Autocatalytic Nitration of Pyrene by Aerated Nitrogen Dioxide in Solution and Comparison with the Nitration on Silica Particles

Article abstract of DOI:10.1246/bcsj.77.147

The nitration mechanism of pyrene (PYH) by aerating NO₂ in solution was studied by varying the factors (concentration of NO₂, addition of HNO₃ gas and H₂O, 1-substituent on PYH, and solvent) affecting the nitration. Only 1-nitropyrene was formed, In acetonitrile, both the decrease in PYH and the increase in 1-nitropyrene depicted sigmoid curves, that is, their changes proceeded abruptly after an induction period, and finally approached to zero, The characteristic feature was reasonably explained by the facts that H⁺ dissociated from HNO₃, accumulated by aerating NO₂ into trace water-containing acetonitrile and released by the nitration, acted as an autocatalyst. The nitration proceeded electrophilically based on the 1-substituent effect of PYH, and accelerated with increasing polarity of the solvents. The nitration was studied kinetically and an ionic mechanism involving NO₂⁺ as an electrophile was proposed. The nitration mechanism of PYH in acetonitrile was different from that in the adsorbed water on silica gel.

Full text of DOI:10.1246/bcsj.77.147

ꢀ
2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 147–155 (2004)
147
Autocatalytic Nitration of Pyrene by Aerated Nitrogen Dioxide
in Solution and Comparison with the Nitration on Silica Particles
ꢀ
Kiyoshi Hasegawa, Hiroshi Kaneko, and Tatsuya Ogawa
Department of Chemical and Biochemical Engineering, Faculty of Engineering, Toyama University,
3190 Gofuku, Toyama 930-8555
Received January 6, 2003; E-mail: hasegawa@eng.toyama-u.ac.jp
The nitration mechanism of pyrene (PYH) by aerating NO2 in solution was studied by varying the factors (concen-
tration of NO2, addition of HNO3 gas and H2O, 1-substituent on PYH, and solvent) affecting the nitration. Only 1-nitro-
pyrene was formed. In acetonitrile, both the decrease in PYH and the increase in 1-nitropyrene depicted sigmoid curves,
that is, their changes proceeded abruptly after an induction period, and finally approached to zero. The characteristic fea-
þ
ture was reasonably explained by the facts that H dissociated from HNO3, accumulated by aerating NO2 into trace water-
containing acetonitrile and released by the nitration, acted as an autocatalyst. The nitration proceeded electrophilically
based on the 1-substituent effect of PYH, and accelerated with increasing polarity of the solvents. The nitration was stud-
þ
ied kinetically and an ionic mechanism involving NO2 as an electrophile was proposed. The nitration mechanism of
PYH in acetonitrile was different from that in the adsorbed water on silica gel.
Most of the nitration of an aromatic ring has been carried out
in strongly acidic, polar media, and under these conditions the
nitrating species is generally regarded as a nitryl cation. Al-
N2O4, HNO3 (excess), and HNO2 under conditions which are
relevant for determining the relative reactivity of PAH toward
nitrogen compounds in the atmosphere, and noted that nitration
follows a pseudo first-order kinetics to the concentration of
PAH. The aeration of gaseous nitrating species, such as NO2,
HNO3, and HNO2, on PAH adsorbed on SPM is an important
model for elucidating the nitration process of PAH in the at-
mosphere. Using a simulation apparatus, we have recently re-
ported that the nitration of pyrene (PYH) adsorbed on silica
1
though less frequently discussed in textbooks, nitration can also
be carried out by NO2/N2O4 in organic solutions.2 Despite
the numerous studies devoted to the nitration of polycyclic ar-
omatic hydrocarbons (PAH), the nitration of PAH with NO2/
–10
3
5,7
N2O4 is fairly complex and often controversial. Hitherto,
5,6
3–5,7
free-radical attack, ionic electrophilic substitution,
elec-
and nitrosation mechanisms have been
proposed. The operating reaction mechanisms depend upon the
2,8,9
10
16–18
tron-transfer (ET),
particles proceeds autocatalytically and electrophilically.
These behaviors were reasonably explained by the facts that
2
5–7
7
þ
substrates, solvent polarity, temperature, and the existence
of an acid and water.7
dissociated H of HNO3, accumulated by the reaction of
NO2 with the adsorbed water on silica particles, acts as an au-
6,7
ꢁ
þ
þ
The nitration of PAH using NO2/N2O4 is very mild, but not
explored fully for synthetic purposes. It is especially useful for
tocatalyst, and the resultant HNO2 /HN2O4 attacks at car-
bon atoms having a high electron density. It is reasonable to as-
sume that adsorbed water forms a water layer on the surface of
silica particles, and that a micro solution environment is pre-
pared for nitration. Thus, the nitration can be considered to
be a reaction occurs in the water layer. Considering these facts,
þ
easily oxidizable aromatics and is more selective than NO2 ef-
4
fected ones. Furthermore, it does not require a subsequent
strong acid work-up. The nitration of PAH in polar solvents
3,6,7
is known to be catalyzed by the addition of protons
7
and
Lewis acids, however, the rate is slow for nonactivated aro-
matic compounds, such as benzene, naphthalene, and triphen-
þ
it is expected that the H -catalyzed nitration of PAH with NO2
can also occur in organic solutions containing trace water. Ac-
tually, a similar nitration path was observed between in solution
and on silica particles.
This paper describes the nitration of PYH by NO2 in aceto-
nitrile, factors (concentration of NO2, addition of HNO3 gas
and H2O, 1-substituent on PYH, and solvent) affecting the ni-
tration, chemical species formed during the nitration, kinetics,
and mechanism. The results are compared with the nitration of
PYH on silica particles.
