of fast reacting hydrocarbons will increase the reactivity of the slower reacting hydrocarbons. Literature Cited
Altshuller. A. P.. Bufalini. J. J.. Photochem. Photobiol. 4. 97-146 (1965). Altshuller, A. P., Cohen, I. R., Intern. J . Air Water Pollution 7,787 (1963). Altshuller, A. P., Cohen I. R., Intern. J . Air Water Pollution 8, 611 (1964). Altshuller A. P.. Cohen I. R.. Purcell. T. C.. Can. J . Chem. 44,2973 (i966j. Altshuller, A. P., Klosterman, D. L., Leach, P. W., Hindawi, I. J.. Siasbv. - J. E.. Intern. J . Air Water Pollution 10, 81 (1966). Benson, S. W., Axworthy, A. E., J. Chem. Phys. 26, 1718 (1957). Benson, S. W., J. Am. Chem. SOC.87, 972 (1965). Bufalini, J. J., Altshuller, A. P., Can. J . Chem. 43, 2243 (1965). Bufalini, J. J., Stephens E. R., Intern. J . Air Water Pollution 9, 123 (1965). Cohen, I. R., Purcell, T. C., presented at the Symposium on Photochemical Aspects of Air Pollution, 152nd Meeting, ACS, New York, N. Y., September 1966. Cvetanovic, R. J., J . Chem. Phys. 30, 19 (1959). _
I
Ford, H. w . , Endow, N., J . Chem. PhYs. 27, 1156 (1957). Johnston, H. S., fnd. Eng. Chem. 48, 1488 (1956). Jones. W. M.. Davidson. N.. J . Am. Chem. Soc. 84. 2868 (1962). Kaufman, F., Kelso, J. R., J. Chem. Phys. 40, 1162 (1964). McMillan, G. R., Calvert, J. G., in “Oxidation and Combustion Reviews,” Vol. 1, C. F. H. Tipper, Ed., p. 106, Elsevier, New York, 1965. Nicksic, S. W., Harkins, J., Fries, B. A., J. Air Pollution Control Assoc. 14, 158 (1964). Sato, S., Cvetanovic, R. J., Can. J . Chem. 36, 970 (1958). Sato, S., Cvetanovic, R. J., Can. J. Chem. 37, 953 (1959). Schuck, E. A., Doyle, G. J., Air Pollution Found. (Los Angeles) Rept. No. 29, 1959. Schuck, E. A., Stephens, E. R., Schrock, R. R., presented at the 59th Annual Meeting of APCA, San Francisco, Calif., June 2C-24, 1966. Thomas, S. S., Calvert, J. G., J . Am. Chem. SOC.84, 4207 (1962). Wei, Y. K., Cvetanovic, R. J., Can. J. Chem. 41, 913 (1963). Zaslowsky, J. A., Urbach, H. B., Leighton, F., Wnuck, R. J., Wojtowicz, J. A., J . Am. Chem. Soc. 82, 2682 (1960). Receiced for reciew December 16, 1966. Accepted February 6, 1967. Mention of commercial products does nor constitute endorsement by the Public Health Sercice.
Quantum Yields During Dilute Gas Phase Photolysis of Ethyl Nitrite in the 3500 A. Region E. A. Schuck and E. R. Stephens Statewide Air Pollution Research Center, University of California, Riverside, Calif.
rn Parts per million concentrations of ethyl nitrite (EtONO) were photolyzed in the absence and presence of added nitrogen or NO. The light source employed had its intensity maximum a t 3500 A. with 85 % of its energy confined to the 3300 to 3750 A. region. The was usually much less than unity and decreased as the concentration of EtONO or added gas was increased. Above a n initial EtONO of 20 p.p.m., or in the presence of added gases, the reaction followed a pseudo first-order rate law. I n the absence of added gases and at initial EtONO concentrations, which were much less than 20 p.p.m., the reaction tended to follow a second-order relationship. A mechanistic approximation of the results can be obtained b y assuming the existence of two equally important primary processes. These are cleavage a t the RO-NO bond and formation of an excited state. Recombination of the initial fragments from RO-NO bond cleavage and collisional deactivation of the excited state appear to be important secondary reactions. The choice between these two primary steps may involve the rotational isomers. Thus, intramolecular rearrangement of the excited cis-isomer may be the major source of the C H 3 C H 0 .
