hydrogen bromide and oxygen, or by photolytic dissociation of the halide) is the source of reaction centers, the use of bromo ketone suggest itself as another interesting mode for catalyzing (‘abnormal” addition. Minute amounts of bromine released in accord with the equilibrium (1):
+ HBr S CH3COCH3 + Brz
CH3COCH2Br
should supply new centers for chain initiation. To test this theory, bromoacetone was prepared according to the directions given in Organic Syntheses (7). The product was vacuum distilled, transferred to storage vessels, and carefully degassed on the high vacuum line. Propylene (3.4 cc.), hydrogen bromide (1.9 cc.), and bromoacetone (0.2 cc.) were distilled into a Pyrex bomb tube a t -78” and sealed off in the absence of air. The tube was shrouded from even diff’use light and after ten minutes, during which time the contents warmed from liquid air temperature, the tube was cooled in solid carbon dioxide and the product removed. iifter washing with dilute caustic and water, the material was dried and distilled. A quantitative yield of pure n-propyl bromide was obtained (n:’, found, 1.4340; theory, 1.4341).
496
F. F. RUST AND W. E. VAUGHAN
The same procedure was followed with butene-1 (4.4 cc.), hydrogen bromide (3.2 cc.), and bromoacetone (0.1 cc.), except that the total reaction time was reduced to five minutes. Again the yield was quantitative and apparently the product was pure n-butyl bromide (n;’, found, 1.4400; theory, 1.4398). As a check on the foregoing experiments, the same procedure was again followed with respect to a mixture of acetone (0.3 cc.), propylene (3.8 cc.), and hydrogen bromide (2.1 cc.), except that the reaction time was lengthened to two hours. In this case only isopropyl bromide was formed (n;’, found, 1.4252; theory, 1.4251). These results demonstrate conclusively that bromoacetone can provide initial centers for the “abnormal addition” of hydrogen bromide. SUMMARY
1. The presence of large concentrations of oxygen inhibits the photo-reaction of hydrogen bromide and olefins (ethylene and propylene). 2. The products of these retarded reactions include the n-monobromide, dibromide, bromohydrin, and water. In the case of propylene, bromoacetone is also formed. No peroxidic compounds were found. 3. Bromoacetone (and by analogy any a-bromo ketone) acts as a powerful catalyst for the “abnormal” addition of hydrogen bromide to olefins, even in the dark. EMERWILLE,CALIF. REFERENCES (1) ALTSCHUL AND BARTLETT, J . Org. Chem., 6, 623 (1940). (2) BOCEEMULLER AND PFEUFFER, Ann., 637, 178 (1939). (3) COOKAND BATES,J . Am. Chem. SOC.,67, 1775 (1935). (4) Handbook of Chem. and Phys., C. D. Hodgman, Ed., Chem. Rubber Pub. Co., Cleveland, 22nd ed., 1937. (5) See MAYOAND WALLING,Chem. Rev., 27, 351 (1940) for a good review of the work. (6) Merck’s Index, Merck and Co., Inc., Rahway, N. J., 5th ed., 1940, p. 781, No. 2048. (7) Organic Syntheses, H. T. Clarke, Ed., Wiley, N. Y., 1930, Vol. X, pp. 12-13. (8) SIMAMURA, Bull. Chem. SOC.Japan, 16, 292-7 (1940). AND SIMAMURA, Bull. Chem. SOC.Japan, 14,323 (1939). (9) URUSHIBARA (10) See, for example, VAUGHAN AND RUST,J . Org. Chem., 6, 449 (1940). (11) VAUGHAN, RUSTAND EVANS, J . Org. Chem., 7, 477 (1942).
