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Oxidation of Organic Compounds Downloaded from pubs.acs.org by NORTH CAROLINA STATE UNIV on 09/16/18. For personal use only.
Butene Hydroperoxide W I L L I A M F. B R I L L Princeton Chemical Research, Inc., Princeton, N. J. Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, 1-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography. Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. 1-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene.
^ U y l i c hydroperoxides are primary products i n the autoxidation of olefins, and lack of definite information on their reactivity and chemical behavior has hampered efforts to understand olefin oxidation mechanisms (2). This deficiency is most strongly felt in determining the relative rates of addition and abstraction mechanisms for acyclic olefins since assignment of secondary reaction products to the correct primary source is required. Whereas generalizations about the effect of structure on the course of hydroperoxide decompositions are helpful, most questions can be answered better by directly isolating the hydroperoxides involved and observing the products formed by decomposition of the pure compounds. W i t h the development of techniques for preparing and isolating acyclic allylic hydroperoxides, an attempt was made to examine the behavior of butene hydroperoxide and determine which secondary products arise from it during the oxidation of butènes. Butènes were of particular interest since indirect evidence had already indicated that none of the readily identified products can be ascribed unequivocally to a hydroperoxide source (4). 93
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The preparation of acyclic allylic hydroperoxides has been described before (3, 7, 9), but it is not clear how the reactivities differ from the better known saturated hydroperoxides and cyclic allylic hydroperoxides. Dykstra and Mosher prepared allyl hydroperoxide by the reaction of allyl methanesulfonate with hydrogen peroxide and alcoholic potassium hydroxide and purified the hydroperoxide by gas chromatography. It detonated on heating and decomposed on exposure to light but was relatively stable in the cold and dark. The isomeric allylic hydroperoxides formed from the autoxidation of the branched olefin, 4-methyl-2-pentene, have also been isolated and were not abnormally reactive (3). In the present study, cis- and irans-2-butene were photooxidized in the presence of methylene blue as a sensitizer (14), and the product, l-butene-3-hydroperoxide, was isolated by preparative chromatography. 1-Butene proved unreactive and 2-butene-l-hydroperoxide could be formed only by isomerization of the secondary hydroperoxide. Experimental Materials. Olefins were Phillips pure grade, appearing to be better than the reported 99% purity by gas chromatographic analysis on an A g N 0 column. Methylene blue was U.S.P. basic blue N o . 9, and methanol was Baker spectrophotometric reagent. A l l other solvents were either chromatographic or spectroscopic grade. 3
Photosensitized Oxidation. Rapidly stirred solutions of butènes were irradiated in methanol containing 0.5 gram per liter of methylene blue at 20 °C. The reactor was fitted with a Hanovia 200-watt mercury lamp (654A036) placed in a water-cooled borosilicate glass well, and contained separate inlets for saturating the solution with butene and dispersing oxygen. Oxygen uptake was followed with wet-test meters, employing approximately a 100% excess of oxygen over that reacted and passing the effluent through an overhead reflux condenser followed by a trap, both kept at dry ice temperatures. Oxidations were most conveniently run by first dissolving approximately 1 mole of butene in 1 liter of methanol and then irradiating while bubbling oxygen through the solution without adding more butene. Variations with 1-butene involved continuously or periodically saturating the solution with olefin while attempting oxidation in the absence or presence of the reflux condenser. In all cases, only traces of product evolved. Isolation and Identification of Hydroperoxide. A solution produced by oxidizing £rans-2-butene was concentrated at reduced pressure without heating from 0.085M to 1.5M. The hydroperoxide was isolated from the concentrate by preparative chromatography under the following conditions: a 5-foot 3/8-inch column of aluminum containing 10% diisodecyl phthalate on Fluoropak 80; Autoprep 705 with flame ionization detector; carrier, 200 m l . per minute helium split 8 to 1 between trap and detector;
