SOME COMPLICATING FACTORS IN THE PHOTOLYSIS OF ACETONE1

ABSTRACT

The photolysis of acetone has been investigated a t room temperature using low pressures and high intensities. In addition an investigation was made of the photolysis of azomethane-acetone mistures. The re~u l t s~ ind ica te that the

curvature a t low temperatures of Arrhcnius plots of ~ ~ ~ , / ~ ~ ~ ~ ~ [ ~ c e t o n e ] is due to two causes ( a ) a reaction between methyl radicals and adsorbed acetone and (b ) t o the occurrence of the disproportionation reaction

CHa+CHsCO+ CHd+CH2CO. Confirmatory evidence for wall effects was obtained from experiments a t low

pressures and higher temperatures.

INTRODUCTION

The photochemical decomposition of acetone has been thoroughly investi- gated, especially by Noyes and his co-workers. In the temperature region from 120" to 200°C. it has been established that all the methane and ethane formed can be accounted for by the reactions:

CHa+CH3COCH3 + CH4+CHsCOCH3 [I] CH,+CH3 + CDH, PI

I t has, however, been shown (6, 8, 9) that a t lower pressures reaction [2] becomes dependent on a third-body. Whence a t higher pressures

R C H ~ kl = Constant = -

~ $ , ~ , [ ~ c e t o n e ] k?f and a t low pressures

R C H ~ = Constant.

~ ~ , ~ , [ . ~ c e t o n e ] a For the activation energy difference, El-iE?, a value of 9 .7f0 .1 Itcal. has

been generally accepted (12). This value has been deduced from experiments a t temperatures above 100°C., and a t pressures between 25 and 200 inm.

In the lower temperature region there are complications in the kinetics. Curvatlire has been observed in the Arrhenius plot between room temperature and 125°C. (7, 12). Several suggestions have been made as to the cause of the discrepancy (11). In the first place the presence of an appreciable amount of acetyl radicals constitutes the most striking difference between the low- and the high-temperature photolysis. The diffusion of radicals out of the light- beam and possible wall effects have also to be considered. As far as diffusion is concerned, Nicholson (10) has shown that although it is important in some

' iVanuscript received Septenaber 17 , 1954. Contribz~tion froltz tlze Division of P z ~ r e Cl~emistry , National Research Coz~?zcil, Ottawa, Canada.

Issz~ed a s N.R.C. No. 3450. 2National Research Coz~ncil of Canada Postdoclorate Fellow, 1962-54. Present address: Departe-

men1 de Cirimie Playsiqz~e, Universith Laval, Qz~bbec.

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48 CANADIAN JOURNAL OF CI-IEMISTRY. VOL. 33

cases, the effect is still too small to account for the curvature in the Arrhenius plot a t low temperatures.

The purpose of the present work was in the first place to study the reaction of methyl radicals with acetone in the absence of complications due to acetyl radicals. For this purpose azomethane was used as a source of methyl radicals. I t has the advantage of absorbing a t longer wave lengths where acetone is transparent. The second object of the worlc was to investigate the photolysis of acetone under special conditions (high intensities and low pressures), to obtain information regarding wall reactions ancl reactions of acetyl radicals.

EXPERIMENTAL

Azomethane was prepared by Dr. L. C. Leitch as described in previous papers from this laboratory. Acetone was obtained from the Eastman Kodak Co. Both conlpounds were thoroughly degassed and stored behind mercury cutoffs.

Two different types of apparatus were used. Apparatus I was of the con- ventional type which has been described elsewhere (1). A Hanovia S-500 medium pressure mercury arc was used as a light source. The cylindrical quartz reaction cell (5 cm. diameter, 10 cm. long) xvas completely filled with a nearly parallel light beam.

A p p a r a t ~ ~ s I1 xirhich was used for loxv pressure experiments was essentially the same as that described by Dodd and Steacie (6). The lamp xvas a B.T.H. ME/D 250 xir. coillpact source. In some experiments a "paclted" reaction vessel was ilsed. For these the cell which xvas 70 cm. long contained two inner quartz tubes concentrically mounted. Thc sui-face/volume ratio of the paclced cell was 7.1 cnl-I. The cell xvas completely filled by a parallel beam of light. An empty cell was also used, which was 100 cm. long.

In experiments with azomethane-acetone inixturcs a Corning filter No. 7380 was used to limit radiation to wave lengths greater than 3400 A. For other experiments a Corning No. 986 filter was used.

