Reaction of acrolein with acetylperoxyl radicals in the gas-phase

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:4<277::AID-KIN5>3.0.CO;2-A

A rate constant for the epoxidation of acrolein by acetylperoxyl radicals has been determined to be k₄ = (1.3±0.9)×10⁴ dm³mol^-1s^-1 at 383 K, which is anomalously fast in comparison with the epoxidation of alkenes. Abstraction of the acyl hydrogen atom from acrolein by acetylperoxyl radicals at 393 K was found to be at least 60 times slower than from acetaldehyde and at least three orders of magnitude slower than abstraction of the acyl hydrogen atom of the epoxide of acrolein. The fast rate for epoxidation of acrolein and the slow rate for hydrogen abstraction provide an explanation for the anomalously slow rate for the autoxidation of acrolein and suggests that acrolein formed during the autoxidation of alkene will react further to give its epoxide, and not exclusively by abstraction of the acyl hydrogen atom as was previously accepted.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:4<277::AID-KIN5>3.0.CO;2-A

The Reaction of Acrolein
with Acetylperoxyl Radicals
in the Gas-Phase
PETER J. RODEN, MORAY S. STARK, DAVID J. WADDINGTON
Department of Chemistry, University of York, York, YO10 5DD, U.K.
Received 26 August 1998; accepted 10 November 1998
ABSTRACT: A rate constant for the epoxidation of acrolein by acetylperoxyl radicals has been
4
3
Ϫ1 Ϫ1
determined to be k ϭ (1.3 Ϯ 0.9) ϫ 10 dm mol s at 383 K, which is anomalously fast in
4
comparison with the epoxidation of alkenes. Abstraction of the acyl hydrogen atom from
acrolein by acetylperoxyl radicals at 393 K was found to be at least 60 times slower than from
acetaldehyde and at least three orders of magnitude slower than abstraction of the acyl hy-
drogen atom of the epoxide of acrolein. The fast rate for epoxidation of acrolein and the slow
rate for hydrogen abstraction provide an explanation for the anomalously slow rate for the
autoxidation of acrolein and suggests that acrolein formed during the autoxidation of alkene
will react further to give its epoxide, and not exclusively by abstraction of the acyl hydrogen
atom as was previously accepted. ᭧ 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 277–282, 1999
INTRODUCTION
atom abstraction from acrolein (reaction 1), followed
by oxygen addition (reaction 2):
The gas-phase autoxidation of propene has been much
studied recently, with detailed reaction mechanisms
being produced [1–3] to be used as submechanisms
in the simulation of hydrocarbon combustion [4] and
to study the possible production of propene oxide via
propene autoxidation [5].
C H CHO ϩ Xؒ !: ؒC H CB O ϩ XH
(1)
(2)
2
3
2
3
C H CB O ϩ O !: C H C(O)O ؒ
2
3
2
2
3
2
C H C(O)O ؒ ϩ C H !: ؒC H
3
2
3
2
3
6
2
ϩ CO ϩ C H O (3)
2
3
6
Although at lower temperatures the majority of the
propene reacts by radical addition to the double bond
to yield either the epoxide or acetaldehyde and for-
maldehyde, hydrogen abstraction can occur under all
conditions. The resulting allyl radical can yield acro-
lein, and computer simulations have suggested that the
subsequent behavior of the unsaturated aldehyde can
play a significant role in the autoxidation of propene.
For example, during the simulation of the oxidation of
a fuel-rich propene:oxygen mixture at 550 K, ca. 40%
of the epoxide formed was predicted to be produced
by epoxidation of the alkene by the acrylperoxyl rad-
ical (reaction 3) [3], which is formed by hydrogen
Mechanisms describing the further reaction of acro-
lein have tended to assume that attack on the carbon-
carbon double bond was insignificant, with hydrogen
atom abstraction from the aldehyde group dominating
[
1–3], and with the subsequent mechanism and reac-
tion rates chosen by assuming analogous behavior to
that of acetaldehyde, which has been more thoroughly
studied [6].
However, it is possible that the assumption of the
lack of reactivity of the C"C double bond is not
valid. In this work, two reactions of acylperoxyl rad-
icals with acrolein have been examined, namely the
epoxidation of the C"C double bond (reaction 4) and
abstraction of the acyl hydrogen atom (reaction 5) by
acetylperoxyl radicals:
Correspondence to: M. S. Stark
᭧
1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/040277-06

2
78
RODEN, STARK, AND WADDINGTON
CH C(O)O ϩ C H CHO
3
2
и
2
3
CH ϩ CO ϩ H COCHCHO (4)
и
3
2
2
CH C(O)O ؒ ϩ C H CHO !:
3
2
2
3
CH C(O)O H ϩ C H CB O (5)
3
2
2
3
The consequences of these findings on the reactions
of acrolein during the autoxidation of propene are dis-
cussed.
EXPERIMENTAL
Acrolein was distilled and dried over calcium hydride.
Acetaldehyde (Aldrich) was distilled and stored over
molecular sieve. Oxygen, helium (B.O.C.), 1-butene,
and propanal (Aldrich) were used without further pu-
rification. The surface of the reaction vessel (Pyrex,
Figure 1 Consumption of acetaldehyde (᭺) and acrolein
(
[
ϩ) during their co-oxidation: initial concentrations,
CH CHO]/[O ]/[He]/[C H CHO] ϭ 5/5/4/1; total initial
3
2
2
3
3
pressure, 400 mbar; 393 K.
volume ϭ 167 cm , surface : volume ratio ϭ 1.00
Ϫ
1
cm ) was conditioned by performing repeated autox-
idation experiments until a reproducible overall rate
for the autoxidation of acetaldehyde was obtained.
Analysis was mainly by GC (Pye PU4500) with a
Tenax GC column and flame ionization detector (2.5
m length, 3 mm i.d.). Product identification was by
GC-mass spectrometry (VG Analytical Autospec,
electron impact, 6 kV). The concentrations of products
were calibrated using authentic samples (Aldrich), ex-
cept for the epoxide of acrolein, glycidaldehyde, and
peracetic acid. The peracid was determined by allow-
ing some of the reacted gases to flow into an evacuated
vessel, where they were dissolved in water, followed
by titration with potassium iodide [7]. Glycidaldehyde
was prepared by the addition of hydrogen peroxide to
an aqueous solution of acrolein [7,8]. A pure sample
could not be extracted however, so its calibration fac-
tor was assumed to be the same as methylglyoxal, as
it is known that isomeric epoxide and ketone groups
have very similar FID calibration constants [9]. For
example, with the FID that was used, the calibration
constants for acetone and propene oxide differed
by less than the experimental error of a few per-
cent.
CH C(O)O ؒ ϩ CH CHO !:
3
2
3
CH C(O)O H ϩ CH CB O (6)
3
2
3
9
3
Ϫ1 Ϫ1
with a rate constant of k ϭ 1.7 ϫ 10 dm mol s
6
Ϫ1
exp(Ϫ30.1 kJ mol /RT). There is an estimated stan-
dard error for the rate constant of ca. Ϯ65% at 393 K
[11]. During the co-oxidation of acetaldehyde and
acrolein, the acrolein is sufficiently unreactive that it
is unlikely to be a major perturbation to the oxidation
of the acetaldehyde, so the dominant radical will still
be acetylperoxyl. Therefore any acrolein that does re-
act will do so predominantly with acetylperoxyl radi-
cals formed by the oxidation of acetaldehyde. As acro-
lein does not noticeably react away in Figure 1, an
upper bound for the ratio of the rate constants for ab-
straction from acrolein and acetaldehyde (k /k ) can be
5
6
estimated from the rate of consumption of acetalde-
hyde and the upper limit for the rate of removal of
acrolein:
k /k (393 K)
5
6
(
d[C H CHO]/dt) [CH CHO]
2 3 initial 3
initial
initial
ϭ
Ͻ 0.017 (I)
(d[CH CHO]/dt) [C H CHO]
3
initial
2
3
3
3
Ϫ1 Ϫ1
S
k (393 K) Ͻ 2.9 ϫ 10 dm mol
5
RESULTS AND DISCUSSION
Mixtures of acetaldehyde and oxygen with smaller
amounts of acrolein were reacted together at 393 K
As a comparison, acetaldehyde and propanal were
co-oxidised under the same conditions, the propanal
reacts away faster than acetaldehyde (Fig. 2), with
k /k (393 K) ϭ 2.5 Ϯ 0.3, demonstrating that the pres-
ence of the conjugated vinyl group instead of an ethyl
group reduces the rate of hydrogen atom abstraction
(Fig. 1). During the initial stages of its autoxidation,
acetaldehyde is consumed by abstraction of the acyl
hydrogen atom by the dominant radical present, the
acetylperoxyl radical (reaction 6) [10]:
7
6

REACTION OF ACROLEIN WITH ACETYLPEROXYL RADICALS
279
Figure 3 Production of peracetic acid (᭺) and glycidal-
dehyde (ϫ100) (ϩ) during the co-oxidation of acetaldehyde
and acrolein: initial concentrations, [CH CHO]/[O ]/[He]/
Figure 2 Consumption of acetaldehyde (᭺) and propanal
(
[
ϩ) during their co-oxidation: initial concentrations,
CH CHO]/[O ]/[He]/[C H CHO] ϭ 5/5/4/1; total initial
3
2
3
2
2
5
[
C H CHO] ϭ 5/5/4/1; total initial pressure, 400 mbar;
2 3
pressure, 400 mbar; 393 K.
3
93 K.
from the acyl group by a factor of at least ϫ125 at
3
93 K:
To obtain a rate constant for the epoxidation of
acrolein by peracetyl radicals (k ), small quantities of
4
CH C(O)O ؒ ϩ C H CHO !:
acrolein and 1-butene were co-oxidized with acetal-
dehyde (Fig. 4), the ratio of the rate of build up of the
two epoxides allowed the determination of the ratio of
the rate constants (k /k ) for their formation at 383 K:
3
2
2
5
CH C(O)O H ϩ C H CB O
(7)
3
2
2
5
5
3
Ϫ1 Ϫ1
k (393 K) ϭ (4.3 Ϯ 2.8) ϫ10 dm mol s
7
4
9
The build up of the products glycidaldehyde and
peracetic acid produced during the co-oxidation of
acrolein and acetaldehyde are given in Figure 3. The
epoxide, which is only formed in small quantities, is
far more reactive than acrolein itself. Its concentration
decreases when that of peracetic acid becomes signif-
icant, suggesting its consumption by addition to the
peracid to form glycidaldehyde monoperacetate,
which would decompose at this temperature to two
acid molecules (reaction 8). This reaction is analogous
with that of peracetic acid with acetaldehyde, which is
known to be a major source of the large quantities of
acetic acid produced during acetaldehyde autoxidation
at these temperatures [12,13].
CH C(O)O ϩ 1-C H
8
3
2
и
4
CH3 ϩ CO ϩ H COCHC H (9)
и
2
2
2
5
CH C(O)O H ϩ H COCHCHO
3
2
2
H COCHCH(OH)O C(O)CH
CH C(O)OH
3
2
2
3
ϩ H COCHC(O)OH (8)
2
That the epoxide increases in concentration early in
the reaction when the peracid concentration is low
suggests that the formation of the epoxide by the re-
action of acrolein with the peracid is minor (the partial
pressure of the peracid for the early part of the reaction
was lower than the detection limit of ca. 2 mbar).
Figure 4 Production of glycidaldehyde (ϩ) and 1-butene
oxide (᭺) during the co-oxidation of acetaldehyde, 1-butene
and acrolein: initial concentrations, [CH CHO]/[O ]/[He]/
3
2
[C H CHO]/[1-C H ] ϭ 10/10/8/1/1; total initial pressure,
2
3
4
8
400 mbar; 383 K.

2
80
RODEN, STARK, AND WADDINGTON
k4/k10 ϭ [H COCHCHO]/[C H CHO]
2
2
3
ϭ (6.2 Ϯ 0.5) ϫ 10Ϫ3 (III)
The rate constant k measured at 383 K can be ex-
4
trapolated to 403 K by assuming an A factor for the
8
3
Ϫ1 Ϫ1
reaction of 1.3 ϫ 10 dm mol s , which is the av-
erage of epoxidation A factors that have been mea-
6
sured [14], giving k (403 K) ϭ (3.3 Ϯ 2.3) ϫ 10
1
0
3
Ϫ1 Ϫ1
dm mol s , which is three orders of magnitude
faster than abstraction from acrolein. Since the yield
of peracetic acid increases with time, while the yield
of epoxide does not decrease with time, it is unlikely
that consumption of the epoxide by reaction with the
peracid is significant for this experiment.
Figure 5 Production of glycidaldehyde (ϩ) and 1-butene
oxide (᭺) during the co-oxidation of acetaldehyde, 1-butene
and acrolein: initial concentrations, [CH CHO]/[O ]/[He]/
Similarly, for the results described by Figure 4,
epoxidation of acrolein and 1-butene by peracetic acid
is unlikely to be significant, as the rates of formation
of the two epoxides are essentially unvarying, while
the concentration of the peracetic acid increases with
time. This is consistent with previous studies which
found that epoxidation by peracetic acid was always
minor in comparison with epoxidation by acetylpe-
roxyl radicals, in the early stages of the reaction
3
2
[
C H CHO]/[1-C H ] ϭ 10/10/8/1/1; total initial pressure,
2
3
4
8
4
00 mbar; 403 K.
k /k (383 K) ϭ
4
9
(
d[H COCHCHO]/dt) [1-C H ]
2
initial
4
8 initial
[10,15,16].
(
d[H COCHC H ]/dt) [C H CHO]
It is known that rate constants for the epoxidation
2
2
5
initial
2
3
initial
ϭ 1.33 Ϯ 0.31 (II)
of alkenes by peroxyl radicals correlate well to the
ionization energy of the double bond, with a lower
ionization energy giving a faster reaction [15,17]. This
is taken to indicate a degree of charge transfer at the
transition state for the initial addition of the peroxyl
radical to the double bond. The correlation for the ad-
dition of acetylperoxyl radicals to a series of substi-
tuted ethenes from propene to 2-methyl-2-butene
The rate constant for the epoxidation of 1-butene
7
3
Ϫ1 Ϫ1
(
9) has been determined (k ϭ 8.7 ϫ 10 dm mol s
9
Ϫ1
exp(Ϫ28.9 kJ mol /RT) (Ϯ65% at 383 K), [10]).
Hence, the rate constant for the formation of glycidal-
dehyde can be determined as k (383 K) ϭ (1.3 Ϯ 0.9)
ϫ 10 dm mol s . The co-oxidation of acetaldehyde
and acrolein cannot be used over a large enough tem-
perature range for Arrhenius parameters to be deter-
mined for the reaction, as below ca. 370 K the autox-
idation of acetaldehyde is too slow to be useful and
above ca. 400 K the further reaction of glycidaldehyde
4
4
3
Ϫ1 Ϫ1
[10,15,16] is given in Figure 6. Also included is the
rate constant for the epoxidation of acrolein, which
appears to be anomalously fast (by two orders of mag-
nitude) in comparison with extrapolation of the be-
havior of the epoxidation of the alkenes. The second
Ϫ
1
major photoionization peak at 1055 kJ mol has been
used as a measure of the ionization energy of the
C"C double bond, as the first photoionization peak
(
10) is too rapid in comparison with its rate of for-
mation.
Ϫ
1
at 975 kJ mol [18] is thought to be due to removal
of an electron from an orbital situated mainly on the
carbonyl oxygen atom [19]. The anomalously fast rate
for the epoxidation of acrolein may provide an ex-
planation for the autoxidation of acrolein at rela-
tively low temperatures (ca. 513 K) being consid-
erably slower than that of acetaldehyde or propanal
[20]. Chain branching during aldehyde autoxida-
tion is generally thought to be by decomposition of
the acyl hydroperoxide: for example, for acetal-
dehyde,
CH C(O)O ϩ H COCHCHO
3
2
и
2
и
CH C(O)O H ϩ H COCHCO (10)
3
2
2
For example, at 403 K (Fig. 5), the concentration
of butene oxide increases as expected, but that of gly-
cidaldehyde appears essentially constant. This behav-
ior is consistent with the glycidaldehyde being both
produced and consumed by peracetyl radicals (4 and
1
0) and from steady-state analysis:

REACTION OF ACROLEIN WITH ACETYLPEROXYL RADICALS
281
C H C(O)O ϩ C H CHO
2
3
2
и
2
3
и
3
C H C(O)O H ϩ C H CO (14)
2
3
2
2
Likewise, the assumption that any acrolein formed
during alkene autoxidation will react exclusively by
abstraction of the acyl hydrogen is unlikely to be valid,
with epoxidation by peroxyl radicals being a possible
major alternative route. It is also evident that glyci-
daldehyde is much more reactive toward peroxyl rad-
icals than the parent aldehyde, so it would be unlikely
to build up to significant levels during alkene autoxi-
dation. Indeed, in preliminary work on propene autox-
idation, glycidaldehyde was only found in trace quan-
tities [7].
Figure 6 The correlation between the rate constant for
epoxidation of alkenes by acetylperoxyl radicals (k) at 393
K [10,15,16] and the ionization energy [21] of the C"C
double bond of alkenes (•), showing the anomalous behavior
of acrolein (ϩ) (at 383 K). The error limits represent the
standard errors in the ratio of the epoxidation rate constant
to that of the reference reaction with which they were all
measured, the abstraction of the acyl hydrogen atom from
acetaldehyde by acetylperoxyl radicals.
CONCLUSION
The epoxidation of acrolein by acetylperoxyl radicals
has been examined and found to be anomalously fast,
in comparison with the epoxidation of alkenes. The
competing reaction of abstraction of the acyl hydrogen
atom is considerably slower than from acetaldehyde
or propanal. Both results suggest that acrolein formed
during alkene autoxidation will be consumed by epox-
idation, and not solely by abstraction of the acyl hy-
drogen as has been assumed previously. The epoxi-
dation of the acrolein double bond also provides an
explanation for the autoxidation of acrolein being con-
siderably slower than that of acetaldehyde or propanal,
through inhibiting the formation of the acylhydrope-
roxide branching agent by providing an alternative re-
action for the peroxyl radicals.
CH CHO ϩ CH C(O)O ؒ !:
3
3
2
CH CB O ϩ CH C(O)O H (6)
3
3
2
CH C(O)O H !: CH ؒ ϩ CO ϩ ؒOH
(11)
3
2
3
2
It is known that the addition of small quantities of
alkene to reacting mixtures of acetaldehyde and oxy-
gen can greatly inhibit the autoxidation of the alde-
hyde, due to an alternative reaction for the acylperoxyl
radical, epoxidation of the alkene, competing with the
formation of the peroxide (4) [10]. For example, for
the addition of propene:
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