THEPHOTOLYSIS OF ACETONE
Dec., 1960
exists in a colloidal state as a reactive polymeric form of the ion HMn03- with general formula [Mn,02,1]2- reaches about 200 p M , its rate of formation decreases considerably, indicating participation of the product in an oxidizing back reaction Mn(IV) (HMn08-:i
+ OH +Mn(V) + H20 + OH +Ivln03- + H20)
When all OH radicals are consumed in this reaction, as indicated in the second linear section of the yield-dose curve, then the small amount of chain reaction initially occurring at this concentration will be eliminated, and all the H atoms will reduce to Mn(1V) via reactions 1 and 5. Consequently the net radical reduction yield becomes g(H)-g(0H) The true course of the reaction of hydrogen peroxide with permanganate is not known in neutral solution but we shall simply assume that it reduces 2 equivalents, which eventually become stabilized as Mn(1V). The total yield is then the sum of the radical and hydrogen peroxide yields. G( -Mn04-) = 1/3 [g(H) - g(0H) 2g(H202)]. Exact values for the radical and molecular product yields in neutral solution are still uncertain;2l if g(H) g(0H) and g(H202)= 0.75 (substantially the same as in acid solution) then the anticipated value of G [ - Mn 04-]would be 0.5. If g[H506] 0.5 as indicated by the work of Senvar and Hart32 and Adams,33 then the experimental value of G [ -MnOA-] would indicate g(H) - g(OH) 0.6.
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(32) C. B. Senvsr and E. J. H a r t (unpublished results).
(33) G. E. Adams, PI ivate communication.
1847
5. Reduction in Alkaline Solution.-The qualitative features of the reduction in strongly alkaline solution are similar to those of neutral solution, but the quantitative aspects merit some discussion. The stable reduction product here is manganate ion (Mn (VI)) and consequently the decrease in slope of the yield-dose (Fig. 12) when manganate accumulates is attributed to reaction 2. ;lfn042-
+ OH +MnOc- + OH-
For the over-all 1 equivalent reduction we have for the initial yield G[-Mn04-] = G[Mn042-] = g(H) g(0H) 2g(HzOz)= 3.0. For the secondary slope G[Mn042-] = g(H) - g(0H) 29 (H202)= 1.30, so that g(0H) 0.8. It is apparent that determination of G(O)2 would give a value for g(Hz02) and allow determination of g(H). Consequently this system may well allow the determination of all the primary chemical products g(H2),g(Hz02),g(H) and g(0H) over the range of alkalinity in which manganate is stable, and further work is anticipated. Acknowledgments.-This work was commenced a t King's College, Newcastle-on-Tyne, England on the suggestion of Prof. Joseph Weiss. Thanks are due to the Director of A.E.R.E. (Harwell) for support of this part of the investigation and permission to publish a t Argonne. Thanks are due to Miss Vicki R. Meyers for experimental assistance, and to Dr. E. J. Hart for constant encouragement and critical discussion.
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THE PHOTOLYSIS OF ACETONE BY B. DEB.DARWENT, M. J. ALLARD,M. F. HARTMAN AND L. J. LANGE Department of Chemistry, The Catholic University of America, Washington 17, D. C. Received April 1, ID60
The effects of pressure, intensity and surface/volume ratio on RCH,/R'/PC~E~ = 7.have been investigated between 217 a d 327". The plot of y vs. p , the concedration of acetone, are linear but with positive intercepts, the magnitude of which increases with intensity and temperature. This indicates that CH, is formed by the reaction of CHs with a substance, possiblyeCHJCOCHz, whose concentration varies as I1/q and is approximately independent of p , in addition to the accepted reaction of CH, with acetone.
I. Introduction 280", a distinct positive intercept was obtained Although the kinetics of the photolysis of ace- which is not in accordance with equation A. I n tone are complex below 100" and above 350°, addition, previously published results2 are not inwithin that interval they appear to be simple and consistent with the presence of such an intercept. to be adequately represented, at least as far as the The results of the present investigation provide formation of methane and ethane are concerned, further data concerning the reality of the intercept and the effects of temperature, intensity and by the mechanism surface/volume ratio on its magnitude. (CH,)ZCO hv +CHI CH3CO
+
+ + +
(0)
CII,CO +CH; CO (1) CHa -C (CHs)zCO + CH4 CHzCOCHa (2) 2CHd +CzHe (3)
which leads to the relationship y =
~ Z C H J R ' / Z C=~ (kz/k31/2)p H~
(A)
where R is the rate of formation of the product and p the concentration of acetone. Some preliminriry results' showed that, although a linear relationship existed between y and p a t (11 B. deB. I l s r a e n t and J. E. Schingh, unpublished results.
