Nighttime aqueous-phase formation of nitrocatechols in the

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Kristijan Vidovi#, Society.Damjan 1155 Sixteenth Street N.W., Washington, Laši# Jurkovi#, Martin Šala, DC 20036 Published by Grgi# American Ana Krofli#, and Irena


Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01161 • is published by the American Publication Date (Web): 26Chemical Jun 2018

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OH

Page 1 ofEnvironmental 26 ELECTROPHILICScience & Technology OH ROUTE

OH OH

OXIDATION & CONJUGATION

CH3

CH3 NO 2 OH OH

RADICAL Plus Environment ACS Paragon ROUTE

O 2N

CH3

Environmental Science & Technology

1

Nighttime aqueous-phase formation of

2

nitrocatechols in the atmospheric condensed

3

phase

Page 2 of 26

4 5

Kristijan Vidović,† Damjan Lašič Jurković,‡ Martin Šala,† Ana Kroflič,†* and Irena Grgić†*

6 †

7

Department of Analytical Chemistry, National Institute of Chemistry, SI-1000 Ljubljana,

8

Slovenia

9 ‡

10 11

Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia

12 13

*

Corresponding Authors

14

1 ACS Paragon Plus Environment

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15

Abstract:

16

Yellow-colored methylnitrocatechols (MNC) contribute to the total organic aerosol mass and

17

significantly alter absorption properties of the atmosphere. To date, their formation

18

mechanisms are still not understood. In this work, the intriguing role of HNO2 (catalytic and

19

oxidative) in the dark transformation of 3-methylcatechol (3MC) under atmospherically

20

relevant aqueous-phase conditions is emphasized. Three possible pathways of dark 3-methyl-

21

5-nitrocatechol and 3-methyl-4-nitrocatechol formation, markedly dependent on reaction

22

conditions, were considered. In the dominant pathway, HNO2 is directly involved in the

23

transformation of 3MC via consecutive oxidation and conjugated addition reactions (non-

24

radical reaction mechanism). The two-step nitration dominates at pH around the pKa of

25

HNO2, which is typical for atmospheric aerosols, and is moderately dependent on

26

temperature. Under very acidic conditions, the other two nitration pathways, oxidative

27

aromatic nitration (electrophilic) and recombination of radical species, gain in importance.

28

The predicted atmospheric lifetime of 3MC according to the dominant mechanism at these

29

conditions (2.4 days at pH 4.5 and 25 °C) is more than 3-times shorter than via the other two

30

competitive pathways. Our results highlight the significance of catechol oxidation-conjugated

31

addition reaction in nighttime secondary nitroaromatic chromophore formation in the

32

atmosphere, especially in polluted environments with high NOx concentrations and relatively

33

acidic particles (pH around 3).

34



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35

Introduction

36

One of the most important properties of ambient volatile organic compounds (VOC) driving

37

the formation of secondary organic aerosol (SOA) is water solubility. Water-soluble organic

38

pollutants emitted into the troposphere in the gaseous form partition into the atmospheric

39

aqueous phase (clouds, fog, moist aerosols). There they can transform into low-volatility

40

products that remain in the particulate phase even after water evaporates and contribute

41

substantially to the atmospheric SOA mass. 1 Fundamentally, mechanisms of these reactions

42

are not always the same as of those in the gaseous phase. However, gas-phase reactions

43

predominantly proceed through molecular-type intermediates (e.g. activated complexes),

44

whereas aqueous-phase reactions often involve charged molecular forms and hydrated

45

species.2 Besides many short-lived radicals (OH•, NO3•, SO4•−),3 other non-radical oxidants

46

such as hydrogen peroxide (H2O2), organic hydroperoxides (ROOH), ozone (O3)4 and also

47

nitrous acid (HNO2) take an important part in aqueous-phase transformations. Although the

48

role of HNO2 in the atmospheric waters is not yet clear,5-9 its importance as a catalyst in

49

condensed-phase aromatic nitration has already been established.10-16 Furthermore, aromatic-

50

nitration products, particularly abundant methylnitrocatechols (MNC), are significant

51

contributors to atmospheric brown carbon (BrC),17 i.e., organic aerosol fraction that

52

efficiently absorbs solar and terrestrial radiation, and therefore ultimately producing climate

53

forcing.18 However, current understanding of their secondary formation in the atmosphere is

54

still incomplete.

