The NO3 radical-mediated liquid phase nitration of phenols with nitrogen dioxide

Article abstract of DOI:10.1021/es990844m

Analysis of Kyodai nitration, which refers to the mixture of NO₂ and O₃ in nonprotic solvents, may contribute to an understanding of the atmosphere because it can be used in the study of the nitration of aromatic compounds with NO

Full text of DOI:10.1021/es990844m

Environ. Sci. Technol. 2000, 34, 2224-2230
(NO2+). The nitrate radical in gas-phase tropospheric chem-
istry plays an important role as the dominant night-time
oxidizing radical. Its phase transfer into aqueous solutions,
which in the troposphere are represented by the droplets of
clouds, fog, and rain, has been discussed during the past
decade (20) and has most recently been investigated by means
of the wetted-wall reactor technique (21). The results obtained
from this study indicate that the nitrate radical phase transfer
is not limited by the kinetics of phase transfer (reactive
uptake). Further data suggest that because of the poor
solubility of NO3 in water its removal from the gas phase is
only important if NO3 is removed quickly from the equilib-
rium, because of a fast reaction in water (22). Hence, chemical
reactions of the nitrate radical in aqueous solution have been
studied intensively by laboratory techniques (23) and may
be responsible for the formation of nitro compounds as well.
NO3 in aqueous solution may undergo reactions with
aromatic compounds which have been shown to proceed
faster than the corresponding gas-phase reactions by orders
of magnitude (24). From phenol, a mixture of o- and
p-nitrophenol is formed (25). The corresponding reaction in
gas phase has been shown to give essentially the ortho-isomer
(26). Hence, the ortho/ para orientation ratio in the nitration
of phenol could be a useful tool for distinguishing among
these mechanisms.
The NO Radical-Mediated Liquid
3
Phase Nitration of Phenols with
Nitrogen Dioxide
B A R B A R A B A R L E T T A ,
E Z I O B O L Z A C C H I N I , S I M O N E M E I N A R D I ,
M A R C O O R L A N D I , A N D
B R U N O R I N D O N E *
Department of Environmental Sciences, University of
Milano-Bicocca, Piazza della Scienza, 1, I-20126, Italy
Phenols are transformed into nitrophenols by the combined
action of ozonized oxygen and nitrogen dioxide (Kyodai
nitration) in different solvents. Dipole moments of the solvents
were correlated with the ortho/para nitration ratio with
phenol, showing an influence of the polarity of the solvent
on the ortho/para nitration selectivity. A Hammett F of
-0.72 was obtained in the Kyodai nitration of 4-substituted
phenols. These facts suggest the formation of a charge-
transfer complex which evolves to the addition of NO to
3
Previous work shows that the strong electron donor
inductive effect of a phenolic group on an aromatic ring
results in a prevalently ortho/para orientation for the elec-
trophilic attack. Nitration with NO2+ gives a slight preference
for the para position (27).
phenols followed by elimination of nitric acid to give the final
nitrophenols. The position of the nitro group in nitrophenols
found in air samples may be indicative of a gas- or a liquid-
phase nitration occurring in the troposphere.
A good model for the study of the reactivity of the nitrate
radical NO3 with phenols is the use of a mixture of nitrogen
dioxide and ozone in nonprotic solvents (the Kyodai nitra-
tion). This system generates the nitrate radical NO3 which
is in equilibrium with N2O5. Dinitrogen pentaoxide may also
form the nitronium ion NO2+ and nitrate NO3- according to
the following equations:
Introduction
Although several hundreds of organic molecules have been
identified or tentatively identified in urban rainwater, at least
an equal to or greater number of compounds were detected
but not identified (1).
Aromatic compounds are important constituents of
gasoline, automobile exaust, and ambient air measurements
in tunnels and urban atmospheres (2). Some aromatic
compounds such as toluene are also important industrial
solvents. In the atmosphere aromatic compounds react with
OH radicals (3-5) leading to the formation of phenol, cresols,
and dimethylphenols in 10-25% yields (6, 7) which may be
transferred to the atmospheric aqueous phase. Phenols form
nitrophenols, which have been observed in the gas and
particle phase, fogwater, rainwater, snow, and in clouds (8-
11).