2,11
12
ylene. Suzuki et al. have reported that the nitration of non-
activated arenes can be promoted by the addition of O3 to N2O4
in polar and nonpolar solvents, and is more accelerated by the
addition of protons and Lewis acids. Compared with the nitra-
tion of PAH by NO2/N2O4 and O3-added NO2/N2O4 in solu-
tion, none has been reported concerning the nitration process,
kinetics, and mechanism when NO2 is aerated in a solution con-
taining PAH.
On the other hand, the reaction between PAH and NO2 has
received considerable interest as a possible route to the forma-
tion of mutagenic nitro-polycyclic aromatic hydrocarbons
Experimental
Materials. PYH and 1-nitropyrene (1-NO PY) were purchased
2
(
(
NPAH) in the gas phase and on suspended particulate matters
3–15
from Wako Pure Chemical Industries, Ltd. and used without fur-
ther purification. 1-Methylpyrene (1-MePY),16 1-methoxypyrene
(1-MeOPY),17 and 1-bromopyrene (1-BrPY)18 were synthesized
SPM) in the atmosphere.1
Nielsen investigated the nitra-
11
tion rates of 25 PAHs in water–methanol–dioxane containing
Published on the web January 13, 2004; DOI 10.1246/bcsj.77.147

1
48 Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
Autocatalytic Nitration of Pyrene by NO2
by the reported methods. Solvents of acetonitrile, methanol,
chloroform, and dichloromethane were used as purchased from
Wako Pure Chemical Industries, Ltd. The water content of aceto-
nitrile was varied by adding water to acetonitrile with a microsyr-
inge.
Aeration period of NO /min
2
0 2
4
6
14 16
29 31 39 41
4
3
.4
3
A cylinder-loaded NO2 gas (100 ppm in N2) was supplied by
Takachiho Chemical Industry Inc. NO2 gas of various concentra-
tions was prepared by mixing certified NO2 gas with N2 gas with
a flow meter. A Saltzman solution determining the concentrations
of NO2 and NO was prepared according to the reported method.19
HNO3 gas (10.3 ppm) was generated by aerating N2 gas at a flow
2
1
0
ꢂ1
rate of 400 mL min through an impinger containing 10 mL of
ꢃ
concentrated H2SO4 and 0.8 g of NaNO3 at 25 C.
Conversion of NO2 in Acetonitrile. NO2 (100 ppm) was
passed through an impinger containing acetonitrile (20 mL) at a
ꢂ1
flow rate of 400 mL min and the chemical species in the solution
and the effluent gas were determined. First, the species were deter-
mined by 2 minutes passing at a set time, and their material balance
was examined. Second, the concentration change of the species
with time was examined. In both experiments, NO2 and NO in
the solution were ejected by passing N2 through the solutions,
and their concentrations were measured by the Saltzman method.19
On the other hand, after 0.2 mL of the sampled solution was diluted
0
10
20
30
40
50
Discharge time of NO /min
2
Fig. 1. Conversion products of NO2 in acetonitrile and their
material balance. : HNO3 + HNO2, : NO2(Insoluble),
:
NO(Insoluble), : NO2 + NO(Soluble), : Total,
NO2] = 100 ppm.
[
ꢂ
with 1.8 mL of ion-exchanged water, the concentrations of NO3
ꢂ
and NO2 were measured with ion chromatography (IC). UV–vis
and Raman spectra of the nitrogenous species formed by aerating
NO2 in acetonitrile were also measured.
man spectroscope (NRS-1000, Ar laser, excited at 514.5 nm).
Results
Nitration of Pyrene by NO2. NO2 of various concentrations
was passed through an impinger containing an acetonitrile solution
Conversion Products of NO2 and Material Balance.
Figure 1 shows the conversion products of NO in acetonitrile
and their material balance. The aeration time was each 2 mi-
ꢂ3
2
of PYH (5.00 mg in 20 mL = 1.24 mmol dm ) at a flow rate of
ꢂ1 ꢃ
4
00 mL min at 11 C. To supply the decrease in the acetonitrile
nutes during the discharge of NO2 (100 ppm). The quantity
of NO2 was calculated to be 3.4 mmol for 2-minute aeration.
NO2 was converted into HNO3, HNO2, and NO by a reaction
solution, a small amount of acetonitrile was added occasionally.
The solution (0.1 mL) was withdrawn with a microsyringe at timed
intervals and diluted to 5 mL with acetonitrile; the decay of PYH
(
Ct=C0) was then measured with HPLC at ꢀ ¼ 234 nm. The nitra-
with trace water (447 ppm) in acetonitrile according to, at least,
which are well known to oc-
2
cur when NO was absorbed in water:
the following three reactions,2
0,21
tion of 1-substituted PYH was also performed by a method similar
to that of PYH. The products were measured with GC/MS after
concentration of the solution.
Effect of HNO3 and HNO2 Gases, Water, Solvent, and 1-
Substituent of PYH on the Nitration. The effect of previous aer-
ation of HNO3 and HNO2 gases, water addition, and 1-substituent
of PYH on the nitration was examined in acetonitrile (20 mL), and
the solvent effect was also studied by varying the solvent. HNO316
and HNO217 gases were generated by the same methods as those
described previously. After a fixed amount of water and HNO3
2
NO2 þ H2O ꢀ HNO3 þ HNO2;
ð1Þ
ð2Þ
ð3Þ
3NO þ H O ꢀ 2HNO þ NO;
NO2 þ NO þ H2O ꢀ 2HNO2:
2
2
3
The electric conductivity of HNO3 was measured after aerating
HNO3 gas (28 ppm) separately in trace water-containing aceto-
nitrile (20 mL) and water (20 mL) for 30 min at a flow rate of
(
10.3 ppm) and HNO2 (11.1 ppm) gases had been added separately,
ꢂ1
4
mS m for acetonitrile and 82.0 mS m for water at 25
00 mL min . The conductivity was estimated to be 3.34
ꢂ1
PYH was nitrated with NO2 (100 ppm) by the same method as that
described above.