55

The amount of HNO2/nitrite (NO2−) in the atmospheric aerosols strongly depends on

56

the level of local pollution, liquid water content, pH, etc. Because of its dissociation

57

equilibrium (pKa (HNO2) = 3.2)19 and modest water solubility (Henry's law constant, Hcc

58

(HNO2) = 1.2 x 103at 298 K),20 only very low HNO2 concentrations are found in acidic

59

droplets, normally in the range of 10−7–10−6 mol L−1.21, 22 As pH of atmospheric waters can 3 ACS Paragon Plus Environment

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60

range from 1.95 to 7.74 and considering that significant deviations from Henry's law have

61

been observed,9 total nitrite concentrations up to 10−4 mol L−1 have been measured in fog

62

water samples.21 Aromatic compounds, on the other hand, contribute significantly to the

63

budget of atmospheric pollutants.23 Nevertheless, because of the lack of appropriate kinetic

64

and mechanistic data, they are very seldom included into atmospheric models which deviate

65

substantially from field measurements.24 Of great importance are substituted aromatics;

66

catechol, hydroquinone and resorcinol are typical aromatic constituents of biomass-burning

67

emissions.25 According to Hoffmann et. al.26 aromatic compounds with a dimensionless

68

Henry’s law constant (Hcc) equal or greater than 103 are important for atmospheric multiphase

69

chemistry (Hcc (catechol) = 2.03 x 107, Hcc (resorcinol) = 2.5 x 108, Hcc (hydroquinone) = 6.4

70

x 108, Hcc (3-methylcatechol) = 3.3 x 108).26, 27

71

It has been speculated that dark nitration of substituted aromatics in the atmospheric

72

aqueous phase containing HNO2 can be an important source of nighttime BrC.6 The

73

importance of HNO2 in acidic aqueous solution of reactive aromatic compounds was first

74

recognized by Martinsen.28 He showed that nitration of activated aromatics such as phenol

75

was dependent on HNO2; in the presence of nitric acid (HNO3) only, phenol was not nitrated.

76

Already at that time, he concluded that HNO2 had a strong catalytic character. Veibel29 later

77

postulated a reaction mechanism where nitrosated intermediates formed from phenol and

78

HNO2. Bunton15 further proposed the existence of two reaction mechanisms of aromatic

79

nitration in HNO3 solution containing traces of HNO2. According to the first mechanism,

80

nitronium ion (NO2+ or its different carrier, e.g., H2NO3+, N2O5) was the main nitrating agent,

81

while HNO2 acted as a retardant. The second reaction mechanism assumed recombination of

82

aromatics with nitrosonium ion (NO+ or its different carrier, e.g., H2NO , N2O4 or HNO2

83

itself), which led to aromatic nitrosation followed by oxidation to yield nitrated products. In

84

this second case, HNO2 accelerated nitration of activated aromatic compounds. Recently,



Page 6 of 26

85

nitration of 2-methoxyphenol (guaiacol) in the presence of HNO2 under mild atmospheric

86

aqueous-phase conditions has been well described accounting for NO2+ and nitrogen dioxide

87

(NO2•) as nitrating species, and it was shown that the reaction pathways were competitive

88

under nighttime atmospheric conditions.30 In contrast, the majority of existing studies on the

89

atmospheric aqueous-phase nitration only refer to radical mechanisms, ignoring possible non-

90

radical reactions.4 Moreover, studies of non-radical reactions usually report only the identities

91

of formed products, providing very little mechanistic and kinetic information necessary for

92

the implementation in multiphase models.4

93

So far, there have been no aqueous-phase nitration studies performed on

94

methylcatechols being important precursors to yellow-colored MNC, which contribute to the

95

total organic aerosol mass and significantly alter absorption properties of atmospheric organic

96

aerosols.31-33 In this work, we emphasize the intriguing role of HNO2/NO2− in dark

97

transformation of 3-methylcatechol (3MC) under atmospherically relevant aqueous-phase

98

conditions. pH-dependent kinetic studies were performed in acidic solutions of sodium nitrite

99

(NaNO2/H2SO4), which allowed us to deduce the reactivity of 3MC towards HNO2/NO2−

100

with a great deal of confidence. We identify three possible routes of MNC formation under

101

atmospheric nighttime conditions, which are shown to depend markedly on reaction

102

conditions, specifically on pH and temperature.

103 104

Experimental section

105

Materials. Acetonitrile (Sigma-Aldrich, gradient grade, for HPLC >99.9%), formic acid

106

(Kemika), and high purity water (18.2 MΩ cm) supplied by a Milli-Q water purification

107

system were used for mobile phase preparation. Sulfuric acid 98% (H2SO4, EMSURE, p.a.

