THE! PHOTOCHEMISTRY OF 1,Q-DIOXOLANE
2863
The Photochemistry of 1,3=Dioxolane1
by B. C. Roquitte Radiation Research Laboratories, Mellun Inetthde, Pthburgh, Pennaylvania
(Received February 25, 1966)
The gas phase photodecomposition of 1,3-dioxolane with the full light of a medium-pressure mercury arc has been investigated in the pressure range 11-82 mm and at temperatures 27-110". The reaction products found were H2, CO, C02, CH4, C2H6,C2H4, C4HI0,and CH20. Experiments were carried out with various additives (02,C2D4, n-C4Hlo, and i-CdH10). Two primary processes are important in the decomposition CHpCH2 /l
CH2=CH2
+ CO2 + HZ(and/or 2H)
35%
(A)
(B) The azomethane-sensitized decomposition of l,&dioxolane has also been studied briefly. (35%
Introduction As part of a general study of the photochemistry of cyclic molecules containing one or two oxygen atoms in the ring, the present study of the photochemical decomposition of l13-dioxolane was undertaken. It was of particular interest to ascertain whether the photochemical modes of decomposition are analogous to the thermal decomposition2 of 1,3-dioxolane. Therefore attention was focused on the direct unimolecular decomposition processes in this molecule, although its radical-sensitized decomposition was briefly studied. This paper describes the first study of the photochemical decomposition of 1,3dioxolane.
Experimental Section Materials. Eastman Kodak White Label l,&dioxolane was distilled under vacuum and a middle fraction was retained for most of the experiments reported here. Analysis by gas chromatography using a 2-m Carbowax column and a 2-m dinonyl phthalate column showed that the distilled sample contained only one component. Oxygen from the Matheson Co. was used directly from the cylinder. The ethylene-& was obtained from Volk RadioChemical Co. It was degassed and distilled from -160 to -196". Both 2,3-dimethylbutane and 12-
butane (research grade) from Phillips Petroleum Co. were outgassed and distilled under vacuum in the usual way. Apparatus and Procedure. A Hanovia U-type medium-pressure mercury arc was used as a light source in all of the experiments. The light was partially collimated onto a quartz cell of 189-cc volume mounted in an electrically heated furnace. The temperature of the cell could be controlled within k3" during a run. I n most of the runs no filter was used since it was noted that, when using a Corning filter 7910, which transmits light of wavelength longer than 2100 A, the photochemical decomposition was negligible. I n the azomethane-sensi tized decomposition a Corning filter 5860 was used to isolate 3660-A light. The highvacuum system was of standard type provided with two LeRoy traps, a solid nitrogen trap, and a Toepler pumpgas buret for gas analysis. Before photolysis a sample was expanded in the cell and then condensed at liquid nitrogen temperature and degassed. In experiments with added gas the reactant and added gas were thoroughly mixed by means of an all-glass circulating pump before being exposed to the radiation. (1) Supported, in part, by the,U. S. Atomic Energy Commission. (2) W. B. Guenther and W. D. Walters, J . Am. Chem. Soc., 73, 2127 (1951).
Volume 70, Number 9
September 1966
B. C. ROQUITTE
2864
Table I : Photolysis of 1,3-Dioxolane Press., mm
7 -
11.0
20.0
20.0
20.0
46.5
50.0
50.0
2i
27
ucts
129
110
110
27
50.0
50.0
73.0
61.0
81.0
82.0
120
120
27
27
7
27
27
80
27
121.5
120.0
19.0
30.0
58.0
80
110
Exposure time, min--
7
120.7
183.8
122.2
50.0
120.0
#moles
7-
2.90 2.60 0.39 0.26 1.89 2.10 na n a
81
-
7-
Prod-
50.0
Temp, O C
7
3.82 3.45 0.48 0.29 2.51 2.89 2.51
4.72 3.12 0.61 0.27 3.20 3.13 0.31 2.47
5.62 3.49 0.89 0.37 4.48 4.21 0.29 2.64
7.58 4.63 1.34 0.53 5.37 5.44 0.31 n
0.64 0.41 0.09
Trace 0.47 0.64 n 0.74
1.08 0.72 0.12 0.03 0.59 0.80 n n
120.0 7
2.21 1.53 0.27 0.08 1.11 1.52 0.09 1.73
4.63 3.64 0.62 0.20 2.38 3.22 0.17 2.99
7.01 5.86 1.02 0.37 3.49 4.80 0.33 3.96
6.85 4.79 1.12 0.43 4.82 5.40 0.39 3.70
3.36 2.77 0.36 n n n n 2.55
9.60 5.81 1.80 0.70 7.34 7.14 0.28 4.05
7.64 5.29 1.31 0.35 4.48 5.21 n n
n = not analyzed.
