March, 1963
LIQUID
PHASEREACTION OF METHYL RADICALS WITH METHANOL
that ‘ I . . .a positive deviation from Raoult’s law corresponds to local segregation of the components, which might well facilitate diffusion and yet impede viscous flow.” Benaene-diphenyl is a system showing such positive deviations and it is evident from Table I
605
that the viscosity increases more rapidly than the diffusion rate of either component decreases, which is in accord with this prediction. Acknowledgments.-The author is indebted to Professor R. H. Stokes for helpful discussions.
THE LIQUID PHASE REACTION OF METHYL RADICALS WITH METHANOL’ BY MARKCHER Research Department of Atomics International, A Dibision of North American Aviation, Innc., Cunoga Park, Cul. Received August 17,1969 The liquid phase photolysis (X > 2900 A.) of acetone-ds in the presence of CH30H and CD,OH has been studied a t 30.0” by measuring the isotopic composition of the resulting methanes. The results are interpreted in terms of hydrogen abtstraction reactions of methyl-& radicals with substrate molecules. The conclusions are: the rate constant for the abstraction of hydrogen from the methyl group in methanol is (a) 45 times greater than the rate constant for the abstraction of hydrogen from the hydroxyl group in methanol, (b) 8.7 times greater than the rate constant for the abstraction of deuterium from CD:,OH, and (c) 0.56 times as large as the rate constant for the abstraction of deuterium from acetone-&. The disproportionation reaction between methyl and acetyl radicals to form methane and ketene is shown to be unimportant except a t light intensities much higher than that used in the measurements. til the entire sample into the reaction cell. After photolysis the There is general agreement at present that hydrogen mixture was distilled into a Dry Ice trap, the non-condensable atoms are producedwhen methanol is decomposed-by gas was toeplered through a liquid nitrogen trap into a calibrated ultraviolet photolysis,2 mercury phot~sensitiaation,~~~ volume where the pressure was re%d,and then it was transferred or radiolysis.6g6 There is some question, however, as into a sealed container for analyak, using a modified C.E.C. 21620 mass spectrometer. to which of the two types of hydrogen in methanol Materials.--Methanol-& (CD,OH), 99% stated isotopic purity, actually is involved in the subsequent abstraction reacand acetone-&, 99.5% stated isotopic purity, were purchased tion producing hydrogen gas. I n order to answer this from Volk Radiochemical Company. The methanol-& was question directly we have photolyzed mixtures of acechemically pure by gas chromatography, but the acetone-& contained a few per cent of unidentified chemical impurities in some tone-& in both CD30H and in CH30H, and measured but not all of the purchased samples. The CDsH/CDr yield the isotopic composition of the product methane, the ratio from the photolysis or 7-radiolysis of various samples of assumption being that methyl radicals and hydrogen acetone-& alone ranged from 0.093 to 0.28; purification by gas atoms behave similarly in the abstraction from methchromatography of a previously radiolyzed sample decreased the anol. Since our interest was directed mainly in trying CD3H/CD4 yield ratio from 0.23 to 0.084. Both unpurified and purified (by gas chromatography) acetone-ds were used in our to correlate results in the liquid phase radiolysis af measurements; the spread of the data in Fig. 2 probably is due methanol,6 we chose to work first with liquid mixtures. largely to this cause. The methanol was Mallinckrodt reagent Studies of the liquid phase photolysis of acetone, alone’ grade. It was dried with “Drierite” and purified by bulb to and in the presence of various so1vents,8--loalready have bulb distillation a t - 80 ’. indicated that decomposition proceeds by way of free Results radicals with reactions analogous to those known to The main gaseous products of the reaction were CD4, occur in the gas phase. CDaH, and CO. Trace amounts of other isotopic Experimental methanes also were observed. The yields of methane
Because of the high cost of the deuterated materials, about 0.3 cc. was photolyzed in each run. Reaction cells of approximately l-mm. path length were constructed from thin parallel rectangular strips of quartz, 14 mm. wide and 35 mm. long, fused t o one arm of a stopcock. The temperature was maintained at 30.0 i 0.1” by circulating water through glass jacket surrounding the reaction cell. The light source was a Hanovia mercury arc type A. A flat Pyrex glass window was cemented to the wall of the jacket to limit the effective light to wave lengths above 29003000 A., well above the absorption limit of methanol. The experimental procedure was to pipet 0.3 cc. of solution, which had been prepared by weight, t o a tube in the vacuum line, degas by the freeze-pump-thaw technique, and then vacuum dis(1) Work performed under AEC contract AT(Il-l)-GEN-8. (2) R. P. Porter and W. A. Noyes, Jr., J . Am. Chem. SOC.,81, 2307 (1959). (3) M. K. Phibbs and B. de B. Darwent, J . Chem. Phys.. 18, 495 (1950). (4) (a) R. F. Pottie, A. G. Harrison, and F. P. Lossing, Can. J . Chem.. 39, 102 (1961); (b) A. R. Knight and H. E. Gunning, ibid., 39, 1231 (1961). (5) J. H. Baxendale and F. W. Mellows, J . Am. Chem. SOC.,83, 4720 (1961). (6) J. G. Burr, Proceedings of the IAEA Symposium on the Uses of Tritium, Vienna, May, 1961. (7) R. Pieck and E. W. 1%.Steacie, Can. J . Chem., 33, 1304 (1955). (8) D. B. Peterson and G. J. Mains, J . Am. Chem. Soc., 81, 3510 (1959). (9) D. H. Volman azld L. W. Swanson, ibid., 83, 4141 (1960). (IO) R. Doepker and G. (J. Mains, ibid., 83, 294 (1961).