108

grade) and sodium nitrite (NaNO2, Sigma-Aldrich, ACS reagent, ≥97.0%) were used for

109

reaction mixture preparation. Standard substance 3-metylcatechol (3MC) (Sigma-Aldrich, 5 ACS Paragon Plus Environment

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110

98%) was used also as a reactant. Standards of reaction products 3-methyl-5-nitrocatechol

111

(3M5NC) and 3-metyl-4-nitrocatechol (3M4NC) were prepared in the laboratory by the

112

procedures adopted from Palumbo34 and Kitanovski.31 Griess reagent (modified) was used for

113

spectrophotometric determination of total nitrite.

114

Experimental Methods. Nitration of 3MC in acidic NaNO2 solution was investigated under

115

conditions relevant for the atmospheric aqueous phase. Initial concentrations of reactants in

116

the reaction mixture were 1×10−4 and 2×10−4 mol L−1 3MC, and 1×10−4, 2×10−4, 1×10−3 and

117

2×10−3 mol L−1 NaNO2. H2SO4 was used for pH adjustment. For details on experimental

118

conditions applied in each experiment see Table S1. Experiments were reproducible within

119

95%. Concentrations of 3MC and formed nitrated products during the reaction, 3M5NC and

120

3M4NC (500 µL sample was taken each time from 100 mL reaction mixture), were followed

121

by Agilent 1100 Series HPLC System equipped with a UV/vis diode-array detector (Atlantis

122

T3 column (3.0×150 mm, 3 µm particle size, Waters), isocratic elution by acetonitrile/0.1%

123

formic acid, 0.6 ml min–1, 10 µL injection, detection wavelengths: 275 nm for 3MC and 345

124

nm for MNCs). Total nitrite content was determined spectrophotometrically by use of Griess

125

reagent, for nitrite mass balance control.

126

Modeling. In order to elucidate the underlying mechanism of observed MNC formation, a

127

mathematical model was developed. The model explicitly considers three different pathways

128

of product formation presented in Figure 1. Overall, it includes 11 chemical species in 11

129

reactions (Table 1). As some of the reactions are reversible, 14 reaction constants describe

130

the experimental system. By writing down the mass balances of all species, a system of 11

131

ordinary differential equations is obtained, however some further simplifications were made.

132

It was assumed that due to a fast dissociation reaction, HNO2 and NO2− are always in the

133

equilibrium, pKa of 3.2 was used.35 Because of long reaction time-scales compared to oxygen

134

dissolution rate, the concentration of oxygen was assumed constant. We verified the latter 6 ACS Paragon Plus Environment


Page 8 of 26

135

assumption by performing an experiment with constant flushing of the reaction mixture with

136

oxygen (exp. 3 in Table S1) and no substantial deviation from the model was observed

137

(Figure S1a). In its final iteration, the system consists of 8 ordinary differential equations

138

describing 11 reactions altogether (with corresponding reaction rate constants), excluding

139

HNO2/NO2− equilibrium. From the model, time-dependent concentrations of all species were

140

obtained and compared with the experimental data.

141

The reaction system was solved in MATLAB® environment (MathWorks, Natick,

142

MA). Due to a high variance in rate constants (over several orders of magnitude), the system

143

of equations was rather stiff. “Ode15s” solver was utilized to solve it, which is a variable-

144

order backward-differentiation-formula based solver that uses a quasi-constant step size

145

allowing to solve stiff systems in a relatively short time-frame. To describe the experiments

146

in the best possible way, regression analysis was performed on all reaction constants

147

considering all experiments performed at 25 °C simultaneously, except k10 and k11 which

148

were taken from the literature and kept fixed during the regression analysis.36, 37 MATLAB’s

149

optimization scheme “fminsearch” utilizing the Nelder-Mead simplex algorithm was used for

150

this matter. The objective function that was minimized was the sum of squares of differences

151

between all experimental and model concentrations. In order to reduce the error of solver

152

getting stuck in local minima and to get the solution as close as possible to the global

153

minimum of the objective function, the regression was repeated numerous times with

154

different initial values. Overall, a very good agreement between the experimental data and the

155

model was achieved with the final set of rate constants values. The resulting set of best-fit

156

kinetic rate constants is gathered in Table 1.

157

Confidence intervals were calculated with the intent of quantifying the reliability of the

158

fitted rate constants. Herein the confidence interval is defined as a maximum allowed relative

159

change in the reaction constant that keeps the relative difference between model outputs with


Page 9 of 26


160

the original and modified sets of constants within 1%. The difference was defined as a sum of

161

relative differences in the concentrations of both products in the model outputs, and the

162

average value of all experiments was taken.

163

In order to investigate temperature-dependent behavior of the reaction system,

164

additional experiments were performed at 15 and 5 °C (experiments 17 and 18 in Table S1).

