Kinetics and mechanisms of reactions of the nitrate radical (NO 3) with substituted phenols in aqueous solution

Article abstract of DOI:10.1039/b412933d

Second order rate constants were obtained for the reactions of the nitrate radical (NO₃) with substituted phenols in aqueous solutions at 298 K and pH = 0.5. The following compounds were investigated and the corresponding rate constants are rep

Full text of DOI:10.1039/b412933d

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R E S E A R C H P A P E R
Kinetics and mechanisms of reactions of the nitrate radical (NO3)
with substituted phenols in aqueous solution
Paolo Barzaghi and Hartmut Herrmann*
Leibniz-Institut fu¨r Tropospha¨renforschung, Leipzig, Germany
Received 23rd August 2004, Accepted 20th October 2004
First published as an Advance Article on the web 4th November 2004
Second order rate constants were obtained for the reactions of the nitrate radical (NO ) with substituted phenols
3
in aqueous solutions at 298 K and pH ¼ 0.5. The following compounds were investigated and the corresponding
8
ꢂ1 ꢂ1
s
s ]; (4) 2-methoxyphenol [k
rate constants are reported: (1) 2-hydroxyphenol [k ¼ (5.6 ꢀ 0.8) ꢁ 10 M
]; (2) 2-methylphenol [k ¼ (8.5
4
1
2
8
ꢂ1 ꢂ1
8
ꢂ1 ꢂ1
ꢀ
ꢁ
M
0.2) ꢁ 10 M
s
]; (3) 2-ethylphenol [k
3
¼ (6.7 ꢀ 0.4) ꢁ 10 M
¼ (1.1 ꢀ 0.1)
8
10 M
ꢂ1 ꢂ1
7
ꢂ1 ꢂ1
8
s
]; (7) 2-fluorophenol [k
]; (5) 2-nitrophenol [k ¼ (2.3 ꢀ 0.4) ꢁ 10 M
s ]; (6) 2-cyanophenol [k ¼ (3.1 ꢀ 0.3) ꢁ 10
]; (8) 2-chlorophenol [k
5
6
ꢂ1 ꢂ1
8
ꢂ1 ꢂ1
ꢂ1 ꢂ1
8
ꢂ1
s
7
¼ (5.5 ꢀ 0.4) ꢁ 10 M
s
8
¼ (2.9 ꢀ 0.3) ꢁ 10 M
ꢂ1
8
8
ꢂ1 ꢂ1
s
]; (9) 2-bromophenol [k ¼ (2.7 ꢀ 0.1) ꢁ 10 M
s
]; (10) 2-phenylphenol [k10 ¼ (2.4 ꢀ 0.4) ꢁ 10 M
s ];
9
8
ꢂ1 ꢂ1
8
ꢂ1 ꢂ1
(
(
(
11) 4-hydroxyphenol [k11 ¼ (8.8 ꢀ 0.5) ꢁ 10 M
s
]; (12) 4-aminophenol [k12 ¼ (8.1 ꢀ 0.3) ꢁ 10 M
s ];
9
ꢂ1 ꢂ1
9
ꢂ1 ꢂ1
13) 4-chlorophenol [k13 ¼ (1.7 ꢀ 0.2) ꢁ 10 M
s
]; (14) 4-bromophenol [k ¼ (1.0 ꢀ 0.4) ꢁ 10 M
14
s ];
s ]. Moreover, the temperature dependence of the
8
ꢂ1 ꢂ1
15) 4-hydroxybenzen ethyl ester [k15 ¼ (8.0 ꢀ 0.4) ꢁ 10 M
reaction of NO with four ortho-substituted phenols, (a) 2-methylphenol, (b) 2-ethylphenol, (c) 2-hydroxyphenol
3
and (d) 2-methoxyphenol were investigated in the temperature range of 278 r T r 318 K and the corresponding
activation parameters were obtained for the first time. A comparison of the reactivity of NO towards other
aromatics present in the tropospheric aqueous phase such as substituted benzenes and substituted benzoic acid
was undertaken.
3
setup to determine the initial concentration of the thus formed
radicals and to observe their time dependent decay for kinetic
studies.
