Kinetics of the Anaerobic Reaction of para-Substituted Phenols with Nitrogen Dioxide

Article abstract of DOI:10.1002/kin.21054

para-Substituted phenols in aqueous solution under anaerobic conditions readily react with nitrogen dioxide (NO₂ ^●) over a wide range of experimental conditions. The rate and rate law of the process were dependent on phenol concentration and solution pH. The kinetic order in phenol changed from one (low concentration) to zero (high concentration), a result attributable to total NO₂ ^● capture. Initial consumption rate (r₀) of phenols versus pH plots showed parabolic behavior with a minimum rate at pH ca. 5. On the other hand, the maximum rate took place at high pH (pH>10) and involved the protonated phenols. The reaction rate of para-substituted phenols with NO₂ ^● correlated with the bond dissociation energy and with Hammett's parameter. Based on such results and also supported by analysis of products carried out by HPLC-MS/MS, our data conclusively show that, in spite of the fast acid–base interchanges of phenols and the interconversion of the different nitrogen oxides, the mechanisms of phenols nitration mediated by NO₂ ^● or HONO are clearly different.

Full text of DOI:10.1002/kin.21054

Kinetics of the Anaerobic
Reaction of
para-Substituted Phenols
with Nitrogen Dioxide
1
1
2
1,3
´
´
´
JAEL REYES, EDUARDO LISSI, CAMILO LOPEZ-ALARCON, MARıA A. RUBIO
1Facultad de Qu´ımica y Biolog´ıa, Universidad de Santiago de Chile, Santiago, 8320000, Chile
2Facultad de Qu´ımica, Pontificia Universidad Cato´lica de Chile, PUC, Santiago, Chile
3CEDENNA, Universidad de Santiago de Chile, Santiago, 8320000, Chile
Received 18 July 2016; revised 3 October 2016; accepted 4 October 2016
DOI 10.1002/kin.21054
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: para-Substituted phenols in aqueous solution under anaerobic conditions readily
react with nitrogen dioxide (NO2•) over a wide range of experimental conditions. The rate
and rate law of the process were dependent on phenol concentration and solution pH. The
kinetic order in phenol changed from one (low concentration) to zero (high concentration),
•
a result attributable to total NO2 capture. Initial consumption rate (r0) of phenols versus
pH plots showed parabolic behavior with a minimum rate at pH ca. 5. On the other hand,
the maximum rate took place at high pH (pH>10) and involved the protonated phenols. The
•
reaction rate of para-substituted phenols with NO2 correlated with the bond dissociation
energy and with Hammett’s parameter. Based on such results and also supported by analysis
of products carried out by HPLC-MS/MS, our data conclusively show that, in spite of the fast
acid–base interchanges of phenols and the interconversion of the different nitrogen oxides, the
•
ꢀC
mechanisms of phenols nitration mediated by NO2 or HONO are clearly different.
2016
Wiley Periodicals, Inc. Int J Chem Kinet 1–9, 2016
reactive species are capable of reacting with a wide
range of organic comp•ounds. Among the compounds
able to neutralize NO2 , phenols appear as one of the
most important substrates [6–11]. The reaction be-
INTRODUCTION
Nitrogen oxides constitute a family of free radicals
(NO•, NO2 , NO3 ) characterized by a rich chem-
istry with relevance in a variety of fields, ranging from
air chemistry to biological processes [1–5]. In partic•-
ular, the chemistry of the reactions involving NO2
has attracted the attention of researchers, since such
•
•
•
tween phenols and NO2 takes place due to the rel-
atively weak strength of the oxygen–hydrogen bond in
these compounds, allowing their participation in hy-
drogen transfer processes, such as that depicted in Re-
action (R1) [12].
Correspondence to: M. A. Rubio; e-mail: maria.rubio@usach.cl.
2016 Wiley Periodicals, Inc.
Ph − O − H + NO•2 → Ph − O• + HONO (R1)
ꢀC