165

Although the collected experimental data are not sufficient for the exact analysis of

166

temperature dependence of the whole set of reaction rate constants, temperature dependence

167

of an apparent first-order reaction rate in regards to 3MC ( = [3MC]; kapp being the

168

apparent first-order rate constant) was studied by fitting the Arrhenius pre-exponential factor

169

(A) and activation energy (Ea); R and T are the gas constant and temperature, respectively

170

(Equation a).

171

=

(a)

172

Pseudo first-order kinetics was chosen because it was in-line with the proposed reaction

173

mechanism and very good matching was finally obtained (Figure S7).

174 175

Results and Discussion

176

Reaction mechanism. Figure 1 represents the proposed reaction scheme that resulted from

177

the extensive combined experimental and modeling investigation of dark 3MC nitration in

178

aqueous solution under acidic conditions relevant for the atmospheric aqueous phase. In the

179

presence of HNO2/NO2−, the dominant reaction mechanism is the conversion of 3MC to its

180

nitrated derivatives in two consecutive processes: oxidation and nitration by addition

181

(pathway I).7 HNO2 first oxidizes 3MC into the corresponding 3-methyl-o-quinone (4,

182

3MoQ) in two one-electron steps (k2 and k3). 3MoQ then undergoes a conjugate addition

183

reaction with NO2−, in which two isomeric nitro products form (k4 and k5). It should be

184

emphasized that this reaction pathway is strongly pH dependent; as both HNO2 and NO2− are 8 ACS Paragon Plus Environment


Page 10 of 26

185

the reactive species, nitration does not occur at pH far from the pKa of nitrous acid (i.e.,

186

3.2).19 At very low pH, the addition reaction is disfavored and at relatively high pH, HNO2 is

187

not present in its molecular form necessary for the oxidation of 3MC.

188 189

Figure 1. Proposed competitive reaction pathways of nighttime 3MC nitration in

190

NO2−/HNO2-containing atmospheric aqueous phase; 1 3-methylcatechol (3MC), 2 charge

191

transfer complex (CTC), 3 phenoxy radical (PhO•), 4 3-methyl-o-quinone (3MoQ), 5 3-

192

methyl-5-nitrocatechol (3M5NC), and 6 3-metyl-4-nitrocatechol (3M4NC). 5’, 6’ and 5’’, 6’’

193

are protonated forms of 3-methyl-o-quinones and sigma complexes (intermediates in Pathway

194

II) corresponding to 3-methyl-5-nitrocatechol and 3-methyl-4-nitrocatechol, respectively.

195

Pathway I summarizes the dominant reaction mechanism under the applied conditions.

196

Figure 2a shows that in the dark, 3MC can be well nitrated into two isomeric nitro

197

products, major 3M5NC and minor 3M4NC, even in the absence of any additional oxidant

198

(besides dissolved oxygen from the air). In slightly acidic aqueous solution of NaNO2 (pH

199

above 4.5) more than 97% 3MC is converted into the two identified products (see all product

200

yields in Table S1). However, we found that the ratio between the two products depended on

201

the total concentration of HNO2 in the reaction mixture (trace concentration or in excess to

202

3MC). This suggests that competitive routes to pathway I exist that prevail under distinct 9 ACS Paragon Plus Environment

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203

reaction conditions, which are presented in Figure 1. Specifically, NO+ in charge transfer

204

complex (CTC) with 3MC can also take part in an oxidative electrophilic nitration (pathway

205

II); besides another nitration mechanism is possible, involving combination of phenoxy

206

radical (PhO•) with NO2• (pathway III).30, 38, 39 Both competitive mechanisms are only active

207

if the solution is aerated and do not proceed in the absence of oxygen.

208 209

Figure 2. Experimental data (symbols) and calculated concentration profiles according to the

210

proposed reaction scheme (solid lines) of dark 3-methylcatechol (3MC) nitration in slightly

211

acidic NaNO2/H2SO4 solution at 25 °C: a) in the presence of O2 (exp. 2), b) in the absence of

212

O2 (exp. 4) and c) at trace HNO2 concentration (exp. 7). The following reaction products

213

were identified: 3-methyl-5-nitro-catechol (5, 3M5NC) and 3-methyl-4-nitro-catechol (6,

214

3M4NC).

215

Catalytic role of nitrous acid. Besides its direct role in the aromatic nitration (pathway II),

216

dissolved O2 is also capable of oxidizing the released NO• within 3MC oxidation by HNO2.40

217

This way, NO2• is produced in aerated reaction mixture, which after the recombination with

218

another NO• recovers HNO2.41



219

2NO∙ + O 2NO∙

220

NO∙ + NO∙ + H O 2HNO

Page 12 of 26

(1)

(2)

221

The values for the rate constants k10 and k11 were taken from the literature36, 37 that seemed to

222

be the most relevant for the atmospheric aqueous phase. In order to reduce the number of

223

fitting parameters, these two constants were treated as non-adjustable. Note that even if k11

224

had been for two orders of magnitude higher (these values also appear in the literature) it

225

would not significantly change the modeling results. This is also evident from the confidence

226

interval determined for this constant (see Table 1).