The N2-CH4 fractions of the products were removed a t liquid nitrogen temperature and analyzed by the mass spectrometer. On a few occasions checks were made for CO in the NZ-CHd fraction fi-om the photolysis of azomethane- acetone mixtures by passing the gas over hot copper oxide. The quantity of CO was invariably negligible. The CO-CH4 fractions from the acetone photolysis xvere analyzed by conlbustion over hot copper oxide. The ethane fraction as separated a t - 175OC. and occasionally checlted with the mass spectrometer.

RESULTS FROM RUNS AT NORMAL PRESSURES USING APPARATUS I

The results for the photolysis of azomethane and of azomethane-acetone mixtures are given in Tables I and 11. In Fig. 1 Arrhenius plots are given for kl/kz: and k3/kz:, where reaction [3] is

CHaf CH3NNCHa -, CHefCH2NNCI-13. [31 The units of k throughout are cn1.5 molecules, and sec.

Some experiments were also carried out in which acetone was photolyzed a t room temperature and various intensities. The results are given in Table 111.

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AUSLOOS A N D STEACIE: PHOTOLYSIS OF ACETOKE

TABLE I

Teinp., Pressure, Time, RN? R,CH~ R C ? H ~ "C. cm. rnin. cc./rnln. x 10.1 k3/k24 x loL3

TABLE I1 PHOTOLYSIS OF AZOMETHANB-A4CETONE MIXTURES

Temp., Pressure, cm. Time, RN? R C H ~ R C ~ H C "C. Azomethane =Iceto= min. cc./rnin. X lo4 kl/kt: X 1014

I

FIG. 1. Plot of log R ~ ~ ~ / R & ~ ~ ~ [ A ] against l / T for the photolysis of azornethane-acetone mixtures.

Curve A-photolysis of azomethane alone. C ~ ~ r v e B-azomethane-acetone mixt~~res . The dotted line is an cstrapolation to low temperatures of the r e s ~ ~ l t s of Trotman-Diclcenso~~

and Steacie.

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CANADIAN JOURNAL OF CHEMISTRY. VOL. 33

T A B L E 111 PHOTOLYSIS OF ACETONE AT ROOM TEJIPERATURE

AKD VARYING INTENSITY Temperature 27°C.

Relative Pressure, Tiqle, R R RCZHp +CH~+C?HG intensity cm. mln. cc./min. X lo4 Rca4 XI014

CO ~ b , ~ , [ ~ c e t o n e ]

The relative intensity was varied by a factor of 165, where an intensity of 1 corresponds to approximately 5 X 10" quanta per cc. per sec. The light intensity was varied by means of neutral density filters of chrome1 on quartz. The higher intensities are much higher than those used in most previous investigations a t low temperatures. The values of ~ ~ ~ , / ~ ~ , ~ , [ ~ c e t o n e ] have been plotted in Fig. 2.

Intensity

FIG. 2. T h e photolysis o f acetone at 27'C. and 57 mm. with varying intensity.

In addition to the products listed, the highest intensity runs produced a very small amount of a inaterial of intermediate volatility, which could be separated a t - 125°C. In order to obtain a larger amount of this fraction a run was made a t the highest possible incident intensity using a cylindrical cell of 1 liter volume. Mass-spectrometer analysis of the - 125°C. cut from this run showed that it consisted mainly of ketene and acetaldehyde.

DISCUSSION

The methane and ethane formed by photolyzing azomethane in the presence of acetone may be accounted for by the following reactions:

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AUSLOOS AND STEACIE: PHOTOLYSIS OF ACETONE

From this scheme

kl - R ~ ~ 4 - ka[Azomethane] --

I

~ 2 % acetone] k2t[Acetone] '

The values of k3/kzt required in the calculations have been taken from an experimental Arrhenius plot for azomethane (2). A few runs were repeated in the low-temperature region to check again the absence of curvature a t low temperatures in the Arrhenius plot for azomethane. Fig. 1 shows the complete absence of curvature and the results coincide exactly with those previously obtained (2).

