;PA-600/3-77-081
ugust 1977
Ecological Research Series
REACTIONS OF
RADICALS
WITH NITROGEN OXIDES
HBTECTION
Environmental Sciences Research Laboratory
Office of Research and Development
Research Triangle Park, North Carolina 27711
-------
RESEARCH REPORTING SERIES
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This document is available to the public through the National Technical Informa-
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-------
EPA-600/3-77-081
August 1977
Reactions of Isopropoxy Radicals with Nitrogen Oxides
by
G. R. McMillan
M. J. Kaiserman
Department of Chemistry
Case Western Reserve University
Cleveland, Ohio 44106
Grant No. R800659
Project Officer
J. J. Bufalini
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, N. C. 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
LIBRARY
U. S. ENVIRONMENTAL PROTECTION AGENC|
N, J.
-------
DISCLAIMER
This report has been reviewed by the Environmental Sciences Research Lab-
oratory, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of trade
names or commercial products constitute endorsement or recommendation for use.
11
-------
ABSTRACT
Information about the reactions of isopropoxy radicals with nitrogen
dioxide and nitric oxide was sought in an investigation of the gas-phase
co-photolysis of isopropyl nitrite and nitrogen dioxide. Mixtures of these
two compounds and mixtures of these two compounds with added inert gas were
illuminated in small quartz cylinders and in a long path infrared gas cell.
Certain rate constant ratios were calculated from the quantum yields and
product distribution on the basis of a partial mechanism.
Alkoxy radicals with a-hydrogen atoms are believed to react with nitro-
gen dioxide by two processes. Combination occurs yielding an alkyl nitrate.
A parallel process, disproportionation, yields a carbonyl compound and ni-
trous acid. Comparing the present results with literature values for the
relative probability of these two processes shows that disproportion of iso-
propoxy and nitrogen dioxide is considerably less important than expected
from the general appreciation of alkoxy/nitrogen dioxide reactions.
Re-evaluation of some published results in the light of the present
findings suggests that the quantum yields of photodissociation of alkyl ni-
trites are likely to be greater than currently accepted values and that dis-
proportionation of alkoxy radicals with nitric oxide is probably less impor-
tant than is currently accepted.
This report was submitted in fulfillment of Grant Number R800659 by Case
Western Reserve University under partial sponsorship of the Environmental
Protection Agency. Work was completed as of June, 1976.
111
-------
CONTENTS
Abstract iii
Tables vi
Acknowledgements vii
1. Introduction 1
2. Conclusions 6
3. Recommendations 7
4. Experimental Methods 8
5. Results 10
6. Discussion 17
References 24
-------
TABLES
Number Page
1 Values of k
-------
ACKNOWLEDGEMENTS
Case Western Reserve University provided support and assistance for
this work in several ways. Thanks are also due to Dr. A. J. Sumodi for con-
sultation on infrared techniques and to Mr. C. S. Kan for his experiments
on generation of thermally-equilibrated alkoxy radicals.
VII
-------
SECTION 1
INTRODUCTION
For alkoxy radicals possessing an alpha hydrogen, the expected reac-
tions with nitrogen dioxide are disproportionation and recombination.1
k,
R'RCHO- + N02 —>R'RCO + HN02 Disproportionation
k
R'RCHO- + N02 —^R'RCHONOa Recombination
Neither these nor other gas-phase reactions of alkoxy radicals have been
studied using a direct detection method, and in fact few absolute rate con-
stants of alkoxy radicals are available. No value for kj or kr is known to
within a factor of about 100.2 On the other hand, k,/k ratios should be
determinable by use of methods now available. The rate constant, kr, can be
calculated in principle by using the method of Benson,3 but the kinetic in-
formation needed for such a calculation is not always available. These ra-
tios are also directly useful for kinetic analysis of other reaction sys-
tems.'* For photochemical smog formation, the ratio is of interest because
disproportionation, unlike recombination, is not likely to be chain termina-
ting.5 The data available for the ratio kj/kr are collected in Table 1.
The state of knowledge of these ratios is unsatisfactory. Although al-
ky 1 peroxides are considered to be suitable sources of alkoxy radicals, there
are clearly problems in using these alkoxy radical sources in the nitrogen
dioxide reaction. One such problem may be surface effects similar to those
reported by Ludwig for pyrolysis of isopropyl peroxide-nitric mixtures.6 In
any case, for application to smog formation, it is desirable to have k
-------
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(CH3)2CHONO > a(CH3)2CHO«* + (1-a)(CH3)2CHO* + NO (1)
There is evidence that the alkoxy radicals produced behave chemically as if
there were two types present -- a vibrationally-excited radical and a radical
at thermal equilibrium.9>l° At low pressures, all or nearly all the excited
isopropoxy radicals dissociate according to step (2).6
(CH3)2CHO'* k\ CH3CHO + CH3' (2)
Unexcited isopropoxy radicals will undergo a complex sequence of secon-
dary reactions.11* However, if photolysis occurs in the presence of an ex-
cess of nitric oxide, this complex sequence of secondaryreactions can be re-
duced. All the unexcited isopropoxy radicals will be scavenged as described
by the disproportionation, equation (3), and recombination, equation (4),
steps.6'13
(CH3)2CHO- + NO Ji3-* CH3COCH3 + HNO (3)
(CH3)2CHO« + NO -^ (CH3)2CHONO (4)
The rate constant ratio, k3/k4, independent of temperature, has been reported
to be as low as 0.15 and as high as 0.22.11*"17
When isopropyl nitrite is photolyzed in the presence of nitrogen dioxide,
the isopropoxy radicals are expected to react as follows:7'18