227

The set of Reactions 1–2 nicely demonstrates that in 3MC nitration, the role of HNO2

228

is not only oxidative, but also catalytic, which is additionally supported by the experiments

229

performed in the absence of O2 (Figure 2b, Figure S1b).11, 15, 16, 42 Even if O2 is completely

230

expelled from the reaction mixture, 3MC nitration proceeds to a certain extent; until the

231

accumulated NO• in the reaction mixture moves the equilibria in pathway I towards the

232

reactants at the expense of PhO• and CTC. Note, to avoid NO loss, each solution (i.e., NaNO2

233

and 3MC stock solutions) was purged with N2 before mixing and the reaction mixture was

234

held under inert atmosphere during the experiment.


Page 13 of 26


235 236

Figure 3. Calculated concentration profiles of inorganic and organic reactive species in the

237

reaction mixture at pH 4.7 and 25 °C initially containing 1x10−3 mol L−1 NaNO2 and 1x10−4

238

mol L−1 3MC: a) and c) in the presence of O2 (exp. 2), b) and d) in the absence of O2 (exp. 4).

239

Calculated concentration profiles for NO•, NO2• and HNO2 (always in equilibrium with

240

NO2−) resulting from the proposed reaction scheme in the absence and presence of O2 are

241

shown in Figure 3a-b. In aerated solution (Figure 3a), NO• is scavenged by O2 yielding NO2•

242

in Reaction 1, which results in the lower NO• concentration in comparison to non-aerated

243

conditions. The concentration of NO2• is always lower than that of NO•, indicating its slower

244

formation and/or more numerous consumption routes (see below). PhO• and NO• compete for

245

NO2• in the solution. As the concentration of the former is always several orders of

246

magnitude lower (compare Figure 3a and c), the dominant sink of NO2• is the

247

disproportionation reaction with NO• to HNO2 (Reaction 2). The observed low PhO•

248

concentration in comparison to NO• results from the fast oxidation of PhO• by HNO2,

249

yielding 3MoQ and NO• (k3). However, if the concentration of HNO2 was too low to



Page 14 of 26

250

efficiently oxidize PhO• to 3MoQ, PhO• would become the dominant sink of NO2•. Similar

251

has been proposed for phenol.8, 43-46

252

Modeling results. Model function derived on the basis of the reaction scheme in Figure 1

253

describes the experimental data very well (see Figure 2 and Figures S1–S6). Parity plot

254

comparing experimental data with the calculated values is shown in Figure S8. Global fitting

255

to all experimental data at 25 °C at a time allowed us to determine kinetic parameters

256

describing the proposed reaction pathways gathered in Table 1 with a fair amount of

257

confidence. From the computed confidence intervals, it is however evident that even a

258

significant change in particular reaction rate constants has a negligible effect on product

259

formation. The least reliable constants are those denoting products formation by the

260

electrophilic and radical reaction pathways, which can be attributed to their minor roles in the

261

overall transformation of 3MC. Nevertheless, it can still be concluded that in pathways II and

262

III the formation of 3M4NC is favored in comparison to pathway I. With the most reliable

263

kinetic rate constants, the reaction of 3MC oxidation followed by the addition of NO2−

264

(pathway I) is recognized as the most significant pathway. Relative importance of

265

competitive nitration pathways depends on reaction conditions, which is elaborated in the

266

following sections.

267 268

Table 1. Best-fit kinetic rate constants (ki) with confidence intervals as defined above, valid

269

at 25 °C. k10 and k11 are taken from the literature and were kept fixed during the regression

270

analysis. Product

ri

2

k1[3MC][HNO2] – k−1[CTC]

3

k2[CTC] – k−2[PhO•][NO•]

4

k3[PhO•][HNO2] – k−3[3MoQ][NO•]

5

k4[3MoQ][NO2−]

Const.

ki

k1 k−1 k2 k−2 k3 k−3 k4

9.18·10−1 2.7·101 4.3·107 1.24·1010 1.90·108 1.46·106 1.25·100

Conf. int. [+/- %] 0.84 14.91 7.44 7.29 4.88 4.76 1.23

Units L mol−1 s−1 s−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1


Page 15 of 26


6 5 6 5 6 NO2• NO2−

k5[3MoQ][NO2−] k6[CTC][O2] k7[CTC][O2] k8[PhO•][NO2•] k9[PhO•][NO2•] k10[O2][NO•]2 k11[NO2•][NO•]

k5 k6 k7 k8 k9 k10 k11

1.25·10−1 1.0·106 2.1·105 6.2·108 1.2·108 6.0·106 3.0·107

1.33 10.32 7.17 14.15 9.24 15.31 15.67

L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 L mol−1 s−1 L2 mol−2 s−1 L mol−1 s−1