ꢂ1
ꢃ
C.This result shows that HNO3 formed was partially dissoci-
þ
Analysis. HPLC was performed by a JASCO HPLC system
equipped with a PU-980 pump, a 970 UV–vis detector, and a
Mightysil RP-18 column. A mixture of CH3CN:H2O = 75:25
was used as a mobile phase. The structures of the products were
examined with a GC/MS (JEOL 700 Q) equipped with a fused-sili-
ca capillary column (HP-5, 30 m long, 0.32 mm i.d.). The column
ated in polar acetonitrile and could afford H continuously as
it was consumed.
Since each sampled aliquot was diluted with water before an
IC measurement, the molar amounts of NO3 and NO2 can be
regarded as being the same as those of HNO3 and HNO2, re-
spectively. The total molar amount of HNO3 and HNO2 is larg-
ꢂ
ꢂ
ꢃ
ꢃ
ꢃ
ꢂ1
temperature was raised from 130 C to 270 C at 5 C min . IC of
ꢂ
ꢂ
er than each molar amount of NO and NO in the effluent gas,
2
while the total molar amount of NO2 and NO in acetonitrile is
NO3 and NO2 was performed by a JASCO HPLC system equip-
ped with a Shodex CD-5 electron conductivity detector and a TSK
Anion-PW column (eluent: IC-Anion-A). The water content in
acetonitrile was measured with a Karl-Fisher meter (Kyoto Elec-
tronics, MKC-510N). Electric conductivity was measured with a
HORIBA D24 electrode. The UV–vis spectrum was taken on a
SHIMADZU 1600 and Raman spectrum on a JASCO micro-Ra-
much smaller than each molar amount of NO2 and NO in the
effluent gas. The material balance is fairly good between the
amount of NO2 (3.4 mmol) in the influent gas and the total
amount of HNO3, HNO2, NO2, and NO in acetonitrile and
the effluent gas.

K. Hasegawa et al.
Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
149
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
1
0.8
0
0
.6
.4
0.2
0
0
60
120
180
240
300
0
30
60
Reaction time/min
Fig. 2. Change in the conversion products of NO2 with aer-
90 120 150 180
Reaction time/min
Fig. 4. Effect of added HNO3 gas on the decay of PYH by
ꢂ3
NO2. [PYH] = 1.24 mmol dm
,
: HNO3 (49 ppm),
:
ation time in acetonitrile. : HNO3, : HNO2, : NO2,
NO, [NO2] = 100 ppm.
:
NO2 (50 ppm), : NO2 (50 ppm) + HNO3 (5 ppm).
curves become nearly parallel regardless of the concentration
of NO2. An attractive point is that there is an induction period,
which is shortened with the increasing concentration of NO2.
The increase in 1-NO2PY also depicts sigmoid curves, which
are opposite to the decay curves of PYH.
1
.8
.6
.4
.2
0
1
0
0
0
0
0.8
As already shown in Fig. 2, HNO3 is accumulated linearly in
acetonitrile with the aeration of NO2. It is expected that the
HNO3 acts as a catalyst and accelerates the nitration of PYH
by NO2. The catalytic effect of acids has been reported in the
reaction of PAH with N2O4 previously added to dichloro-
methane,3 however, nothing has been mentioned about the
autocatalytic behavior.
0.6
0
.4
0.2
0
,6,7
Effect of HNO3 on the Nitration of Pyrene. Figure 4
shows the effect of added HNO3 gas on the decay of PYH by
NO2. The decay rate increases by the addition of HNO3 gas.
The aeration of only HNO3 gas (49 ppm) gave only 0.9% of
1-NO2PY after 300 minutes. These results give a clear demon-
stration that HNO3 never serves as a nitrating species, but a cat-
alyst. The increase in the decay rate can be explained by the in-
creased accumulation of HNO3 in acetonitrile by the addition
of HNO3 gas.
0
60
120
180
240
300
Reaction time/min
Fig. 3. Effect of NO2 concentration on the decay of PYH
and on the increase in 1-NO2PY with time. [PYH] =
ꢂ3
1
.24 mmol dm , [NO2]/ppm:
75,
,
: 100, PYH: Black, 1-NO2PY: White.
: 10,
,
: 50,
,
:
,
Figure 2 shows the change in the conversion products of NO2
in acetonitrile with aeration time. The molar amount of each
product increases in the order: HNO3 > HNO2 ꢄ NO2 >
NO. The amount of HNO3 increases linearly with time, and
the increase becomes slow after 150 minutes, while that of
HNO2 reaches at a constant at 60 minutes. On the other hand,
the molar amount of NO2 and NO increases with time, and is
saturated after about 20 minutes. Hayashi et al.20 reported that
the exposure of a NO2–NO–N2 gas to distilled water rapidly in-
creased the concentrations of NO3 and NO2 , and was fol-
lowed by a slow increase in their concentrations with time.
Nitration of Pyrene by NO2. The nitration product of PYH
was only 1-NO2PY by HPLC and GC/MS measurements.
Figure 3 shows the decay of PYH and the increase in 1-NO2PY
with time by varying the concentration of NO2. The increase in
the concentration of NO2 increases the decay rate. As obviously
shown when NO2 of more than 75 ppm is aerated, the variation
depicts sigmoid curves. At the initial stage, the decay increases
slowly, then abruptly increases and finally slowly approaches to
zero. The initial decay rate increases with the increasing con-
centration of NO2, whereas at the rapid decay stage the decay
Furthermore, to make sure the catalytic effect of HNO3 on
the nitration of PYH, we previously aerated HNO3 gas (10.3
ppm) into acetonitrile and then reacted with NO2 (100 ppm).