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injection on column; temperature of injector, column, detector, and collector, 80°. Retention times were 14 minutes for l-butene-3-hydroperoxide, 1.5 minutes for methanol, and 3 to 4 minutes for other products, which under the best conditions produce an area amounting to less than 10% of the hydroperoxide peak. The trapped product gave an immediate test with K I in acetic acid. Its infrared spectrum was similar to that of 3-butene-2-ol with major absorption peaks at 3, 8.7, 9.5, 10.3, and 10.8 microns and minor peaks at 6.3, 7.2, 7.7, 11.6, 12.4, and 12.6 microns. There was no absorption arising from carbonyl. In a 25% solution of hydroperoxide in carbon tetrachloride, the hydroperoxide proton gave rise to a broad band at 8.7 p.p.m. (referred to T M S ) in the N M R spectra. The hydroperoxide could also be isolated i n purities above 90% by careful distillation at low temperatures at 8 mm. Methanol was first removed through an 8-inch Vigreux at a 4 to 1 reflux ratio and a pot temperature of 20°C. A sample of the methanolic oxidation product was treated directly with powdered sodium borohydride. Water was added, the solvent allowed to evaporate, and the aqueous solution extracted with carbon tetrachloride. Methyl vinyl carbinol was shown to be the major product, but about 10% impurities were present. Reduction by stirring overnight with 1.0M sodium sulfite followed by extraction with carbon tetrachloride produced a solution which analyzed as methyl vinyl carbinol with about 1 % impurities. Experiments w i t h 1-Butene-3-hydroperoxide. F r o m 0.5 to 2 m l . of solvent was added directly to the traps in which about 0.1 m l . of hydroperoxide had been collected by preparative gas chromatography, and the solutions were transferred to heavy-walled 8-ml. tubes and sealed with Teflon-lined caps. In several instances, it was necessary first to centrifuge the solutions to remove moisture apparently accumulated during the collection period. The rate of thermal decomposition of a 0.070M solution i n benzene at 100° was determined by following the decrease in the hydroperoxide area by gas chromatography, using the diisodecyl phthalate on a Fluoropak column. The isomerization in hexane was allowed to proceed at room temperature until the ratio of the area of l-butene-3-hydroperoxide (13.5 minutes) to the new peak (at 25.5 minutes) became constant i n 14 days. The combined peak areas for a constant sample size (30 /Jiters) showed little decrease on longer standing. The isomerized mixture (0.7 ml.) was stirred with 0.3 m l . of 1.0M sodium sulfite for 16 hours. The hexane layer was separated, and the aqueous layer was saturated with sodium sulfate and extracted twice with 0.5-ml. portions of hexane. The combined extracts were found to contain 8 1 % 3-butene-2-ol and 19% crotyl alcohol. Analysis. Hydroperoxide was analyzed by gas chromatography under the sample conditions described for the isolation procedure; only
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10- to 30-/xliter samples were injected. Decomposition products were analyzed using a 12-foot, 1/8-inch column of 10% Carbowax 2 0 M terephthalic acid on Gas Chrom Q. The temperature was programmed from 60° to 190°C. at 8° per minute. Orders of the retention times for the solvents and more important possible products were: acetaldehyde, propionaldehyde, acetone, acrolein, methanol-methyl ethyl ketone, carbon tetrachloride, benzene-methyl vinyl ketone, water-2,3-butandione, crotonaldehyde, methyl vinyl carbinol, allyl alcohol, crotyl alcohol, and the "dimer" of methyl vinyl ketone. Since the quantities available d i d not allow isolation of the products, each peak was confirmed by adding the known compound to the solution being examined and observing the increase i n peak height or the appearance of a new peak. Di-2-ethyl sebacate absorbant was used to separate benzene and methyl vinyl ketone and to provide additional confidence in product identity. Hallcomid separated benzene, methyl vinyl ketone, and methyl vinyl carbinol but gave poor resolution of the lower boiling carbonyl compounds. Reactions in Methanol. By dropping 434 grams of the deep blue solution from the photosensitized oxidation of frans-2-butene onto glass beads heated to 200°C. in a Claisen flask, 414 grams of clear distillate were collected over 7 hours. Iodometric titration (15) showed the oxidate to be 0.089M and the distillate to be 0.073M in hydroperoxide, indicating that 2 1 % of the hydroperoxide was lost in the flash distillation. Decomposition of the distillate i n the vaporizer of the gas chromatograph produced the normal amounts of methyl vinyl ketone and methyl vinyl carbinol and a trace of crotonaldehyde. Considerable butene also remained in the distillate. A 0.09M oxidation solution was refluxed in the presence of 1.0 mole % of cobaltous chloride hexahydrate for 20 hours. The concentration of hydroperoxide was reduced to 0.007M. The products consisted of 3 % acetone, 13% methyl vinyl ketone, 10% crotonaldehyde, 