(CH3)2CHO- + N02 Ji^.CH3COCH3 + HN02 (5)
(CH3)2CHO- + N02 _£l-» (CH3)2CHON02 (6)
With (k5 + k6) likely to be greater than or equal to 109'5 £/mole-sec,2 no
other reactions of unexcited isopropoxy are expected to be important at early
stages of the photolysis provided that the concentration of nitrogen dioxide
is not too low. If acetone and isopropyl nitrate are formed only in steps
(5) and (6), and if no subsequent removal of the products occurs, and if cer-
tain other conditions are met, then R = ks[N02][(CH3)2CHO«] and
(A)
R(CH3)2CHON02 = k6[N02][(CH3)2CHO.]. This leads to:
[CH3COCH3]t ks
[(CH3)2CHON02]t k6
The concurrent photolysis of nitrogen dioxide complicates this simple
analysis. The extinction coefficient of this compound at 3660 A is about
155 £/mole-cm,19 compared with approximately 50 A/mole-cm for isopropyl ni-
trite. 20Leighton gives a detailed description of the photochemistry of ni-
trogen dioxide.21 The main steps at wavelengths shorter than 3980 A are:
0(3P) (7)
-------
0(3P) + N02 -^ NO + 02 (8)
0(3P) + N02 + M ks> > N03 + M (8')
Since the primary quantum yield is 0.95,22 nitrogen dioxide will photodecom-
pose efficiently under the proposed conditions. In view of the products of
this photodecomposition, at least two additional steps which form acetone
must be considered.23'21*
(CH3)2CHONO + 0(P) -4 _> CH3COCH3 + NO + HO- ? (9)
(CH3)2CHO + 02 kl°> CH3COCH3 + H02* (10)
If the ratio k^/k.,, is to be simply extracted, the constraints of the
system place the following limits on the range of pressures of nitrogen diox-
ide at which the system may be studied:
1. The pressure of nitrogen dioxide must be high enough to scavenge the
isopropoxy radicals in order to avoid the formation of acetone and isopropyl
nitrate through reactions of isopropoxy other than (5) and (6) .
2. The pressure of nitrogen dioxide must be high enough to avoid deple-
tion in the reaction zone.
3. The pressure of nitrogen dioxide must be low enough to avoid appreci-
able photolysis which would lead to significant product formation through re-
actions (9) and (10) .
4. The pressure of nitrogen dioxide must be low enough to minimize
product- forming reactions of nitrogen tetroxide, which is formed in the ni-
trogen dioxide-nitrogen tetroxide equilibrium.
The results of the experiments to determine ks/k6 (k(j/kr) may also lead
to an independent check of the published value of the primary quantum yield
for dissociation of isopropyl nitrite, which is the only value available for
this or any secondary alkyl nitrite. Ludwig reported (j>i to be 0.36 at 26°C
and at 3660 A on the basis of product yields and a proposed mechanism for
the isopropyl nitrite-nitric oxide system.17 This method requires accurate
knowledge of the ratio of k3/kit. If k3/kit is as low as 0.15 or as high as
0.22,16 ! can vary from 0.49 to 0.36. The primary quantum yield also re-
quires accurate measurement of a minor product, acetone((i> . ~~~, = 0.060).17
L,n3LUL,ri3
A confirmation of the value for the primary yield cj>i reported by Ludwig
seems desirable. For the isopropyl nitrite-nitrogen dioxide system, consid-
eration of reactions (2), (5) and (6) leads to:
= WHO + (1 + ksA6) V3)2CHON02
-------
Acetaldehyde is no more than a. trace product,17 thus the determination of j
requires the accurate measurement of ks/k6 as well as the quantum yield of
isopropyl nitrate formation. If 4>i can be obtained, Ludwig's data may be re-
interpreted to give a new value of k3/kit.
In summary, several reasons led to the decision to study the photolysis
of isopropyl nitrite in the presence of nitrogen dioxide.
1. No detailed information is available for reactions of secondary al-
koxy radicals with nitrogen dioxide. Furthermore, no value of k^/k^, for any
alkoxy radical is well established.
2. Alkyl nitrites are a good source of alkoxy radicals and the methods
developed in this study can be extended to primary as well as other secondary
alkoxy radicals.
3. The result obtained for the primary quantum yield will be a good
check of the value obtained by Ludwig from the photolysis of isopropyl ni-
trite-nitric oxide mixtures. Ludwig's data may then be reinterpreted on the
basis of the new primary quantum yield.
-------
SECTION 2
CONCLUSIONS
The importance of the disproportionation reaction between the iso-
propoxy radical and nitrogen dioxide is less than would be expected from
literature data on alkoxy radical/nitrogen dioxide reactions. The ratio
of rate constants for disproportionation and combination of isopropoxy and
nitric oxide is found to be smaller than any of the five literature values.
Alkoxy radical/NO reactions usually have been studied using alkyl per-
oxides as thermal or photochemical radical sources. If the present study
is supported by further work, the peroxide technique and results obtained
using the peroxide technique will have to be reconsidered carefully. Most
literature data on the photolysis rates of alkyl nitrites must be recalcu-
lated.
-------
SECTION 3
RECOMMENDATION
None of the reactions discussed in this report are believed to be crit-
ical in smog formation nor in laboratory smog chamber processes, at least
under most conditions. These reactions and their rate constants do appear
in a generalized way in most reaction sets used for computer simulations of
smog. Appropriate modification of rate constant input should be considered.
-------
SECTION 4
EXPERIMENTAL
MATERIALS
Isopropyl nitrite was prepared by reaction of isopropyl alcohol with
nitrous acid. The product was treated with anhydrous K2COs and distilled
at atmospheric pressure. Nitrogen dioxide from the Matheson Chemical Com-
pany was passed over PgOs and fractionally distilled. Both isopropyl ni-
trite and the nitrogen dioxide were stored on the vacuum line at -78° in
the dark. Nitrogen was passed over hot copper to remove oxygen and through
traps cooled to -196° to remove condensable impurities.