Figure 5a shows the effect of the previous aeration time of
HNO3 gas on the decay of PYH by NO2. The ratio (c=a) of
ꢂ3
the initial concentration of HNO3 (c mol dm ) to PYH (a
ꢂ3
mol dm ) is given for each curve. The decay rate increases
with increasing the initial concentration of HNO3. The decay
ꢂ
ꢂ
plots express the sigmoid curves, which are characteristic fea-
The decay rate rapidly in-
tures of autocatalytic reactions.2
2–25
creases after the induction period, which is shortened as the in-
itial concentration of HNO3 increases. A unique feature of this
kinetic behavior is that the decay rate exhibits a maximum
(rmax) along the reaction coordinate (Fig. 5b), which corre-
sponds to the rate at the inflection point of the sigmoid curve.
The autocatalytic decay profile of PYH is similar to the profiles
ꢂ
of the HNO2-autocatalyzed oxidation of ferroin by NO3 in
23
acidic media and the chlorine–thiourea reaction in acidic me-
dia,24 and the nitration of halogen- and carboxyl-substituted
benzenes by N2O5 in aprotic solvents.25

1
50 Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
Autocatalytic Nitration of Pyrene by NO2
[
H SO ]/a = 2
2 4
1
.8
.6
.4
.2
0
1
0.8
0.6
c/a = 0.82
.54
.27
(a)
0
0
0
0
0
0
0
.14
0
0
0
.4
.2
0
0
60
120
180
240
300
0
60
120
Reaction time/min
Fig. 7. Solvent effect on the nitration of PYH by NO2.
180
240
300
Reaction time/min
0
:
CH3CN, : CH3OH, : CH2Cl2, : CHCl3, [PYH] =
1.24 mmol dm , [NO2] = 100 ppm.
-
-
-
-
-
-
0.003
0.006
0.009
0.012
0.015
0.018
ꢂ3
(b)
1
0
0
0
0
.8
.6
.4
.2
0
Fig. 5. Effect of previous aeration time of HNO3 gas and
addition of H2SO4 in place of HNO3 gas on the decay of
PYH by NO2. : 0 min, : 20 min, : 40 min, : 80
ꢂ3
min, ꢅ: 120 min, : H2SO4 (2.48 mmol dm ), [PYH]
ꢂ3
= 1.24 mmol dm , [NO2] = 100 ppm, [HNO3] = 10.3
ppm.
0
60
120
180
240
300
Reaction time/min
1
.8
.6
.4
.2
0
Fig. 8. Substituent effect on the nitration of PYH by NO2.
: 1-MeOPY, : 1-MePY, : PYH, : 1-BrPY, : 1-
0
0
0
0
ꢂ3
NO2PY, [PYH] = 1.24 mmol dm , [NO2] = 100 ppm.
ter. The water content was varied from 180 ppm to 4200 ppm.
All of the decay plots express the sigmoid curves, and the decay
rate decreases with increasing water content.
Effect of the Solvent on Nitration. Figure 7 shows the sol-
vent effect on the nitration of PYH. The decay rate increases
with increasing the dielectric constants (") of the solvents:
CH3CN (37.5) > CH3OH (32.6) > CH2Cl2 (7.77) > CHCl3
0
60
120
180
240
300
Reaction time/min
(
4.8). In a hydrophilic CH3OH solvent as well as CH3CN, nitra-
Fig. 6. Addition effect of water to acetonitrile on the nitra-
tion of PYH by NO2. Water content/ppm: : 4200,
tion was accelerated via the induction period, whereas nitration
did not substantially occur (1-NO2PY = 0.5% at 300 min) in
hydrophobic CHCl3, although it contained water to react with
aerated NO2. These results can be explained by the fact that
in CHCl3 having a much lower dielectric constant than CH3CN,
CH3OH, and CH2Cl2 the dissociation of the accumulated
HNO3 is prevented. The solvent and the HNO3-catalyzed ef-
fects expect that an ionic electrophilic nitration mechanism is
:
: 180, [PYH] = 1.24
3
000,
: 1500,
ꢂ3
: 447,
mmol dm , [NO2] = 100 ppm.
Figure 5a also shows the addition effect of concentrated sul-
furic acid on the decay of PYH. PYH did not decrease exponen-
tially with time, even when an excess amount of H2SO4
26
5–7
(
[H2SO4]/a ¼ 2), stronger acid than HNO3, was used. Millen
more probable than a radical nitration mechanism.
reported that N2O4 in concentrated sulfuric acid was ionized to
Effect of 1-Substituents of Pyrene on Nitration. The ni-
tratin of 1-substituted PYH by NO2 was also investigated under
the same conditions as those of PYH. Figure 8 shows that the
decay plots for PY, 1-MePY and 1-MeOPY express the sig-
moid curves, except for the slow decay of 1-NO2PY and 1-
þ
þ
produce NO2 and NO ions.
Effect of Water on Nitration. Figure 6 shows the addition
effect of water to acetonitrile on the nitration of PYH. Commer-
cially available acetonitrile was found to contain 447 ppm wa-

K. Hasegawa et al.
Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
151
BrPY. The decay rate increases in the order: 1-MeOPY > 1-
MePY > PYH > 1-BrPY > 1-NO2PY. This result shows that
the electron-donating groups promote the nitration rate, where-
as the electron-withdrawing groups retard it. The substituent ef-
fect is a typical feature for an electrophilic reaction, such as an
NO2 attack on substituted benzene. From a GC–MS measure-
ment, two mono nitro derivatives were obtained from 1-MeO-
PY and three nitro derivatives were formed from 1-MePY and
0
1
2
3
4
5
-
-
-
-
-
þ
2
.9
4
.5
1
-BrPY. Although we could not determine the substituted posi-
7
.6
5
.7
tion of a nitro group because of the lack of authentic chemicals
and also the difficult separation of the isomers by TLC, the ni-
tration positions for 1-MePY and 1-BrPY were estimated to be
c/a= 10.1
3
, 6, and 8, but for 1-MeOPY, only the positions of 6 and 8 were
0
20
40
60
80
100
expected to be possible from a calculation of the HOMO elec-
tron density of carbon atoms of 1-substituted pyrenes using the
LCAO method.