corresponded to not more than 201, acetone decomposition. The CD4/CD3Hratio was determined from the heights of the mass peaks 20 and 19 after suitable corrections for the C-13 isotope contribution and on the assumption of equal spectrometer sensitivity. The CDJCDSH ratio was plotted against the initial acetone/methanol concentration ratio with results as shown in Fig. 1 and 2. An attempt to increase the gas yields by substituting a Vycor filter for the Pyrex filter resulted in anomalously high yields of CD4. Consequently, the effect of varying the light intensity on the CD4/CD,H yield ratio in the photolysis of a mixture of CDdCOCDs and CH3OH was studied using the Vycor filter by inserting wire screens of varying transmittance between the lamp and the cell. The average rate of gas production was taken as a measure of the relative light intensity. The results are shown in Fig. 3. The experimental points denoted by solid circles are based on the photolysis experiments using the Pyrex filter, and were estimated by interpolation from the data shown in Fig. 1.
606
Vol. 67
MARKCHER
0.20 0.25
I
0.5
0.4
-E t -6u c c 3
0.3
I
\
2
0.05
0.1
I
vI
I
I
I
I
I
0
2
4
6
8
10
12
01
0.2
1
14
100 X [CDaCOCDal/[CHaOHI.
Fig. 1. -The isotopic composition of methane in the photolysis of CDaCOCD3-CHsOH mixtures: 0, unpurified acetone-&; 0, acetone-& purified by gas chromatography.
26 I
0
I
. 22
h
6
-'
18
v
\
14 10
4
1 0
2
I I I I I I 4 6 8 10 12 14 100 X [cDsCoCDa]/[CDsOH].
I
16
I 18
Fig. 2.-The isotopic composition of methane in the photolysis of CD3COCD3-CD30H mixtures: 0, unpurified acetone-&; 0 acetone-de purified by gas chromatography. Kotice that the scale of the Y-axis is different from that of Fig. 1.
Discussion In the photolysis of mixtures of acetone-& and methyl-d3 alcohol the only important reactions leading to the formation of CD, and CD3Hare assumed to be
6 1
0
I 2
I
1
4
6
I 8
I
10
12 100 X [CDsCOCDal/ [CDsOH].
I
I
I,
14
16
18
Fig. 4.-Plot of the function y / ( l - c u ) os. the acetone-&/ methanol-& concentration ratio: 0,unpurified acetone-&; 0 , acetone-& purified by gas chromatography.
we must assume the following reactions, analogous to ( 2 ) and (3) k7
+ CH30H +CD3H + CHzOH CD3 + CHBOH + CD3H + CH3O
CD,
(7)
k3
IC1
+ CDJCOCD3+CD, + CDzCOCD3 CD3 + CDsOH +CD, + CDzOH CD3 + CDEOH + CD3H + CD3O k'l CD3 + CDZHCOCD, --+ CDdH + ki CDZCOCD, CDZH + CDaCOCD, + CDZH + kz CDZCOCD, CDzH + CDaOH CDZH + CDzOH CD3
(I) (2)
(8) I n the case of CD30H, consideration of reactions 1 through 6 gives for the ratio of the rates of production of CD4 to CD3H
IC3
(3)
+
where 2 is the mole fraction ratio of acetone (d6 d6) to methanol, the constants a, b, and c are defined as (4)
(5) ----t (6) Reactions 4, 5, and 6 are included because of the presence of acetone-ds in the acetone-de. They are needed to account for the decrease in the slope in Fig. 2 as the acetone concentration is increased, and also to account for the finite CD4/CD3H yield ratio in the photolysis of acetone alone. If light methanol is substituted for methanol-da,
+ a)-'(ki/kz) + a/@ + a) a[(k,/kd + 1/(2 + a ) ]
a = (1
b
=
c =
(k3/k2)
and a is the acetone-d5/acetone-da concentration ratio. I n deriving eq. 9 we assume that le6 = k,, k6 = lcz, and also that the CD*H/CDg concentration ratio is a ) , since acetone-ds can decompose given by a / ( 2 to form either CD3 or CDzH with nearly equal probability. Equation 9 may be rearranged to give the linear form
+
March, 1963
LIQUID P H A S E
y/(l - cy)
=
( b - c)-1
REACTION O F METHYL RADICALS
+ a(b - c)-%
(10)
inasmuch as the value of c can be estimated from the CD4/CD3H yield in “pure” acetone-d6. Figure 4 shows the data of Fig. 2 plotted according to eq. 10, usingc = 0.085. With solutions containing CH30H, consideration of reactions 1, 4, 5, 7, and 8, and assuming k8 = k3, gives the expression y == px(1
+ pcx)-l
(11)
where p
= (1
+ a ) - l w ( h + k3)