The phytotoxicity of nitro- and dinitrophenols is well
documented (12, 13) and with dinitrophenols is strongly
dependent on the position of the nitro groups (14).
Nitrophenols may be formed by tropospheric gas-phase
reactions (15), or they may be formed within the tropospheric
aqueous phase. It has been observed that the decay rate for
N2O5 is 4 × 10-5 - 10-3 s-1 in the cap cloud of the Great Dun
Fell (16) and the hydrolysis of N2O5 at relative humidity above
80% was found to be a main source of nitrate in cloud (17).
If only a minor portion of this N2O5 reacts with aromatic
compounds, then this corresponds to an important source
of nitroaromatics.
NO2 + O3 f NO3 + O2
NO3 + NO2 h N2O5
-
N2O5 h NO2+ + NO3
The Kyodai nitration has been used for the nitration of
acetanilides, alkylbenzenes, halogenobenzenes, ketones,
acetals, acylals, benzoic acid derivatives, benzene, carboxylic
acid salts in solid phase, adamantane, and styrenes (28). The
study of the Kyodai nitration could be relevant for the
atmosphere since it allows for the study of the nitration of
aromatic compounds with NO3 under nonacidic conditions.
Here we report the Kyodai nitration of phenols to give
potentially phytotoxic nitrophenols.
Experimental Section
Reagents. All reagents and solvents used were commercial
products reagent grade. Dichloromethane and trichlo-
romethane were dried by distillation over calcium chloride.
Acetonitrile and tetrahydrofuran were HPLC grade. Nitrogen
dioxide and oxygen were Air Liquide products.
Instrum ents. An ozone generator Ozone Electronics was
used, producing ozone at a rate of 15 mg/ L, with an oxygen
flow of 30 L/ h under an applied voltage of 70 V. Ozone
production was measured monitoring the absorbance at 260
nm. The HPLC system pump was a Waters 600E, and the
Diode Array Detector used was a HP-1040A. The stainless
These processes may also be driven by reactions of the
nitrate radical NO3 in gas phase (18) or after transfer to the
liquid phase (19) or by the reaction of the nitronium cation
* Corresponding author phone: +39 02 64474302; fax: +39 02
64474300; e-mail: Bruno.Rindone@unimi.it.
9
2 2 2 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000
10.1021/es990844m CCC: $19.00
2000 Am erican Chem ical Society
Published on Web 04/29/2000

TABLE 1. Reaction Yields from Phenol 1 and 4-Methylphenol 7 in Different Conditionsc
conversion of
phenol 1 (%)
reaction products
isomer ratio
ortho/para (%)
conversion of
4-methylphenol 7 (%)
reaction products
from 4-methylphenol 7 (%)
reagent
HNO3/H2O
HNO3/H2SO4/H2O
HNO3/CH2Cl2
NO2-O3/CHCl3
NO2-O3/CH2Cl2
NO2-O3/THF
NO2-O3/CH3NO2
NO2-O3/CH3CN
from phenol 1 (%)a,b
66 ( 14
94 ( 7
74 ( 2
44 ( 7
54 ( 8
39 ( 8
89 ( 6
83 ( 9
2: 26; 3: 34; 4: 6
2: 28; 3: 58; 4:6
2: 42; 3: 48; 4: 7; 5: 3
2: 18; 3: 24; 4:2
2: 23; 3: 28; 4: 2
2: 17; 3: 21; 4: 2
2: 46; 3: 43
0.76 ( 0.10
0.78 ( 0.15
0.88 ( 0.40
0.72 ( 0.10
0.81 ( 0.08
0.85 ( 0.08
1.06 ( 0.08
1.20 ( 0.10
100
100
99 ( 1
79 ( 5
72 ( 2
8: 83; 9: 11; 10: 4; 11: tr
8: 90; 9: 5; 10: 4; 11: tr
8: 86; 9: 13
8: 43; 9: 16
8: 38; 9: 8
2: 45; 3: 36; 4: 1
a
b
c
No m -nitrophenol was detected. Dinitrophenols 5 and 6 are som etim es form ed, and their yield is ca. 0.5%. Five runs were perform ed for
each reaction.