11. Experimental The reaction vessel was a cylindrical Pyrex cell, 5 cm. X 10 cm., fitted with a corex window, transparent to X > , 2800 A., connected to a trap and to conventional storage, vacuum and analyt.ica1systems. The cell was surrounded by a cylindrical furnace, the temperature of which was manually controlled and did not vary by more than 1" along the length of the cell or between experiments at a constant stated temperature. The volumes of the reaction cell and of the total (2) (a) A. F. Trotman-Dickenson and E. W.R. Stesoie, J . Chsm. Phya.. 78, 3986 (1951); (b) R. H. Linnell and W. A. Noyes, J . Am. Chsm. Soc., 18, 1097 (1950).
B. DEB.DARWENT, AI. J. ALLARD,M. F. HARTMAN AND L. J. LANGE
1848
Vol. 64
curate mass spectrometric analysis and Mandelcorn and SteacieS have shown that ethylene becomes of some importance a t temperatures in the
45
TABLE I
THEPHOTOLYSIS OF ACETQNE
t
I
I
I
I
I
2 3 4 [GH3COCH3] x IO6 mole c r y ?
I
1
5
6
Fig. 1.-The relationship between y and p. The temperatures are as indicated. The broken lines represent the relationship corrected for CzH4. reaction system were 180 cm.3 and 335 ~ m . respectively. ~, Spectroscopically pure acetone was used after purification by trap-to-trap distillation in vacuo. The active light was obtained from a Hanovia Type S medium pressure mercury arc; the unfiltered light was approximately collimated by a quartz lens and completely filled the reaction cell. At completion of the photolysis, the acetone was condensed at -80" in a trap in the reaction system and the products separated into two fractions, CH, f CO a t -196' The CH, 4- CO fraction was and Czhydrocarbons a t - !60'.. analyzed by hot CuO oxldation and this was checked occasionally by mass spectrometric analysis. The following values are representative of the percentage of CH, as measured by the two procedures
I I1 I11 IV v Mass spectrometer 48.4 64.1 61.1 57.9 60.7 CUO 49.4 64.7 61.2 59.0 60.5 No significant amount of Cz hydrocarbon was ever found in this fraction. The Cz hydrocarbon fraction was usually much less than CO. I n fact, the volumes of CZobtained, althe CH, though large enough for accurate measurement, were too small for reliable mass spectrometric analysis. Such analyses showed that the CZfraction occasionally contained small amounts of acetone, always less than 595, but it was not possible to obtain a reliable estimate of the amount of ethylene present. I n some experiments the surface/volume ratio was increased, approximately 12-fold, by nearly filling the reaction cell with thin walled Pyrex tubing of 6 mm. diameter. This increased the surface area 8-fold and decreased the volume from 180 to 120 cm.3; i t also resulted in a decrease in the incident intensity.
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111. Results The experimental results obtained in this investigation are presented in Table I together with the estimated production of CzHsfrom the measured amounts of the C p fraction. As mentioned previously, the Cz fraction v7as too small for ac-
Reaction cell, 180 cm.3; reaction system, 335 CHaCOCHa] moles ern. - 8 x 109
Duration (aec.)
--Products CO
CHI
(moles X 106)c2
CZHG
0.79 2.19 2.78 3.74 5.93 6.11
Temp. = 217" 4.4 2.4 1950 8.1 6.0 1207 920 7.2 6.0 7.1 6.4 640 630 8.8 8.8 660 9.3 9.4
2.14 2.92 2.31 2.04 1.91 2.08
2.14 2.92 2.31 2.04 1.91 2.08
0.96 0.99 1.10 1.20 1.92 2.05 3.01 3.27 4.13
2400 3195 3300 3370 2400 2475 1555 1200 1505
Temp. = 246" 7.2 6.3 8.2 7.1 9.4 8.6 10.4 10.3 12.5 12.8 13.0 13.7 11.9 11.9 9.9 11.5 12.2 14.9
2.05 2.23 2.41 2.57 2.65 2.60 1.74 1.71 1 .54
2.05 2.23 2.41 2.57 2.65 2.60 1.74 1.71 1.54
0.78 0.79 0.95 1.41 2.08 2.14 2.49 2.67
6240 4290 4135 1800 1500 2445 1825 1770
Temp. = 285" 15.0 17.4 8.4 11.6 12.2 13.7 7.8 8.6 8.8 11.8 12.6 15.6 12.2 16.5 12.7 16.2
2.29 1.75 1.59 0.92 0.85 1.13 1.33 1.11
2.29 1.75 1.44 0.85 0.75 1.00 1.15 0.96
0.48 0.59 0.75 0.88 1.05 1.14 1.27 1.42 1.63 1.74 I .93 1.97