APPARATUS
Conventional mercury-free high vacuum lines were used for storage,
sample preparation, and preliminary separation.
The source of 3660 A radiation was a Hanovia 673A medium pressure mer-
cury arc, used with a Corning 7-37 filter in order to reproduce the spec-
tral intensity distribution realized by Ludwig.17 The reaction cell used
for quantum yield measurements was a quartz cylinder, 10-cm long, with plan-
ar end faces. Reactants were allowed to mix in the cell by diffusion. The
absorbed light intensity, based on ferrioxalate actinometry was integrated
over the course of an illumination. Depending upon relative pressure, the
intensity of light absorbed by nitrogen dioxide was 2-12% of that absorbed
by isopropyl nitrite.
Photolysis experiments with a long-path gas cell allowed in situ ana-
lysis of the reaction mixture by infrared spectrophotometry (LPIR). The
aluminum cell, obtained from Beckman Instruments, Inc., has a volume of
12.3£ and an optical path variable between 0.1 and 20 m. Photolyzing
light enter the cell through quartz windows. The light source and filter
used was the same as that used in the small cell experiments.
ANALYSIS
The reaction mixture from photolyses in the small cell was analyzed by
gas chromatography on a 3-m long, 0.6 cm o.d. stainless steel column con-
taining 20% tricresyl phosphate on Chromasorb W. The reaction mixture was
transferred on the vacuum line to a thin capillary. The sample was injected
into the chromatograph by breaking the capillary in a teflon tube through
which carrier gas was flowing.
8
-------
In the LPIR experiments, isopropyl nitrate was measured from the absor-
bance at 1282 cm"1. Acetone was measured from the total carbonyl absorbance
at 1742 cm"1 by subtracting the contribution from acetaldehyde, using quantum
yield data of Ludwig.17
-------
SECTION 5
RESULTS
THE REACTION IN SMALL QUARTZ CELLS
Illumination of mixtures of isopropyl nitrite vapor and nitrogen diox-
ide in small cells led to three carbon-containing products detectable by gas
chromatography and/or long-path infrared spectrophotometry-acetone, acetalde-
hyde and isopropyl nitrate.
Reproducible yields of the carbonyl products were not obtained by gas
chromatography. Large, irreproducible amounts of acetone were observed in
the absence of light. Exhaustive tests, with variation in the analytical
procedure, failed to provide a solution to the interference. It was conclu-
ded that acetone is formed by thermal processes upon condensation of the re-
action mixture in the vacuum line and in the first section of the GC column.
The quantum yields of isopropyl nitrate appear in Tables 2 and 3. The
photolyses, of 2.5-min duration, were done in the 10-cm cell at a tempera-
ture of 25°. The percent conversion of isopropyl nitrite was kept constant
at approximately 0.06%, but the percent conversion of nitrogen dioxide was
as high as 10% for small initial concentrations of this reactant.
The data in Table 2 illustrate the effect of changing the concentra-
tions of the components of the photolysis mixture. The variation of the
pressure of either isopropyl nitrite or nitrogen dioxide produced no signi-
ficant change in the results. The average quantum yield of isopropyl ni-
trate is 0.54 ± 0.02 (s.d.).
The data in Table 3 show the effect of the addition of an inert gas, ni-
trogen, to photolysis mixtures of constant nitrogen dioxide pressure and vary-
ing isopropyl nitrite pressures. The quantum yields were not significantly
different from those measured for the nitrogen-free case.
A number of experiments were carried out in which mixtures of isopropyl
nitrite (at about 7.0 Torr) and the nitric oxide (at about 30-40 Torr) were
illuminated for 6 hrs. The quantum yields of acetone were: 0.040, 0.046,
0.046, 0.038, and 0.036. Isopropyl nitrate was also observed in trace a-
mounts with quantum yields in the range of 0.001-0.002.
10
-------
TABLE 2. QUANTUM YIELDS OF ISOPROPYL NITRATE
P TATT*
K(CH3)2CHONO' 10rr
7.2
7.3
7.4
7.6
7.7
7.3
7.6
7.9
7.7
3.8
3.9
3.9
1.8
1.7
1.9
PN02' T°rr
0.57
0.57
0.29
0.29
0.29
0.14
0.14
0.07
0.07
0.14
0.14
0.14
0.14
0.14
0.14
10 I , Quanta sec~
3.
0.35
0.34
0.38
0.37
0.40
0.37
0.37
0.38
0.31
0.20
0.20
0.21
0.10
0.09
0.10
$(CH3)2CHON02
0.56
0.54
0.52
0.57
0.53
0.52
0.51
0.52
0.56
0.57
0.54
0.54
0.54
0.55
0.50
11
-------
TABLE 3. QUANTUM YIELDS OF ISOPROPYL NITRATE IN THE PRESENCE OF NITROGEN
P(CH3)2CHONO' T°rr
7.6
7.0
3.9
3.8
1.9
1.8
PN02' T°rr
0.14
0.14
0.14
0.14
0.14
30
25
42
24
8
0.14 12
Quanta sec
0.34
0.34
0.20
0.22
0.08
0.09
VWHO»0
0.51
0.54
0.50
0.58
0.52
0.51
12
-------
THE REACTION IN THE LONG PATH INFRARED (LPIR) CELL
No dark reactions were observed. The results of photolysis of isopropyl
nitrite/nitrogen dioxide/nitrogen mixtures at 25° are shown in Table 4. The
ratio of yields ¥„„ „»„,. /Y..,., , ~ur.Mri is found to be constant, all values
falling in the range 0.06 ± 0.01 with variation in isopropyl nitrite pressure
(1.5-6.8 Torr) , nitrogen dioxide pressure (0.08-0.78 Torr) , and nitrogen pres-
sure (3-580 Torr) .
