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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
undergoes a type of polymerization to produce a more complex molecule which has perhaps some unsaturated character. Subsequent reactions of this complex molecule may then produce CZ and CH by sufficiently exothermic processes. The excitation of these species may then either be due to the exothermicity of these reactions or to subsequent processes involving other active species in the flame. In the absence of more detailed experimental information with regard to the production of these species it does not seem possible at the present time t o decide between these two alternatives. ACKNOWLEDGMENT
The authors wish to express their thanks to A. G. Gaydon and
F. T. McClure for their valuable suggestions in connection with this paper. LITERATURE CITED
Daly, D. F., and Sutherland, G. B. a.M., “Third Symposium on Combustion and Flame and Explosion Phenomena,” p. 530, Baltimore, Williams & Wilkins Go., 1949. Fowler, A., and Gaydon, A. G., Proc. Roy. SOC.(London), A142, 562 (1933). Gaydon, A. G., Quart. Revs., 4, 1 (1950). Gaydon, A. G., “Spectroscopy and Combustion Theory,” London, Chapman and Hall, Ltd., 1948. Gaydon, A. G., Trans. Faraday SOC.,42, 292 (1946). Gaydon, A. G.,and Wolfhard, H., Proc. Phys. Soc. (London), 6 4 ~ 3irr , (19~)
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Gaydon, A. G., and Wolfhard, H., “Third Symposium on Combustion and Flame and Explosion Phenomena,” p. 504, Baltimore, Williams & Wilkins Co., 1949. Griffing, V. F., and Laidler, K. J., Ibid., p. 432. Herman, R. C.,and Hornbeck, G. A., J . Chem. Phys., 17, 1344 (1949).
Herman, R. C., Hornbeck, G. A.. and Laidler, K. J., Science, 112,497 (1950).
Hornbeck, G . A., J . Chem. Phvs., 16, 845 (1948). Hornbeck. G.A..and Herman, R. C.. Ibid.. 17, 842 (1949). . .
Ibid., 18,763 (1950). Hornbeck, G. A’., and Hopfiold, H. S.,Ibid., 17, 982 (1949). Kondratjew, V.,Acta Physicoch,iin. U.R.S.S., 2, 120 (1935). Kondratjew, V., and Ziskin, M . , Ibid., 7, 65 (1937). Laidler, K.J., J . Chem. Phys., 17, 221 (1949). Laidlbr, K.J., and Shuler, K. E., Chem. Revs., 48, 153 (1951). Oldenberg, O.,and Rieke, F. F., J . Chem. Phys., 6,439 (1938). Plyler, E.K.,J . Research Natl. Bur. S’tundards. 40,113 (1948).
Polanyi, M.,“Atomic Reactions,” London, Williams and Norgate, 1932. Shuler, K. E., J . Chem. Phys., 18, 1221 (1950). Ibid., 18,1466(1950);19,888 (1951). Silverman, S,,J . Optical Soc. Am., 39,275 (1949).
Silverman, S., Hornbeck, G. A., and Herman, R. C. J . Chem. Phys., 16,155 (1948).
Steacie, E. W. R., “Atomic and Free Radical Reaotions,” New York, Reinhold Publishing Corp., 1946. Vaidya, W. M., Proc. R o y . SOC.(London),.A147, 513 (1934). RECEIYED January 26, 1951. The work described in this paper was 8upported in part by the Bureau of Ordnance, U. S. Navy, under Contract NOrd-7386.
PREFLAME COMBUSTION OF HYDROCARBONS Spectroscopic Studies o f Reaction Intermediates J. R. THOMAS
AND
H. W. CRANDALL
California Research Corp., Richmond, Calif.