The laser flash photolysis set-up consists of a multigas
excimer laser (Mod. Compex 201, Lambda-Physik) operated
at l ¼ 248 nm (KrF gas fill, W ¼ 700 mJ) with pulse durations
of 10–40 ns. The optical detection system comprises a helium–
neon laser (Mod. 35–2, Spindler and Hoyer) operated at
1
. Introduction
Since the industrial revolution, a strong anthropogenic influ-
ence on the tropospheric composition has taken place. Many
gases, originating entirely from anthropogenic sources, are at
the parts per trillion levels but may still have significant effects
on the environment. Some trace gases control or affect the
1
,2
Earth’s climate and habitability.
The gas phase oxidation of the aromatic hydrocarbons by
light, OH, NOx and O3 leads to products with increased
polarity that might favour a phase transfer to the tropospheric
liquid phase. Moreover, the low vapour pressure of many of
6
32 nm the light of which was passed into a combination of
6
two dielectrically coated mirrors in White configuration and
folded 32 times through the reaction cell resulting in a total
optical pathlength of d ¼ 192 cm. The electrical output of the
photodiode was amplified and fed to a digital storage oscillo-
scope (Mod. Delta Classic, Gould) connected to a computer.
The timing of the experiment was controlled by a trigger
generator. The thermostated reaction cell used in the present
work consisted of a glass cylinder with an inner diameter of
3
,4
these products initiates particle formation.
Most of the
substituted benzenes have been identified as components of
the atmospheric aerosol. These aromatic hydrocarbons are
particularly interesting among the other volatile organic com-
2
pounds (VOCs) due to their physical and chemical properties.
Multiphase aromatic conversion mainly driven by radicals
5
1
0 mm and an inner length of 60 mm with a total volume of
18 ml. The front windows of the cell are plane plates made
3
such as OH and NO could play an important role in multi-
5
phase tropospheric chemistry. Such chemistry might be in-
volved in the multiphase formation of nitroaromatics following
NO emissions. Aromatic VOCs such as nitroaromatic com-
of fused silica. The experimental set-up is described in more
7–9
detail elsewhere.
x
3
The formation of NO following the flash photolysis of
pounds might have adverse effects on ecosystems and on
2
human health.
In the present work, the rate coefficients for reactions of the
nitrate anions (0.01 M, pH ¼ 0.5 with HClO
4
) at l ¼ 248
8
nm occurs as follows:
ꢂ
1
NO3 þ hn (l ¼ 248 nm) þ H - OH þ NO2 (R-a)
OH þ HNO - NO þ H O (R-b)
nitrate radical (NO ) with a number of ortho- and para-
3
substituted phenols have been determined for the first time
using the laser flash photolysis-long path laser absorbance
3
3
2
(
LFP-LPLA) technique. Rate constants for reactions of nitrate
The decays of NO in the presence of variable amount of excess
3
radical in aqueous solution have been studied mainly at 298 K
and in some cases the temperature dependency of the rate
constants have also been investigated.
reagent were measured under pseudo-first order conditions,
therefore the reactant concentrations were chosen to be at least
ten times higher than the initial maximal radical concentra-
ꢂ7
tions, typically [NO
3
]
0
¼ 1 ꢁ 10 M. Normally, eight first
2. Experimental
order decay rate constants were determined and averaged for
every reactant concentration. The errors stated throughout this
work are statistical errors for a confidence interval of 95%.
A laser flash photolysis technique was used to produce NO
aqueous solution coupled with a long path laser absorption
3
in
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
5379

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In Fig. 1a a typical decay trace of nitrate radical in the
6
presence of 2-bromophenol (2 ꢁ 10 M) at 298 K and pH ¼ 0.5
is shown.
As can be seen from the logarithmic decay reported in Fig. 1b,
in the time range of the kinetic measurements a good linearity is
obtained, and under pseudo-order conditions the first order rate
constant could be obtained from the regression line.
All solutions were freshly prepared before every single
experiment and the reagents used in this study were purchased
from Sigma-Aldrich at the highest purity available (499.9%).