Table 5 shows the results from photolysis of isopropyl nitrite/nitrogen
dioxide mixtures. No nitrogen or other inert gas was present. The yield ra-
tio was unaffected by halving the photolysis time at a constant pressure of
nitrogen dioxide. This effect was observed at a nitrogen dioxide pressure of
0.29 Torr at times of 10.0 and 5.0 min and at a nitrogen dioxide pressure of
0.15 Torr at times of 5.0 and 2.5 min. The yield ratio was also unaffected
by a five-fold change of nitrogen dioxide pressure at a constant isopropyl ni-
trite pressure of 6.7 Torr and a constant photolysis time of 2.5 min. There
was, however, a change in the yield ratio when the pressure of nitrogen diox-
ide was held constant and the pressure of isopropyl nitrite was decreased be-
low a certain level. At isopropyl nitrite pressures of 6.7 and 8.8 Torr, the
same yield ratio was observed as in the presence of added nitrogen, but at
pressures of isopropyl nitrite of 4.8 Torr and below, the yield ratio decreas-
ed with decreasing pressure. This decrease of the yield ratio was observed at
pressures of nitrogen dioxide of 0.15 and 0.08 Torr. Furthermore, at a given
low pressure of isopropyl nitrite, the yield ratio was about the same for the
two pressures of nitrogen dioxide.
13
-------
TABLE 4. PRODUCT YIELDS IN THE PRESENCE OF NITROGEN FROM PHOTOLYSES IN THE
IN THE INFRARED GAS CELL
P(CH3)2CHONO'
Torr
6.8
6.8
6.8
6.8
4.8
4.7
3.8
3.8
3.8
3.8
1.5
1.5
1.5
1.5
6.7
6.8
6.8
6.8
3.8
3.8
PN(Y
Torr
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.47
0.47
0.78
0.78
0.26
0.26
Torr
250
180
120
84
150
110
580
110
90
3
18
18
18
18
11
12
11
11
15
15
YCH COCH
Micromol
0.84
0.60
0.67
0.61
0.37
0.50
0.34
0.42
0.43
0.37
0.14
0.15
0.15
0.23
0.46
0.67
0.63
0.43
0.47
0.60
, YaCH
Micromol
13
13
10
11
7.8
8.8
5.9
7.4
6.4
7.8
2.8
2.7
3.1
3.4
9.7
12.0
9.5
6.4
9.9
9.0
/ YCH3COCH3
Y(CH3)2CHON02
0.07
0.05
0.07
0.06
0.05
0.06
0.06
0.06
0.07
0.05
0.05
0.06
0.05
0.07
0.05
0.06
0.07
0.07
0.05
0.07
Product yields from photolysis of 2.5 min duration.
14
-------
TABLE 5. PRODUCT YIELDS IN THE ABSENCE OF NITROGEN FROM PHOTOLYSES IN THE
INFRARED GAS CELL
Photolysis
Time, min
10
10
10
10
10
10
5
5
5
5
5
5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
P(CH3)2CHONO'
Torr
6.8
6.8
6.8
6.7
6.7
6.7
6.7
6.7
6.7
6.7
6.7
6.7
6.8
6.8
6.8
6.7
6.7
6.7
6.7
6.7
8.8
Torr
0.30
0.30
0.28
0.29
0.29
0.29
0.15
0.15
0.15
0.15
0.15
0.28
0.15
0.15
0.15
0.08
0.08
0.08
0.39
0.38
0.15
YCH COCH '
Microraol
3.0
3.1
3.8
3.3
2.9
2.6
1.4
1.4
1.3
1.8
1.9
2.1
0.73
0.73
1.0
0.72
0.72
0.81
0.95
1.0
0.88
YacH
Micromol
52
55
49
58
63
55
25
25
22
23
25
32
16
16
18
15
15
14
20
18
19
'2 VCH3C°CH3
Y
ffU ^ f^Ur\\J^
'3^ 22
0.06
0.06
0.08
0.06
0.05
0.05
0.06
0.06
0.06
0.08
0.08
0.07
0.05
0.05
0.06
0.05
0.05
0.06
0.05
0.06
0.05
(continued)
15
-------
TABLE 5. (continued)
Photolysis
Time, min
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
P(CH3)2CHONO'
Torr
8.8
4.8
4.8
3.4
3.4
8.8
8.8
4.8
4.7
3.8
3.8
3.4
3.4
2.8
2.8
Torr
0.15
0.15
0.15
0.15
0.15
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
YCH COCH '
Micromol
0.72
0.41
0.35
0.25
0.25
0.67
0.68
0.33
0.27
0.23
0.16
0.23
0.27
0.13
0.13
Ya
(CH3)2CHON02'
Micromol
15
11
13
9.5
9.5
14
14
13
9.6
8.5
9.7
8.5
9.6
7.4
7.7
HC°«3
Y(CH3)2CHON02
0.05
0.04
0.03
0.03
0.03
0.05
0.05
0.03
0.03
0.03
0.02
0.03
0.03
0.02
0.02
Product yields from photolysis of 2.5 min duration.
16
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SECTION 6
DISCUSSION
PRELIMINARY DISCUSSION OF THE MECHANISM
Of the many elementary chemical processes which are expected to be in-
volved in the photolysis of isopropyl nitrite - nitrogen dioxide mixtures at
3660 A, the following need to be considered if the results are to be inter-
preted.
