FORMATION O F ETHANE IN THE GAS-PHASE PHOTOLYSIS OF AZOMETH.4NE
867
Intramolecular Formation of Ethane in the Gas-Phase Photolysis of Azomethanel
by S. Toby and J. Nimoy School of Chemistry, Rutgers, The State University, K e w Brvnswisk, New Jersey 08905 (Received October 4, 1965)
The photolysis of gas-phase azomethane was reinvestigated in order to ascertain the importance of the intramolecular formation of ethane. The quantum yields of the intramolecular formation of ethane at 100, 135, and 180" had an average value of 0.007 & 0.001. These values show good agreement with literature values from experiments done in the presence of radical scavengers and mixed isotopes. It has already been shown that oxygen has no appreciable effect on the primary radical split and this work confirnis that the primary molecular split is also unaffected by oxygen. At the usual intensities employed, the fraction of the ethane from the intramolecular split is negligible below 80". However, a t 180" half of the total ethane comes from the intramolecular split. The identity of the precursor of the intramolecular split was considered and postulated to be an azomethane triplet. The effect on the Arrhenius parameters is generally to increase the measured activation energy of abstraction by approximately 0.3 kea1 mole-' so that for accurate work the intramolecular split should be taken into account.
Introduction The photolysis of gaseous azomethane (A) was first studied quantitatively by Jones and Steacie.2 The main steps in the mechanism are A
+ hv +2CHa + N2 2CH3 --t C2Ha
CH3
+A
---+
CHI
+ -CHzNzCH3
(A) (1)
(2)
from which it follows that a = RM/RE'/'[A] = k2/kl'/' (units I.'" mole-'/' sec-'" throughout) where Rnf = d [CH,]/dt, RE = d [CzH6]/dt,and a is introduced for convenience. Toby and Weiss found, however13 that a showed marked dependence on azomethane concentration, particularly at and above 100" and at pressures far too high for the third-body restriction on methyl combination to be important. Their results suggested a second source of ethane in the azomethane photolysis but were not consistent with a simple intramolecular split A + CzHs N2 and so a tentative mechanism invoking the CH3N2- radical was suggested. Rebbert and Ausloos4 have photolyzed mixtures of azomethane and azomethane-ds in the presence of oxygen as a radical scavenger. They obtained evidence of the occurrence of the intramolecular spl& in the gas phase. More recent work by Rebbert and
+
Ausloos5 gave stronger evidence for the occurrence of the intramolecular formation of ethane and cast doubt on the reactions involving CH3Xz-. This doubt is well founded, for a subsequent esr study of the photolysis of azomethane6 showed no sign of the CH8K2-radical a t temperatures as low as - 196". Since azomethane is an extensively used source of methyl radicals, it is important to establish a t least the main features of its photolysis. The present work was done with the intention of repeating and extending Toby and Weiss's work using the photolysis of azomethane alone. This has the advantage of avoiding any possible secondary effects due to the presence of the oxygen scavenger and it also avoids any possible ambiguity in the results due to isotope effects.
Experimental Section The apparatus and techniques used have been pre(1) Presented a t the Physical Chemistry Division, 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. (2) M. H. Jones and E. W. R. Steacie, J . Chem. P h y s . , 21, 1018 (1953). (3) S. Toby and B. H. Weiss, J . P h y s . Chem., 66,2681 (1962). (4)R. E. Rebbert and P. Ausloos, ibid., 66, 2253 (1962). (5) R. E. Rebbert and P. Ausloos, ibid., 67, 1925 (1963). (6) P. B. Ayscough, B. R. Brooks, and H. E. Evans, ibid., 68, 3889 (1964).
Volume 70, ATumber S March 1966
868
S. TOBYAND J. SIMOY
viously described.3 An Osram HBO-75W high-pressure mercury arc was powered by a constant-wattage transformer. The output was made parallel with a quartz lens and filtered with a Corning 7-37 filter to give light mostly in the 3660-A region (filter A). One series of experiments was performed using the more monochromatic filter described by Kasha' which eliminates the 3340-A lines (filter B). The intensity was varied with neutral density filters. Photolysis products were partially separated with a low-temperature still and the methane, ethane, and nitrogen were analyzed gas chromatographically using a silica gel column at 25".
'Oo2
Results Experiments were done in the range 50-1800. Figure 1 shows a plot of LY us. azomethane pressure. There is no dependence on azomethane pressure at 50" but there is increasing dependence as the temperature is raised. The effect on LY of varying the incident intensity at constant azomethane pressure is shown in Figure 2 where, for convenience, intensity is shown as rate of nitrogen formation. If we now assume a second primary process
A
+ h~ +C2H6 + Nz
, s:
,005[5~0.