T h e work described was initiated to obtain a better understanding of the reactions occurring during the preflame combustion of hydrocarbons. A recording ultraviolet spectrophotometer was employed for kinetic studies. The time dependence of the optical density was measured using light of varying wave lengths. It was found that the rate of formation of formaldehyde gradually increased during the induction period and that there was rapid production of formaldehyde during the cool flame. Compounds absorbing at 2050 A. were also formed autocatalytically during the induction period and were produced extremely rapidly in the cool flame. The intermediates which give rise to the 2600 A. absorption peak and referred to by Ubbelohde as com-
pound X were identified as @-dicarbonyl compounds. These materials are consumed during the cool flame. A considerable fraction of the oxidized hydrocarbon may go through a @-dicarbonylstructure at some time during its oxidation and such compounds may be critical materials in cool flame formation from many hydrocarbons. The tendencies of several possible intermediates to produce @-dicarbonyls were measured. I t was established that a simple aldehyde or ketone does not contribute to P-dicarbonyl formation. A mechanism to explain p-dicarbonyl formation is proposed which involves no molecular intermediates. This mechanism explains the fact that cyclopentane does not form any P-dicarhonyl compound during its preflame reactions.
OUTSTANDING characteristic of hydrocarbon reactions is their autocatalytic nature. Because of the long induction periods which may be observed in these reactions it seems necessary to attribute the catalytic effect to molecular intermediates rather than to short-lived atoms or molecular fragments (3). Consequently, the identification of these intermediates and a study of their kinetic behavipr is essential to the understanding of the mechanism of combustion. This communication describes aainvestigation of the formation of reaction products and intermediates, as determined by changes in the ultraviolet light absorbing properties of mixtures of oxygen and pure hydrocarbons during preflame reactions.
EXPERIMENTAL M E T H O D S
A Cary Model 11 recording ultraviolet spectrophotonieter was used to follow the oxidation reaction. Because of the rapid response time of the machine (0.5 second for full scale deflect@), it was possible to follow accurately the absorption a t any given wave length even in rapid reactions. I n slower reactions, it was posbible to scan the spectrum from 4000 to 2000 A. a t 45 second intervals. A quartz absorption cell, 2 em. in diameter and 10 em. in len th, was used as a reaction chamber. The cell was wrapped wit\ a Nichrome heater supplied by a constant voltage transformer. A thermocouple located next to the outside surface of the cell was used to measure the temperature. The heated cell was placed in ita normal position in the spectrophotometer and
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N-BUTANL TEMP-282.C. PRESS - 4 6 C M
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Vol. 43, No. 12
an external lead connected to the usual gas handling system containing pumps and a mercury manometer. Appropriate fueloxygen mixtures were admitted to the evacuated cell from an external reservoir to give the desired pressure of reactants. In all of the experiments reported here, the molar ratio of fuel to oxygen was unity. The hydrocarbons employed in this work were Phillips’ research grade. RESULTS
LO
3.0
The kinetic behavior of combustion intermediates is as important as their identification. The phenomenon of cool flames ( 2 ) associated with the slow combustion of most hydrocarbons affords a particularly important and interesting opportunity for kinetic studies. Typical tracings of absorption spectra from 4000 to 2000 A. taken a t various times during the combustion of n-butane and of isobutane are shown in Figure 1. To obtain each t r w r or spectrogram required 45 seconds. The time in minutes accompanying each curve corresponds to the time after the beginning of the reaction a t Jvhich scanning was started. These spectrograms show three general regions of absorption. The irregular or fine structure a t long wave lengths corresponds to formaldehyde absorption, this is superimposed upon the absorption due to 2000 3000 4M0 2000 3000 4000 higher aldehydes and ketones. .4t the shorter wave lengths the WAVE LENGTH-A. absorption is presumably due to peroxides, acids, aldehydw, and possibly to conjugated unsaturates. The absorption peak a t Figure I. Cool Absorption Spectra as a Function of Time