2
REYES ET AL.
•
While the reaction of phenols with NO2 is not rele-
vant in the gas phase, in aqueous solutions is partic-
ularly favored by the inclusion of electron-donating
groups in the para position of such derivatives [10].
This is directly associated with the capacity of phe-
nols to stabilize the secondary free radicals (phenoxyl
radicals) generated as a consequence of the hydro-
EXPERIMENTAL
Reagents
Trolox
(6-hydroxy-2,5,7,8-tetramethylchromane-
2-carboxylic acid), PGR (pyrogallol red), ABTS
(2,2ꢁ-azino-bis(3-ethylbenzothiazoline-6-sulfonic
acid)diammonium salt), N-(1-naphthyl) etilendi-
amine, sulfanilamide, phosphoric acid, phenol,
para-hydroxyphenol, para-methoxyphenol, para-
nitrophenol, coumaric acid (trans-4-hydroxycinnamic
acid), ferulic acid (4-hydroxy-3-methoxycinnamic
•
gen abstraction triggered by NO2 [10]. Conversely,
at high pHs (higher of phenols pKa), the energeti-
cally favorable oxidation of phenolates to phenoxyl
radicals is promoted by the concomitant reduction of
•
NO2 to nitrite in a plain electron transfer process
acid),
and
sinapic
acid
(3,5-dimethoxy-4-
(Reaction (R2):
hydroxycinnamic acid) were purchased from
Sigma Aldrich (St. Louis, MO). Powdered metallic
cooper, sodium nitrite (primary standard), nitric acid,
hydrochloric acid, phosphorous acid, and sodium
chloride were purchased from Merck (Darmstadt,
Germany). Nitrogen was purchased from Linde Gas
(Santiago, Chile).
Ph − O− + NO2• → Ph − O• + NO−2
(R2)
The rate constant of Reaction (R1) increases when the
strength of the H–O bond in the phenols decreases [10],
whereas the rate constant of Reaction (R2) is de-
termined by the oxidation potential of the phenolate
ion. In para-substituted phenols, the rate of the elec-
•
NO2 Generation
NO2 was produced by the oxidation of metallic Cu0
•
•
tron transfer to NO2 will be then determined by the
induced by HNO3, according to Reaction (R3):
value of the sigma (σ+) parameter of the substituent
group.
Cu0(s) + 4 HNO3 → (Cu II) + 2 NO3− + 2 NO2• + 2 H2O
Reactions (R1) and (R2) are simple processes that
represent elementary steps and follow bimolecular
kinetics whose rate constants have been previously
determined by time-resolved pulse radiolysis [13].
However, steady-state measurements of these reac-
tions are difficult to interpret. This is mainly due to
the fast interchange•among reactive nitrogen species
(R3)
To this purpose, 10 mg of powdered metallic Cu0 was
suspended in 20 mL of water, and 30 mL of concen-
trated HNO3 was added. The produced NO2• was trans-
ferred to the reactor (volume ca. 500 mL) by a nitrogen
stream (Fig. 1). The total flow of gases was kept at ca.
such as NO•, NO2 , NO2−, NO3 , HONO, N2O3,
•
•
and N2O4 [14]. Additionally, the descriptions of such
reactions are complex since each reactive species
is distributed between the aqueous solution and
the gas phase [4,14]. To get more insight into the
influence of the chemical structure of the phenol on
the kinetics of the above-mentioned reactions, in the
present work we studied the consumption of para-
substituted phenols, bearing electron donor and ac-
45 mL/min. The NO2 reaching the reacting cell was
•
ceptor groups, triggered by NO2 , and the results
were compared with those previously reported em-
ploying HONO as a reactant. For this purpose, so-
lutions containing phenols were kept at known ex-
perimental conditions (concentration, ionic strength,
and pH) and bubbled with a continuous flux of a
•
gas mixture of nitrogen and NO2 [14] or exposed
Figure 1 Scheme of the system employed to generate
NO2•. Concentrated HNO3 was added to a suspension of
metallic Cu0 in water. The suspension was exposed to a con-
stant flux of nitrogen, employed as carrier gas, adjusted at a
flux of 45 mL/min. A mixture of N2 and NO2• was continu-
ously bubbled into a reaction cell containing the sample.
to HONO. In addition, and taking into account the im-
portance of cinnamic acid derivatives in the antioxidant
field, we compared results obtained employing para-
substituted phenols with coumaric, ferulic, and sinapic
acids.
International Journal of Chemical Kinetics DOI 10.1002/kin.21054