Temp. = 327'; full intensity 11.8 0.91 7.6 4880 4.9 5.9 .35 2400 8.1 .54 2330 6.7 .54 6.6 10.0 2390 1.25 19.0 23.4 4845 11.3 0.58 2400 9.2 10.4 14.4 0.68 2395 31.6 1.54 4820 24.0 9.6 0.43 1380 6.7 1.17 31.6 34.1 4680 1200 9.G 0.40 7.1 10.0 0.43 1200 6.8
0.64 .26 .39 .38 .90 .41 .47 1 .os 0.29 .83 .26 .28
Temp. = 327"; packed cell"; intermediat,e intensity 0.70 4440 4.2 5.7 0.37 0.23 0.90 4080 5.0 7.0 .41 .25 1.67 3000 6.4 9.3 .37 .22 2.04 2400 5.8 8.6 .R5 .18 Temp. = 327'; low intensity 5.5 0.16 0.099 25200 3.7 0.48 ,113 6.5 .22 30000 4.5 0.50 .48 ,131 8.6 13.3 1.26 21600 .50 .121 10.5 16.1 1.66 21720 .44 ,099 9.8 15.2 1.85 20400 .50 ,104 17.0 18780 11.0 2.10 a Reaction cell = 120 ~ m . ~ . (3) L. Mandelcorn and E. W. R. Steacie, Can. J. Chem., 32, 331 (1954).
THEPHOTOLYSIS OF ACETONE
Dee., 1960
vicinity of 300". Since they measured the effects of temperature, concentration and intensity on the rates of formation of CzHe and CzH4 it was possible, by interpolation within the temperature range covered, to obtain the ratio CzH4/CzHe a t two concentrations and two intensities a t the temperatures used in the present investigation, thus permitting the amount of ethane produced to be calculated. The relationships derived from Mandelcorn and Steacie's data and used for correcting the present results were
1849
'7
CzHa/CzH6= 1.13 X 10-8 pla-O.* a t 327" = 6 . 1 X 10-4pat2850
where p is the concentration of acetone (moles c m . 3 and I , the rate of formation of CO (moles C M . - ~ sec.-l). The ratio was quite negligible at the lower temperatures but was large a t 327" and especially so a t the lower intensities. The relationship between y and p is shown, for the high intensity experiments, in Fig. 1. The corrected values are also shown, as broken lines, for the two highest temperatures. The effect of intensity on that relationship, using the "corrected" values of y , a t 327" is shown in Fig. 2. Those data demonstrate quite clearly that positive intercepts are required if y is assumed to be a linear function of p. The effect of intensity on the slopes and intercepts obtained a t 327" (Fig. 2) is shown in Fig. 3. The intercepts are seen to be dependand the slopes on ent on IO.54 Le., the inf and the slopes tercepts vary approximately as d are approximately independent of the intensity. The relationships between y and p a t three lowest temperatures are a t least reasonably sound. However, at 327" and especially a t the lowest intensity the corrections for CzH4 were large and those data should be accepted only with reservation. IV. Discussion The data obtained in this investigation demonstrate the following facts: (a) The quantity y is a linear function of p but positive intercepts are obtained. (b) The slopes and intercepts of the y vs. p plots increase with increasing temperature and, at 327", the intercepts increase with intensity. (e) The rate of formation of CO (identified with I,) is a linear function of p . This requires the fraction of the incident light absorbed to be small. Hence the intensity was approximately constant throughout the cell, indicating that diffusion of the radicals was not an important consideration. (d) The rate of formation of CO is independent of temperature, a t constant p and I , so that there was no significant contribution from chains. (e) I n the experiments with the packed cell the values of y are just about as expected from the change in intensity resulting from the packing. Accordingly the change in surface/volume ratio, by a factor of 12, did not have any detectable effect on y . Hence surface effects are probably not important. The relationship between y and p is of the type =
ap $. bI.'/S
(B)
where a and b are constants independent of pressure,
0
05
IO [CH,COCH,~
15 x
20
25
In6
mole ~ m : ~ .
Fig. 2.-The
effect of intensity a t 327". Relative percentage intensities as indicated.
2 0,
I
1.8-
9 +
1.6 -
zz E n
5c - l
-
1.4
-
0
pl
1.2 *
' OCb8
1'0
;1 Id 1'6 Log Relative Intensity.