SCHEME 1
steel column was an Alltima RP C18, 250 mm length, 4.6 mm
internal diameter, and 5 µm particle size (100 Å). Isocratic
elution was carried out using a two solution mixture: solution
A/ solution B ) 55:45 where A ) 90% phosphate buffer (0.05
M KH2PO4) and 10% CH3CN, while B ) 25% phosphate buffer
(0.05 M KH2PO4) and 75% CH3CN. The flow was 1 mL/ min.
The Diode Array Detector (DAD) was used monitoring the
wavelength at 230 nm.
Results and Discussion
The Kyoda i Nitration of Phenol. Phenol 1 gave with the
Kyodai reaction a mixture of mono- and polynitrophenols
with moderate to high conversion and high yields (Table 1).
The reactions were carried out in different solvent at 0 °C
and stopped after 1 min to avoid polynitration. o-Nitrophenol
2 and p-nitrophenol 3 (Scheme 1) were major reaction
products, together with small amounts of p-benzoquinone
4, 2,4-dinitrophenol 5, and 2,6-dinitrophenol 6. The o/p-
nitrophenol ratio was different from that observed in
electrophilic nitrations with HNO3.
Some other solvents were tested but gave poor results;
2-chloro-2-methylpropane and 2-chlorobutane gave very low
yields and formation of secondary products. In methanol
there was no reaction also at room temperature and with a
higher reaction time. In ethyl acetate and diethyl ether it was
not possible to obtain a good reproducibility of the ortho/
para selectivity. Poor solubility of the reactant gases and the
volatility of the solvent are some of the reasons of the failure
to perform the Kyodai nitration in these solvents.
Kyoda i Nitrations. All the reactions were carried out in
five runs at 0 °C with five different concentrations of sub-
strate, in a range varying from 4 × 10-3 M to 10-2 M, in a
three-necked flask. One neck was fitted with a gas inlet tube
(Y shaped), and the flow was maintained very low to
avoid the stripping of solvents. The flow rate of the gases
was 30 L/ h for the mixture oxygen/ ozone and 0.9 L/ h for
nitrogen dioxide: in these conditions the molar ratio O3:
NO2:phenol was 1:0.6:1. Another neck was fitted with a tube
used to quench the reactions by bubbling nitrogen after 1
min, in such a way the unreacted gases were stripped out the
solution.
The Kyoda i Nitration of 4-Methylphenol 7. 4-Meth-
ylphenol 7 gave with the Kyodai reaction in the same
conditions 2-nitro-4-methylphenol 8 and 2,6-dinitro-4-
methylphenol 9 (Scheme 2). Nitration occurred only in
position ortho because of the presence of the methyl group
in position para. Table 1 shows the quantitative results thus
Product Studies. All the reactions products were identified
by by HPLC-DAD in comparison with authentic reference
compounds. The quantitative analysis was performed with
the aid of a calibration curve.
Kinetic Experim ents. The solution were prepared by
dissolving 0.72 mmol of phenol and 0.72 mmol of a second
phenolic substrate in 30 mL of acetonitrile. The NO2/ O3
mixture was then bubbled under the liquid surface. Samples
were withdrawn after 30 s at 0 °C, then flushed with nitrogen
in order to strip the gaseous reactants, and immediately
analyzed by HPLC.
+
obtained. The electrophilic nitration with NO2 gave again
compounds 8 and 9, but also Pummerer’s ketone 10 and the
dimer 11 were obtained under these conditions. Conversions
were generally higher than those observed with phenol 1.
Solvent Effect in the Kyoda i Nitration of Phenol 1. The
results of the Kyodai nitration of phenol 1 in nitromethane
or acetonitrile were different from those obtained in chlo-
roform, methylene chloride, tetrahydrofuran, or in the
electrophilic nitration with NO2+. In the latter conditions the
ortho/para ratio was 0.7-0.8. No interconversion among the
three isomeric nitrophenols was noticed in these experiments.
The ortho/para ratio in the Kyodai nitration of phenol 1
was plotted vs the dipole moment of the reaction solvent
(Figure 1). A linear correlation with a determination coef-
ficient of 0.977 was obtained showing that the ortho/ para
ratio increases when increasing the polarity of the solvent.
This allowed for calculating the ortho/para ratio of 0.86 for
the Kyodai nitration in water.