about 2600 A. found with n-butane and higher hydrocarbons, Cool name combustion of n-butane and irobutdne but absent in isobutane and propane, is due to an unknown intermediate. This peak appears to be identical with that observed by Egerton and Pidgeon ( 1 ) . Ubbelohde (4)attempted unsuccessfully to characterize this intermediate. The isolation and identification TEMPERATURE 304% of the compound responsible for the band a t 2600 A. PRESSURE 37 CM. HG. will be discussed in detail in the following paper. The behavior of formaldehyde as a function of time (measured by absorption a t 3390 A.) during a typical preflame oxidation of n-butane is shown in Figure 2. As nearly as could be determined the abrupt change in optical density occurred a t the same time as the pressure pulse characteristic of cool flame formation. During the induction period preceding the cool flame, the formaldehyde was produced at a slow but accelerating rate. At the time of the occurrence of the cool 0 io io 0 80 IO0 lio I& 160 Is0 TIME-SECONDS flame, there was rapid production of formaldehyde. Following this, the formaldehyde concentration reFigure 2. Optical Density at 3390 A. as a Function of Time mained constant. Cool name combustion of n-butane The behavior of the unknown intermediate during a typical cool flame combustion of n-butane, as measured by its absorption a t 2575 A. is shown as a function of time in Figure 3. I n this instance, two successive cool flames occurred as inTEMPERATURE -300% PRESSURE-38 CM. HG. dicated by the apparently instantaneous decreases in optical density. As in the case of formaldehyde, n 4 C O O L FLAME the rate of formation of this material also increased during the induction period. However, simultaneously with the occurrence of the cool flame, the optical density a t 2575 A. decreased abruptly. Similar results were obtained during the cool flame combustion of n-pentane, as shown in Figure 4. It has been noted in many experiments, both with these and other hydrocarbons, that when a 2600 A. absorption peak is encountered there is always an increase in the optical density at this wave length prior to cool flame occurrence. It is therefore concluded that for a cool flame to occur in a hydrocarbon that produces a 2600 A. peak, the concentration of this intermediate must increase. I DO Ibo ti0 l i 0 I30 A 0 Figure 5 illustrates the behavior of the optical density a t 2050 A. during the cool flame combusFigure 3. Optical Density at 2575 A. as a Function of Time tion of propane. Here it is seen that the concenCool flame combustion of n-butane
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tration of the inproducts are formed. The details of the mechanisms whereby these compounds are produced are difficult to prove and are termediates having absorption in known in only a few instances. It is hoped that investigations now in progress will furnish further evidence concerning the this region inTEMPERATURL-I&?*C. combustion mechanism. creases rapidly PRLSSURE-42.5 CM.’HG. One of the most interesting observations made in this work is during the cool 2.5 the remarkable decrease in the concentration of the unknown flame reaction. intermediate which absorbs a t about 2600 A. during the cool As is discussed below, peroxides flame. All other reaction intermediates which include formprobably account aldehyde and peroxides appear to increase in the cool flame. for a major part Although it is not necessary that the concentration of a critical of the absorpintermediate decrease during the pressure pulse, the nature of the tion observed at cool flame phenomena, and in particular the feature of flame 2050 A. periodicity, may find explanation in an intermediate whose conAlthough it is centration increases to a critical point resulting in the occurrence not intended to of a cool flame and consequent destruction of the intermediate. The fact that hydrocarbons which give rise to the 2600 A. abexamine the 2050A. absorption sorption peak only produce cool flames when this band is inregion in detail creasing can be taken as an indication that the substance rea t this time, it sponsible for this peak may be a critical intermediate. On the seems appropriate other hand, these cool flame phenomena may result from the t o discuss i t periodic formation and destruction of an inhibitor which reacts with active intermediates or they may be caused merely by the briefly. Peroxides exothermic nature of the reaction. are the most interesting compounds which absorb SUMMARY Figure 4. Optical Density at 2 6 2 5 A. strongly a t this as a Function of Time wave length. Employing a recording ultraviolet