C
leavage of the RO-NO bond is considered to be the major primary process in nitrite photolysis (Calvert and Pitts, 1966). The nature of the products formed and an of unity indicates this is the most probable path during photolysis of certain alkyl nitrites. 138 Environmental Science and Technology
The purpose of this investigation is to assess the importance of this cleavage to EtONO photolysis in the parts per million by volume concentration range. Both MeONO and EtONO have been postulated as intermediates (Leighton, 1961) which occur during formation of photochemical air pollution products. Furthermore, photolysis of EtONO in oxygen results in the formation of peroxyacetyl nitrate, which is an important phytotoxicant and eye irritant found in photochemical air pollution mixtures. Thus, nitrite photolysis may be closely related to an understanding of this latter perplexing problem. Experimental
The experiments were carried out in a 60-liter reaction cell which also housed the optics for a 40-meter folded path infrared cell. Infrared energy entered and left the cell by means of KBr lenses mounted in one end of the 3-fOOt long 10-inch diameter cylindrical cell. The cell was constructed of low carbon steel and the interior surfaces were coated with porcelain. Borosilicate glass windows mounted in the cell wall allowed entrance of the irradiating light. A Perkin-Elmer Model 221 G Infrared Spectrophotometer was used to monitor the concentrations of reactants and products. Four 20-watt General Electric black light fluorescent tubes provided the irradiating light. The peak energy of these lamps occurs at 3500 A. with 8 5 % of their energy confined to the 3300 to 3750 A. region. The average light intensity in the cell was 9.2 i 0.4 X l O I 4 quanta cm.-*sec.-I as measured with a nitrobenzaldehyde solid state actinometer (Pitts er al,, 1964) and by gas phase photolysis of NO2 (Schuck er al.. 1966). A vacuum rack
equipped with an oil diffusion pump allowed evacuation of the reaction cell to to mm. Hg. Compressed nitrogen was obtained from the Liquid Carbonic Co. and nitric oxide from the Matheson Co. The nitrogen was used without treatment and the traces of nitrogen dioxide in the nitric oxide were removed by passage through an “Ascarite” filter. Ethyl nitrite in mixture with 8 to 15% EtOH was obtained from Mallinckrodt. The nitrite was admitted to the reaction cell by vacuum distillation from a frozen sample of the EtONO mixture. Concentrations are stated in parts per million. One part per million is equivalent to 4.09 X 10+ mole l i t e r 1 . Operation of this system below 1 p.p.m. EtONO
results in large experimental errors. This is partially a result of the limiting infrared absorptivity of the nitrite, An additional factor is the observed sensitivity of the reaction to third body effects. Thus, at very low concentrations the rate of reaction is influenced by the vacuum attainable in the system and thus the ability to measure this vacuum becomes a limiting factor. The initial conditions, @, and product data are shown in Tables I, 11, and 111. The @ values were obtained by use of the equation: d(RONO)/dr k,(RONO)
@ =
Table I. Photolysis of EtONO Alone Expt No.
1 2 3 4 5 6 7 8 9 10 11 12
Initial Concn. EtONO, P.P.M. 0.77 2.40 5.75 5.96 8.22 17.1 18.1 21.2 40.0 86.0 87.5 Sl.1
Initial
CHSCHO
@
-0,991 0.576 0.582 0,516 0.518 0.422 0.406 0.419 0.348 0.280 0.131 0.129
Products, P.P.M. EtOH NO
Recovery, Nz0
C
N
4.4 4.8
1.4 2.1
5.8 6.0
1.3 1.2
100 115
145 140
14.1
4.8
14.0
1.1
111
95
16.8
4.8
15.4
1.9
102
91
Table 11. Photolysis of EtONO in Presence of N2 Expt . No.
12 13 14 15 16 17 18
Expt. No.
Initial 15.3 15.5 16.8 13.3 18.5 19.7 20.0
RONO
19 20 21 22 23 24 25 26 27 28 29
21.2 20.3 19.4 19.7 17.7 20.4 16.5 17.5 22.3 18.1 22.9
Helium
CHaCHO.