271 272

Experiments 17 and 18 (Table S1) were additionally used to quantify the temperature

273

dependence of the apparent first-order reaction rate with respect to 3MC. The best-fit values

274

of A and Ea obtained were 5.0 x 103 s−1 and 46.4 kJ mol−1, respectively. An observation was

275

made that the global pseudo first-order kapp considered for this purpose (kapp = 3.69 x 10-5 s−1

276

at 25 °C) is comparable to the reaction constant k1 multiplied with the initial HNO2

277

concentration yielding a pseudo first-order kinetic rate constant valid at the given conditions

278

(k1’ (exp. 2) = 3.83 x 10-5 s−1). This signifies the importance of this reaction step in

279

controlling the overall reaction rate under applied experimental conditions. Note also the

280

confidence interval of this reaction in Table 1 which is very narrow.

281

Pathway I: Initial oxidation step. As supported by our experimental-modeling study, the

282

mechanism of 3MC oxidation is likely to proceed via several reaction steps.11 When HNO2

283

approaches 3MC, HNO2 acts as an electron acceptor and 3MC as an electron donor and they

284

form a charge transfer complex also known as π-complex.42,

285

electrons from the aromatic system, it easily loses OH− (that is readily solvated in water) and

286

3MC-NO+ complex (2) remains in solution. In such CTC, 3MC is possibly subjected to a

287

one-electron oxidation process. The electron transfer between 3MC and NO+ yields an aryl

288

cation radical and nitrogen oxide (NO•), and after the loss of a proton from the phenolic

289

group (note the cation radical is a very strong acid, pKa = −1.9)51 it rapidly tautomerizes to a

290

more stable PhO• (3).12, 13, 16, 52 When HNO2 is in excess, PhO• is readily oxidized to 3MoQ

291

(4) through similar reaction steps as just described, leaving behind another NO•.12, 13

47-50

After HNO2 withdraws



Page 16 of 26

292

There are a few research papers investigating atmospheric processes where the

293

formation of quinone is reported.5, 6 However, it has always been considered as a side product

294

and little or no information is offered about the mechanism of its formation, particularly in

295

respect of its possible central role in the aqueous-phase nitration of aromatic pollutants.

296

HNO2 is not a strong oxidant; nevertheless, NO+ being liberated from HNO2 during its

297

interaction with the reactive aromatics can act as an oxidant as well. The redox potential of

298

NO+/NO• (E° = 1.325 V vs. Ag/AgCl)53 is substantially higher than the reversible oxidation

299

potential of 3MC/3MoQ at pH 4.5 (Eox = 0.3 V vs. Ag/AgCl), which means that an

300

exothermic electron transfer between them can occur.

301

Pathway I: Nitration of the corresponding quinone via conjugate addition reaction.

302

From 3MoQ, nitration proceeds by a unique conjugate addition reaction also known as 1,4

303

addition to diketone.54 Although the oxidative addition was discovered already two centuries

304

ago,55 its importance in the atmospheric processes has not been discussed before. To react

305

with NO2−, 3MoQ needs to be first activated towards the nucleophile, i.e., its protonation is

306

necessary. Depending on which carbonyl oxygen the protonation takes place, two different

307

isomeric nitro products can form. If C1-carbonyl oxygen is protonated (5’ in Figure

308

1),3M5NC forms upon the nucleophilic attack of NO2−. As this carbonyl oxygen is much less

309

sterically disturbed, more 3M5NC product forms in comparison to 3M4NC, which results

310

from the protonation of C2-carbonyl oxygen (6’ in Figure 1). This is in accordance with our

311

experimental observations.

312

For comparison, Khalafi and Rafiee7 investigated the nitration of 3MC under similar

313

conditions. They used different modeling approach and determined second-order rate

314

constants of consecutive oxidation (kox = 5.15 L mol−1 s−1) and nitration (knitr = 8.29 L mol−1

315

s−1) steps at 25 °C. Nematollahi56 calculated the homogenous second-order rate constant

316

(k = 7.3 L mol−1 s−1) for 1,4 addition reaction of electrochemically generated


Page 17 of 26


317

3MoQ with NO2− in aqueous phosphate buffer (pH 6.5, c = 0.15 mol L−1). These constants

318

are of the same order of magnitude as the rate limiting constants in pathway I determined in

319

our work (k1 = 0.918 L mol-1 s-1, k4 = 1.25 L mol−1 s−1).

