Environ. Sci. Technol.
2000, 34, 2224
-
2230
3
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
F
-
3
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
N
2
O
5 is 4 × 10
-
5 -
10
-
3 s
-
1 in the cap cloud of the Great Dun
Fell (16) and the hydrolysis of N
2
O
5 at relative humidity above
80% was found to be a main source of nitrate in cloud (17).
If only a minor portion of this N
2
O
5 reacts with aromatic compounds, then this corresponds to an important source of nitroaromatics.
These processes may also be driven by reactions of the nitrate radical NO
3 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.
2224
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000
(NO
2
+ ). The nitrate radical in gas-phase tropospheric chemistry 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 NO
3 in water its removal from the gas phase is only important if NO
3 is removed quickly from the equilibrium, 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.
NO
3 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.
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 electrophilic attack. Nitration with NO
2
+ gives a slight preference for the para position (27).
A good model for the study of the reactivity of the nitrate radical NO
3 with phenols is the use of a mixture of nitrogen dioxide and ozone in nonprotic solvents (the Kyodai nitration). This system generates the nitrate radical NO
3 which is in equilibrium with N
2
O
5
. Dinitrogen pentaoxide may also form the nitronium ion NO
2
+ and nitrate NO
3
according to the following equations:
NO
2
+ O
3 f
NO
3
+ O
2
NO
3
+
NO
2 h
N
2
O
5
N
2
O
5 h
NO
2
+ +
NO
3
-
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 NO
3 under nonacidic conditions.
Here we report the Kyodai nitration of phenols to give potentially phytotoxic nitrophenols.
Reagents. All reagents and solvents used were commercial products reagent grade. Dichloromethane and trichloromethane were dried by distillation over calcium chloride.
Acetonitrile and tetrahydrofuran were HPLC grade. Nitrogen dioxide and oxygen were Air Liquide products.
Instruments. 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
10.1021/es990844m CCC: $19.00

2000 American Chemical Society
Published on Web 04/29/2000

c reagent
HNO
3
/H
2
O
HNO
3
/H
2
SO
4
/H
2
O
HNO
3
/CH
2
Cl
2
NO
2
-O
NO
NO
2
2
-O
NO
2
-O
-O
NO
2
-O
3
/CHCl
3
3
/CH
2
Cl
2
3
/THF
3
/CH
3
NO
2
3
/CH
3
CN conversion of phenol 1 (%)
66
83
(
(
14
94
(
7
74
(
2
44 ( 7
54
(
8
39 ( 8
89
(
6
9 reaction products from phenol 1 (%)
2: 26; 3: 34; 4: 6 a,b
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
2: 45; 3: 36; 4: 1 isomer ratio ortho/para (%)
0.76
1.20
(
(
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
0.10
conversion of
4-methylphenol
100
100
99
(
1
79 ( 5
72
(
2
7 (%) reaction products from 4-methylphenol 7 (%)
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 a No m-nitrophenol was detected.
each reaction.
b Dinitrophenols 5 and 6 are sometimes formed, and their yield is ca. 0.5%.
c Five runs were performed for
steel column was an Alltima RP C18, 250 mm length, 4.6 mm internal diameter, and 5 µ m particle size (100 Å ). Isocratic elution was carried out using a two solution mixture: solution
A/solution B
M KH
2
PO
4
) 55:45 where A
) and 10% CH
3
) 90% phosphate buffer (0.05
CN, while B ) 25% phosphate buffer
(0.05 M KH
2
PO
4
) and 75% CH
3
CN. The flow was 1 mL/min.
The Diode Array Detector (DAD) was used monitoring the wavelength at 230 nm.
Kyodai Nitrations. All the reactions were carried out in five runs at 0 ° C with five different concentrations of substrate, 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 O
3
:
NO
2
: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.
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 Experiments. 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 NO
2
/O
3 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.
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 NO
2
+ /phenol varied in a range between
100:1 and 1:1. After 1 h a sample was injected in HPLC after dilution.
The Kyodai 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/pnitrophenol ratio was different from that observed in electrophilic nitrations with HNO
3
.
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.
The Kyodai Nitration of 4-Methylphenol 7. 4-Methylphenol 7 gave with the Kyodai reaction in the same conditions 2-nitro-4-methylphenol 8 and 2,6-dinitro-4methylphenol 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 obtained. The electrophilic nitration with NO
2
+ 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 Kyodai Nitration of Phenol 1. The results of the Kyodai nitration of phenol 1 in nitromethane or acetonitrile were different from those obtained in chloroform, methylene chloride, tetrahydrofuran, or in the electrophilic nitration with NO
2
ortho/para ratio was 0.7
+ . In the latter conditions the
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 coefficient 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.
VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2225