25
50
~
75 100 Pozo,mmHg
Figure 1. Semilogarithmic plot of pressure a t various temperatures.
cy
, 125
I50
s1:
us. azometharie
I
(B)
.OlOL
+
where @A @B = 1 from Jones and Steacie's work12 then the rate law becomes a-' = RE[A]~/RM' =
h/h'-k
@BRN[AI'/Rx~
A graph of a - 2 vs. RN[ A ] 2 / R ~is~ 2shown in Figure 3. The value of @n a t 180" shows a slight pressure dependence. I f this is ignored, then we obtain for @B at 100, 135, and 180" values of 0.0071 f 0.001, 0,0081 f 0.0005, and 0.0052 f 0.0005, respectively. At 50" the scatter is too great for a meaningful slope to be Obtained* This is because the split is relatively unimportant at this temperature as will be discussed later. I n order to ascertain the importance of hot-radical effects, a series of experiments was performed at 50" with the more monochromatic filter B. If hot radicals were present with filter A, the effect of jilter B would be to increase the values of Actually, filter B reduces the values of slightly and shows that @B has a slight dependence on wavelength, as Rebbert and Ausloos noted.4
Discussion Although the value of +B appears to be independent of temperature, the fraction of the total ethane from the intramolecular split varies considerably with both temperature and absorbed intensity. As the intensity The Journal of Physical Chemistry
Figure 2. Semilogarithmic plot of CY US. absorbed intensity (expressed as rate of nitrogen formation) at the temperatures and azomethane pressures noted.
decreases, the intramolecular split becomes relatively more important and we may write lim
@E = @B
Ia+O
This is shown graphically in Figure 4. At sufficiently low intensities all curves extrapolate to a value of @B of 0.006. However, at the higher intensities normally used, the fraction of intramolecular ethane is about 50% at 180" but only 1.5% at 50". This accounts for the scatter shown in Figure 3 at 50" which is imposed by the limitations of analyticai errors. It should be noted that the results shown in Figure 3 cannot be obtained a t constant incident intensity and I
(7) M. Kasha, J . Opt. SOC.A m . , 38, 929 (1948).
FORMATION OF ETHANE IK
THE
GAS-PHASE PHOTOLYSIS OF AZOMETHANE
869
.300----I50"
CAI :4. Ix IC3m/I
100" tA1~2.8~10~~3m/l
,100
135' EA1=2.8x 10-3m/l
',010
0
3
0
180" tAl.2.8x1d3m/l E
t
cw, I I I
.003'
5 8
R&R;
x
cw)
various Figure 3. Plot of' a-2 as a function of R N [ A ] ~ / R aMt Z temperatures and azomethane pressures.
110
2Io R ~ X30I O (m/ ~ O410 1-5)
5b
20
Figure 4. Quantum yield of ethane as a function of absorbed intensity (expressed as rate of nitrogen formation).
.015r
I
,010
this explains why Toby and Weiss's results3 do not give a value for +B. It is of interest to compare the present work with work done by Rebbert and Ausloos with CH3N2CH3-CD3X2CD3mixtures and in the presence of oxygen.*a5 Good agreement is obtained, as shown in Figure 5, with an estimated value for +B of 0.007 0.001 independent of temperature. Increase of azomethane pressure appears to increase +B slightly. Work on the photooxidation of azomethane8 has shown that primary process A is not appreciably affected by the presence of oxygen and the comparison shown in Figure 5 shows that primary process B is also not affected. Role of Electronically Excited States of Azomethane. The thermal decomposition of axomethane has been extensively investigated by Forst and Riceg a t about 300" and they concluded that the reaction had no significant molecular component. It therefore seems likely that any intramolecular rearrangement in the photolysis occurs via an electronically excited state. Certainly, if steric effects alone were important, one would expect a significant split in the photolysis of the isoelectronic molecule, acetone, where the methyl radicals are closer together, and this has not been observed. Although cis-axomethane has recently been isolated in the liquid and solid phases,l0 gaseous azomethane is
01 ' 1 I
1 1 I !