19 60 200 10 3 98 1 1.4 X lo4 108
Recovery, %-
Products, P.P.M.
0
CHaCHO
EtOH
NO
NzO
c
N
0.413 0.363 0.362 0.213 0.272 0.160 0,089
12.6 11.8 13.9 11.1
4.1 3.5 5.7 3.8
14.5 10.4 15.8 6.2
1.4 1.2 0.9 0.3
109 99 117 112
113 83 105 51
Table 111. Photolysis of EtONO in Presence of NO and Other Gases Initial Concn., P.P.M. Initial Products, P.P.M. NO Other @ CHCHO EtOH 9.6 20.2 60 206 336 769 861 4160 1 . 4 X lo4 1 . 4 X 10“ 42b
0.371 0.303 0.260 0.259 0.218 0.224 0.225 0.162 0.164 0.173 0.280
NzO
Recovery, C
19.5
3.7
1.1
120
18.3 20.2
1.4 1.6
2.0
113 107
18.9
0.0 0.0
3.0 2.5
108
Volume 1, Number 2, February 1967 139
In this equation, k, is the specific absorption rare. Because of the low concentrations under study, this system can be classified as weakly absorbing (Leighton, 1961); therefore, k , is approximately the product of K, the absorption cross section, and the measured light intensity. K is the absorption coefficient at 3500 A. in terms of cm.2 molecule-'. For EtONO, the numerical value of K is 1.49 X Table IV lists the principal wave numbers employed for concentration measurements. In certain experiments, gas chromatographs using flame ionization detectors were used to check for C1-C6 hydrocarbons and a chromatograph equipped with electron-capture detector was used as an additional check for presence and concentration of ethyl and methyl nitrate. A typical procedure following start of photolysis involved repetitive scanning from 4000 to 600 cm.-1 with the infrared spectrophotometer. The time period of a single scan varied from 10 minutes for a reaction requiring a few hours for completion to an hour for those reactions requiring more than 24 hours. Thus, each experiment yielded 15 to 40 time concentration spectra. At the end of photolysis, the cell was pressurized to atmospheric and a final scan made. This final step provided a more accurate measure of the simple compounds such as H20,NO, CO, etc., since the absorptivities of these compounds is a function of pressure. This pressure effect is an added aid to identification of simple components. The use of a light source with a bandwidth of at least 450 A. obviously does not correspond to monochromatic light. However, the expected detrimental effects on experimental results due to the use of this source are not realized as evidenced by the measurement of light intensity by two totally different techniques. One basic reason for this lack of effect is associated with use of a weak absorption system both in the use of NO, as an actinometer and in photolysis of EtONO. Such a system cannot be affected by variations in absorbed light since the fraction of light absorbed is always negligible. Thus, for example, changes in absorber concentration in a weakly absorbing system do not affect the fraction of light absorbed. Another factor which is responsible for lack of error is that the absorption curves of o-nitrobenzaldehyde, NO?, and EtONO are well known and, more important, exhibit a regular and thus definable variation with wavelength. These variations when combined with the well defined con-
Table IV. Wave Number Used to Measure Reactants, Products, and Principal Suspected Products Compound Wave Number (ctn.-9
EtONO EtOH CHaCHO NO NO? N20
co con
EtONO? MeON02 MeN02
780, 930, 1050. 1680 1045, 3660 1760, 2738 1905 1620 2245 2175 2365 910 850, 1675 1580, 1595
140 Environmental Science and Technology
tinuum of the light source allow definition of parameters which apply to the wavelength region covered. Results and Discussion The primary dissociative process in the gas phase photolysis of tert-butyl nitrite and rert-pentyl nitrite is considered to be cleavage of the RO-NO bond (Calvert and Pitts, 1966) with a quantum yield of unity. This is somewhat surprising since cleavage of the RO-NO bond, in addition to forming alkyloxyl radicals, yields NO which has been widely used as a radical trap. Thus, the observed quantum yield of unity is unexpected since the recombination reaction should act to reduce this yield. This study was an attempt to assess the contribution of RO-NO bond cleavage to ethyl nitrite photolysis. The initial experiments were concerned with photolysis of parts per million concentrations of ethyl nitrite in the absence of added gases. As shown in Figure 