adduct ipso, 18 ortho, 20 meta, 19 para, 21 conformation
1
2
1
2
3
2
3
1
1
2 angle θ
-
1.34
-
130.04
129.95
1.93
-
149.87
140.07
-
0.011
143.19
0.001
143.69
-
1
-
a angle
O
114.45
120.81
120.84
178.59
10.77
172.31
-
-
3.50
3.09
2.65
-
1.39
UHF/6-31G**
B3lyp
-
1.31
5.48
-
-
5.48
4.41
10.0
-
-
6.22
-
2.71
4.84
4.07
-
6.48
The Hammett Plot for the Kyodai 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-chlorophenol 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 reaction (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, indicated a low sensitivity of the reaction to electronic effects, suggesting that a radical mechanism is occurring. In fact, the electrophilic nitration of phenols by NO highly negative Hammett F value of
2
+ had the
6.38 using Hammett
σ (30).
NO
The Active Species in the Kyodai Nitration of Phenols.
3
, NO
2
, NO
2
+ , and N
2
O
5 could be the active species in the
Kyodai nitration of these phenols.
(1) NO
3
. Phenols are known to react with NO
3 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 NO
3 with electron-rich aromatics (23). On the contrary, strongly deactivated aromatic compounds react with NO
3 via an addition elimination mechanism such as that depicted in Scheme 3 (32). Four regioisomeric adducts 18
-
21 are
2226
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000 formed which will then add NO
2 zenes 22 to give the dihydroben-
-
28. Elimination of HNO
3 will form the final nitrophenols.
(2) NO
2
. Also the nitration of phenols with NO
2 is believed to occur ultimately through the formation of a phenoxy radical 17 followed by addition of NO
2 leading to nitrocyclohexadienones, which rearrange to nitrophenols (33). This reaction is of practical importance since NO
2 may be formed in the photolysis of nitrate anion in advanced oxidation processes (34).
(3) N
2
O
5
. 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 NO
2
+ as the active species (36).
(4) NO
2
+
. 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 NO
2
Hammett
F value of
-
+ thus generated showed a
6.8 using Brown’s σ
+
(37). This value and the use of Brown’s σ
+ of the substituent 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 product from the dimerization of a phenoxy radical [4-Me-
C
6
H
4
-
O]
‚
(42). Here, the phenoxy radical derives from the deprotonation of the cation radical [4-Me-C
6
H
4
-
O]
+‚ initially 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 followed 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 radical. 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
-
Elimination Mechanism in the Kyodai
Nitration of Phenols. These facts suggest that a radical pathway such as the addition of NO
3 followed by addition of NO
2 and elimination of HNO
3 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 NO a Hammett F value of 6.8 using Brown’s σ
2
+
+ resulted in
(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 value and the use of Brown’s σ
+
F suggest a resonance stabilization of a transition state bearing a positive charge on the aromatic substrate.
On the contrary, the Kyodai nitration of phenols presented in this paper showed the much lower Hammett
F value of

FIGURE 1. Correlation between
selectivity and dipole moment of the solvents.
FIGURE 2. Hammett plot for the
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: k
+ k meta
+ k para
ADD
) k ipso
+ k ortho
. 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
VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2227

a mechanism different from the electrophilic nitration was occurring with this substrate.
This behavior is not unusual for the reaction of NO
3 with aromatic compounds. In fact, the reaction of 4-substituted toluenes with NO
3 in gas phase has been suggested by us
(19) to occur via a rate-determining equilibrium addition of
NO
3 to give four regioisomeric adducts (the ADD mechanism). This reaction had a
F value of
-
4.3 using Hammett’s σ . Moreover, this mechanism has been postulated in other reactions of NO
3 in solution. In fact, in the reaction of
NO
3 with several toluenes substituted with electron withdrawing 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 NO
3 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 NO
2
, 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.
Computational Methods in the Kyodai Nitration of
Phenols. The failure to find the corresponding m-nitrophenols leads to some considerations on the product-determining steps depicted in Scheme 3. After the reversible addition of NO
3 to phenol 1 to give four different regioisomeric adducts
2228
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 11, 2000
18 21 a further equilibrium addition of NO
2 to give several regioisomeric diastereoisomeric dihydrobenzene nitronitrates 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 components in the equilibrium and to predict the relative abundance of the final products assuming a fast elimination reaction of nitric acid from the regioisomeric dihydrobenzenes 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 C s
O and C s
O bonds was calculated (C
)
φ
-
C
-
N
-
O
)
θ ; C
-
C
-
O
-
H
). This allows for establishing the most stable conformation.
Three stable conformations were noticed with the ipso
NO
3
-adduct 18 and the ortho NO
3
-adduct 20, two stable conformations constituted the meta NO
3
-adduct 19 and the para NO
3
-adduct 21 (Table 2). The heat of formation of all

these adducts ranged from 1.6 to 10.0 Kcal mol 1 . The para NO
3
-adduct 21 is the most stable. Next is the orthoadduct 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 NO
3
(15, 26). Compounds 2 and
3 may derive from the ipso NO
3
-adduct 18, the ortho NO
3
adduct 20, and the meta NO
3
-adduct 19 but not from the para NO
3
-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.
Stereochemical Factors Influencing the Elimination 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 E
2 mechanism is occurring in these reactions, the stereochemical requirements of this mechanism should determine the reaction rate. The
E
2 mechanism occurs via either a concerted loss of the two groups being eliminated (here H and NO
3
) (E
2 cis) or by a nonconcerted loss involving their antiperiplanar arrangement
(E
2 trans).
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 NO radical adducts 18 -
2 to the
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 shows the mechanism of the elimination of nitric acid (E
2 cis) from the diastereoisomers of intermediates 23 and 26, deriving from the ipso adduct 18 and from the ortho adduct 20, respectively.
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 NO
3
(15, 26). Fast equilibrium addition of NO
2 will give the dihydrobenzenes 22 and 23 and the concerted (E
2 cis) 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 intermediates.
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 (E
2 trans).
Hence, the elimination reaction will be more difficult, and, accordingly, the selectivity will be lower.
The Effect of the Solvent on the Ortho/Para Ratio in the
Kyodai Nitration of Phenols. In the nitration of phenol 1,
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).
Atmospheric Implications. The Kyodai nitration is a good model for the nitration of phenols in the tropospheric aqueous phase. Moreover, the formation of both the paraand 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.
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.
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 contribute 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).
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.
VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
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Received for review July 26, 1999. Revised manuscript received January 11, 2000. Accepted February 15, 2000.
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