Reaction time/min
Fig. 10. Plot of lnðCt=C0Þ versus time for the nitration of
PYH by NO2 in the presence of excess HNO3. [PYH] =
ꢂ3
Discussion
1.24 mmol dm , [NO ] = 100 ppm, [HNO ] = 3.6–12.5
2 3
ꢂ3
mmol dm
.
Proton Autocatalyzed Nitration of Pyrene. A reaction is
autocatalytic if one of its products increases the rate of the re-
23
mation rate of HNO3 increases in the order: path A > path C >
path B and that via path A is 3.2-times larger than that of path
C.
action. For example, an autocatalyst can accelerate its own
production. In this case, some product must be initially present,
since otherwise the reaction will never start. In the nitration of
þ
þ
Kinetics. As qualitatively shown in Figs. 3 and 5, increas-
ing the concentration of either NO2 or HNO3 increases the de-
cay rate of PYH, namely the formation rate of 1-NO2PY. How-
ever, a regular determination of the partial kinetic order with
the reactants is not possible under the sigmoid decay of PYH.
To simplify the kinetics, the effect of the reactants on the rate
was investigated under an excess of one reactant, that is, under
a pseudo zero-order condition. Figure 10 shows a plot of
lnðC =C Þ versus the reaction time for the nitration of PYH
PYH by NO2, although H is not present initially, H that was
formed by the dissociation of HNO3 accumulated by aerating
NO2 (path A) starts the reaction and accelerates the nitration
þ
by collaborating with the H that was released with the forma-
tion of 1-NO2PY (path B). The previous aeration of HNO3 gas
(path C) shortens the induction period. Accordingly, the proton
autocatalyzed nitration of PYH can be expressed as follows:
þ
H
þ
t
0
PYH þ NO2 ꢂꢂꢂꢂꢂꢂꢂꢂ! PY{NO2 þ H :
Paths A, B, and C
ð4Þ
by NO2 in the presence of excess HNO3. Even in the case of
3
using 10.1-times HNO to PYH, a short induction period is ob-
served. All of the decay curves obey a linear relationship be-
Before considering the nitration kinetics of PYH, the amount
of HNO3 accumulated with time was estimated. Figure 9 shows
the time course of the amount of HNO3 accumulated by paths
A, B, and C. The amount of HNO3 via paths A and C increases
linearly, whereas that via path B increases sigmoidally. The for-
tween lnðCt=C0Þ and the reaction time after the induction peri-
od. This result suggests that after an excess amount of the ni-
trating species was accumulated by aerating NO2, PAH de-
cayed according to first-order kinetics under aerating NO2.
þ
Next, to elucidate the order of H , the initial concentration of
excess HNO3 was varied, and then the relation between kobs
and the concentration of HNO3 was examined after the induc-
tion period. As shown in Fig. 11, a linear relationship having
5
4
3
2
1
0
þ
slope k is found, also suggesting first order for H . Further-
more, as shown in Fig. 3, the decay of PYH is almost irrespec-
tive of the concentration of NO2 after the induction period.
Therefore, after the induction period the decay rate of PYH
can be expressed as
þ
ꢂ dx=dt ¼ k½PYHꢆ½H ꢆ ¼ kða ꢂ xÞðc þ x þ yÞ;
ð5Þ
where a is the initial concentration of PYH, x is the concentra-
tion of PYH that has reacted after t minutes, y is the concentra-
0
30
60
90
120 150 180
þ
tion of H that has accumulated by the aeration of NO2 for t
þ
Reaction time/min
minutes, and c is the concentration of H that is present by
the previous aeration of HNO3. When HNO3 gas is not previ-
ously aerated, c may be set as zero in Eq. 5. When c is smaller
than a, the decay rate slowly increases, and ultimately c þ x þ
y can be approximated by x þ y as x þ y increases beyond c.
Eq. 5 is expressed as
Fig. 9. Time course of the concentration of HNO3 formed
by Paths A, B, and C. : Path A, : Path B, : Path C,
Path A: [NO2] = 100 ppm, Path B: [PYH] = 1.24
mmol dm , [NO2] = 100 ppm, Path C: [HNO3] = 10.3
ppm.
ꢂ3

1
52 Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
Autocatalytic Nitration of Pyrene by NO2
tiating Eq. 5 was unsuccessful because xc (xc ¼ ða ꢂ c ꢂ yÞ=2)
involves y, which is a function of t. Alternatively, xc was esti-
mated from the value of Ct=C0, which corresponds to ða ꢂ xÞ=a,
showing the rmax of each curve in Fig. 5b. Next, the induction
period (ti) of PYH was estimated by drawing a line tangent to
each curve at xc. Then, the total concentration of HNO3
0
0
0
0
0
0
0
.30
.25
.20
.15
.10
.05
.00
(
c þ xi þ yi) at the induction periods was evaluated from
Fig. 9. As summarized in Table 1, it is found that the decay
of PYH is accelerated when c þ xi þ yi attained the initial con-
centration of PYH, a. In the HNO2-autocatalyzed oxidation of
ferroin by NO3 in acidic media, it has been reported that if
the initial concentration of HNO2 is compatible with that of fer-
roin, the autocatalytic behavior disappears.