After each run the products were condensed at - 196" and then distilled through two LeRoy traps at - 196" and a solid nitrogen trap (-210"). In experiments in which oxygen was added to the reactant, the noncondensable gases were pumped off, while in runs which were performed without oxygen, the noncondensable gases were measured in a Toepler pump-gas buret. The noncondensable fraction which contained Hz, CH4, and CO ~ 7 a sanalyzed by a mass spectrometer. The condensable fraction was transferred quantitatively into an ampoule and then introduced into the inlet system of a dual-column, dual-detector gas chromatograph. The two columns and the detectors were connected in series, and the electrical signals from the detectors were fed into a double-pen recorder. One of the columns was 2 m long, packed with 25 wt % Carbomax 600 on Chromosorb, while the other was 1.5 m long and packed with silica gel. Ethane, carbon dioxide, ethylene, and butane were all resolved very well in the silica gel column. The Carbowax column could resolve acetaldehyde, ethylene oxide, and 1,3dioxolane. The columns were calibrated using known quantities of authentic samples. Since it was not possible to analyze formaldehyde by gas chromatography, it was estimated quantitatively by the color r e a ~ t i o n . ~In order to analyze all of the products, two runs were carried out under identical conditions, of which one was used for analysis of formaldehyde while the other one was used for analysis of all other products. Results Within the decomposition range 0.5-2.5Q/,, the principal products of the reaction were hydrogen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, n-butane, and formaldehyde (Table I). The Journal of Physical Chemistry
Acetaldehyde was not detected as a product, and, if it were present at all, it could not be more than a few per cent of the total products. The product yields at different exposure times are shown in Figures 1 and 2, which demonstrate the leveling off of formaldehyde at longer exposure while the other products were linearly proportional with time except carbon monoxide, the rate of which increased with time. The scavenging effect of ethylene-& (Table 11) on the hydrogen yield is prominent in Figure 3. The purpose of added ethylene4 was dual. First, it acted as a scavenger of the hydrogen atom. Second, it acted as a detector of shorter wavelengths since
7
L
Figure 1. Yield of CO and Hz us. time.
(3) D. Matsukawa, J. Biochem. (Tokyo), 30, 386 (1939).
A
THEPHOTOCHEMISTRY O F 1,3-DIOXOLANE
2865
Table 11: Effect of Addends in the Photolysis of 1,3-Dioxolane" Press., mm
Addends
... ...
0 2
C2D4 n-CaHlo i-CdHlo Products
...
... ...
10.0
...
...
...
...
...
1.5
4.0
9.0
...
...
...
... ... 284.0 101.0
fimoles
c
3.82 3.46 0.48 0.29 2.51 2.89 nb 2.51 a
3.0
n a' 1.81 1.90
n n n a 1.79 1.99
a
a
1.71 3.34 0.41 n n n n
2.50
n
n
n
n
1.47 3.33 0.35 n n n n n
Pressure of 1,3-dioxolane 20.0 mm; exposure time 120.0 min; temperature 27".
*n
1.25 3.17 0.36 n n n n n = not analyzed.
3.28 2.72 0.48 n n n n n
3.71 3.00 0.69 0.26 2.50 2.60 n 2.40
a = absent.
ethylene starts absorbing light at a wavelength of 1900 A. A trace amount of Dz (-0.1%) was detected as a product of runs with added ethylene-&; therefore, it was concluded that wavelengths shorter than 1900 A were not playing an important role in the decomposition of 1,&dioxolane. Product yields were determined with added oxygen as well. I n these runs the reactant pressure (20 mm) and exposure time (120 min) were kept constant. These results are given in Table 11. The effect of temperature on the products of the photolysis with 20 mm of 1,3-dioxolane is shown in Figure 4. All of the products increased with increasing temperature except carbon monoxide; hydrogen increased the most and formaldehyde the least.
The mole ratios H:C, H:O, and C:O for the total observed products are given in Table 111. It appears that, within experimental errors, the material balance was reasonably good. I n Table IV the product yields of the azomethanesensitized decomposition of 1,3-dioxolane are sum-
Figure 2.
Figure 3. Yield of hydrogen us. per cent of added ethylene.
Yield of CHzO u8. time.
vo'olume 70,Number 9 September 1966
2866
B. C. ROQUITTE
Table I11 : Mass Balance of the Products of Photolysis of 1,3-Dioxolane Press.,
Temp,
mm
O C
20.0 20.0 50.0 50.0 61.0 Theoret
~
27 110 27 27 80
...
Exposure time, min
120.0 121.5 58.0 120.7 122.2
...