ANAEROBIC REACTION OF para-SUBSTITUTED PHENOLS WITH NITROGEN DIOXIDE
3
radical (ABTS•+), the NO2 -mediated consumption
of the former and the production of t•he latter were
employed to assess the rate of NO2 introduction
into the reaction cell. In addition, the consumption of
Trolox, a hydrosoluble vitamin E analogue, was also
determined.
•
titrated as nitrite-employing Griess’ assay. To this aim,
3.0 mg of N-(1-naphthyl) etilendiamine was solubi-
lized in 3 mL of bidistilled water (named solution A).
A second solution (named solution B, Griess’ reagent)
was prepared by solubilizing 30 mg of sulfanilamide
in a mixture of 2.8 mL of water and 0.2 mL of phos-
phoric acid. For nitrite quantification, equal volumes
of solutions A and B were added to each sample. After
30 min incubation at 4ºC in the dark, the absorbance
of the sam−ple was measured at 543 nm and the total ni-
trite (NO2 + HONO) concentration was determined
from a calibration plot employing NaNO2 as primary
standard.
In aqueous solutions (pH 7.4), PGR presents an ab-
sorption spectrum with an absorption characterized by
a band with a maximum in absorbance at 540 nm.
The intensity of this band decreases when PGR is
oxidized (forming a quinone•as single product) by
reactive species such as NO2 , hypochlorite, perox-
ynitrite, and peroxyl radicals [15,16]. In the presence
•
of NO2 , it was observed a decrease of the absorbance
of PGR at 540 nm, together with the formation of
a new band with a maximum near 380 nm (Fig. 2).
From kinetic data (such as that shown in Fig. 2), we
determined the initial consumption rate of PGR (r0)
in a wide range of initial concentrat•ions at a fixed
flux (45 mL/min) of the (N2 + NO2 ) gas mixture.
Figure 3 shows the dependence of r0 values with the
initial concentration of PGR. As it is shown, at PGR
concentration lower than 30 µM, r0 was proportional
to the PGR concentration. However, above this con-
centration, r0 values reached a plateau at 0.8 µM/min
(Fig. 3). This behavior implies first- and zero-order ki-
netics in PGR, at low and high PGR concentrations,
respectively. Moreover, the presence of a plateau indi-
cates that all the incoming NO2• molecules are trapped
by PGR. If it is •assumed that one PGR molecule re-
•
Reaction of Phenols with NO2 and HONO:
Kinetics of Phenol Consumption and
Formation of Oxidized Products
To follow the kinetics of the reaction between para-
substituted phenols or cinnamic acid derivatives and
NO2 , aqueous solutions of phenols were bubbled with
•
•
a nitrogen flow enriched in NO2 . At different incuba-
tion times, aliquots were taken and analyzed by high-
performance liquid chromatography (HPLC) coupled
to a diode array detector (DAD). The HPLC–DAD
system (Agilent 1200 series) employed a C-18 col-
umn, and the mobile phase (isocratic) was a mixture of
potassium phosphate (10 mM, pH 2.6) with acetoni-
trile in an 80/20 V/V ratio. As HONO is generated as
a product of the reaction (Reaction (R2), the consump-
tion of phenols elicited by this reactive species was also
measured. Thus, solutions containing para-substituted
phenols (50–100 µM) and nitrite (2–200 µM)
were incubated at pH 2 with 17 mM NaCl solution
and analyzed by the same HPLC–DAD system.
Formation of oxidized products was studied by
HPLC with mass detection (MS/MS). Aliquots of
working solutions were analyzed employing an Eksi-
gent model ultra LC 100-XL with a Triple Quad 4500
detector. An Inersil ODS-4 C18, 3 µm (particle size)
2.1 × 100 mm column size, GL Science (Tokyo, Japan)
was kept thermostated at 40°C. Formic acid 0.1% in
water and acetonitrile was used as a mobile phase, and
mass detection (ESI-MS/MS) was carried out employ-
ing a negative ionization mode.
•
moves two NO2 radicals; an input rate of NO2 ca.
1.6 µM/min can be estimated.
•
ABTS readily reacts with NO2 generating the
cationic free radical (ABT•S•+) as a principal prod-
uct. In the presence of NO2 , the intensity of the UV–
visible band of ABTS at 340 nm decreases, generat-
ing four main bands in the visible region at 413, 646,
734, and 820 nm and an isosbestic point at 380 nm
(Fig. 4). The intensity of the band at 734 nm has been
commonly employed to assess ABTS•+ concentration
(ε734 = 1.3 × 104 M−1 cm−1) [17].. Assuming a total
removal of NO2• from the incoming gas flow, and that
one molecule of ABTS reacts with one molecule of
NO2 , the rate of ABTS•+ formation implies that ca.
•
•
1.6 µM/min of NO2 is introduced into the reaction
cell.
Trolox is a water-soluble phenol that readily re-
acts with free radicals (X·) removing two free rad-
icals per each molecule of Trolox, according to
Reaction (R4).
RESULTS AND DISCUSSION
•
Rate of NO2 Introduction into the
Reaction Cell
Taking advantage of the oxidizing ability of NO2• and
the spectroscopic properties of PGR and ABTS cation
2X• + Trolox → products
(R4)
International Journal of Chemical Kinetics DOI 10.1002/kin.21054