ꢂ
23
2
4
6
8
10
12
14
3
Eberson et al. studied the relative reactivity of six PAH with
[
HNO
3
]
0
/mmol dm-3
N2O4 in the presence or absence of a catalytic amount of meth-
anesulfonic acid in CH2Cl2, and noted that the increase in the
yield of NPAH obeyed approximately a second-order kinetics
in the absence of the acid. Nielsen11 reported that the nitration
of PAH with N2O4 in excess of HNO3 follows a pseudo first-or-
der kinetics to the concentration of PAH. The kinetics of the au-
tocatalytic nitration of PYH is different from those suggested
Fig. 11. Plot of kobs versus initial concentration of HNO3.
PYH]
mmol dm
ꢂ3
[
=
ꢂ3
1.24 mmol dm
,
[HNO3]0
= 3.6–12.5
.
ꢂ dx=dt ¼ kða ꢂ xÞðx þ yÞ:
ð6Þ
3
11
The decay rate becomes independent of c. We can reasonably
explain that the shapes of four curves in Fig. 5a (c=a ¼ 0,
by Eberson et al. and Nielsen. This result would arise mainly
from the difference of previously added N2O4 and aerated NO2.
Nitration Mechanism. Before discussing a nitration mech-
anism, the nitrogenous species formed by aerating NO2 in ace-
tonitrile was examined spectrophotometrically. Figure 12
0
.14, 0.27, 0.54, and 0.82) are similar, except for the induction
period.
An attempt to estimate each inflection point (xc) by differen-
Table 1. Effect of HNO3 Formed by Paths A, B, and C on the Nitration of PYH
ꢂ3
HNO3/mmol dm
PYH
HNO3
ꢂ3
ꢂ3
aÞ
ꢂ3
bÞ
dÞ
cÞ
a/mmol dm
aeration time/min c/mmol dm
c=a xc /mmol dm
ti /min
xi
yi
c þ xi þ yi ðc þ xi þ yiÞ=a
1
1
1
1
1
.24
.24
.24
.24
.24
0
20
0
0
0.48
0.46
0.45
0.42
0.40
47
35
28
18
8
0.060
0.040
0.024
0.016
0.007
1.25
0.96
0.80
0.50
0.21
1.31
1.17
1.16
1.19
1.23
1.06
0.95
0.94
0.96
0.99
0.169
0.337
0.674
1.011
0.14
0.27
0.54
0.82
40
80
120
a) Concentration of PYH at inflection point. b) Induction period. c) and d) Concentration of HNO3 formed by Paths A and B at induc-
tion period, respectively.
Fig. 12. UV–vis spectra of the nitrogenous species (curve a) formed by aerating NO2 in acetonitrile together with those of NO2
(
curve b) and N2O4 (curve c). [NO2] = 100 ppm, aeration time = 60 min.

K. Hasegawa et al.
Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
153
shows the UV–vis spectra of the nitrogenous species (curve a)
together with those of NO2
tionary state.
Previous proposals concerning the mechanism of the N2O4
27,28
(ꢀmax ¼ 410 nm, curve b) and
27
þ þ
N2O4 (ꢀmax ¼ 340 nm, curve c). In hexane, NO2 also repre-
sented the same absorption maximum at 410 nm and an equili-
brium mixture of NO2 and N2O4 represented an isosbestic point
nitration of reactive aromatics (ArH) include NO2 or H N2O4
3,30,33–35
mediated nitration in strongly acidic media
þ
(Eqs. 7 and
8), where NO2 is formed through the equilibrium of Eq. 7:
29
at 357 nm. Fine structure appears in the spectrum of NO2,
whereas no fine structure appears in the spectrum of N2O4.2
The spectrum of the nitrogenous species shows fine structure
þ
þ
þ
H þ N2O4 ꢀ H N2O4 ꢀ NO2 þ HNO2;
ð7Þ
7–29
þ
þ
ArH þ H N2O4 ! Ar NO2 þ H þ HNO2;
(
ꢀ ¼ 337, 347, 358, 372, and 388 nm) at wavelengths slightly
ArH þ NO2þ ! ArNO2 þ Hþ:
ð8Þ
longer than that of N2O4 (ꢀmax ¼ 340 nm) and no absorption
due to NO2. Aeration of NO2 (100 ppm) in acetonitrile (2.48
To confirm the effect of HNO2 on the formation of PYH, the
nitration of PYH by NO2 was performed in acetonitrile sepa-
rately aerated HNO2 gas (11.1 ppm) and added urea (2.48
ꢂ3
mmol dm ) added concentrated sulfuric acid gave the same
spectrum as curve a. The addition of concentrated nitric acid
(1.24 mmol dm
mmol dm ) in acetonitrile, in which NO2 should be formed,
ꢂ3
ꢂ3
30
)
to concentrated sulfuric acid (2.48
þ
mmol dm ) as a HNO2 removing reagent. As shown in
Fig. 14, the decay rate of PYH decreased with increasing con-
centration of HNO2, and slightly increased as HNO2 was re-
moved, showing that HNO2 functions in the equilibrium illus-
ꢂ3
gave no absorption at wavelengths longer than 320 nm, show-
þ
ing that the observed fine structure is not due to NO2 . Based
on these results, it seems reasonable that the species are com-
3
trated in Eq. 7. Since N2O4 is too weak an electrophile, proton-
ation is expected to form a much more effective electrophilic
attacking species,
on the decay of PYH (Fig. 6) can be explained by the idea that
þ
3,30
posed of H N2O4,
acidic condition, and its dissociation product, NO2 , although
which is in equilibrium with N2O4 under
þ
3,34,35
þ
H N2O4, for PYH. The effect of water
the existence was not proved by Raman spectral measure-
þ
2
6,31
ꢂ1
ment
because a line due to NO2 (ꢁN{O ¼ 1400 cm
)
the formed HNO3 is diluted with excess water to decrease the
þ
ꢂ1
overlapped with a strong line (1390 cm ) due to acetonitrile.