Electrophilic Nitration. Reactions with nitric acid were
performed in a two-necked flask, under strong magnetic
stirring, by dropping a water solution of nitric acid in the
aqueous solution containing the substrate. All the reactions
were thermally controlled to 0 °C. The reactions were carried
out in triplicate at three different concentrations of phenol
varying in a range between 2.5 and 2.5 × 10-2 M. Also the
concentration of the acidic solutions varied in a range
between 14.4 and 2.9 M. Solutions were prepared in such a
way that the ratio NO2+/ phenol varied in a range between
100:1 and 1:1. After 1 h a sample was injected in HPLC after
dilution.
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VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 2 2 5

formed which will then add NO2 to give the dihydroben-
zenes 22-28. Elimination of HNO3 will form the final
nitrophenols.
SCHEME 2
(2) NO2. Also the nitration of phenols with NO2 is believed
to occur ultimately through the formation of a phenoxy
radical 17 followed by addition of NO2 leading to nitrocy-
clohexadienones, which rearrange to nitrophenols (33). This
reaction is of practical importance since NO2 may be formed
in the photolysis of nitrate anion in advanced oxidation
processes (34).
(3) N2O5. Dinitrogen pentoxide itself is considered not to
be very reactive with aromatic compounds (35) unless an
+
acidic catalyst in present. In this case it will generate NO2
as the active species (36).
(4) NO2+. The nitronium cation has been considered as
the reagent in many Kyodai nitrations occurring in acidic
conditions (36), particularly in the case of electron-poor
aromatic substrates. This species has been suggested to be
formed by an acid-catalyzed decomposition of dinitrogen
pentoxide. The Kyodai nitration of some electron poor
+
aromatic compounds with NO2 thus generated showed a
Hammett F value of -6.8 using Brown’s σ+ of the substituent
(37). This value and the use of Brown’s σ+ are typical of the
electrophilic aromatic substitution occurring via SET to give
an aromatic cation radicals and a nitrate anion eventually
bound in a charge-transfer complex (CTC). In fact, a CTC
has been proposed as an intermediate in some electrophilic
substitution reactions in order to account for some features
of the photochemical aromatic nitration (38) and was
confirmed by N-15-CIDNP experiments (39). The original
concepts (40) have been recently reviewed in the light of
distinguishing if electron transfer and bond formation take
place in separate steps (41).
Further evidence of the SET mechanism occurring in the
electrophilic nitration of 4-methylphenol 7 with nitric acid
is the highly negative Hammett F value of -6.38 (30) and
the finding of Pummerer’s ketone 10, typical reaction prod-
uct from the dimerization of a phenoxy radical [4-Me-
C6H4-O]‚ (42). Here, the phenoxy radical derives from the
deprotonation of the cation radical [4-Me-C6H4-O]+‚ ini-
tially formed. This result is in accord with the finding of
Pummerer’s ketone 10 in the reaction of tetranitromethane
with 4-methylphenol 7 suggested to occur via CTC fol-
lowed by formation of the phenoxy radical. The high
sensitivity of this reaction to polar effects (F ) -4.25) (43)
derived from the rate-determining SET to the cation radi-
cal. On the contrary, Pummerer’s ketone 10 was never
observed in the Kyodai nitration of 4-methylphenol 7. Hence,
the SET mechanism shown in point 1 and in the second part
of point 4 should be ruled out for the Kyodai nitration of
phenols.
The Addition-Elim ination Mechanism in the Kyoda i
Nitration of Phenols. These facts suggest that a radical
pathway such as the addition of NO3 followed by addition
of NO2 and elimination of HNO3 should be in operation in
the Kyodai nitration of phenols. The occurrence of this
pathway could be studied measuring the influence of the
nuclear substituents in the Kyodai nitration of phenols.
Previous studies had shown that the Kyodai nitration of
alkylaromatics occurring via reaction with NO2+ resulted in
a Hammett F value of -6.8 using Brown’s σ+ (29) of the
substituent (37). This was in line with the fast formation of
a CTC and a rate-determining SET to give, in the case of
phenol 1, the radical cation 16 and its conjugate base, the
phenoxy radical 17. In fact, the highly negative Hammett F
value and the use of Brown’s σ+ suggest a resonance
stabilization of a transition state bearing a positive charge
on the aromatic substrate.