Equations 10 and 11 permit us to determine the numerical, values of the interesting rate constant ratios, namely, k7/ka, k7/k2, and k7/kl, from the initial slope in Fig. 1 [initial slope = p ] and from the slope and intercept in Fig. 4. I n these calculations we take a! = 0.05, as estimated from gas chromatography and mass spectrometer measurements of the acetone-&. The resu Its are as follows: k-i/& = 45 f 12; h / k 2 = 8.7 f 2.8; and k7/k1 = 0.56 f 0.05. An indication of the goodness of the fit may be obtained by noting that the solid curves in Fig. 1 andl 2 were calculated on the basis of eq. 9 and 11 using the constants given above, except that in Fig. 1 the entire curve was shifted upward by 0.038 unit in order to fit the observed intercept. This latter point is discussed below. Qualitatively, the ratio k7/k3 = 45 f 12 means that the abstraction of a hydrogen atom from CHSOH by methyl radicals occurs mainly (98% probability) a t the methyl group. Moreover, this value probably is too low since in our treatment we have neglected entirely the effect of CDZHOH impurity in the CD3OK. This result is consistent with the observation by Porter and Noyes2 that in the gas phase photolysis of CD30H as much as 20% of the product hydrogen is H2, since a high intrinsic probability that photolytic H atoms abstract from the methyl group to produce H D is partially offset by the isotope effect tending to retard abstraction of D atoms relative to H atoms,I1 as suggested below. The ratio Ic7/kz = 8.7 f 2.8 is a measure of the isotope effect corresponding to the abstraction of hydrogen or deuterium from the methyl group in methanol. This (11) I n our experiments the ratio of relatire rates of abstraction by CDI radicals from the methyl and hydroxyl groups in CDaOH is k d k a = 5 2 . Thus we oalculate that 16% of the methane originating in the abstraction from CDaOH is CDaH.
607
W I T H &‘IETHASOL
result may be compared with a calculated valueL2 of 6.0 at 30”, based on published13 C-H and C-D stretching frequencies for CH30H and CD30H. The ratio k?/kl = 0.56 f 0.05 is a measure of the relative rates of abstraction by CDBradicals of hydrogen from the methyl group in methanol and deuterium from acetone-&. This ratio would no doubt be even smaller if light acetone were being compared to methanol. This means that in the liquid phase the rate of abstraction from acetone is greater than that from methanol, whereas in the gas phase the opposite is true.I4 The interpretation of the experimental measurements as presented here is based on the assumption that all the reactions producing CD4 or CD3H have been included in the mechanism.16 There is considerable evidence 7-g that in the liquid phase photolysis of acetone the reaction CD3 CD3CO = CD4 CDzCO (12)
+
+
is of importance. The increase in the CD4/CD3H yield ratio with increasing light intensity constitutes additional evidence for the occurrence of this reaction.16 In our measurements using the Pyrex filter, however, reaction 12 need not be considered in view of the fact that the CD$CD3H yield ratio in Fig. 3 extrapolated to zero intensity, where this reaction is certainly not important, differs from the observed CD4/CD3H ratio in the Pyrex experiments by less than The small but real positive intercept in Fig. 1 may be due to the contribution of reaction 12 occurring partly homogeneously in the solution and partly as a cage reaction. Correction for the excess CD4 would tend to shift all the experimental points downward, and thus the intercept, which mas not predicted by eq. 11, conceivably might be eliminated. Ackn.owledgment.-The author is indebted to Mr. R. A. lvleyer for the mass spectroscopic analyses. (12) L. Melander, “Isotope Effects on Reaction Rates,” Ronald Press Co., New York, N. Y., 1960, p. 20. (13) M. Falk and E. Whalley, J . Chem. Phys., 34, 1554 (1961). (14) A F. Trotman-Dickenson, “Gas Kinetics,” Butterworths Scientifio Publications, London, 1955, pp. 201-202. (15) Isotopic exchange between methanol and acetone-& is not considered important in view of the very small yields of CDzHz. Furthermore, photolysis of liquid mixtures of acetone in Dz0 indicated no deuterium in the methane product. (16) The dependence of the rate of this reaction on intensity implies that the reaction takes place mainly homogeneously in the liquid phase rather than in a “cage” following the primary dissociation step. (17) The fact that the results based on the Pyrex filter experiments fall reasonably close to the straight line drawn through the points based on the Vyoor filter experiments suggests that the essential difference between these two sets of experiments is in the total over-all intensity entering the reaction cell, rather than in the difference in spectral distribution.