Figure 13 depicts the change in the absorbance at 372 nm with
the aeration time of NO2 under the addition and no addition of
H2SO4. The concentration of H N2O4 gradually increases with
time, and saturates in both cases, showing that the formation
rate of H N2O4 is controlled by the amount of aerated NO2.
This result supports that after trace water in acetonitrile is con-
sumed by aerated NO2, NO2 is rapidly dimerized,
concentration of H , and eventually to suppress the formation
þ
of H N2O4.
Two possible nitration mechanisms are suggested in
þ
þ
þ
Scheme 1. An electrophilic attack of NO2 or H N2O4 on
the C-1 position of PYH forms a ꢂ-complex, followed by the
formation of 1-NO2PY along with releasing H . The H acts
þ
þ
þ
as an autocatalyst. Eberson et al.34 reported that at the low acid
29,32
and the
þ
concentration of H N2O4 is accumulated to reach a nearly sta-
1
0
0
0
0
0
0
.06
.05
.04
.03
.02
.01
0
0
0
0
0
.8
.6
.4
.2
0
0
60 120 180 240 300
Reaction time/min
0
50
100
150
200
Fig. 14. Effect of previous aeration time of HNO2 gas and
addition of urea on the decay of PYH by NO2. : 0 min,
: 20 min, : 80 min, : 120 min, : [urea] = 2.48
mmol dm , [PYH] = 1.24 mmol dm , [NO2] = 100
ppm, [HNO2] = 11.1 ppm.
Aeration time/min
Fig. 13. Change in the absorbance at 372 nm with aeration
time of NO2 under addition ( ) and no addition ( ) of
H2SO4. [NO2] = 100 ppm, acetonitrile = 20 mL.
ꢂ3
ꢂ3
+
PYH + NO2
Slow
fast
+
+
PY H-NO2
1-NO PY + H
2
+
PYH + H N O
−HNO , Slow σ-complex
2
4
2
Scheme 1. Two possible nitration mechanisms.

1
54 Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
Autocatalytic Nitration of Pyrene by NO2
þ þ þ
the NO attacks NO2/N2O4 to form NONO2 /NON2O4 as
concentration caused by the hydrolysis of N2O4 by trace water
in polar solvents, a weakly electrophilic reagent, H N2O4, can
þ
17,18
electrophiles.
The results of the proton autocatalytic nitra-
be formed by the protonation of N2O4, which is responsible for
the remarkably mild, efficient and selective nitration of PAH by
N2O4. When an excess amount of H2SO4 ([H2SO4]/a ¼ 2) was
used, as shown in Fig. 5a, the decay curve of PYH fell between
two curves of c=a ¼ 2:9 and 4.5 in Fig. 10. This result shows
that H2SO4, a stronger acid than HNO3, is more potent than
HNO3 for intensifying the autocatalysis. In Eq. 7, an increased
tion of PYH in solutions suggest that the atmospheric nitration
of PAH by NO2 would be promoted on the SPM adsorbed at-
mospheric HNO3 and H2SO4 mists.
þ
In conclusion, we were able to explain the kinetics of the H
autocatalytic nitration of PYH by aerating NO2, and suggested
þ
an ionic electrophilic substitution mechanism involving NO2
as an electrophile. The autocatalytic nitration is the first exam-
ple found for the nitration of PAH carried out in organic solu-
tions containing NO2/N2O4 with or without acids.
þ
þ
concentration of H promotes the dissociation of H N2O4, and
þ
would accelerate the reaction of NO2 with PYH. Considering
the effects of HNO2 and urea on the nitration of PYH (Fig. 14)
þ
and the promotion of the nitration after the accumulation of H ,
þ
References
we consider that the nitration mechanism through NO2 is
more probable. Based on the substituent effect (Fig. 8), an elec-
1 E. D. Hughes, C. K. Ingold, and R. I. Reed, Nature, 158,
448 (1946).
þ
trophilic attack of NO2 on PYH can be regarded as the rate-
determining step.
2
W. A. Pryor, G. J. Gleicher, J. P. Cosgrove, and D. F.
Church, J. Org. Chem., 49, 5189 (1984).
The presence of an induction period for the nitration of PYH
by NO2 can be explained by the accumulation of N2O4 after the
formation of HNO3 (Eqs. 1–3) and the pre-equilibrium generat-
3 L. Eberson and F. Ladner, Acta Chem. Scand., Ser. B, 39,
43 (1985).
3
þ
4
5
L. Eberson and F. Ladner, Acc. Chem. Res., 20, 53 (1987).
G. L. Squadrito, D. F. Church, and W. A. Pryor, J. Am.
ing NO2 , that is, the protonation of N2O4 and the dissociation
þ
25
of H N2O4 (Eq. 7). Ingold et al. studied an autocatalyzed ni-
tration of halogen- and carboxyl-substituted benzenes by N2O5
Chem. Soc., 109, 6535 (1987).
G. L. Squadrito, F. R. Fronczek, D. F. Church, and W. A.
Pryor, J. Org. Chem., 54, 548 (1989).
G. L. Squadrito, F. R. Fronczek, D. F. Church, and W. A.
Pryor, J. Org. Chem., 55, 2616 (1990).
þ
6
in CCl4, and concluded that NO2 arising from the ionizing ac-
tion of HNO3 produced during the nitration is the nitrating spe-
cies. They suggested the pre-equilibrium that NO2 is progres-
sively freed by the solvating HNO3 from the influence of anions
7
þ
8
9
E. Bosch and J. K. Kochi, J. Org. Chem., 59, 3314 (1994).
K. Furukawa, F. Yoshizawa, Y. Akutsu, M. Arai, and M.
þ
ꢂ
(
N2O5 + (x þ y)HNO3 ꢀ NO2 (xHNO3) + NO3 (yHNO3)).