26% methyl vinyl carbinol, and 36% of a less volatile product. The major product appears to arise by the dimerization of methyl vinyl ketone since it also formed readily on refluxing methyl vinyl ketone i n methanol containing cobaltous chloride and was present i n commercial ketone. The refluxed solution was concentrated by removing the solvent through a semimicrocolumn, and the major products were isolated by preparative gas chromatography. Infrared spectra identical to those of authentic samples were obtained for crotonaldehyde and methyl vinyl carbinol. Trapped methyl vinyl ketone yielded a cloudy carbon tetrachloride solution which produced a poor but recognizable spectrum. The "methyl vinyl ketone dimer" absorbed strongly at 5.85, 6.1, and 8.95 microns. Results The photosensitized oxidation of either frans-2-butene or a mixture of cis- and fmrw-2-butene in methanol using methylene blue as a sensitizer produced a single hydroperoxide—3-butene-2-hydroperoxide— cleanly at atmospheric pressure. The hydroperoxide was reduced to 3butene-2-ol by treating the methanolic reaction solution with sodium
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borohydride or sodium sulfite. Isolation of the hydroperoxide by preparative gas chromatography yielded a pure compound whose infrared spectra resembled that for 3-butene-2-ol, and the alcohol was produced by reducing the pure hydroperoxide. Whereas a 7% solution of hydroperoxide i n methanol could be obtained from 2-butene i n a 30-hour oxidation, 1butene failed to yield detectable hydroperoxide on oxidation for 3 days. It was therefore necessary to isomerize l-butene-3-hydroperoxide by allowing a dilute hexane solution to stand at room temperature until an equilibrium mixture with 2-butene-1-hydroperoxide was produced. Direct analysis by gas chromatography showed that the solution contained 86% of the secondary and 14% of the primary isomer. Reduction with sodium sulfite produced 8 1 % of the secondary alcohol and 19% of the primary isomer, crotyl alcohol. N o attempt was made to isolate the primary hydroperoxide, and the behavior of this isomer was not studied directly. Neither the thermal nor the cobalt-catalyzed decomposition of 3butene-2-hydroperoxide i n benzene at 100 °C. produced any acetaldehyde or propionaldehyde. In the presence of a trace of sulfuric acid, a small amount of acetaldehyde along with a large number of other products were produced on mixing. Furthermore, on heating at 100 ° C , polymerization is apparently the major reaction; no volatile products were detected, and only a slight increase i n acetaldehyde was observed. Pyrolysis of a benzene or carbon tetrachloride solution at 200°C. i n the injection block of the gas chromatograph gave no acetaldehyde or propionaldehyde, and none was detected i n any experiments conducted i n methanol. Decomposition of a 0.07M solution of 2-butene-3-hydroperoxide i n benzene at 100°C. had a half-life of 23 hours. The final products were 37% methyl vinyl ketone, 37% methyl vinyl carbinol, and 26% acetone. Attempts to substantiate the identity of the acetone were unsuccessful, but it was the only anticipated product with the correct gas chromatographic retention time on four different absorbents. In the presence of approximately 1 % cobalt naphthenate i n benzene, only 4 hours at 100°C. were required to decompose the hydroperoxide almost completely. The yields of products from decompositions catalyzed by some commonly used cobalt and vanadium ( 1 ) compounds are given in Table I. Polymerization appears to be the major reaction. The cleanest product composition may be effected by decomposition of the pure hydroperoxide or solutions in the injection block of the gas chromatograph. In carbon tetrachloride solution only methyl vinyl ketone and methyl vinyl carbinol were produced, the ratio of ketone to alcohol being 2.9. N o definite traces of products from isomerized hydroperoxide were observed. The hydroperoxide appears to be exceptionally stable i n methanol, no isomer appearing after several months' standing. Flash distillation
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Catalyzed Decomposition of 1-Butene3-hydroperoxide in Benzene
Table I.
a
Theoretical Yields, % Catalyst-Conditions 1.84M hydroperoxide 0.5% Co naphthenate/ 85°C. Vapor phase/ 200°C. 1.45M hydroperoxide 1% Co ( C H 0 ) , 55°C. V O ( C H 0 ) , 55°C.
MVK
MVC
4 53
20 18
22 7
16 20
Other
6
B
5
7
2
7
2
8
f
f c
d
Reaction time in liquid phase, 24 hours. Titrometric (15); calculated from wt. of hydroperoxide, 1.89M. Nuodex 6% cobalt. Approximately 4% acetone. O n injection to gas chromatograph, est. 2-sec. reaction time. Approx. 7% acetone. ' MacKenzie Chemical Works cobaltic acetylacetonate; stirred flask, absorbed 0.3 mole 0 / m o l e peroxide. Saturated sol, RO* + 2CHO + C H C H I
!