The phenols are known to absorb strongly in the UV-region
and they might have a relative absorption maximum also in the
visible depending upon the ring substituent, therefore, before
any kinetic measurements, the UV spectra of the compounds
considered in this study were taken in order to avoid possible
interference with both excimer (l ¼ 248 nm) and analytical
(
l ¼ 632.8 nm) light. To better understand the effect of the
Fig. 2 Measured k1st vs. [reactant] for the reaction of NO
2
3
with: (a)
substituent on the reactivity of the phenolic moiety, a larger
number of compounds was considered in this study. Tentative
determinations of the second order rate constant for the
reactions of polysubstituted phenols such as 2,6-dimethyl-
phenol, 2,6-dimethoxyphenol and 1,2,3-trihydroxybenzene
were performed. An offset of about 10% was observed for all
three compounds and this might lead to an underestimation of
the observed rate constant. Hence, the presented experimental
data all refer to phenols which do not absorb light significantly
at l ¼ 248 and l ¼ 632.8 nm.
-methylphenol; (b) 2-ethylphenol.
The reaction pathways, which are generally possible in
reactions of NO with aromatics in aqueous solution, involve
four different competitive classes of reactions, i.e. (a) H-
3
abstraction at the side-chains or phenolic H-atom, (b) H-
abstraction at the aromatic ring, (c) addition–elimination and
(d) direct single electron transfer (SET) from the aromatic
7
,10
ring.
The electronic effect of the substituents could influence (a)
reactions occurring according to an electron transfer mechan-
ism by changing the electron density of the aromatic ring as
well as (b) the OH bond strengths of the phenolic moiety in the
case of the ortho- and para-substituted phenols.
3
. Results and discussions
3
The rate coefficients for the reactions of NO with a number of
ortho- and para-substituted phenols have been determined
using the LFP-LPLA technique.
3
3
.1. Reactions of NO with ortho-substituted phenols at 298 K
First order rate constants were measured for 5 different
ꢂ6
reactant concentrations (generally in the range from 1 ꢁ 10
ꢂ5
to 1 ꢁ 10 M) to obtain second order rate constants from
plots of k1st against the reactant concentration as shown in
Figs. 2 and 3. The investigations were performed at 298 K and
pH ¼ 0.5 (with HClO
4
).
In Fig. 3 the rate coefficients determined for the three ortho-
halogenated phenols show that the reaction proceeds faster for
the phenol containing the most electronegative substituent and
follows the series F 4 Cl 4 Br. However, the formation of a
radical cation carrying a positive charge as reaction intermedi-
ate is expected in an electron transfer reaction. Therefore,
electron withdrawing substituents might destabilise the positive
charge carried by the aromatic ring resulting in a smaller
rate constant for the 2-fluorophenol. Moreover, electron
Fig. 1 (a) Intensity radical decay trace and (b) logarithm of abso-
rbance decay of nitrate radical (l ¼ 632.8 nm) from the flash photolysis
of nitrate in the presence of 2-bromophenol (pH ¼ 0.5, T ¼ 298 K,
3
Fig. 3 Measured k1st vs. [reactant] for the reaction of NO with:
(a) 2-fluorophenol; (b) 2-chlorophenol; (c) 2-bromophenol.
8
averages).
5380
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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withdrawing substituents, such as F, Cl and Br, are considered
to withdraw electron density from the O–H bond and thereby
increase its strength. Hence, a possible contribution from an H-
abstraction mechanism could not explain the order of reactiv-
ity observed in this work.
3
Table 1 Second order rate constants for the reactions of NO with
ortho- and para-substituted phenol in aqueous solution at 298 K
and pH ¼ 0.5
ꢂ1 ꢂ1
Reaction no.
Compound
k
2nd. 298 K/M
s
Thus, it can be concluded that for the case of ortho-haloge-
nated phenols the steric effect prevails over the electronic effect
in the reaction of NO (see also Section 3.5 and Table 3).