spectrophotometer, the time Cool Rams combustion of n-pentane U n f o r t u n a t e I y, dependence of the optical density during cool flame reactions however, acids, of hydrocarbons has been measured using light of varying wave lengths. An intermediate of unknown structure, absorbing a t higher aldehydes, and compounds containing conjugated about 2600 A., was found to be consumed during the pressure double bonds also absorb in this region of the spectrum, and a pulse while all other materials absorbing in the ultraviolet range, resolution of the gross absorption curve into these components including formaldehyde and peroxides, appeared to be generated. is impossible a t this time. It seems likely that the decrease in It is possible that the unknown intermediate may be a critical optical density following the cool flames, Figure 5, can be attribmaterial in cool flame formation from many hydrocarbons. uted to the decomDosition of peroxides. The observed decrease in the optical density corresponds to a decrease in the peroxide pressure of from 1 to 4 em. of mercury depending on whether the extinction coefficient of the peroxide is the same as hydrogen peroxide or ethyl hydroperTEMPLRATURC- 332.C. oxide, respectively. If this pressure decrease during the reaction continues until the apparent decomposition is complete, the residual absorption could be accounted for by an acetic acid pressure of approximately 25 em. of mercury or by a propionaldehyde pressure of approximately 8 em. of mercury, but it seems doubtful that either compound is that abundant. Very little contribution to the optical density can be attributed to straightrchain mono-olefins. Compounds containing conjugated double bonds, such as diolefins, absorb very strongly a t 2050 A. 1,3-Butadiene, for example, a t a pressure of about 0.1 mm. of mercury, could account for the residual optical density shown in Figure 5, although the spectrum of the oxidation products, following the termination of the reaction did not show the characteristic butadiene 14.5 15.0 15.8 16.0 18.5 TIME- MINUTES bands a t 2200 to 2100 A. However, both butadiene and benzene were identified by their spectra in the prodFigure 5. Optical Density at 2 0 5 0 A. as a Function of Time ucts from a higher pressure (1 atmosphere) propane Cool flame combustion of propane cool flame, and it seems possible that other unsaturated comDounds might - account for some of the residLITERATURE CITED ual optical density shown in Figure 5. Unfortunately, be(1) Egerton, A. C., and Pidgeon, L. M., PTOC. Roy. SOC.(London), cause of excessive deposit formation on the walls, the high presA142,26 (1933). sure flames could not be studied directly in the spectropho(2) Jost, W., “Explosion and Combustion Processes in Gases,” p. tometer cell. 437, New York, McGraw-Hill Book Go., 1946. (3) Semenoff, N., “Chemical Kinetics and Chain Reactions,” p. 68, DISCUSSION Oxford, England, Oxford University Press, 1935. Roy. SOC.(London), A152,378 (1935). (4) Ubbelohde, A. R., PTOC. In view of the complexity of combustion reaction mechanisms, it is not surprising that small amounts of a wide variety of RECEIVED July 9, 1951. 3.0
.
(PREFLAME COMBUSTION OF HYDROCARBONS) Identification o f 8-Dicarbonyl Compounds M. R. BARUSCH,
H. W. CRANDALL, J. Q. PAYNE, AND
J. R. THOMAS
California Research Corp., Richmond, Calif.
T
HOilIAS and Crandall ( 6 ) have established the potential importance to the cool flame reaction of the compound which absorbs light a t 2600 A. The present paper concerns its identification. Il-hile investigating the slow oxidation of n-butane by means oi ultraviolet absorption spectroscopy, Egerton and Pidgeon (3) observed the growth of a strong band a t about this wave length, indicating the appearance of an unknown reaction intermediate Further investigations by Ubbelohde (71, who termed this intermediate compound X, and by Egerton and Young (4)failed to idwtify the compound responsible for the absorption band, although the former author eupiessed the opinion that the conipound probably resulted from decomposition of a peroxide and that its s t r u c t q e might resemble that of ascorbic acid. Egerton and Young stated that this material was probably a complicated peroxide possessing a ring structure which might arise from polymerization or condensation reactions between aldeh] des and alkyl peroxides or hydrogen peroxide. Ubbelohde ('7) reported that compound X, which absoibed a t 2600 A, did not appear unless the hydrocarbon giving iise to the cool flame contained a t least a four-carbon chain. The fact that the material being studied in the present work likewise showed both of these chaiacteristics left little doubt that thc authors were working with Ubbelohde's compound X. Identification of the agents absoibing a t 2600 8.nas made by investigating the spectra of a variety of conjugated systems of the appropriate molecular weight, since the 2600 A. band indicated such a structure. 