THE PHOTOLYSIS OF BIACETYL1

Transcription

1 THE PHOTOLYSS OF BACETYL1 ABSTRACT The photolysis of biacetyl has been reinvestigated. The results are, in general, in excellent agreement with those of Blacet-and Bell. Curvature occurs at low temperatures in theilrrheni~ls plot of ~cn.,/~~~~~[~iacet~l], and this is attributed to wall reactions, and to the disproportionation reaction C-,+C-,CO-> C-,+CH,CO. Azomethane-biacetyl mistures have been photolyzed to give further information on these points. An activatio~l energy of 8.5 lical. has been found for the reaction of methyl radicals with biacetyl. NTRODUCTON The photolysis of biacetyl has been thoroughly investigated by Blacet and Bell (4, 3). t has, however, bee11 thought to be worth reinvestigating the reaction, especially in the low temperature region, to see if there were anomalies similar to those which occur with acetone. n particular we were interested in the possibility of complications due to wall effects and to the possible occurrence of the disproportionation reactions CH,+CHsCO -+ CH,+CH,CO and CHjCO + CHjCO -+ CH3CHO + CHQCO. n addition azomethane was photolyzed in the presence of biacetyl to obtain a further check on the activation energy of the reaction of methyl radicals with biacetyl. EXPERMENTAL The reaction cell was a quartz cylinder 10 cm. long and 5 cm. diameter, with a volume of about 170 cc. A Hanovia S-500 medium pressure mercury arc was used for most of the experiments. For a few runs a B.T.H. ME/D 250 w. compact source lamp was used to obtain higher intensities. The reaction cell was completely filled by a nearly parallel light beam. The intensity was varied by means of neutral density filters of chrome1 deposited on quartz. No other filters were used, except where mentioned. The remainder of the apparatus was essentially similar to that described in previous papers from this laboratory. Reagent grade biacetyl (Eastman white label) was used, and was distilled in vacuum with the rejection of a large head and tail fraction. The analysis of the products mas done in the usual way by taking off the CO, CH4 or CO, CH, Nq fraction at liquid nitrogen temperature. n CO, CHA samples the CO was determined by hot copper oxide. The samples containing nitrogen were analyzed with a mass spectrometer. The CzHa fraction was Manuscrikt received September 17, Contributzon front the Division of Pure Chemistry, National Research Coz~ncil, Ottawa, Canada. ssued as N.R.C. No ZNational Research Coz~ncil of Canada Postdoctorate Fellow, Present address: Deparlement de Chintie Physique, Universitd Laval, Qzlbbec. 39

2 TABLE PHOTOLYSS OF BACETYL CH~CO RCH o Relative Temp., Pressure, RCO R ~ ~ Rc~H~ 4 RCH~CO X 1013 intensity "C. cm. cc. per min. X lo4 RCH, R~,H,[CH~COCOCHJ]? 2

3 TABLE 1 PHOTOLYSS OF AZOMETHANE-BACETYL MXTURES Temp., Time, Pressure, crn. - Products, cc./min. X lo4 "C. min. Azomethane Biacetyl Nz CH, CzHs CO TABLE 111 Rate of kll Products, cc./min. X 10' 2 Relative Pressure, Time, R ~ ~ 4 X lor3 formatiorl of - z intensity cm. min. CO CH CzHs CHzCO R~,~,[CH~COCOCH~] excess CH kl& k35 m

4 42 CANADAN JOURNAL OF CHEMSTRY. VOL. 33 taken off at - 17S0C., and the ketene fraction at - 13j C. Both fractions were occasionally checked with a mass spectrometer. RESULTS Table gives the results of runs at temperat~~res between 27 and 200 C., and relative intensities varying by a factor of 20, where a relative intensity of 1 corresponds to an absorbed intensity of about 2 X 1014 qquanta/sec. The amount of ketene formed was measured in only a few experiments. Velocity constants throughout are expressed in units of c1n.5 nmolecules, sec. Table 1 gives the results obtained by photolyzing azolnethane in the presence of biacetyl. For these runs a Corning 738 filter was used, which cut off wave lengths below 3400 A. The results in the last columns of Tables and 11, together with the results of Blacet and Bell, are plotted in Fig.. Table 111 and Fig. 2 show the effect of varying the intensity by a factor of 200 at 27OC. and constant biacetyl concentration. FG. 1. Arrhe~lius plot of k4/k3 for the biacetyl photolysis. 0 Photolysis-relative intensities-lower curve,. --upper curve, 20. A Photolysis-results of Blacet and Bell. Photolysis of azomethane in presence of biacetyl. DSCUSSON To explain their results Blacet and Bell proposed the following secondary reactions :

5 ALSLOOS.4ND STHACE: PHOTOLYSS Activation Energy of the Abst~action Reaction f ethane and methane are formed only by reactions [3] and [4], then so that a straight line may be expected on plotting the L.H.S. against l/t. The results given in Table and Fig. 1 confirnl this for the high-temperature region. itowever, at low temperatures a curvature becomes apparent in the Arrhenius plot. We have also shown Blacet and Bell's results in Fig. 1. Although their original plot was not drawn so as to show curvature, it is evident that if the two highest temperature points are ignored it is possible to draw a line through their remaining points ~vhich is striltingly similar to ours. The two plots thus drawn have the same slope and lead to an activation energy difference E4-+E3 of 8.5 lccal., or assuming E3 = 0, E4 = 8.5 ltcal. Wall Reactions Fig. 1 also shows the results of experiinents in which azomethane was photolyzed in the presence of biacetyl. The effective wave length was 3660 A. Biacetyl also absorbs slightly under our conditions, but the anlount decomposed was negligible compared with azomethane decomposed. This is shown by the very small amount of CO formed at rooin temperature. At this temperature CO is a measure of the amount of biacetyl photolyzed because of the unimportance of reactions [5] and [6]. At higher tenlperatures CO is formed by [5] and [6] from radicals resulting from the photoiysis of azomethane. When azomethane is photolyzed in the presence of biacetyl, the following reactions have to be talten into account to explain the formation of methane and ethane. CH3NNCH3+lzv --t 2CHafNz [91 2CH3 --t C2--6 C31 Cit3+CH3NNCH, --t CH4fCHzNNCH3 [lo] CH3fCH3COCOCH3 --t CH4fCH2COCOCH3 [41 whence The values of kla/k3' have been taken from an experimental plot for the photolysis of azomethane alone (1). From Fig. 1 it is evident that the results agree