II
I
R O — C H — C H — R ' - » R'CHO + R O C H * 2
2
(2)
I I (3)
OO f the many studies of the autoxidation of butènes, few (5, 11) have emphasized methyl vinyl ketone and methyl vinyl carbinol as major products. In the cumene hydroperoxide-initiated oxidation of 1-butene at 105°C. with 60 atm. of air, Chernyak (5) reported an average hourly rate of production of these two products approximately equal to the combined rates of formation of hydroperoxide and epoxide. The reported rates for hydroperoxide plus vinyl ketone and alcohol indicate that 60% of the products occur by abstraction, in agreement with V a n Sickle (17). The observed half life at 100 °C. of 23 hours for a dilute solution of hydroperoxide in benzene indicates that significant decomposition may occur i n the autoxidation of butene, depending on reaction conditions. No reliable evaluation can be made because of the known complications introduced on hydroperoxide decomposition by the effect of the solvent, the hydroperoxide concentration (2), the presence of oxygen (12), and the possibility of a strong acceleration in rate in the presence of oxidizing olefin, observed in at least one system (8). However, using the data reported by Bateman for a benzene solvent at 100 °C. in the presence of air (2), l-butene-3-hydroperoxide decomposes 13 times faster than cyclohexene hydroperoxide, a product which may be formed in extremely high yield by the oxidation of cyclohexene. The free radical isomerization of allylic hydroperoxides (3) in dilute nonpolar solvents appears to give a reliable measure of their relative
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stabilities since almost no by-products form i n this process. It was previ ously concluded from the isomerization of the hydroperoxides from 4-* methyl-2-pentene that tertiary and secondary allylic hydroperoxide are equally stable. It now appears that a primary hydroperoxide represents the least stable allylic structure. The unfortunately large difference i n the reactivity of 1-butene and 2-butene to photosensitized oxidation is not surprising i n view of the previous observed structural effects i n such oxidations (10). Competitive oxidation experiments are underway to determine the exact magnitude of the rate difference involved and to evaluate the effect of impurities which might quench the reaction. Literature Cited (1) Allison, K., Johnson, P., Foster, G . , Sparke, M . , Ind. Eng. Chem. Prod. Res. Develop. 5, 166 (1966). (2) Bateman, L . , Hughes, H., J. Chem. Soc. 1952, 4594. (3) Brill, W. F., J. Am. Chem. Soc. 87, 3286 (1965). (4) Brill, W . F., Barone, B. J., J. Org. Chem. 29,140 (1964). (5) Chernyak, B. L , Koucher, R. V., Nikolayevsky, A. N., Neftekhemiya 4 (3), 452 (1964). (6) Davies, A. G., "Organic Peroxides," Butterworths, London, 1961. (7) Dykstra, S., Mosher, H. S., J. Am. Chem. Soc. 79, 3474 (1957). (8) Hiatt, R., Gould, G. W., Mayo, F., J. Org. Chem. 29, 3461 (1964). (9) Hoffman, J., J. Org. Chem. 22, 1747-1749 (1957). (10) Kopecky, K. R., Reich, H . J., Can. J. Chem. 43 (1965). (11) Koucher, R. V., Tsherniak, Β. I., Nikolayevsky, A. N . , Abadsheva, R. N . , International Oxidation Symposium, I-75, 1967. (12) Morse, Β. K., J. Am. Chem. Soc. 79, 3376 (1957). (13) Muller, Α., Ber. 5, 1142 (1921). (14) Schenck, G. O., Angew. Chem. 69, 579 (1957). (15) Siggia, S., "Quantitative Organic Analysis," 2nd ed., Wiley, New York, 1954. (16) de Roch, I. Seree, Bull. Soc. Chim. France 1962, 1774. (17) Van Sickle, D . E., Mayo, F. R., Arluck, R., Syz, M . G., J. Am. Chem. Soc. 89, 967 (1967). RECEIVED
October 2, 1967.
Discussion J. H . Knox: In liquid-phase olefin oxidation, epoxides increase with temperature relative to cleavage products and soon exceed them i n yield. In gas phase oxidation at about 300°C. epoxides are minor products
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(