TABLE 2. Heat of Formation (Kcal mol-1) of Adducts 18-21
from Phenol 1 with a Full Ab-Initio Calculationa
UHF/6-31G**
adduct
conformation
angle θ
angle O
B3lyp
ipso, 18
1
2
3
1
2
3
1
2
1
2
-1.34
130.04
-129.95
1.93
149.87
-140.07
-0.011
143.19
0.001
114.45
-120.81
120.84
178.59
10.77
-1.31
-5.48
-5.48
-4.41
-10.0
-6.22
-2.71
-4.84
-4.07
-6.48
ortho, 20
172.31
-3.50
m eta, 19
para, 21
-3.09
-2.65
143.69
-1.39
The Ham m ett Plot for the Kyoda i Nitration of Phenols.
The sensitivity of the Kyodai nitration to the electronic effect
of the substituent was checked measuring relative reaction
rates in the Kyodai nitration of 4-methylphenol 7, 4-chlo-
rophenol 12, 4-bromophenol 13, 4-nitrophenol 14, and
4-phenylphenol 15 (Scheme 2). Equimolecular mixtures of
phenol 1, used as the reference compound, and one of
substrates 12-15 were submitted to the Kyodai nitration.
After 30 s reaction the mixture was submitted to HPLC
analysis, and the ratio between the conversion of the two
substrates was measured. Figure 2 shows the plot of the log
of this ratio vs the parameter describing the electronic
contribution of the substituent at position para to the reac-
tion (Hammett σ) (29). The resulting Hammett plot shows
a poor linear correlation (R ) 0.81). However, the value of
the slope of this correlation (the Hammett F), -0.72, indi-
cated a low sensitivity of the reaction to electronic effects,
suggesting that a radical mechanism is occurring. In fact,
+
the electrophilic nitration of phenols by NO2 had the
highly negative Hammett F value of -6.38 using Hammett
σ (30).
The Active Species in the Kyoda i Nitration of Phenols.
NO3, NO2, NO2+, and N2O5 could be the active species in the
Kyodai nitration of these phenols.
(1) NO3. Phenols are known to react with NO3 in aqueous
acidic conditions via a Single Electron Transfer (SET) (31)
giving a cation radical such as 16 which is in prototropic
equilibrium with the phenoxy radical 17 and a nitrate anion
(Scheme 3). This behavior is also observed in the reaction
of NO3 with electron-rich aromatics (23). On the contrary,
strongly deactivated aromatic compounds react with NO3
via an addition elimination mechanism such as that depicted
in Scheme 3 (32). Four regioisomeric adducts 18-21 are
On the contrary, the Kyodai nitration of phenols presented
in this paper showed the much lower Hammett F value of
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2 2 2 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000

FIGURE 1. Correlation between ortho/para selectivity and dipole moment of the solvents.
FIGURE 2. Hammett plot for the Kyodai nitration of 4-substituted phenols.
) -0.72 using Hammett’s σ of the substituent. This very low
sensitivity to polar effects indicates a radical pathway not
involving charges in the transition state. A fast formation of
a CTC followed by recombination to give radical adducts
such as 18-21 in the case of phenol 1 could explain this
behavior.
The poor correlation coefficient is not unexpected, since
the rate constants k plotted against the Hammett’s σ are
actually the sum of the four rate constants related to the four
regiochemistries of the addition reaction: kADD ) kipso + kortho
+ kmeta + kpara. The Hammett relationship here used is based
on the assumption that the relative ratio of these rate
constants is independent from the nature of the substituent
in position 4. This assumption does not account for steric
effects, which are probably the origin of the poor correlation
coefficient.
This mechanistic shift from the electrophilic nitration to
a radical pathway in Kyodai nitrations on increasing the
electron-donating properties of the substrate is apparent also
in the Hammett correlation reported for the Kyodai nitration
of substituted benzenes (37). Here in fact the electron-rich
methoxybenzene did not fit the correlation, suggesting that
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VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 2 2 7

SCHEME 3
a mechanism different from the electrophilic nitration was
occurring with this substrate.
18-21 a further equilibrium addition of NO2 to give several
regioisom eric diastereoisom eric dihydrobenzene nitro-
nitrates 22-28 is expected to occur. The formation of
hydroxy-peroxydihydroarenes from the reaction of OH with
arenes followed by the reaction of the resulting radical with
dioxygen has been recently been demonstrated to be an
equilibrium reaction (45).