50
100
T,
150
I "
zoo
OC'
Figure 5. Quantum yield of the intramolecular formation of ethane as a function of temperature. Data: 0 , Rebbert and Ausloos,4 0, Rebbert and A ~ s l o o s ;a, ~ this work.
almost certainly entirely trans. The probable instability of gaseous cis-azomethane could account for the intramolecular ethane formation, the isomerization occurring via rotation about the N-N bond in the triplet state of azomethane. However, the triplet must be quite short-lived since oxygen has no effect on the intramolecular split. Indirect evidence for a triplet intermediate comes from calculations by Kearns on the potential barrier to photoisomerization in axoalkanes." He showed that an n-a* singlet intermediate will lead to an appreciable activation energy for azoalkane photoiso(8) G. R. Hoey and K. 0. Kutschke, Can. J . Chem., 33, 496 (1955). (9) W. Forst and 0. K. Rice, ibid., 41, 562 (1963). (10) R. F. Hutton and C. Steel, J. Am. Chem. Soc., 8 6 , 745 (1964). (11) D. R. Kearns, J . Phys. Chem., 69, 1062 (1965).
Volume 70,Number 3 March 1966
870
S.TOBYAND J. NIMOY
merization. However, an n-x* triplet would lead to an activation energy of zero in accordance with the temperature independence found in this work. Rebbert and Ausloos have found’? that small quantities of azomethane quenched the phosphorescence of acetone and biacetyl but not the fluorescence. They found that the triplet azomethane molecules either gave txyo methyl radicals or were collisionally deactivated to the ground state. However, a small contribution from an intramolecular split would have been hard to detect. I n addition, as they point out, an excited azomethane molecule produced by photosensitization may have considerably less vibrational energy than one produced by direct photolysis. Thus, the precursor of the intramolecular split appears to be a triplet aeornethane molecule although it is uncertain whether or not the triplet is in its ground vibrational state. If spin is conserved, then the triplet cis-azomethane may produce two methyl radicals in such close proximity that combination occurs before scavenging is possible. Effect of the Intramolecular Split on Arrhenius Parameters. Taking the intramolecular split into account one obtains k2/kl1/? = RAI/ [A](RE - #JBRK)”*
The dBRNterm corrects for the intramolecular ethane and it is of interest to test the effect of this correction on the literature values for the Arrhenius factor ratio A2/,41”2 and the abstraction activation energy Ez. I n order to apply the correction, one needs the values of RN and, unfortunately, only two of the relevant papers give these values. Table I gives a summary of the results. The parameters found in the present work are higher than any other reported values. However when Jones and Steacie’s (footnote a in Table I) results are corrected for the intramolecular split, the parameters show agreement with the present work within the combined experimental errors which are ~ t 0 . 3kcal mole-’ in all cases. Toby’s data (footnote d in Table I) were obtained a t lower temperatures where the effect of the intramolecular split is negligible. It was shown that the methyl abstraction reaction had
The Journal of Physical Chemistry
a heterogeneous component a t low temperatures which probably accounts for the low value of E*. The other data given in Table I were obtained at higher temperatures, but the magnitude of the intramolecular split correction would depend on the light intensities used.
Table I : Effect of Intramolecular Split on Arrhenius Parameters in Photolysis of Azomethane
-1.’/2
A*/A,’/~, mole-’/z set?-’/& Uncor Cor
480 310 1700 160 600
1070
160 2000
---E*,
kcal mole-1Cor
Uncor
7.6 7.3 8 4 6 9 7.8
Ref
8 2
a b
6 9
d
8 7
This work
C
e
a 31. H. Jones and E. W. R. Steacie, J . Chem. Phys., 21, 1018 (1953). * P. Ausloos and E. W. R. Steacie, Can. J . Chem., 32, 593 (1954). S. Toby and K. 0. Kutschke, ibid., 37, 672 S.Toby, J . Am. Chem. SOC.,82, 3822 (1960). e P. (1959). Gray and J. C . Thynne, Trans. Faraday SOC.,59, 2275 (1963).
The effect of the intramolecular split on the measured abstraction activation energy for the photolysis of azomethane in the presence of added substrates was considered. Again, however, there was a dearth of articles in which the values of RN were given. Sample calculations showed that the effect of the intramolecular split was not large and generally increased the measured abstraction activation energy by approximately 0.3 kcal mole-*, which was within the experimental errors. For future work where greater accuracy is desired the intramolecular split should be taken into account. Acknowledgments. We wish to thank the National Science Foundation for its support of this work. J. N. acknowledges with gratitude a Lever Brothers Fellowship. (12) R. E. Rebbert and P. Ausloos, J. Am. Chem. Soc., 87, 1847
(1965).