4
REYES ET AL.
•
Figure 2 Bleaching of PGR (5 µM) induced by NO2 after 0 and 20 min of reaction in phosphate buffer (75 mM) at pH 7.4
and 25°C. Arrows indicate the decrease or increase of the corresponding band intensity.
•
Role of HONO in the Reaction of NO2 with
Para-Substituted Phenols
Irrespective of the initial nitrogen bearing compound
(NO2• or HONO), a large number of substances (NO•,
•
NO2 , HONO, NO2−, N2O3, N2O4) are present along
the reaction and can contribute to the phenol modi-
fication and consumption [18]. To assess this point,
and taking into account the formatio•n of nitrite as
product of the reaction between NO2 and phenols,
we measured the initial consumption rate of para-
•
methoxyphenol following HONO or NO2 anaerobic
addition over a wide range of pH (2–12). Also, we
compared the relative rate of reaction for the family of
cinnamic acid derivatives composed by coumaric, fer-
ulic, and sinapic acids and the effect of electron donor
and electron acceptor groups in para position.
Figure 3 Dependence of the initial consumption rate (r0) of
PGR elicited by NO2 addition. The solution was incubated
•
in phosphate buffer (75 mM at pH 7.4) and 25°C. r0 values
were determined from the slope, extrapolated at zero reaction
time.
•
Reaction Rate of Para-Methoxyphenol with NO2 :
Effect of pH. Figure 6 presents the dependence of the
initial consumption rate of para-methoxyphenol, pro-
moted by NO2 , with the pH of the medium. This pro-
•
At high concentrations, Trolox traps all the free radicals
introduced into the system. Thus, during its exposure to
the gas mixture, all the NO2• can react forming quanti-
tatively nitrite as one of the main reaction products (at
pH 7.4). The titrating of nitrite by Griess assay showed
file can be explained by the formation of nitrite, and its
conjugated acid (pKa = 3.14), and the presence of the
para-methoxyphenol and its phenolate (pKa = 10.2).
In this context, the left branch of the plot would im-
ply that HONO is more reactive than NO2− toward the
protonated phenol. Similarly the right branch would
indicate that phenolates are more reactive than the pro-
tonated compounds.
•
that NO2 was introduced to the system at a rate of
1.7 µM/min (Fig. 5). Remarkably, this value is close to
those determined from the consumption of PGR or the
formation of ABTS•+ radical cation.
International Journal of Chemical Kinetics DOI 10.1002/kin.21054