Computational methods could be used in the scope of
determining the relative abundance of the individual com-
ponents in the equilibrium and to predict the relative
abundance of the final products assuming a fast elimination
reaction of nitric acid from the regioisomeric dihydroben-
zenes to give the final products. In fact, the position of the
equilibrium in the formation of adducts in the reaction of
OH with toluene has been recently calculated using an ab
initio approach in order to predict the fate of aromatic
compounds in the atmosphere, and the ortho adduct has
been shown to be the most stable (46). This explains the
observed predominance of the formation of o- over p-cresol
in this reaction (47).
In the case of phenol 1, some ab initio calculations were
performed with a 6-31G** basis set and a hybrid density
function Becke3LYP (48). Because of the small difference in
energy (only a few kcal‚mol-1) among the different isomers,
the potential energy surface for rotations around the CsO
and CsO bonds was calculated (C-C-N-O ) θ;C-C-O-H
) æ). This allows for establishing the most stable conforma-
tion.
Three stable conformations were noticed with the ipso
NO3-adduct 18 and the ortho NO3-adduct 20, two stable
conformations constituted the meta NO3-adduct 19 and the
para NO3-adduct 21 (Table 2). The heat of formation of all
This behavior is not unusual for the reaction of NO3 with
aromatic compounds. In fact, the reaction of 4-substituted
toluenes with NO3 in gas phase has been suggested by us
(19) to occur via a rate-determining equilibrium addition of
NO3 to give four regioisomeric adducts (the ADD mech-
anism). This reaction had a F value of -4.3 using Ham-
mett’s σ. Moreover, this mechanism has been postulated in
other reactions of NO3 in solution. In fact, in the reaction of
NO3 with several toluenes substituted with electron with-
drawing groups the rate-determining ADD in acetonitrile
was suggested by a F value of -3.2 using Hammett’s σ. In
this paper the initial formation of a CTC was postulated
(32). The ADD channel was shown to compete with SET in
a recent study on the reaction of several aromatics with NO3
in water (24).
The idea of a CTC undergoing recombination to give, in
an equilibrium reaction, four regioisomeric radical adducts
such as 18-21 for phenol 1 has been suggested also in the
Kyodai nitration of toluene, anisole, and chlorobenzene to
account for the finding that the major product at the start
of the reaction is the meta-nitrocompound, postulated to
derive from the reaction of the ortho adduct analogous to 20
with NO2, followed by elimination of nitric acid (44). However,
in our experiments no trace of meta-nitration was obtained
in the Kyodai nitration of phenols 1 and 7.
Com putational Methods in the Kyoda i Nitration of
Phenols. The failure to find the corresponding m-nitrophe-
nols leads to some considerations on the product-determin-
ing steps depicted in Scheme 3. After the reversible addition
of NO3 to phenol 1 to give four different regioisomeric adducts
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2 2 2 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000

these adducts ranged from -1.6 to -10.0 Kcal mol-1. The
para NO3-adduct 21 is the most stable. Next is the ortho-
adduct 20. The following step forms nitrophenols 2, 3, and
29 via elimination of nitric acid from the diastereoisomeric
dihydrobenzenes 22-28.
o- 2 and p-nitrophenol 3 but not m-nitrophenol 29 were
formed in the Kyodai nitration of phenol 1. Moreover,
o-nitrophenol 2 was the major product in the gas-phase
reaction of phenol 1 with NO3 (15, 26). Compounds 2 and
3 may derive from the ipso NO3-adduct 18, the ortho NO3-
adduct 20, and the meta NO3-adduct 19 but not from the
para NO3-adduct 21. In fact, the latter can form neither o-
2 nor p-nitrophenol 3 but will eventually form m-nitrophenol
29. This latter was never found in Kyodai nitrations, thus
implying that if the ADD mechanism is occurring adduct 21
does not evolve to the products. Hence, if the influence of
the solvent in the stabilization of the adducts is negligible,
their stability as resulting from the ab initio calculations
seems not to be the reason of the selectivity in the Kyodai
nitration.
Two other factors could influence the selectivity in the
formation of o- 2 and p-nitrophenol 3 and not m-nitrophenol
29: the electron density at the various positions in adducts
18-21 and the mechanism of the elimination of nitric acid
from the adducts to give 2 and 3.