6t
H:C
H:O
c:o
1.90 1.93 2.0 1.95 1.93 2
2.99 2.78 3.00 2.94 2.96 3
1.57 1.44 1.50 1.50 1.53 1.5 I 50
I 60
I 70
I
I
80
90
I 100
I
1
110
120
Figure 4. Effect of temperatures on the product yields.
DX press., mm
r
20.0
52.5
53.5
QQ.0
Azomethane press., mm 2.0
3.5
964.8
968.4
957.6
906.3
0.12
0.33 12.48 0.25 4.96 7.43 5.51 0.99 n 0.60
0.44 6.76 0.06 3.06 3.68 3.13 0.71 n
0.41 5.86 0.13 4.25 4.20 9.88 0.87 n 0.85
1.5
2.0
Exposure time, min Products
I 40
Temperature 'C
Table IV : Methyl Radical Sensitized Decomposition of 1,3-Dioxolane"
0.05 0.27 nb n n n 0.09 a
I 30
20
~~
Wavelength 3600 A; temperature 27".
rl
' n = not analyzed.
wavelength is absorbed by the air. Therefore, the wavelengths which are most effective lie in the region 2000-1900 A. Table V"
Filter
Product, pmoles
Rei a m t of decompn
Vycor Interference None
0.081 0.311 7.49
3.8 92.5
1
Exposure time 120 min.
Photocheinically E$ective Wavelength. It appears that the ultraviolet absorption spectrum of 1,3-dioxolane has not been determined previously. Owing to the limitation of the Cary 14 spectrophotometer, it was not possible for us to take the complete spectrum down to 1900 A. Nevertheless, it appears certain that 1,3-dioxolane starts absorbing around 2000 A. I n the absence of a complete absorption spectrum, the following experiments were performed to determine the photochemically effective wavelength. The decomposition of l ,3-dioxolane was carried out using the same light source in combination (1) with a Vycor filter, (2) with an interference filter (band width 100 A with maximum at 2100 A), and (3) without filter. The results are given in Table V where the total amounts of noncondensable products are listed. I n a medium-pressure mercury arc the effective light is largely continuous with a few superimposed lines in the region 2000-1849 A. Owing to the presence of a long air path through which the light has to travel before entering the reaction cell, most of the 1849-A The Journal
0.f
Physical Chemistrv
Discussion The experimental evidence suggests that at least two primary processes which take place are CH2-CH2
CHFCH~
+ COZ+ HS (and/or 2H) CH2O + CH, + CO + H
35%
(A)
65%
(B)
The quenching effect of addends n-butane and isobutane on the product yields (Table 11)does not appear to be significant. This perhaps suggests that excited molecules do not play an important part in the decomposition of l13-dioxolane. I n the present system, t,he formation of hydrogen atoms and molecules seems unquestionable. The decrease in the yields of hydrogen with increasing amounts
THEPHOTOCHEMISTRY
OF
of ethylene was due to the addition of the hydrogen atom to the ethylene double bond. As is evident from Figure 3, the hydrogen yield was reduced to about 55% by the addition of 7.5% of ethylene. Extrapolation of the linear part of the curve in Figure 3 to infinite concentration of ethylene showed that about 33% of hydrogen from 1,3-dioxolane decomposition was produced as molecular hydrogen and therefore was unscavengable by ethylene. Both ethylene and carbon dioxide were produced in abundant quantities in the presence of oxygen (Table 11). This indicated that a major portion of ethylene and carbon dioxide was formed in a direct molecular process. The persistent production of formaldehyde in large amounts, even in the presence of oxygen and ethylene, indicated that perhaps it was formed by a direct molecular process and may, in fact, be the same process which yielded CH3, CO, and part of the hydrogen atom. The presence of methyl radical in the photolysis was obvious. The formation of methane and ethane in the pure 1,3-dioxolane and removal of CzHa by the addition of oxygen can only be explained by the reactions of methyl radical. Since no acetaldehyde was detected as a product, the diradical (!JH2-CH2-0, a possible intermediate formed after the elimination of formaldehyde, could decompose after the migration of a hydrogen atom into CH3, CO, and H. On the other hand, the diradical could split to give CH2= CHz and the 0 atom. I n the flash photolysis of ethylene oxide we have observed such a p r o c e ~ s . ~I n addition, the data of Table I show that, at a pressure of 50 mm of dioxolane and at 27", the ethylene yield is greater than CO2. This may indicate that excess ethylene might have come from such a process as CHz--CHz-d
---it
CHFCHZ
+0
Then the number of oxygen atoms is given by [CzHd] [CO,]; at higher conversion this difference is a large number. Thus, all of these oxygen atoms could react with the substrate by some radical mechanism to produce the observed products. The decrease in the formaldehyde rate with reaction time (Figure 1) indicates consumption of the product formaldehyde in secondary reactions. Since the carbon monoxide rate increased with time, it is certain that a portion of CO has been formed by the secondary decomposition of formaldehyde HCHO