(CH3)2CHONO -^- a(CH3)2CHO«* + (1-a) (CH3) 2CHO- + NO (1)
(CH3}2CHO'* -^-> CH3CHO + CH3- (2)
(CH3)2CHO' + NO ks > CH3COCH3 + HNO (3)
(CH3)2CHO- + NO -^±+ (CH3)2CHONO (4)
(CH3)2CHO- + N02-^-> CH3COCH3+ HN02 (5)
(CH3)2CHO' + N02-^- (CH3)2CHON02 (6)
N02 -^-^ NO + 0(3P) (7)
0(3P) + N02 -£*-> NO + 02 (8)
0(3P) + N02 + M ks' > N03 + M (81)
(CH3)2CHONO + 0(3P) J^i> -> CH3COCH3-t- NO + HO' ? (9)
(CH3)2CHO' + 0 -^i^- CH3COCH3 + H02• (10)
Other processes would have to be considered if the extent of reaction was
greater than perhaps 1%. Under the scavenging conditions of the experiments,
bimolecular reactions involving isopropoxy or oxygen atoms having activation
energies greater than about 3 kcal/mole should be too slow to compete.
Steps (1) and (7)
Reaction (1) is generally accepted as the only primary dissociative pro-
cess in the photolysis of alkyl nitrites.1 Discussion of the possible effect
of nitrogen dioxide on the probability of this process is deferred. Step (7)
will occur concurrently with step (1). However, the relative rates of these
17
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photodecompositions may be varied by changing the ratio of concentrations of
nitrogen dioxide to isopropyl nitrite. Thus the rate of (7) is calculated to
be 3-23% of the rate of (1) in the various experiments.
Step (2)
This is a minor process shown by Ludwig to occur at about 9% of the rate
of step (1). Nitric oxide is an inefficient quencher of excited isopropoxy.17
Even if nitrogen dioxide is highly efficient, there would be no great change
in the steady-state concentration of isopropoxy.
Steps (5) and (4)
These are established reactions at high nitric oxide pressures,17 but
they should not become important in the present system until nitric oxide
formed in reactions (1), (7), and (8) has built up to appreciable concentra-
tions after long photolysis times.
Steps (5) and (6)
As ks + k6 ty 1010 £/mole-sec, these are likely to be the principal initial
fates of isopropoxy radicals at nitrogen dioxide pressures greater than 0.05
Torr.
Steps (8), (8') and (9)
Demerjian, Kerr, and Calvert report that k8 = 3.3 x 109 &/mole-sec at 25°
C.k Step (8f) will always accompany step (8) with the relative rates depend-
ing on the nature and pressure of the third body present. No information is a-
vailable in the literature about reactions of nitrogen trioxide with alkyl ni-
trites, but the presence of this intermediate cannot be ignored a priori. The
possible effects of nitrogen trioxide in the system will be discussed later.
No data are available for step (9), but acetone seems to be a likely ulti-
mate product.21f From the data of Davidson and Thrush, the rate constant for
the reactions of 0(3P) with ethyl nitrite is calculated to be 7.65 x 106 £/
mole-sec at 25°C.21* Assuming the same value for kg, it can be estimated that,
in the experiments in which acetone was measured, the rate of step (9) was 25%
of the rate of step (8) at the highest ratio of isopropyl nitrite to nitrogen
dioxide and 5% of the rate of step (8) at the lowest ratio. Based on these
calculations, the question of a possible contribution of step (9) to acetone
formation must be left open.
Step (10]
Heicklen estimates that the rate constant of the reaction of molecular
oxygen with methoxy is 103"2 S,/mole-sec.25 Assuming the same value for kio>
a comparison of rates of steps (10) and (5) shows that the contribution of
step (10) to the formation of acetone is negligible.
The discussion and interpretation of the acetone and isopropyl nitrate
18
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yields will be organized around steps (1) - (8) . The possible influence of
the other processes will be discussed later.
ESTIMATE OF k5/k6
The mechanism predicts that, for small extents of reaction, the yield
ratio (acetone/isopropyl nitrate) should be independent of irradiation time,
nitrogen dioxide pressure, isopropyl nitrite pressure, and inert gas pressure.
Indeed, the yield ratio was 0.06 ± 0.01 (s.d.), independent of all parameters
except isopropyl nitrite pressure. In the absence of added nitrogen the lim-
iting value 0.06 was found only at 6.8 and 8.8 Torr of isopropyl nitrite. As
seen in Table 5, the yield ratio decreases at lower isopropyl nitrite pres-
sures, reaching 0.02 at 2.8 Torr of isopropyl nitrite.
When nitrogen is initally present in the reaction mixture, the yield
ratio reaches the limiting value even at low isopropyl nitrite pressures
(Table 4) . Comparison of data in Tables 4 and 5 shows that the acetone
yield is normal at low isopropyl nitrite pressures, and that the decrease in
the yield ratio is due to the apparent extra production of isopropyl nitrate.
This effect is unanticipated, and no explanation for it can be advanced with
confidence. Any such explanation must take into account the following two ob-
servations. First, a pressure of nitrogen as low as 3 Torr is sufficient to
give the high-pressure limiting value of the yield ratio. Second, no decrease
in the quantum yield of isopropyl nitrate is observed in the small cell.
The limiting value of the yield ratio, 0.06 ± 0.01 (s.d.), is assigned to
the ratio of rate constants ks/k6 .