Stereochem ical Factors Influencing the Elim ination of
Nitric Acid. Also the stereochemistry of the elimination of
nitric acid could be relevant for the ortho- and para-selectivity
in the Kyodai nitration of phenol 1. If the E2 mechanism is
occurring in these reactions, the stereochemical requirements
of this mechanism should determine the reaction rate. The
E2 mechanism occurs via either a concerted loss of the two
groups being eliminated (here H and NO3) (E2cis) or by a
nonconcerted loss involving their antiperiplanar arrangement
(E2trans).
Moreover, intermediates 22-28 exist as the cis- and the
trans-isomer, having different stability. Both diastereoisomers
will be formed in the equilibrium addition of NO2 to the
radical adducts 18-21, but the higher is the difference in
stability between the diastereoisomers in a cis-trans pair,
the higher will be the difference between their equilibrium
concentrations. For instance, PM3 calculations showed that
the cis-isomer of intermediate 23 deriving from the ipso
adduct 18 is more that 1 Kcal mol-1 more stable than the
trans-isomer. Hence, its concentration at the equilibrium
will be higher than that of the trans-isomer.
FIGURE 3. Diastereoisomerism in dihydrobenzene intermediates.
the shift toward a higher ortho/para ratio on increasing the
dipole moment of the solvent is an indication of the initial
formation of a CTC involving some complexation of the
nitrate anion with the phenolic group. In fact, it is known
that CTCs are more stable in solvents with low dipole moment
(49, 50). Hence, CTC formation and recombination to the
radical adducts 18-21 is a concerted process in solvent with
a poor stabilizing ability of the CTC (polar solvents).
Atm ospheric Im plications. The Kyodai nitration is a good
model for the nitration of phenols in the tropospheric
aqueous phase. Moreover, the formation of both the para-
and the ortho nitration product, but not the meta, allows for
distinguishing gas phase nitration, which is known to give
predominantly the ortho nitration product from aqueous
phase nitration, for which the ortho/ para nitration ratio may
be estimated.
Figure 3 shows the mechanism of the elimination of nitric
acid (E2cis) from the diastereoisomers of intermediates 23
and 26, deriving from the ipso adduct 18 and from the ortho
adduct 20, respectively.
The phase transfer of the nitrate radical from the gas
tropospheric phase into droplets of clouds, fog, and rain and
the subsequent reaction with electron rich aromatics could
be an important sink for phenols.
Only from both isomers of intermediate 23 a concerted
loss of nitric acid via a six-membered transition state is
possible. This may be one of the reasons for the formation
of o-nitrophenol 2 in the gas-phase reaction of phenol 1
with NO3 (15, 26). Fast equilibrium addition of NO2 will give
the dihydrobenzenes 22 and 23 and the concerted (E2cis)
gas-phase elimination of nitric acid from 23, especially from
the more stable cis diastereoisomer, to give o-nitrophenol 2
will be a much faster process than the nonconcerted loss of
nitric acid from 22 and the other dihydrobenzene interme-
diates.
The lower portion of Figure 3 shows the shapes of the two
diastereoisomers from the ortho adduct 20. Here, no
concerted loss of nitric acid is possible, and only one
conformer of the cis diastereoisomer has the antiperiplanar
arrangement of the two groups to be eliminated (E2trans).
Hence, the elimination reaction will be more difficult, and,
accordingly, the selectivity will be lower.
A chemical model of tropospheric nitration is important
because nitrophenols are directly emitted from car exhaust
and are formed in a polluted urban environment. A chemical
mechanism (CAPRAM 2.3+ARO) has been developed to
better identify different nitration pathways of aromatics in
the troposphere. Field measurements in accordance with
modeling indicate that tropospheric formation may con-
tribute about one-third of the mononitroaromatic total source
flux. Moreover, the majority of dinitroaromatic compounds
observed in the urban atmosphere result from atmospheric
processing in the aqueous phase rather than from primary
emission (51).
Acknowledgments
Financial support of this work from the European Community
for Scientific Research (No. ENV4-CT97-0411) and the
National Research Council is gratefully acknowledged. We
thank Mr. Paolo Barzaghi for technical and scientific support.
The Effect of the Solvent on the Ortho/Pa ra Ratio in the
Kyoda i Nitration of Phenols. In the nitration of phenol 1,
9
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