1'8
%O
'
Fig. 3.-The effect of intensity on the slopes and intercepts of the y us. p plots at 327": line A, slopes; line B, intercepts.
intensity and surface/volume ratio; they are presumably ratios and products of rate constants. The results also show that the term bI,'/2 is by no means negligible compared with up a t temperatures in excess of about 250" and at pressures and intensities commonly used in studies of the photolysis of acetone. Hence, the previously adopted relationship (A) is erroneous and data derived on that basis are questionable. It is possible that the constant a is, actually, the ratio (kz/k3:/7) and b includes the rate constant for a reaction CH,
+ HX+
CHa
+X
between CH, and some substance (HX), whose con-
Since r is a function of p , the slope of the y us. p plots will not necessarily provide a correct value of the ratio lc,/k,'/2. Accordingly, r should he eliminated from the above relationship. Attempts to eliminate r by the use of steady-state equations have not been successful because of the complexity of the equations. One alternatire is to obtain an expression for T in terms of a measurable quantity, e.g., the rate of formation of methyl ethyl ketone or diacetonyl. If ethane and methyl ethyl ketone are produced only by reactions 3 and 5, respectively, and if they do not disappear after they have been formed, we have Fig. 4.-Test of suggested mechanism: 0 , 184' (Mandelcornandsteacie); A, 217"; 0,241" (Mandelcornand Steacie); 0, 246"; v, 285'.
centration is independent of the concentration of acetone, or approximately so, and varies as The results appear to preclude the possibility of H X being absorbed acetone. Also CH&O is too unstable a t the temperatures under consideration for its stationary concentration to be at all appreciable. Hence it is not likely that CH&O contributes to the formation of CH, in a reaction of the type suggested. An alternative explanation, based on the third body restriction to the combination of CHs, does yield an equation of the correct type for the relationship between y and p . However it is inadequate since (a) it requires y to be independent of I, a t constant p and T; (b) the effect should not be a t all detectable over the range of pre,wires involved. The suggestion that H X is the acetonyl radical is much more plausible. That radical 'is stable and disappears by combination with either methyl radicals4 of itself. The suggested methane-producing reaction
This allows the y us. p relationship to be modified to
dz.
CHs
+ CHzCOCHz +CHa + CH2COCH2
(4)
is actually a disproportionation of free radicals which is considered to be of low activation energy and of comparable rate to the combination of radicals. I n the present case reaction 4 will be in competition with CHs + CHsCOCH?+CH3COCSH5 (5) 2CHaCOCHn +CHaCOCH&HzCOCH*
(6)
Accordingly, the rate of formation of CH4 by reaction 4 should be of the same magnitude as that of the formation of methyl ethyl ketone (MEK) and Mandelcorn and Steacie have shown4 that the rate of formation of MEK is not much lower than R c H , over a wide range of temperatures. Hence, it is likely that, reaction 4 will contribute significantly to the format,ion of CH4in the photolysis of acetone. The introduction of reaction 4 leads to the relationship (4) L. Mandelcorn and E. W. R. Steacie, ibid., 32, 79 (1951).
The rate of formation of MEK was not measured in the present investigation and the published data are not sufficiently extensive to allow the suggested mechanism to be tested. However, it is possible to calculate RMEKfrom the stoichiometry of the reaction and values thus obtained have been used. The relationship between ( R C H J R M E K and ) ( R ' / Z ~ n ~ JisRshown, ~ ~ ~ in) Fig. p 4, for our experiments at 217, 246 and 255" and for the experimentally determined values of Mandelcorn and Steacie a t 184 and 241". Our data at 326' were much too scattered to be of any significance. The results are not in disagreement with equation D but the data do not extend over a wide enough range to provide a strict test of that equation. The intercepts (k4/k6) are subject to a considerable uncertainty but a value of 0.25 f 0.25 is not unreasonable; there does not appear to be any consistent effect of temperature on the intercept so that E4 = Eg = 0, The following values have been obtained for the slopes T (OC.) 184 217 241 246 255 Slope 1 . 3 5 f 0 . 1 5 2 6 5 0 . 4 4.5 5 . 4 z k 0 . 6 9 . 5 h l . O
The uncertainty appears to be approximately =t 10% over the range of temperature investigated. The Arrhenius plot of the slopes is not inconsistent with a straight line, the slope of which suggests an activation energy (Ez - l / Z E?) of 10.0 5 1.0 kcal. mole-'. Assuming Ps = 1.0 and collision freqoencies of 10'4 mole-1 set.-' we find P , = 7 X with an uncertainty of a factor of about 10. The valiies of E2 and Pz obtained do not differ significantly from those based on equation A. However, our results are based on calculated and corrected data and so can be only approximately correct. The reaction should be reinvestigated under conditions such that ethane and methyl ethyl ketone may be accurately measured.