2,4Pentanedionc gave an almost exact match of the absorption shown by the product from n-pentane oyidation. Firm proof of the identity nas established by concentrating the appropriate fraction from the cool flame product of ?&-pentane and converting it to the copper enolate. The copper enolate from the n-pentane cool flame product was shown by x-ray diffractionto be identical nith a sample prepared from known 2,4-pentanedione. Application of the above technique to the identification of the cool flame product from n-butane nas not practicable because of the instability of butanal-3-one. Honever, synthetic butanal-3one prepared and kept in low concentration in a dry nonaqueous solvent gave an absorption spectrum quite similar to that of 2,4pentanedione and to the 2600 A. intermediate band produced in the cool flame from n-butane. By analogy to the case of 2,4pentanedione from n-pentane established above, the source of the 2600 A. band absorption from n-butane can be attributed to butanol-3-one. Some corroboration of the assignment was obtained by showing that the rate of deconiposition by water of an iso-octane solution of known butanal-3-one matched the rate of decomposition similarly induced by adding 7%ater to the isooctane solution of the cool flame product of n-butane. The production of butanal-3-one from n-butane suggests that wine pentanal-3-one might be produced in addition to the 2,4pentanedione from the cool flame combustion of n-pentane. This P-ketoaldehyde should contribute to the 2600 9. absorption band. However, i t would not be as stable as the @-diketoneand probably for this reason was not isolated from the combustion product. Since all 8-diketones and p-ketoaldehydes will probably show similar absoiption spectra, it is expected that the 2600 A. absorption band observed during the combustion of higher paraffiic hydrocarbons is due to one or both of these types of structures. The compounds 1,3-propanediol and 2-methyl-l,3-propanediol,
possible intermediates in propane and isobutane oxidation, respectively, do not appear to hare been prepared. The failure to observe a 2600 A. absorption band during the combustion of propane and isobutane may be due to the instability of thcsc compounds. EXPERIMENTAL
2,~PENT.lh-EDIOS'iC1N A comparison of thp ultraviolet absorption spectra of 2,4-pentanedione in the vapor phase and in iso-octane solution with the cool flame combustion products of n-pentane is presented in Figure 1. The solid curves represent the pure vapors in both cases and the broken lines represent the iso-octane solutions. The data obtained for thc solution of 2,4-pentanedione are in agreement with those of Rasmussen, Tunnicliff, and Brattain (6). The combustion product vapors were produced from equal mole mixtures of n-pentane and oxygen a t 276" C. in the quartz cell of the spectrogmph.as desciibed by Thomas and Crandall(6). rLTRAVI0LET SPECTRAL E V I D E N C E O F
TKY COalBUSTION PRODUCT O F n-PENTANE.
.
3 01
I _ -
I
I
I
V
I
0.1 2000
2200
2400
2600
2800
3000
3200
3400
3600
3800
4
IO
W A V E LFNGTH A
Figure 1.
Spectra of 2,4-Pentanedione and Combustion Product of n-Pentane A. E.
Combustion product in iro-octane 2,4-Pentanedione vapors
D.
Combustion product vapors
C. 2,4-Pentanedione in iao-octane
The combustion product which was dissolved in iso-octane wa8 obtained from the organic phase of the condensate produced in B flow system of the type described by Baruech and Payne ( I ) . The reaction was carried out at 304" i.3" C. employing 150 mm. CJf pentane and 610 mm. of air. Inspection of Figure 1shows that the peaks of the spectra of the synthetic and combustion vapors as well as the corresponding iso-octane solutions are in good agreement. ISOLATIOlV O F 2,4-PEhTTSPSEDIOUE FROM THE C O O L rL4M.E PRODUCTS OF %-PENTANE. The cool flame flow systemmentioned above was used to collect a large sample of combustion product The reaction was again carried out a t 304" f 3" C. The npentane was introduced by carburetion with air maintaining the carburetor a t 0" C. The flow rate was adjusted so that 0.5 ml. of liquid pentane was consumed per minute. The condensate 13 as cooled in a dry-ice trap. It consisted of approximately one part aqueous phase to five parts organic phase. Spectroscopic
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