1 for a typical experiment the time concentration relationship suggested was first order with an @ of 0.42. Product analysis shows that 80% of the carbon was in the form of CHsCHO and 20% as EtOH. Eighty per cent of the nitrogen appeared as NO and 2 0 x as N 2 0 . Thus the @ was less than unity and the yield of N20 was also less than expected. Previous experiments with MeONO show (Leighton 1961) an N 2 0 to NO ratio of 1 in contrast to the ratio of 0.25 found here. This is not due to different reactant species since similar experiments in this laboratory using parts per million concentrations of MeONO also exhibited an N,O to NO ratio of 0.25. The most logical explanation is that the reaction leading to N,O formation is less important at these lower reactant concentrations. Additional information is obtained when the initial EtONO concentration is varied. The results plotted in Figure 2 show that @ was a function of the initial EtONO concentration and that the apparent first-order dependence disappeared at very low initial concentrations. In fact, below an initial EtONO of 5 p.p.m. the data followed an approximate second-order dependence. The @ approached unity as the initial EtONO approached zero. The noted switch in reaction order suggests
3
- 0.6 0.41
o.2
Q
t
I
0
200
=
0.12
I
1
I
400 Time, m i n u t e s
600
800
Figure 1. Rate relationship during photolysis of ethyl nitrite in absence of added gases
0.45
I
Initral (EfONOI=20 pPni
-
Q0.35
0.90 -
0.25
;
5
Figure 3. Effect of added gases of ethyl nitrite photolysis I
0
1
I
1
J
200
400 Time, minutes
600
800
Figure 2. Effect of ethyl nitrite concentration on rate of photolysis in absence of added gases
excited molecule formation a s qualitatively explained by Lindemann (1922). Thus, the governing rate law would have the following form:
Equation 1 reduces to a first order at higher concentrations where k3 (RONO) >> kn, and to a second order at lower concentrations where k3 (RONO)
(12)
H,
(16)
(19) (20)
The rate equation derived from this mechanism (reactions 11 through 17), assuming equal probability of reactions 11 and 12, is given by:
142 Environmental Science and Technology
(24)
Combining Equations 23 and 24 we obtain:
hv
RONO ___
+ 2(ARONO)
@ = I 2[1
+
[(RONO) 2(ARON0)] (RONO) 2(ARON0)]
+
+
(25)
Evaluating Equation 25 in terms of specific experiments does result in an approximate fit of the data. Thus, Equation 25 predicts a 9 at r = 0 of 0.52 compared to an observed @ of 0.42 when 20 p.p.m. of EtONO are photoIyzed in the absence of added gases. The predicted when half the EtONO has reacted is also 0.52. Thus, as observed, a pseudo first-order relationship is predicted. Applying Equation 25 to the case where the initial EtONO is 2.4 p.p.m. gives a predicted @ at t = 0 of 0.65 compared to the observed value of 0.58. When half the EtONO has reacted, the predicted @ is 0.61 indicating a deviation from the pseudo first-order rate, These calculated results show that the mechanism as given by reactions 11 to 14 is a fair approximation of the data. In agreement with observation, this mechanism predicts a @ which is a function of the initial EtONO concentration with a rate law which is approximately first order in the 20-p.p.m. initial concentration range and approximately second order in the 2-p.p.m. initial concentration range. It also agrees with the observation that @ approaches unity as the initial EtONO concentration approaches zero. It is not in agreement with experimental fact
when EtONO is photolyzed in the presence of a large concentration of added gases. If an additional M term is included to account for addition of nitrogen, it would not change the basic form of Equation 25. Thus, the lower limit of @, according to this equaton, is 0.50 while we observe, for example, a @ of 0.1 when EtONO is photolyzed in the presence of an atmosphere of nitrogen. Returning our attention to Equation 21 we now attempt to evaluate the term containing k l j . In this case the experimental data do not furnish a clear-cut decision as t o the relative importance of reactions 15, 16, and 17. One method of approach in this case is to define the extreme limits. The maximum numerical value obtainable from the klj term occurs if reaction 15 is considered to be much faster than reactions 