ANAEROBIC REACTION OF para-SUBSTITUTED PHENOLS WITH NITROGEN DIOXIDE
5
Figure 4 ABTS (20 µM) consumption and ABTS•+ cation formation induced by NO2 between 0 and 30 min of reaction in
phosphate buffer (75 mM) at pH 7.4 and 25°C. Arrows indicate the decrease or increase of the band intensity.
•
•
Figure
5
Concentration of NO2 generated by the
Cu0/HNO3 system. NO2 concentration was determined by
the formation of nitrite ions (NO2−) during the reaction of
NO2• with an excess of Trolox. NO2− was determined by the
Griess methodol. Trolox solutions were incubated in phos-
phate buffer (75 mM, pH 7.4 and 25°C•) and bubbled with
•
Figure 6 Dependence of the initial consumption rate (r0) of
•
para-methoxyphenol (10 µM) induced by NO2 at different
pHs.
the gas mixture comprising N2 and NO2
.
At low pH (<5), the protonated phenol is removed
faster by HONO (excess) than by the nitrite anion. The
process can take place through a complex mechanism
(such as proton-coupled electron transfer) [19] and can
be represented by
The faster rate of the phenol/NO2• interaction takes
place at pH >10 and hence must involve the depro-
tonated HONO and para-methoxyphenol and can be
represent•ed by Reaction (R2). It is interesting to note
−
that NO2 to NO2 reduction increases its water solu-
bility, favoring the nitration of the phenols.
HONO + PhOH → NO• + PhO• + H2O (R5)
International Journal of Chemical Kinetics DOI 10.1002/kin.21054

6
REYES ET AL.
Figure 7 Dependence of the log of initial consumption rate (log r0) of para-methoxyphenol, elicited by HONO, at different
pHs. Measurements were carried out under continuous N2 bubbling.
Figure 6 shows that more than one mechanism is opera-
•
tive in the reaction of phenols with NO2 . Furthermore,
•
the difference in the profiles obtained employing NO2
(Fig. 6) and HONO (Fig. 7) shows that, in spite of the
fast conversions between different forms of nitrogen
oxides, the mechanism of NO2• and HONO promoting
nitration of phenols is clearly different. These differ-
ences remain even when HONO reaction is carried out
under continuous nitrogen bubbling (anaerobic condi-
tion).
The occurrence of different mechanisms at different
•
pHs is a frequent feature of NO2 + HONO reactions
[18,20,21] The data of phenol reaction with HONO
represented in Fig. 7 show that the fastest rate takes
place at low pH according to Reaction (R5) [14,22].
Conversely, the slowest reaction takes place at pH ࣈ11
involving nitrite and phenolate anions as reactants.
•
Figure 8 Kinetics profiles of phenol consumption by NO2
addition. Solutions of phenol (ꢀ), para-hydroxyphenol (ꢁ),
and para-methoxyphenol (ࣻ) at 10 µM were incubated under
a NO2 flow (1.8 µM/min) at pH 7.4 and 25°C.
Consumption rate of Cinnamic Acid
Derivatives: Dependence of the Phenol
Concentration and their Characteristics
Figure 8 shows typical results of the kinetic profiles
of phenol consumption elicited by NO2 . From these
para-substituted phenols (Fig. 9A) and cinnamic acids
(Fig. 9B), respectively. These plateaus were indepen-
dent of the compound reactivity, suggesting total trap-
ping of the NO2• present in the gas flow. This indicates
that secondary reactions barely contribute to the total
rate of the process, at least for monophenols [23]. It
is interesting to note that the comparison of the value
•
kinds of data, initial consumption rates (r0) were deter-
mined and plotted as the function of phenol concentra-
tion (Fig. 9). As this figure shows, at low concentration,
the rate depends of the compound considered and its
concentration. However, at high concentrations, r0 of
the studied compounds (except coumaric acid) reached
a plateau at similar values ࣈ0.9 and 0.4 µM/min for
•
in the plateau with the rate of incorporation of NO2
(ࣈ1.7 µM/min) leads to the conclusion that nearly two
International Journal of Chemical Kinetics DOI 10.1002/kin.21054