Fig. 1 also gives an Arrhenius plot for kl/k23, calculated from results with azomethane-acetone mixtures. The dotted line in the figure was obtained by

i extrapolation of the high-temperature results of Trotman-Dickenson and 1 Steacie on the photolysis of acetone itself. I t is evident that the results agree I exactly a t high temperatures, but that there is appreciable curvature a t low

temperatures in the results obtained with azomethane-acetone mixtures. At 27OC. a mean value of about 1.6 X 10-l4 was found for kl/k2;. This is in excel- lent agreement with the value of 1.55 X 10-l4 found by Nicholson (10) in

/ the direct photolysis of acetone a t similar intensities. Since no acetyl radicals are present in the photolysis of azomethane-acetone mixtures, the curvature I in the plot a t low temperatures and low intensities cannot be due to reactions ' involving acetyl. Diffusion of radicals out of the light beam is not a possible explanation in the present case, since the cell was filled with light and the cell volume mas used in the calculations. I t is therefore suggested that wall reactions

1 are involved, and specifically that methyl radicals react with adsorbed acetone to form methane. I t may be mentioned that analogous curvature in Arrhenius plots has been found when azomethane was photolyzed in the presence of biacetyl (3) and methyl ethyl ketone (5).

Fig. 2 shows that in experiments on the photolysis of acetone alone the ratio ~ ~ ~ , / ~ ~ , ~ , [ ~ c e t o n e ] does not vary with intensity in the low intensity region. Most previous work mas done a t still lower intensities. The constant low- intensity value (2 X 10-14) lies well above the value obtained by extrapolating Trotman-Dickenson and Steacie's Arrhenius plot (12) to lower temperatures. The extrapolated value is in good agreement, however, with the value of 1.6 X 10-l4 found by photolyzing azomethane-acetone mixtures. Interference by reactions of acetyl is excluded in this intensity range as the cause of the curvature in the Arrhenius plot. The evidence again strongly favors wall re- actions as the cause of the discrepancy.

At higher intensities, however, a sharp increase of R ~ H , / R ~ , H , [ A c ~ ~ o ~ ~ ] becomes apparent. This suggests the formation of methane in a radical- radical reaction. Further, since the effect only occurs a t low temperatures, the

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52 C:\hr.ADIAS JOURNAL O F CEIEbIISTRY. VOL. 33

participation of acetyl is strongly indicated. I t is therefore suggested that the additional methane arises by the disproportionation reaction,

CH,+CH3CO -+ CH,+ CHZCO. NI The intensity dependence of methane formation can be explained on this basis, as well as the ltetene formation a t high intensities. The small amounts of acetaldehyde found by mass-spectrometer analysis may be accounted for by the similar reaction

2CH3CO -+ CH?CO+CH3CHO. [.I Evidence for both these reactions has been fou~ld also in the photolysis of biacetyl (3). Reaction [5] has also been suggested in the photolysis of acetal- dehyde (4).

RESULTS FROM EXPERIMENTS AT LOW PRESSURES USING APPARATVS I1

The results of a fen? experiments a t low pressures using a "packed" reaction vessel are given in Table IV, ancl are plotted in Fig. 3 and compared with the

TABLE IV PI-IOTOLYSIS OF ACETONE AT LOW PRESSURES

"Packed" cell

Rate, ~l~olecules/ R ~ ~ I +CH,+C~HG Pressure, Ti~!le, - cc./sec. X 10-lo XlOj --

I ~ I I ~ . m ~ n . CO CI-1.1 C ~ H G ~ ~ ~ ~ , [ i \ c e t o ~ ~ c ] ~ CO @CO

results of Dodd and Steacie (6). For these experiments the incident illtensity was kept constant and the pressure was varied from 25 to 0.03 mm. The values of +co in the last column of Table IV were determined in the usual waj- a t higher pressures. The absorbed intensity a t lower pressures was obtained by a Beer's law extrapolation.

Table V gives the results of esperi~nents a t 27°C. and varying intensities a t four different pressures (from 0.G to 4.7 mm.). The results are given in Fig. 4 in the form of a plot of log R ~ ~ ~ / R ~ ~ ~ ~ against the logarithm of the incident

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AUSLOOS A N D STEACIE: PHOTOLYSIS OF ACETONE

PRESSURE mm.

FIG. 3. The photolysis of acetone a t higher temperatures and low pressures. Dotted curve-resolts of Dodd and Steacie in an "~~npackecl" cell. 'Triangles-Dodd and Steacie "paclced" cell. Circles-present ~vork in "pacl<ed" cell.