It is known that the nitration mechanism of PAH with NO2/
Tamura, Kayaku Gakkaishi, 59, 160 (1998).
N2O4 depends on the polarity2,3,5,6,9 and dielectric constant of
solvents. In CCl4, the exclusive pathway is a free-radical,5
while in CH2Cl2, the ionic reaction pathway predomi-
7
10
76.
B. G. Gowenlock, B. King, and J. Pfab, J. Chem. Res., 2000,
–7
2
1
1
1
2
T. Nielsen, Environ. Sci. Technol., 18, 157 (1984).
T. Mori and H. Suzuki, Synlett, 1995, 383.
2
,3,5,7,8
3,6,7
nates
and is subject to efficient acid catalysis.
Pryor
et al. suggested that the more easily ionized PAH undergo ni-
2
13 T. Ramdahl, A. Bjꢀrseth, D. M. Lokensgard, and J. N. Pitts,
tration of the ET mechanism by NO2/N2O4 in CH2Cl2. The ET
Jr., Chemosphere, 13, 527 (1984).
14 C. H. Wu and H. Niki, Environ. Sci. Technol., 19, 1089
(1985).
15 K. Inazu, T. Kobayashi, and Y. Hisamatsu, Chemosphere,
35, 607 (1997).
11
mechanism was also suggested for the reaction of PAH and
alkylbenzenes with NO2/N2O4 in CH2Cl2. Eberson and Lad-
8,9
3,4
ner pointed out that the mechanism of NO2/N2O4 nitration is
fairly complex, and more work is needed to establish its mech-
anism with any degree of certainty. Compared with the nitra-
16 H. Wang, K. Hasegawa, and S. Kagaya, Chemosphere, 39,
1923 (1999).
þ
tion mechanisms described above, the H autocatalyzed nitra-
tion mechanism proposed by us is the first found for the nitra-
tion of PAH using NO2 as a nitrating reagent.
1
7
H. Wang, K. Hasegawa, and S. Kagaya, J. Environ. Sci.
Health, Part A, 35, 765 (2000).
H. Wang, K. Hasegawa, and S. Kagaya, Chemosphere, 41,
479 (2000).
1
8
Comparison with the Nitration on Silica Particles. We
have previously reported that heterogeneous nitration of PYH
with NO2 is reasonably explained by the facts that dissociated
1
1
2
9
0
B. E. Saltzman, Anal. Chem., 32, 135 (1960).
Y. Hayashi, T. Kawanishi, N. Shimizu, Y. Fujiwara, and T.
þ
H of HNO3, accumulated by the reaction of NO2 with the ad-
sorbed water on silica particles, acts as an autocatalyst and a re-
Nakase, J. Jpn. Soc. Atmos. Environ., 31, 75 (1996).
Y. N. Lee and S. E. Schwartz, J. Phys. Chem., 85, 840
21
1981).
þ
þ
sultant HNO2 /HN2O4 attack at a carbon atom having a high
(
16
electron density. The decrease in PYH proceeded along sig-
moid curves, however, we could not kinetically explain the ni-
tration process. In the nitration of PYH in acetonitrile, the ef-
fects of the NO2 and the added HNO3 concentrations were sim-
ilar to those on silica gel, however, both nitration mechanisms
appear to be different because the nitration on silica gel was ac-
celerated by coexisting HNO2 gas (6 ppm/N2) in NO2 (6 ppm/
N2). In the presence of HNO2 gas, we suggested another mech-
2
2
D. Lavabre, V. Pimienta, G. Levy, and J. C. Micheau, J.
Phys. Chem., 97, 5321 (1993).
G. Bazsa, I. P. Nagy, I. Lengyel, and D. A. Franz, J. Chem.
Educ., 68, 863 (1991).
R. Irving, K. Kustin, and R. H. Simoyi, J. Phys. Chem., 96,
2
3
2
4
5
852 (1992).
25 V. Gold, E. D. Hughes, C. K. Ingold, and G. H. Williams, J.
Chem. Soc., 1950, 2452.
26 D. J. Millen, J. Chem. Soc., 1950, 2600.
27 T. C. Hall, Jr. and F. E. Blacet, J. Chem. Phys., 20, 1745
þ
anism, that H dissociated from accumulated HNO3 trans-
þ
formed HNO2 into NO (HNO2 + H ꢀ NOþ + H2O) and
þ

K. Hasegawa et al.
Bull. Chem. Soc. Jpn., 77, No. 1 (2004)
Chem. Soc., Faraday Trans., 91, 1439 (1995).
155
(
1952).
S. Voigt, J. Orphal, and J. P. Burrows, J. Photochem.
Photobiol., A: Chem., 149, 1 (2002).
W. A. Pryor, L. Castle, and D. F. Church, J. Am. Chem.
Soc., 107, 211 (1985).
R. O. C. Norman, W. J. E. Parr, and C. B. Thomas, J. Chem.
Soc., Perkin Trans. 1, 1974, 369.
H. G. M. Edwards, J. M. C. Turner, and V. Fawcett, J.
2
8
32 T. F. Redmond and B. W. Bradford, J. Phys. Chem., 72,
1626 (1968).
2
9
33 L. A. Pinck, J. Am. Chem. Soc., 49, 2536 (1927).
34 L. Eberson and F. Ladner, Acta Chem. Scand., Ser. B, 39,
357 (1985).
3
0
35 F. Effenberger, W. Kurtz, and P. Fischer, Chem. Ber., 107,
1285 (1974).
3
1