6 44 CANADAN JOURNAL OF C-EMSTRY. VOL. 33 excellently with those from the photolysis of biacetyl itself. t is also apparent that curvature is still present in the Arrhenius plot, although there is none in the plot for azomethane itself. t is suggested that the curvature results from wall effects at lower temperatures where the diffusion of radicals to the wall is of more importance. More conclusive evidence for this will be discussed in a forthcoming paper (2). Disfiroportionation of Acetyl t is suggested that the reactions CH3fCH3CO -3 CH,+CH2CO [11 and 2CH3CO -+ CHzCO+ CH3CHO [12 also occur. f this is so, the ratio RCH~/R$~,,[CH~COCOCH~] will become intensity dependent, i.e. To check this two series of runs were carried out at relative intensities which differed by a factor of 20. Fig. 1 shows that variation in intensity has little effect at temperatures above 100 C. At 27" ~ OW~V~~R~~~/R~,~,[CH decreases appreciably with intensity as may be expected if reaction [ll] occurs, since CH3CO radicals will be more stable at room temperature. The amount of lretene formed was also measured at a few temperatures. From 200" to 137 C. a sharp decrease in lcetene was observed (Table ) in agreement with the results of Blacet and Bell who measured the lcetene formed at 200, 150, and 100 C. They explain the formation of lcetene by reaction [5]. f this has an appreciable activation energy the drop in lcetene with decreasing temperature is to be espected. f reactions 1111 and [12] are neglected, the ratio talces the form Since E5>> E7, this predicts a drop in the ratio of ketene to methane with decreasing temperature, as is observed from 200" to 137OC. There are, however, three anomalies all of which point to the occurrence of [ll.] or [12]. (a) There is always an increase in the ratio CHZCO/CH4 with increasing intensity. (b) At temperatures below 137"C., the rate of lcetene formation begins to increase again. At room temperature the ratio CH,CO/CH4 is larger than unity. (c) Mass-spectrometer analyses indicated the presence of acetaldehyde as a reaction product. n order to obtain more definite information on these points, a series of runs were made in which the intensity was varied over a wider range (by a factor of 200). For these runs the temperature was 27f1 C., and the biacetyl pressure was 2.3 cm. The results are given in Table 111 and Fig. 2. Fig. 2 indicates a considerable increase in the ratio R~,,/R,&~,[cH~cococH~] with increasing

7 AUSLOOS AND STEACE: PHOTOLYSS o NTENSTY FG. 2. Effect of B.T.-. lamp. Pressure 2.3 cm. 0 -anovia lamp. Temperature 27 C. intensity. f the curve in Fig. 2 is extrapolated to zero intensity a value of 0.65 X 10-l3 is obtained for the ratio. This is higher than the value which would be expected from an extrapolation of the straight-line portion of the Arrhenius plot in Fig. 1. The curvature in Fig. 1 is thus in part but not solely due to disproportionation of acetyl radicals. The residual curvature is probably due to diffusion effects accolnpanying wall -eactions. The fact that the ratio CHZCO/CH4 becomes considerably higher than 1 at high intensities indicates that there is a further source of 1:etene in addition to reaction [ll]. As pointed out previously the presence of acetaldehyde suggests reaction [12]. The direct production of acetaldehyde in the primary step CH~COCOCH~+ZV -+ CH3CHO +CH?CO is unliltely on the basis of Blacet and Bell's results in the presence of iodine. Also, the fact that acetaldehyde has been observed in the acetone photolysis as well (2) suggests a common origin such as reaction [12]. A rough quantitative check on the validity of assuming reactions [ll and [12] can be made as follows. From reactions [ll],[12], and [3] the relationship can be deduced where RgH4 represents the amount of excess methane necessary to account for the increase in the ratio RcH,/R~,H,[CH3COCOCH3] above the limiting value 0.65 X Although the acetaldehyde concentration could not be quantitatively determined, it is evident that RCH~CHO = RcH~co-R&~. The values of kll/klz+ k3* calculated in this way are given in the last column of Table 111, and are independent of intensity.

8 46 CAYAD.4N JOURXAL OF C-EMSTRY. VOL. 33 n conclusion, it may be pointed out that the results given here indicate complications at low temperatures in all systems in which acetyl radicals are present, and in particular that such complicating processes will be intensity dependent. The analogous case of acetone is discussed in a forthcoming paper (2) - REFERENCES 1. Aus~oos. P. and STEACE. E. \T. R. Call. 1. Chem. 32: AUSLOOS; P. and STEACE; E. W. R. Can. j. Chem. 33: BELL, W. E. and BLACET, F. E. J. Am. Chern. Soc. n press. 4. BLACET, F. E. and BELL, W. E. Discussio~ls Faraday Soc. 14: