Nitrous Oxide as a Radical Scavenger

Full text of DOI:10.1021/jo971559t

4
494
J . Org. Chem. 1998, 63, 4494-4496
Nitr ou s Oxid e a s a Ra d ica l Sca ven ger
Sch em e 1
Yong-Tae Park* and Kwang-Wook Kim
Department of Chemistry, Kyungpook National University,
Taegu 702-701, Korea
Nam Woong Song and Dongho Kim*
Spectroscopy Laboratory, Korea Research Institute of
Standards and Science, Taejon 305-600, Korea
trapping effect with nitrous oxide. The reaction mixture
was analyzed by HPLC. The only three peaks at reten-
tion times 9.2, 14.0, and 16.9 min on theHPLC chromato-
gram were observed at the reaction condition and corre-
spond to benzene (solvent), unreacted benzoyl peroxide,
and biphenyl, respectively. The yields of biphenyl after
Received August 21, 1997
Nitrous oxide is known as a trapping agent for solvated
electrons.1 It has been used as a blocking agent for
2
-4
comparing reaction rates
as well as for selecting
reaction pathways.5 We recently and unexpectedly ob-
served that a photochemical, radical-mediated reaction
was retarded to some extent by the presence of nitrous
oxide.6 In view of this finding, it seemed important to
clarify the possibility of nitrous oxide as a radical
trapping agent by studying the effect of nitrous oxide on
well-known radical-mediated reactions such as the ther-
mal and photochemical reactions of benzoyl peroxide.
It has been reported that the thermal decomposition
of a benzene solution of benzoyl peroxide produces a
mixture of biphenyl, carbon dioxide, and benzoic acid in
yields less than 50%, in addition to high-boiling materi-
als.7 There is no doubt that the radical species, such
as benzoyloxyl, phenyl, and phenylcyclohexadienyl radi-
cals, are involved in the decomposition.7 It has been
also reported that photochemical decomposition of ben-
3
0 min reaction times were 7.4, 3.3, and 2.5% under
argon, nitrous oxide, and oxygen, respectively. The
formation of biphenyl was retarded by 60% and 66%
under nitrous oxide and oxygen, respectively. The rel-
evant results of the thermal reaction are summarized in
Table 1. Reaction of a dilute benzene solution of benzoyl
-4
peroxide (1.0 × 10 M) shows that the biphenyl yield is
also reduced to 60% both under nitrous oxide and oxygen.
The ratio of biphenyl yields under nitrous oxide and
oxygen is 1.3, indicating that the inhibition by nitrous
oxide is a little less effective than oxygen.
,8
The reaction mechanism of the peroxide decomposition
8,10c
is shown in Scheme 1 as described elsewhere.
The
,8
thermal decomposition produces radical intermediates,
phenyl σ and phenylcyclohexadienyl radicals. Nitrous
oxide presumably inhibits the formation of biphenyl by
reacting competitively with benzene solvent for the same
intermediate radicals to give R-NdN-OH. Several
attempts to detect the adducts of the nitrous oxide radical
such as benzenediazotic acid have failed in the thermal
reaction by LC/MS and GC/MS. Probably the adduct of
nitrous oxide radical is too labile to be detected by GC/
MS and LC/MS.
-
3
zoyl peroxide (10 M) in Freon 113 produced 8% of
benzene and 7% of phenyl benzoate in the absence of
cyclohexane and 35% of benzene in the presence of
cyclohexane (0.06 M).9 It has also been shown that
phenyl and benzoyloxyl radicals are involved in the
mechanism of formation of products in the photochemical
reaction of benzoyl peroxide.10
-
3
A benzene solution of benzoyl peroxide (1.0 × 10 M,
2
5 mL) in a 50 mL, three-neck flask was heated under
-2
A methanol solution of benzoyl peroxide (1.0 × 10
reflux for 30 min during purging with argon, nitrous
oxide, or oxygen. Oxygen which is known as a radical
M, 2 mL) was irradiated by using monochromatic light
(band-pass 274 ( 6 nm) from a Xe-lamp (450 W), and
scavenger1
0c,11
was used to compare the extent of the
the reaction mixture was analyzed by HPLC. The HPLC
profile (not shown) showed several products. Peaks with
retention times of 9.2, 12.7, 14.0, and 16.9 min on a C-18
(
1) Dainton, F. S.; Peterson, D. B. Nature 1960, 186, 878. (b)
Dainton, F. S.; Peterson, D. B. Proc. R. Soc. (London) 1962, A267, 443.
2) Matheson, M. S.; Mulac, W. A.; Weeks, J . L.; Rabini, J . J . Phys.
Chem. 1966, 70 , 2092.
(
column [4.6 × 250 mm, CH
3
CN:water (3:1) as eluent] are
identified as benzene, phenyl benzoate, benzoyl peroxide,
and biphenyl, respectively. The yields of benzene, phenyl
benzoate, and biphenyl produced in the photochemical
reaction under argon were 2.6, 1.2, and 0.04%, respec-
tively. The ratio of benzene yields formed under argon
and nitrous oxide is 0.7. The formation of benzene was
retarded about 30% in the presence of nitrous oxide
(
(
3) Cercek, B.; Kongshaug J . Phys. Chem. 1970, 74, 4319.
4) Elisei, F.; Favaro, G.; G o¨ rner, H. J . Photochem. Photobiol. A.
Chem. 1991, 59, 243.
5) Bernhardt, P. V.; Lawrance, G. A.; Sangster, D. F. Polyhedron
991, 10, 1373.
6) Park, Y.-T.; Song, N. W.; Kim, Y.-H.; Hwang, C.-G.; Kim, S. K.;
Kim. D. J . Am. Chem. Soc. 1996, 118, 11399. (b) Park, Y.-T.; Kim,
Y.-H.; Kim, S. K.; Song, N. W. Bull. Korean Chem. Soc. 1994, 15, 607.
(
1
(
(
7) Hey, D. H.; Liang, K. S. Y.; Perkins, M. J . Tetrahedron Lett. 1967,
(Table 1). The formation of benzene in the presence of
1
1
1
477.
(
8) Swern, D., Ed. Organic Peroxide; Wiley: New York, 1970, Vol.
, pp 573-575; 1971, Vol. 2, pp 838-844.
9) Chateauneuf, J .; Lusztyk, J .; Ingold, K. U. J . Am. Chem. Soc.
oxygen was extensively reduced, presumably because
oxygen acted not only as the radical inhibitor but also
as the triplet state quencher. The formation of other
minor products such as phenyl benzoate and biphenyl
was also reduced in the presence of nitrous oxide or
(
988, 110, 2886. (b) Chateauneuf, J .; Lusztyk, J .; Ingold, K. U. J . Am.
Chem. Soc. 1988, 110, 2877.
10) Brown, D. J . J . Am. Chem. Soc. 1940, 62, 2657. (b) McClure, J .
H.; Robertson, R. E.; Cuthbertson, A. C. Can. J . Res. 1942, 20B, 103.
c) Pryor, W. A. In Introduction to Free Radical Chemistry; Prentice-
(
(
Hall: Englewood Cliffs, NJ , 1966, pp 8-25 and pp 94-95. (d) Bradley,
J . D.; Roth, A. P. Tetrahedron Lett. 1971, 3907. (e) Scaiano, J . C.;
Stewart, L. C. J . Am. Chem. Soc. 1983, 105, 3609.
(11) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemisrty; Harper & Row Publishers: New York, 1987; pp
779-782.
S0022-3263(97)01559-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/11/1998

Notes
J . Org. Chem., Vol. 63, No. 13, 1998 4495
Ta ble 1. Effects of Nitr ou s Oxid e on th e F or m a tion s of th e P r od u cts in th e Th er m a l a n d P h otoch em ica l Rea ction s of
Ben zoyl P er oxid e
product yield (Y, %) under
yield ratio
concn benzoyl peroxide
reaction (solvent)
thermal at 80°C
(M) (reaction duration, min)
product
biphenyl
Ar
N2O
3.3
O2
YN O/YAr
YN O/YO2
2
2
1.0 × 10- (30)
3
7.4
2.5
0.4
1.3
(benzene)
-
4
1
.0 × 10 (30)
biphenyl
benzene
phenyl benzoate
biphenyl
benzene
2.5
2.6
1.2
0.04
21.5
8.9
0.7
1.0
1.7
1.1
0.03
20.4
8.1
0.9
b
1.1
0.02
b
0.4
0.7
0.9
0.8
0.9
0.9
0.9
1.1
-
photochemicala
1.0 × 10 (30)
-2
(methanol)
1.0
1.5
-
3
1
.0 × 10- (30)
phenyl benzoate
biphenyl
7.2
0.4
1.1
1.5
0.6
a
Monochromator light (274 ( 6 nm) from Xe-lamp. b Less than detecting limit.
benzoyloxyl radical.13 The lifetime of benzoyloxyl radical
is also quenched by 40% in the presence of nitrous oxide
in the right inset of Figure 1.
Thus, we conclude that nitrous oxide is a moderate
radical-trapping agent, based on the results of the
thermal and photochemical reactions as well as the
results of laser flash photolysis: thermal and photo-
chemical decompositions which are shown as radical-
mediated reactions are retarded by the presence of
nitrous oxide; the lifetimes of the transients assumed to
be phenyl and benzoyl radicals (albeit not identified
completely) are reduced by the presence of nitrous oxide.
The trapping reaction might be similar to the known
1
4
trapping reaction of N-tert-butyl phenylnitrone which
is structurally similar to the nitrous oxide:
F igu r e 1. Transient absorption spectra 5 µs after the laser
-
4
pulse for benzoyl peroxide (1.0 × 10 M) in methanol under
argon. Inset: Temporal profiles of transients at 260 nm (left)
and 295 nm (right) from the photolysis of benzoyl peroxide;
curve a, under argon; curve b, under nitrous oxide.
oxygen. A similar result to the above was obtained for a
-
3
dilute solution of benzoyl peroxide (1.0 × 10 M). It
seems that the quenching selectivity of nitrous oxide
toward carbon- and oxygen-centered radicals is not clear,
although oxygen prefers a carbon-centered radical to an
oxygen-centered one.
Exp er im en ta l Section
Benzoyl peroxide (Fluka) was purified by recrystallization
(
methanol, twice). All solvents used were reagent grade unless
otherwise specified. High-performance liquid chromatogaphy
HPLC, Rainin) was performed on a Rainin C-18 column for the
(
To examine the effect of nitrous oxide on the transients,
benzoyl and phenyl σ radicals, benzoyl peroxide was
irradiated using laser flash photolysis. A transient
absorption spectra was obtained at a 5 µs delay after
pulsing in the laser flash photolysis12 of a methanolic
thermal reaction mixtures. Argon was used as received from
the supplier. Nitrous oxide was prepared by heating ammonium
nitrate (50 g) to 250 °C in 200 mL in a three-neck flask. Water,
an unwanted product in the reaction, was eliminated by passing
the generated nitrous oxide gas through a Dewar flask contain-
ing dry ice and acetone and then through a tube of calcium
chloride. A trace amount of nitric oxide (NO) side product was
eliminated by passing the gas mixture through a aqueous ferrous
sulfate solution in line before the Dewar flask.
-
4
solution of benzoyl peroxide (1.0 × 10 M). There are
two absorption peaks at about 260 and 295 nm, and the
lifetimes of these transients are 405 and 155 µs, respec-
tively (Figure 1). They are regarded as different species
because their lifetimes differ. The transient absorbing
at 260 nm is assigned to the phenyl σ radical because
the absorption wavelength and lifetime are similar to a
Th er m a l Rea ction .
A benzene solution of benzoyl peroxide (10 M, 25 mL) in a
0 mL, three-necked flask was deaerated with argon, nitrous
-3
5
oxide, and oxygen after purging with argon, for 30 min. After
refluxing under the desired gas using a long introducing line of
the gas (i.d., 5 mm; length, 1.5 m) for 30 min, the reaction
mixture (1.0 µL) was chromatographed on Rainin C-18 column
reported phenyl-type radical of an N-benzylpyridinium
salt.6b The temporal profiles of phenyl σ radical at 260
nm under argon (curve a) and nitrous oxide (curve b) are
compared in the left inset of Figure 1. The lifetimes of
phenyl radical in methanol are 405 and 136 µs under
argon and nitrous oxide, respectively; the lifetime is
shortened by about 66% in the presence of nitrous oxide.
It is assumed that the transient species at about 295 nm
is the benzoyloxyl radical because the lifetime and
absorption wavelength are similar to those reported for
of Rainin HPLC (column size, 4.6 × 250 mm; mobile phase, CH
3
-
CN:H O(3:1); flow rate, 0.7 mL/min) to show three peaks at
2
retention times of 9.2, 14.0, and 16.9 min with mixture eluent
(CH CN and H O, 3:1) as a mobile phase. The peaks of retention
times of 9.2, 14.0, and 16.9 min are assigned to solvent benzene,
unreacted benzoyl peroxide, and biphenyl, respectively. The
measurement of the yield of biphenyl was done by using a
calibration curve of authentic sample.
3
2
(
13) Wang, J .; Tateno, T.; Sakuragi, H.; Tokumaru, K. J . Photochem.
(12) The laser flash photolysis study should be safely carried out
Photobiol. A. Chem. 1995, 92, 53.
because of the decomposition property of benzoyl peroxide.
(14) J ansen, E. G. Acc. Chem. Rev. 1971, 4, 31.

4
496 J . Org. Chem., Vol. 63, No. 13, 1998
Notes
P h otoch em ica l Rea ction . A stock methanol solution of
Research Sys. SR250) was used to record the photomultiplier
output (Hamamatsu R955) which is attached on the exit slit of
the monochromator. The temporal profile of the transient
absorption signal was monitored by using a 500 MHz digital
storage oscilloscope (Hewlett-Packard HP54503A). The sample
solutions were circulated from a Teflon bottle (2.5 L in volume)
to the fluorometer quartz cuvette of 10 mm in path length (flow
type, Helma QS1.0) to reduce the effect by the accumulation of
the reaction product and dilution of the reactant in the photolysis
cell.
-
2
benzoyl peroxide (10 M, 2 mL) in a UV cuvette (1 cm path, 3
mL cuvette) was irradiated for 30 min using a monochromatic
light (274 ( 6 nm) from Xe-lamp (450 W). The reaction mixture
was chromatographed as the above thermal reaction to give
several product peaks. The products, benzene, phenyl benzoate,
and biphenyl, were assigned for the peaks of retention times
9
.2, 12.7, 14.0, and 16.9 min, respectively, in reference to peaks
of authentic samples. The yields of benzene, phenyl benzoate,
and biphenyl were determined using calibration curves of
authentic samples.
La ser F la sh P h otolysis. A detailed description of the
experimental setup can be found elsewhere. The fourth har-
6
Ack n ow led gm en t. This research was supported by
the Basic Science Research Institute Program, Ministry
of Education, 1997 (Project No. BSRI-97-3402) (Y.-T.P),
and Center for Molecular Science through KOSEF
(D.K.). We thank Professor David D. Lightner for
disscussion.
monic (266 nm) output from a Q-switched Nd:YAG laser (Spec-
tron SL803G) was used as an excitation source (5 ns, 40 mJ /
pulse). A cw Xe-arc lamp (Atago Bussan Co. XC-150) was used
as a probe light source for the transient absorption measure-
ment. The spectral resolution was obtained by using a 320 mm
monochromator (J obin-Yvon HR320) after the probe light passed
through the sample solution. A boxcar signal averager (Stanford
J O971559T