2CH0
2867
1,3-DIOXOLANE
+ CHO CH20 + CO
H --f
An approximate estimate indicates that at least 35% of the decomposition proceeds via process A and the
rest via process B, provided, of course, no other primary processes are involved. The decrease in the yield of COz and C2H4 by the addition of 3 mm of oxygen in 20 mm of 1,3-dioxolane suggests that perhaps the triplet state was the precursor of about 28% of the COZ and CzH4formed in the system. Further addition of oxygen (10 mm) did not quench the products COZ and CZH4 anymore within experimental errors. This is in contrast to the decomposition of acetone5where very small amounts of oxygen were very effective in removing the triplet state. It is possible that the major part of COz and CzH4 originated via singlet state and that is why they are not quenched completely. However, it has been indicated that the diagnostic test of triplet by the use of oxygen is not effective in every case.6 The existence of diradicals in the present system cannot be ignored and requires some comment. It is possible that the initial act of a photon in this system might lead to the formation of diradicals such as cHz--CH2-0-CHz-0 (I) and/or 0-CH2-CHz-O-CH2 (11) and these could decompose to yield observed products or re-form the starting compound. Since the C-0 bond in 1,3-dioxolane is of the order of 75-80 kcal/ mole' and since the radiation absorbed corresponds to 144 kcal/mole or greater, the diradical formed will be highly energetic and might decompose to give the observed products. The temperature dependence of the product yield (Figure 4) indicates that the decomposition of diradical I into CZH4, COz, and HZ (and/or 2H) was favored by higher temperatures whereas the breakdown of diradical I1 into CH20, CO, CH3, and H atom was almost independent of temperature in spite of the fact that in both cases the rupture of C-0 bond(s) was essential. Owing to the polychromatic nature of the light source, photons with varying amounts of energies are absorbed in the system. Thus, it is possible that the diradicals I and 11, formed by primary process, have different lifetimes and consequently behave differently with increasing temperature. Since quantum yields are not measured, it was virtually impossible to ascertain as to what extent these diradicals re-formed 1,3-dioxolane. Finally the formation of n-butane in this system could be explained as the result of the reaction of ethyl radicals which were formed by addition of H atom to ethylene
Roquitte, J . Phys. C h . ,70,2699 (1966). (5) G. W. Luckey and W. A. Noyes, Jr., J . Chem. Phys., 19, 227 (1951); J. Heicklen, J . Am. Chem. SOC.,81, 3863 (1959). (6) D. W. Setser, D. W. Placaek, et al., Can. J . Chem., 40, 2179 (1962). (7) P.Gray and A. Williams, Chem. Rev., 59, 239 (1959).
(4) B. C.
Volume 70,Number 9 September 1966
B. C. ROQUITTE
2868
H
+ C2H4 2CzH5
+C2HS --j
C2H4
IC4H10
+ CzHs
Methyl Radical Sensitized Decomposition. Magram and Taylor* have pointed out that an oxygen atom will affect the ease of removal of the hydrogen atom on an adjacent carbon atom. It may be expected from this that the hydrogen atoms on the methylene group flanked by two oxygen atoms will be more reactive than those of the other two methylene groups in 1,3dioxolane. Since methyl radicals and hydrogen atoms are formed in the primary process(es), it is quite likely that they will react with the labile hydrogen atoms
r
+
CH4(H2) CHZ-~H
I
I
0 0
v CH2
v CH
In order to decide the fate of the radicals X and Y, the methyl radical sensitized decomposition of l13-dioxoIane was undertaken. It is postulated that these radicals decomposed to give the observed products (Table IV). It should be noted that the ratio COz:CH20is much higher than that in the direct photolysis of 1,3-dioxolane. This may be interpreted to mean that, because the hydrogen atoms in CHZ flanked by oxygen atoms are more reactive, radical Y was produced at a higher frequency than the X radicals, and the former decomposed to give C02 and ethylene and H atom. It may be pointed out that yield of CzH4 in the methyl radical sensitized decomposition was much too small compared to the yield of COz and this probably was due to removal of ethylene by some secondary reactions. In conclusion, the present study provides evidence that in the photolysis of 1,3-dioxolane two primary processes account for most of the products. It is interesting to note that, whatever the detailed mechanism may be, the end products in both the thermal and photochemical decompositions are surprisingly similar.
Acknowledgment. The author is grateful to Dr. K. 0. Kutschke for his comments on this work. (8) S. J. Magram and H. A. Taylor, J . Chem. Phys., 9 , 755 (1941).
The J o u r 4 of Physkd Chemistry