320

Pathway II: oxidative aromatic nitration. Aromatic compounds can also be subjected to a

321

two-electron process, i.e., electrophilic aromatic substitution.30,

322

nitrosation products were found in the reaction mixture, which can be attributed to low

323

nitrosation rates resulting from significant energy barriers for N-protonated nitroso

324

derivatives formation from CTC.48,

325

autoxidized gaining nitrated products that are highly reminiscent of those from electrophilic

326

aromatic nitration.61 CTC oxidation (only possible in the presence of O2 or other oxidative

327

species) is believed to proceed through several rearrangement steps,60 each of them being

328

accompanied by a certain energy barier.59 Therefore, such oxidative nitration only happens if

329

the electron transfer in CTC is prohibited or highly reversible as proposed in our case.

330

Individual rearrangement steps, e.g., sigma complex formation (5’’ and 6’’ in Figure 1), are

331

not taken into account in our model study. To our knowledge, oxidative aromatic nitration

332

has never been considered in atmospheric processes, although some evidence has recently

333

been published by our group.30

50

57-60

Interestingly, no

However, in the presence of O2, CTC can be

334

If electron transfer in CTC is reversible, nitration mechanism through consecutive one-

335

electron processes (pathway I) cannot be easily distinguished from the more conventional

336

two-electron electrophilic nitration;62 especially if the aryl cation radical (or PhO•)

337

intermediate is not detectable.63 Usually, substituted aromatics exhibit higher degree of

338

charge transfer and the corresponding substituted CTC are more prone to electron transfer

339

resulting in the favorable nitration according to pathway I.62 Nevertheless, results of our

340

combined experimental and modeling approach show that in the case of 3MC, thermal

341

decomposition of CTC via one-electron transfer giving aryl cation radical and NO• competes



Page 18 of 26

342

with the electrophilic two-electron process following the autoxidation of 2 to 3MC-NO2+.

343

Once CTC is formed (its formation is considered rate limiting), it rather undergoes

344

thermolysis than oxidation with O2 (k2 >> k6, k7), supporting the domination of pathway I.

345

However, as 2 and 3 are both (trans)formed reversibly, their preferential reaction route

346

depends on applied conditions. When back-electron transfer (k−2[PhO•][NO•]) prevails over

347

the oxidation of phenoxy radical (k3[PhO•][HNO2]; i.e., at low HNO2 concentrations, Figure

348

2c), electrophilic substitution is preferred. Oxidative aromatic nitration of 3MC is thus

349

disfavored at high HNO2 concentrations (Figure 2a), but prohibited in the absence of O2

350

(Figure 2b and Figure S1b).

351

Pathway III: recombination of radical species. Following the existing literature64-68

352

another possible pathway was included in our model. 3MC can be nitrated via PhO•

353

combination with NO2• which could also result in the distinct ratio between 3MNC isomers.

354

As NO• is not present in the solution nor is expected to form during the studied reaction, O2 is

355

needed to oxidize NO• to NO2• and allow for this reaction pathway.

356

Product distributions. Numerical integration reveals (Figure 4) that the proposed

357

competitive pathways II and III to the dominant oxidation-addition mechanism contribute

358

more to the nitration in para position (i.e., 3M4NC formation), which actually allowed us to

359

distinguish between them and the dominant pathway in the model. As the formation of

360

reactive NO2+ occurs within the CTC (note NO2+ is 1014 times more reactive than NO+ in

361

electrophilic aromatic substitution reaction and reacts with all reactive aromatics at the same

362

rate),64,

363

observed as to particular MNC isomer. Similar explanation can be given for the radical

364

recombination. In contrast, the conventional pathway I yields for an order of magnitude more

365

3M5NC than 3M4NC. Compare the ratios k4/k5 with k6/k7 and k8/k9 in Table 1.

69, 70

3MC and NO2+ are encountered without diffusion and less preference is


Page 19 of 26


366 367

Figure 4: Product distributions (3-methyl-5-nitrocatechol (5, 3M5NC) with pattern and 3-

368

methyl-4-nitrocatechol (6, 3M4NC) without pattern) by mechanism (colors) at different

369

reaction conditions: with O2 (exp. 2) and without O2 (exp. 4) both at 1·10−3 mol L−1 HNO2,

370

and with O2 at 1·10-4 mol L−1 HNO2 (exp. 7, with O2).