PRIMARY DISSOCIATIVE QUANTUM YIELD OF ISOPROPYL NITRITE
Assuming that for short duration photolyses steps (2) , (5) , and (6) ac-
count for the isopropoxy radicals generated in Step (1), the primary disso-
ciative yield, i, is given by:
*' = WHO + C1 + k5/k6)$(CH3)2CHON02 (C)
= 0.031 + (1 + 0.06) (0.54)
= 0.60 ± 0.03
The acetaldehyde yield is that obtained by Ludwig.6'17
The experimental primary quantum yield is much greater than the value of
0.36 calculated by Ludwig from results of photolyses of mixtures of isopropyl
nitrite and nitric oxide at 26°C and 3660 A.17 In that work, isopropoxy radi-
cals were proposed to disappear via steps (2) , (3) , and (4) . The primary dis-
sociative yield was calculated from:
*' = WHO + C1 + ocH3
If the Ludwig mechanism is correct and if 4>i is much greater than reported
by Ludwig, the main source of error is likely to lie either in $ mru or
19
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Wk3.
(CH3)2CHO- + NO —^-> CH3COCH3+ HNO (3)
(CH3)2CHO + NO —^ (CH3)2CHONO (4)
During the course of the present study, a few attempts were made to re-
produce the value of $ rnru obtained by Ludwig.17 Values about 30% lower
were obtained. This change in value is in the wrong direction to explain
the discrepancy in ! and probably reflects the difficulty in analyzing for
a compound both produced in small yield and present as an impurity in the
starting material. The yields obtained by Ludwig should be considered the
more reliable.
If the value for $_„ -,_.„„ determined by Ludwig is correct, then accor-
LH3UJLn3
ding to equation C, ka/ki* would be 0.12 if $i is 0.60. The published values
of k3/ki, obtained from studies of the photolysis and pyrolysis of isopropyl
peroxide - nitric oxide mixtures are presented in Table 6. The value of 0.12
is outside the range of reported values but not much so.
TABLE 6. RATE CONSTANT RATIOS (k3/ki,) OF ISOPROPOXY AND NITRIC OXIDE
Radical Source ka/k.* T°C Ref.
Photolysis (2300-2500A)
Pyrolysis
Pyrolysis
Pyrolysis
0.15
0.15
0.21
0.22
26,77
121-159
104-149
126-180
14
15
16
17
Neither the pyrolysis nor the photolysis systems is without complication.
Ludwig observed a surface contribution towards production of acetone at temper-
atures below 150°C.6 If, in the pyrolysis of isopropyl peroxide - nitric ox-
ide mixtures, some isopropyl nitrite produced by step (4) undergoes this sur-
face catalyzed conversion, the apparent ks/kit will be too high. It should be
noted that ratios of rate constants k,. .. .. /k , . . , if in
disproportionation combination
error, are expected to be too high. This is because there are several side
reactions that may form the disproportionation product, but few that may form
the combination product. All published values of k3/ki, were obtained from re-
sults of systems wherein isopropoxy was formed by photolysis or pyrolysis of
isopropyl peroxide. It is suggested on the basis of the present study that
all these values (including two published by the principal investigator) are
too high. This further suggests that there is some problem with the peroxide
method. Since nearly all information about disproportionation and combination
of alkoxy radicals and NO are based on the use of peroxides as radical sources,
A
20
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it is concluded that published k,. . /k , . ratios must
r disproportionation combination
be accepted only with serious reservations.
POSSIBLE INFLUENCE OF OTHER REACTIONS ON THE VALUE REPORTED FOR k5/k6
Influence of Nitrogen Dioxide on the Primary Dissociative Yield of Isopropyl
Nitrite
It has been assumed that the primary dissociative yield of isopropyl ni-
trite is not affected by the presence of nitrogen dioxide. The data seem to
support this assumption. Simple quenching of electronically excited isopropyl
nitrite molecules is ruled out by the lack of variation of the quantum yield
of isopropyl nitrate with change in nitrogen dioxide pressure. Indeed, there
seems to be no evidence in the literature for quenching of excited alkyl ni-
trite molecules by any molecule. High quantum yields have been calculated
for photolysis of n-octyl nitrite in solution.26 Ludwig observed no quench-
ing by nitric oxide at pressures up to 40 Torr.17 More to the point, in view
of the high dissociative quantum yield observed in this study, there is the
possibility of some reaction such as (11), which might produce isopropoxy
radicals.
(CH3)2CHONO* + N02 kll> (CH3)2CHO- + N203 (11)
This reaction is somewhat analogous to the well-known process:27'28
(CH3)3CONO + CH3- > (CH3)3CO- + CH3NO
Step (11) can probably be excluded, however, in light of the observed lack
of dependence of the quantum yield of isopropyl nitrate on nitrogen dioxide
pressure.
Influence of Nitrogen Trioxide
As previously stated, no account can be given of the fate of nitrogen
trioxide formed in the process (81), which always will proceed concurrently
with process (8).
0(3P) + N02 ks ) NO + 02 (8)
0(3P) + N02 + M —^-U N03 + M (8')
On the basis of bond energy and activation energy correlations, Demerjian,
Kerr, and Calvert predict k12 to be about 2.5 x 105 A/mole-sec.4 If the
rate constant for nitrogen trioxide reacting with isopropyl nitrite is
N03 + CH3CHO kl2 > HN03 + CH3CO (12)
comparable, such a reaction may compete in the present system, with acetone
or isopropyl nitrate being the ultimate products. It seems possible, how-
ever, to rule out such a process on the basis of comparison of computed
values of the maximum nitrogen trioxide quantum yields with the actual prod-
uct quantum yields.
2.1
-------
For nitrogen as a third body, the rate constant ke equals 2.2 x 1010
H2/mole2-sec.^ For isopropyl nitrite as a third body, k8 is probably a-
bout 3.2 x 10u £2/mole -sec, based on the estimated relative efficiencies
of nitrogen and isopropyl nitrite as third bodies for iodine atom recombi-
nation. 2? Thus, the rate of step (81) is predicted to be about 5% of the
rate of step (8) under typical experimental conditions and about 13% of the
rate of step (8) under conditions favoring the third body reaction.