16 and 17. Using this restriction, the k15 term will approach a constant value of 0.5. This numerical value will seriously affect the magnitude of values obtained from the previous consideration of the ki*term. However, being a constant under these assumptions the k I 5term does not disturb the trends in @ obtained from the klr term consideration. Employing both terms in Equation 21 under the assumed conditions, we obtained a predicted @ of 0.5 when fractional part-per-million concentrations of EtONO are photolyzed alone and a @ which approaches zero when EtONO is photolyzed in the presence of a large concentration of inert gas. Thus, inclusion of this kl: term does lead to a better approximation of the data a t the extreme conditions but, a t the same time, leads to serious numerical deviations when applied to experiments involving photolysis of EtONO alone a t intermediate concentrations. Re-examination of the product data shows that reaction 17 is important even though reaction 15 is much faster. Reaction 17 must be considered since, in this proposed mechanism, it is the sole source of EtOH as a product which, by experimental fact, accounts for 20z of the EtONO initially present. There are, of course, many other reasons why the mechanism in Equations 11 through 17 does not numerically precisely fit the data. For example, a closer fit can be obtained by assuming an unequal probability of the two primary steps, reactions 11 and 12. Just as likely is the existence of additional reactions or third body effects not taken into account by the mechanism. Reactions 1 1 through 20 were chosen so as to agree with product data. The addition of excess NO, in addition to functioning as a third body in reaction 14, will increase the probability of reactions 15 and 16, reducing the probability of reaction 17. The over-all result, therefore, would be an increase in N 2 0 production and a decrease in EtOH production. These effects are in agreement with observation.
Thus, the @ data favor a mechanism in which both excited molecule formation and RO-NO bond cleavage are important as primary reactions. This suggests the interesting possibility that the choice of initial reaction path is influenced by the rotational isomers. The cis-isomer may be more favorable to intramolecular rearrangement while the trans-isomer might favor RO-NO bond cleavage. These rotational isomers are important t o MeONO photolysis in a solid matrix (Brown and Pimental, 1958); however, this could be due to packing considerations (Calvert and Pitis 1966) within the matrix. This involvement of the cis-isomer in the rearrangement reaction (No. 13) should result in formation of the H O N molecules rather than HNO. Experimentally H N O has been observed while H O N has not. Disproportionation (reaction 16) of the initial fragments in the matrix has been suggested as the major source of the aldehyde and HNO. However, the mechanism which most closely approximates this current study indicates that disproportionation of the fragments can be responsible for only a fraction of the aldehyde.
Acknowledgmenl
The authors express their appreciation to P. A. Leighton for his thought-provoking discussion concerning the theoretical aspects of this investigation and acknowledge the technical assistance of F. R. Burleson, M. A. Price, and R. A. Sorensen.
Literature Cited
Brown, H. W., Pimentel, G. C., J. Chem. Phys. 29, 883 (1958). Calvert, J. G., Pitts, J. N. Jr., “Photochemistry,” Wiley, New York. UU. 48C-1 and 597 (1966). Dalby, F. W.’,ACan.J. Phys. 36, 1336 (1958). Hanst, P. L., Calvert, J. G., J. Phys. Chem. 63, 71,2071 (1959). Leighton, P. A.. “Photochemistry of Air Pollution,” Academic Pyess, New York, pp. 27-8, 65-7 and 70 (1961). Lindemann F. A. Trans. Faraday Soc. 17, 598 (1922). Pitts. J. N.. Jr.. Wan. J. K. S.. Schuck, E. A., J. Am. Cheiiz. Soc. 86, 3606 (1964). Schuck, E. A., Stephens, E. R., Schrock, R. R., J. Air Pollution Conrrol Assoc. 16, 695 (1966). Thompson, H. W., Purkis, C. H., Trans. Faraday Soc. 32,1466 (1936).
Received for reciew Noceniber 7, 1966. Accepted January 30, 1967. This study was supported in part b y Grant AP 00289, Dicision of Air Pollution, Bureau of State Sercices, U. S . Public Health Sercice.
Volume 1 , Number 2, February 1967
143