371

Environmental Relevance. The presented investigation suggests that HNO2 is an important

372

mediator in dark aqueous-phase transformations of particular substituted aromatic pollutants

373

under mild environmental conditions. Its catalytic and oxidative roles in nitration of catechols

374

are emphasized. HNO2 is shown to be directly involved in the transformation of 3MC within

375

the non-radical reaction pathway including consecutive oxidation and conjugated addition

376

reactions, which can be a significant source of BrC and potential SOA compounds.

377

Considering atmospherically relevant aqueous-phase concentrations of 3MC (taken as an

378

upper limit of less water-soluble aqueous-phase atmospheric methoxyphenols)71 and total

379

HNO2/NO2− to be 10−5 and 10−4 mol L−1, respectively, we predicted the atmospheric lifetime

380

of 3MC according to this mechanism to be 2.4 days in the dark (at pH 4.5 and 25 °C;

381

compared with about 8 days for the other two competitive pathways, i.e., II and III, see Table

382

S2). Furthermore, 3MC nitration in the dark was found moderately dependent on

383

temperature.



Page 20 of 26

384

Dark two-step nitration pathway is expected to dominate at pH around the pKa of

385

HNO2, which is typical for atmospheric aerosols. Note that shorter atmospheric lifetimes of

386

3MC at lower pH values gathered in Table S2 do not result in faster production of nitrated

387

products; quinone and other intermediate products rather accumulate in the solution under

388

those reaction conditions (compare atmospheric lifetimes of 3MC and their nitrated products

389

formation equivalents in brackets). Because of the acid-base equilibrium, there is a lack of

390

NO2− under very acidic conditions, therefore the slower oxidative aromatic substitution and

391

radical nitration pathways gain in importance at low pH. Moreover, due to its rapid mass

392

transfer to the gaseous phase (because of relatively high HNO2 volatility and large surface-to-

393

volume ratio of small aerosol particles), the concentration of HNO2 in acidic aerosol water is

394

often very low, which would result in extremely long atmospheric lifetimes of 3MC

395

according to this transformation pathway; i.e., at pH 3.5 and 10−6 mol L−1 HNO2/NO2−

396

(constant concentration in the aerosol water shell is assumed in this case) only about 10% of

397

3MC would be consumed in 10 days. Long atmospheric lifetimes are also expected in cloud

398

water (higher pH and more diluted solutions), as the necessary initial oxidation step is

399

prohibited at those conditions. It is important to mention that benzene does not react at all

400

under these conditions,5 phenol barely gets nitrated (unpublished data), while guaiacol (2-

401

methoxy-phenol) has about 3-times longer expected lifetime.24

402

To support the conclusions made above, absorption properties of the reaction mixture

403

were also investigated (Figure 5). The two products formed, 3M5NC and 3M4NC, as well as

404

the reaction mixture all absorb in near-UV (300–400 nm) and Vis ranges (>400 nm,) and are

405

believed to contribute significantly to BrC and/or absorptive properties of the atmospheric

406

gas. MNC have been identified in PM1032 and PM2.517 samples and it is well known that they

407

are among the major components of HULIS,33 the most widespread chromophoric substance

408

in the atmosphere. Due to the global impacts of absorbing tropospheric aerosols, direct by


Page 21 of 26


409

absorbing solar irradiation and indirect by influencing the cloud formation, the mechanisms

410

of BrC formation are of utmost importance for the atmospheric science community. In this

411

regard we summarize our study and conclude that catechol two-step oxidation-conjugated

412

addition reaction mechanism can be an important pathway of dark secondary nitroaromatic

413

chromophore formation in the atmosphere, especially in polluted environments with high

414

NOx concentrations and relatively acidic particles (pH around 3).

415 416

Figure 5. Absorption spectra of 3-methylcatechol (3MC), 3-methyl-5-nitrocatechol

417

(3M5NC), 3-methyl-4-nitrocatechol (3M4NC) and the reaction mixture after 15h (exp. 2)

418

measured at pH 4.6.

419 420

Associated content

421

Supporting Information

422

Additional 2 tables and 8 figures. The supporting information is available free of charge via

423

the Internet on the ACS Publications website at http://pubs.acs.org.

424 425

Author information

426

Corresponding Authors



Page 22 of 26

427

*Phone: +386 (1) 4760 361 (IG) and +386 (1) 4760 384 (AK); fax: +386 (1) 4760 300; e-

428

mails: [email protected] (IG) and [email protected] (AK).

429

Notes

430

The authors declare no competing financial interest.

431 432

Acknowledgments

433

This work was supported by the Slovenian Research Agency (Contract Nos. P1-0034

434

and P2-0152), which is gratefully acknowledged.

435

436

References

437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466

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Nighttime aqueous-phase formation of nitrocatechols in the

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