At an isopropyl nitrite pressure of 6.8 Torr and a nitrogen dioxide
pressure of 0.08 Torr, the quantum yield of nitrogen trioxide is calculated
to be about 3% of the quantum yield of acetone. Thus, even if every nitro-
gen trioxide molecule ultimately formed acetone, the amount of acetone for-
med from this source would be negligible compared to the amount formed by
the disproportionation of isopropoxy radicals and nitrogen dioxide. At
high nitrogen dioxide pressures, the quantum yield of nitrogen trioxide
would be about 30% of the quantum yield of acetone. Again, assuming that
every nitrogen trioxide molecule would react to produce acetone, the total
amount of acetone would be approximately 30% higher than in the previous
case. This is not experimentally observed and it is thus concluded that
nitrogen trioxide does not react with isopropyl nitrite to form an appre-
ciable amount of acetone.
Influence of Step (9)
It was shown earlier that step (9) cannot be ignored in interpretation
of the data. No information on step (9) has been found in the literature.
If the interpretation by Davidson and Thrush of the reaction of oxygen atoms
(CH3)2CHONO + 0(3P) ^ ^ CH3COCH3 + NO + HO- ? (9)
with methyl and ethyl nitrites is accepted, step (9) would be assumed to form
acetone, nitric oxide, and hydroxy radical in a single step.21* If this as-
sumption is temporarily adopted, a steady state treatment of steps (1), (5)-
(8), and (9) leads to:
RCH3COCH3 *i P k9[(CH3)2CHON03(l+k5/ke) ^
R(CH3)2CHON02 " *» Vk8[N02]+k9[(CH3)2CHONO]> + k6 l J
where R = rate of formation of the product
(J>, I = primary quantum yield and rate of light absorption of isopropyl
nitrite
', I' = primary quantum yield and rate of light absorption of nitrogen
dioxide
In this work, the yield ratio (acetone/isopropy1 nitrate) has been as-
sociated with ks/ke. This is incorrect unless the first term in expression
(E) is negligible with respect to 0.06. The magnitude of the first term de-
pends critically on k9, a quantity for which no value is available. Previ-
ously, a tentative value was presented assuming that the value for kg is
22
-------
equal to the rate constant reported by Davidson and Thrush for the reaction
of atomic oxygen with ethyl nitrite.24 Limits for the value of kg may be
fixed in the following sense. If kg is smaller than a certain lower limit,
step (9) can have no effect on the yield ratio because the first term of ex-
pression (E) will be negligible. If kg is larger than a certain value, ex-
pression (E) predicts a variation in the ratio of rates of formation with
variation of the pressure of either nitrogen dioxide or isopropyl nitrite
that is not observed experimentally. In fact, if step (9) is to be signi-
ficant for our results, the value of kg must lie between the value estima-
ted from the work of Davidson and Thrush24 and three times that value. Even
if the value of kg fortuitously lies in this range, the effect of step (9)
can be no greater than to make the reported value of k5/k6 20% too high.
There is another point of view that tentatively suggests that step (9)
is not important in producing acetone in the present system. It is probably
unrealistic to assume that kg for oxygen atom attack on isopropyl nitrite is
the same as that for attack on ethyl nitrite. Davidson and Thrush24 found
that the Arrhenius parameters for oxygen atom attack on methyl and ethyl ni-
trites were the same as those determined for oxygen atom attack on primary
and secondary hydrogen atoms of alkanes.30 By extending this analysis to
isopropyl nitrite, it is found that kg is predicted to be about 6.4 x 107
£/mole-sec. This value, if substituted into expression (E), leads to a
prediction of a large variation in the yield ratio. Under the experimental
conditions of this work, no such variation has been observed. One may then
speculate that the (CH3)2CONO radical formed initially in step (9) does not
inevitably form acetone under the experimental conditions even though de-
composition of this radical is certainly exothermic.21*
23
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REFERENCES
1. Calvert, J. G., and J. N. Pitts, jr. Photochemistry. John Wiley and
Sons Inc., New York, New York, 1966. 899 pp.
2. Mendenhall, G. D., D. M. Golden, and S. W. Benson. The Very-Low-Presure
Pyrolysis (VLPP) of n-Propyl Nitrate, tert-Butyl Nitrite, and Methyl Ni-
trite. Int. J. Chem. Kinet., 7_, 725-737 (1975).
3. Benson, S. W. Thermochemical Kinetics. John Wiley and Sons Inc., New
York, New York, 1968. 223 pp.
4. Demerjian, K. L., J. A. Kerr, and J. G. Calvert. The Mechanism of Photo-
chemical Smog Formation. Adv. Environ. Sci. and Technol., £, 1-262
(1974). ~
5. Kan, R. 0. Organic Photochemistry. McGraw-Hill Inc., New York, New York,
1966. 293 pp.
6. Ludwig, B. E. The Primary Process in Photolysis of Isopropyl Nitrite.
Ph.D. Thesis. Case Western Reserve University, 1968. 84 pp.
7. Wiebe, H. A., A. Villa, T. M. Hellman, and J. Heicklen. Photolysis of
Methyl Nitrite in the Presence of Nitric Oxide, Nitrogen Dioxide, and Ox-
ygen. J. Am. Chem. Soc., 97, 7-13 (1973).
8. Baker, G., and R. Shaw. Reactions of Methoxyl, Ethoxyl, and t^-Butoxyl
with Nitric Oxide and Nitrogen Dioxide. J. Chem. Soc., 6965-6970
(1965).