ANAEROBIC REACTION OF para-SUBSTITUTED PHENOLS WITH NITROGEN DIOXIDE
7
Figure 10 Dependence of log of the initial consumption
rate (log r0) of para-substituted phenol derivatives with their
Hammett`s sigma parameters (σ). Solutions were incubated
at 25°C at pH 5.0 in the presence of a NO2• flow (R = 0.997;
p = 0.00276).
ing the weakest bond in sinapic acid nearly 50 times
more labile than that of coumaric acid. However, it
•
must be taken into account that both, NO2 and phe-
nols, are, particularly at low pHs, distributed between
the gaseous and liquid phases of the reactor (Fig. 1),
precluding a sound quantitative evaluation of the aque-
ous phase rate constants [4,14].
The other critical parameter controlling the rate of
the reaction between a para-substituted phenol and
NO2 is the electrophilicity of the substituent group.
•
Figure 9 Dependence of the initial consumption rate (r0) of
phenolic compounds (graphic A) and cinnamic acid deriva-
This factor can be quantified by the value of Hammett’s
parameter (σ). This effect is clearly evidenced in the
data depicted in Fig. 10, which show a dependence of
•
tives (graphic B) elicited by NO2 with its initial concen-
tration. Solutions of phenol (ꢀ), para-hydroxyphenol (ꢁ),
para-methoxyphenol (ࣻ), sinapic acid (◦), ferulic acid (ꢂ),
and coumaric acid (ꢃ) were incubated in phosphate buffer
(75 mM, pH•7.4, at 25°C) and bubbled with a gas mixture of
N2 and NO2 at 1.8 µM/min.
•
the phenol consumption rate (r0), induced by NO2 ,
with the charge density in the aromatic ring. The neg-
ative slope (–2.0) emphasizes the electroaffinity of the
process. Other values reported in closely rela−ted works
were –1.23 for HONO [14] and –3 for NO2 [24], in
which the largest slope corresponds to a simple elec-
tron transfer processes.
•
NO2 molecules were consumed by each reacted phe-
nol. In the cases of•sinapic and ferulic acids, near four
molecules of NO2 react with each derivative. How-
ever, at low concentrations, the relati•ve reactivity of
cinnamic acid derivatives toward NO2 was similar to
that reported for their reactivity toward peroxyl radi-
cals [23]. This is in agreement with the fact that the
bond dissociation energy (BDE) of the H–O bond fol-
lows the trend:
The rate of the reaction of para-substituted phe-
nols with NO2 correlated with the BDE: para-
•
methoxyphenol (81.3 kcal/mol), para-hydroxyphenol
(81.7 kcal/mol), phenol (87.5 kcal/mol), and para-
nitrophenol (91.7 kcal/mol) [25] and with Ham-
mett’s parameter (σ): para-methoxyphenol (–0.3),
para-hydroxyphenol (–0.4), phenol (0.0), and para-
nitrophenol (0.8). This emphasizes the relationship be-
tween BDE and charge density at the aromatic ring (R
= 0.951; p = 0.048).
Sinapic acid (76.9 kcal/mol) < ferulic acid
(81.9 kcal/mol) < coumaric acid (84.3 kcal/mol) mak-
International Journal of Chemical Kinetics DOI 10.1002/kin.21054

8
REYES ET AL.
The rate of the reaction of para-substituted phenols
with NO2 correlated with their BDE and Hammett’s
σ parameter
Products•of the Reaction between Phenols
and NO2
•
Phenol. NO2• and, in a smaller degree phenols, are dis-
tributed among the gas and liquid phases present in the
reactor. This mimic what take place in the atmosphere
where they are distributed between the gas phase an
wet particles and/or micrometeors [26,27]. This pre-
cludes a quantitative evaluation of the kinetics of the
chemical processes. In spite of that, HPLC coupled to
MS/MS can provide an insight of the main products
produced in the nitration/oxidation of the phenols at
different stages of the reaction. In fact, the MS/MS
characterization of the obtained product of the reac-
This work was supported by FONDECYT (grants
no 1141142), DICYT (Projects Numbers 091541LG
and 041501RC) and Centers of Excellence with
Basal/CONICYT financing, Grant FB0807, CEDENNA.
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•
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rendering
3,4-dihydroxy-5-methoxy-3-methyl,2-
main
nitrophenyl,2,3-dinitropropanoic acid as
a
product. This reaction path emphasizes the capacity of
•
NO2 to add to activated double bonds [6,28].
CONCLUSIONS
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Alarcon, C. Int J Chem Kinet 2014, 46,143–150.
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