TABLE V EFFECT OF INTENSITY AT 27'C. .4ND LOW PRESSURES

Unpacked cell

Rate, rnolecules/cc./sec. R C H ~ Incident Pressure, Time, X lo-''' 7 x intensity mm. min. co c 1-1 .I C?HG R c , ~ a

intensity. The lowest incident intensity is about eight tiines the lowest intensity used for the runs a t usual pressures (Table 111).

Since we were interested in possible wall reactions, the expel-iinents given in Table IV were done. I t is evident from Fig. 3 tha t the results agree excelle7tly with those of Dodd. They confirm the fact that while the ratio Rc,,/R&,, [ ~ c e t o n e l f falls with decreasing pressure in agreement with the assumption that the recombination of methyl radicals is becoming pressure-dependent,

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CANADIAN JOURNAL O F CI-IEMISTRY. VOL. 33

Incident In tens i ty

FIG. 4. Photolysis of acetone a t 27'C. and low pressures, with varying intensity. The figures on the curves refer t o the pressure in mm.

nevertheless a t low pressures the ratio rises again instead of becoming constant. This increase commences a t higher pressures in the packed cell, and there is no doubt that it is due to a wall reaction which produces methane. The only reasonable possibility seems to be a reaction between methyl radicals and adsorbed acetone. The results appear to indicate that the ratio decreases again a t the lowest pressures, but it is difficult to be sure that this effect is real.

I t may be mentioned that there is a gradual drop in the ratio (+CH4+CzH,)/ CO with decrease in pressure in the paclted vessel. No such effect was observed in the unpacked cell. We are unable to offer any convincing explanation of this.

The experiments a t high intensities and room temperature, given in Table V, were done to obtain more information about the occurrence of reaction [4].

In this temperature region we are primarily concerned with the reactions: CH3+CH3COCH3 -+ CH4+CH2COCH3 [I]

2CH3 --, C?Hs [2] CH3+CHsCO -+ CH4+CHzCO 141

2CH3CO -+ CHaCHO+CH?CO PI CH;+CHsCO -+ CH3COCH3 [6]

2CH3C0 -+ CH3COCOCH3 [7] As a very rouglz approximation, a t constant temperature and pressure but

varying intensity, we may put [CHSCO] [CHa], and thus [CH,CO] I,,:. Whence, if all methane is formed by [I] and [4], we have at low pressures

R C H ~ k4 ke - [Acetone] + - [CH,CO]

R $ C2H6 k$ k 24

so that a t constant low pressure and varying intensity

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AUSLOOS AND STEACIE: PHOTOLYSIS OF ACETONE 55

Since for constant acetone pressure I.,., R,,,/R,~,,, will become constant a t low incident intensities and proportional to 1% a t high intensities. Fig. 4 gives a plot of log R ~ E , / R $ , ~ , against log intensity for four different low pressures. At high intensities a series of straight lines is obtained with a slope of 4, while a t low intensities the ratio approaches a constant value. This furnishes further strong support for the postulation of reaction [4].

REFERENCES 1. A u s ~ o o s , P. and STEACIE, E. W. R. Bull. soc. chim. Belges, 63: 87. 1954. 2. A u s ~ o o s , P. and STEACIE, E. W. R. Can. J. Chem. 32: 593. 1954. 3. A u s ~ o o s , P. and STEACIE, E. W. R. Can. J. Chem. 33: 39. 1955. 4. A u s ~ o o s , P. and STEACIE, E. W. R. Can. J. Chem. 33: 31. 1955. 5. A u s ~ o o s , P. and STEACIE, E. W. R. Can. J. Chem. In press. 1955. 6. DODD, R. E. and STEACIE, E. W. R. Proc. Roy. Soc. (London), A, 223: 283. 1954. 7. DORFMAN, L. M. and NOYES, \V. A., Jr. J . Chem. Phys. 16: 557. 1948. 8. I<ISTIXKOWSKY, G. B. and ROBERTS, E. K. J . Chem. Phys. 21: 1637. 1953. 9. LINNEL, R. I-I. and NOYES, W. A., JR. J. Am. Chem. Soc. 73: 3986. 1951.

10. NICHOLSON, A. J. C. J. Am. Chem. Soc. 73: 3981. 1951. 11. NOYES, W. A., JR. J. Phys. & Colloid Chem. 55: 925. 1951. 12. TROTMAN-DICKENSON, A. F. and STEACIE, E. W. R. J. Chem. Phys. 18: 1097. 1950.

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