9. McMillan, G. R. Photolysis of Alkyl Nitrites. I. tert-Butyl Nitrite.
J. Am. Chem. Soc., 84_, 4007-4011 (1962).
10. Durant, D., and G. R. McMillan. Energy Distribution of Photochemically
Generated t>Pentoxy Radicals. J. Phys. Chem., 70, 2709-2713 (1966).
11. Tarte, P. Photolysis of Alkyl Nitrites. Bull. Soc. Roy. Sci. Liege,
_22, 226-237 (1953) .
12. Thompson, H. W., and F. S. Dainton. The Photochemistry of Alkyl Nitrites.
III. Trans. Faraday Soc., 33_, 1546-1555 (1937).
13. McMillan, G. R., J. Kumari, and D. L. Snyder. The Photolysis and Photo-
oxidation of Alkyl Nitrites. In: Chemical Reactions in Urban Atmospheres,
C. S. Tuesday, ed. American Elservier Publishing Co., Inc., New York,
24
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New York, 1971. pp. 35-43.
14. McMillan, G. R. Photolysis of Diisopropyl Peroxide. J. Am. Chem. Soc.,
_8_3, 3018-3023 (1961) .
15. Yee Quee, M. J., and J. C. J. Thynne. Reactions of Isopropoxyl Radicals.
Trans. Faraday Soc., 6£, 1296-1303 (1968).
16. Hughes, G. A., and L. Phillips. The Kinetics of Disproportionation-Com-
bination Reactions Between the Isopropoxyl Radical and Nitric Oxide, and
of the Pyrolysis of the 0-0 Bond in Di-isopropyl Peroxide. J. Chem. Soc.
(A), 894-897 (1967).
17. Ludwig, B. E. and G. R. McMillan. Primary Quantum Yields in Photodissoci-
ation of Isopropyl Nitrite. J. Am. Chem. Soc., 91, 1085-1088 (1969).
18. Levy, J. B. The Thermal Decomposition of Nitrate Esters. II. The Effect
of Additives on the Thermal Decomposition of Ethyl Nitrate. J. Am. Chem.
Soc., 76_ 3790-3793 (1954).
19. Leighton, P. A. Photochemistry of Air Pollution. Academic Press, New
York, New York, 1961. 300 pp.
20. Ungnade, H. E., and R. A. Smiley. Ultraviolet Absorption Spectra of
Nitroparaffins, Alkyl Nitrates, and Alkyl Nitrites. J. Org. Chem., 21,
993-996 (1956).
21. Reference 19, p. 52.
22. Jones, I. T. N., and K. D. Bayes. Photolysis of Nitrogen Dioxide. J.
Chem. Phys. 59_, 4836-4844 (1973).
23. McMillan, G. R., and J. G. Calvert. Gas Phase Photo-oxidation. In:
Oxidation and Combustion Reviews, Vol I, C. F. H. Tipper, ed. Elsevier
Publishing Co., Amsterdam, 1965. pp. 84-135.
24. Davidson, J. A., and B. A. Thrush. Reaction of Oxygen Atoms with Methyl
and Ethyl Nitrites. J. Chem. Soc., Faraday Trans., I, 71, 2413-2420
(1975).
25. Heicklen, J. Gas-Phase Reactions of Alkylperoxy and alkoxy Radicals. In:
Advan. Chem. Ser. No. 76, 1968. pp. 23-39.
26. Kabasakalian, P., and E. R. Townley. Photolysis of Nitrite Esters in So-
lution. I. Photochemistry of n-Octyl Nitrite. J. Am. Chem. Soc., 84,
2711-2716 (1962) .
27. Gray, P. and P. Rathbone. The Abstraction of Nitric Oxide from t_-Butyl
Nitrite by Methyl Radicals. Proc. Chem. Soc., 316-317 (1960). ~~
28. Jest, B., and L. Phillips. The Reaction of Methyl Radicals with Methyl
Nitrite. Proc. Chem. Soc., 73-74 (1960).
25
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29. Johnston, H. S. Gas Phase Reaction Rate Theory. The Ronald Press Co.,
New York, New York, 1966. 362 pp.
30. Herron, J. T. and R. E. Huie. Rates of Reaction of Atomic Oxygen. II.
Some C0 to CQ Alkanes. J. Phys. Chem., 73, 3327-3337 (1969).
2 o —
31. Batt, L., R. D. McCulloch, and R. T. Milne. Thermochemical and Kinetic
Studies of Alkyl Nitrites (RONO)_D (RQNO), the Reactions Between RO and
NO, and the Decomposition of RO. Int. J. Chem. Kinet. Supp., 7_, 441-461
(1975).
26
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-77-081
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE ANDSUBTITLE
REACTIONS OF ISOPROPOXY RADICALS WITH NITROGEN OXIDES
5. REPORT DATE
August 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
G. R. McMillan
M. J. Kaiserman
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Chemistry
Case Western Reserve University
Cleveland, Ohio 44106
10. PROGRAM ELEMENT NO.
1AA008 21AKC21 (FY 74)
11. CONTRACT/GRANT NO.
R-800659
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTP, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Information was sought concerning reactions of isopropoxy radicals with nitric
oxide and nitrogen dioxide. Isopropyl nitrate was photodissociated in the
presence of oxides of nitrogen and an inert gas. The reaction was found to
be less important than the alkoxy radical NO reactions. The ratio of dis-
proportion to recombination of isopropoxy and NO was found to be smaller than
the published value (0.12 versus .15).
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS \C. COSATI Field/Group
*Air pollution
*Photochemical. reactions
*Nitrogen oxides
*Reaction kinetics
13B
07B
07E
07D
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
35
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
27
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