8
R-1
R-2
2-Hydroxyphenol
2-Methylphenol
(5.6 ꢀ 0.8) ꢁ 10
(8.5 ꢀ 0.2) ꢁ 10
(1.1 ꢀ 0.3) ꢁ 10
(6.7 ꢀ 0.4) ꢁ 10
(1.1 ꢀ 0.1) ꢁ 10
(2.3 ꢀ 0.4) ꢁ 10
(3.1 ꢀ 0.3) ꢁ 10
(5.5 ꢀ 0.4) ꢁ 10
(2.9 ꢀ 0.3) ꢁ 10
(2.7 ꢀ 0.1) ꢁ 10
(2.4 ꢀ 0.4) ꢁ 10
(8.8 ꢀ 0.5) ꢁ 10
8
9 a
8
3
The phenol with the nitro group as a substituent shows the
lowest rate constant for the reaction with NO . The obtained
rate coefficient is 100 times slower than for the reaction of NO
R-3
R-4
R-5
R-6
R-7
R-8
R-9
R-10
R-11
2-Ethylphenol
2-Methoxyphenol
2-Nitrophenol
8
3
7
3
8
with the unsubstituted phenol and one order of magnitude
slower than the other investigated phenols here. The two main
reasons for this are: (a) a high bond dissociation energy (BDE)
2-Cyanophenol
2-Fluorophenol
2-Chlorophenol
2-Bromophenol
2-Phenylphenol
4-Hydroxyphenol
8
8
ꢂ1 11
8
for the phenolic hydrogen (366 kJ mol
) which leads to a
8
very slow H-abstraction reaction and (b) the deactivating effect
of the nitro goup on the aromating ring.
8
9
8
9
9
8
a
ꢂ1
(
1.6 ꢀ 0.6) ꢁ 10
A DGR of ꢂ72.5 kJ mol was calculated (see Section 3.5.2
R-12
R-13
R-14
R-15
4-Aminophenol
4-Chlorophenol
(8.1 ꢀ 0.3) ꢁ 10
(1.7 ꢀ 0.2) ꢁ 10
(1.0 ꢀ 0.4) ꢁ 10
(8.0 ꢀ 0.4) ꢁ 10
3
and eqn. (3)) for the electron transfer reaction of NO with 2-
nitrophenol which is the lowest among the phenols considered
in the present work, as presented in Table 3.
In further experiments, the rate coefficient for the ortho-
substituted phenol with a cyano group as substituent was
obtained (Fig. 3). In contrast to expectations, the 2-cyano-
phenol reacts as fast as the other slightly activated phenols.
4-Bromophenol
4-Hydroxybenzene ethyl ester
a
Ref. 8.
The high bond dissociation energy of the phenolic hydrogen
ꢂ1
The rate constants for the reaction of NO with 4-hydro-
3
xyphenol (R-11, Table 1) obtained here turn out to be two
(
371.3 kJ mol , see Table 3), limits the contribution (o7%) of
8
times slower than the value obtained by Umschlag et al. This
the H-abstraction channel to the observed rate constant.
No direct comparison with literature data is possible since
almost all of the reported rate constants here were determined
disagreement can be explained by the different experimental
conditions applied in the two studies. In their study, Umschlag
et al. determined the second order rate constants using the flash
photolysis of nitrate at l ¼ 248 as in the present work but at
for the first time. Very few literature data are available for the
reaction of NO
8
with substituted phenols. This may be due to
3
the fact that former studies of nitrate radical kinetics in
solution used strong oxidizing precursors which tend to oxidise
pH ¼ 0 (with HClO ) instead of pH ¼ 0.5 and a concentration
4
1
2,13
phenol and its derivatives.
However, ortho-substituted phenols were shown to react
rapidly with NO radicals. All the ortho-substituted phenols
considered in the present work present smaller rate coefficients
with respect to the reaction of NO radicals with the unsub-
3
3
8
stituted parental compound. Umschlag et al. have recently
9
ꢂ1 ꢂ1
s for
obtained a second order rate of (1.8 ꢀ 0.3) ꢁ 10 M
the reaction of NO
3
with phenol at 298 K and pH ¼ 0.
Furthermore, the investigated compounds are shown to
react more slowly than the corresponding para-substituted
phenol, as shown in Table 1.
3
.2. Reactions of NO
3
with para-substituted phenols at 298 K
with
The second order rate constants for the reaction of NO
3
five different para-substituted phenols were obtained. All ex-
periments were carried using the same experimental protocol as
reported above. In Fig. 4 the corresponding plots are shown
and the obtained coefficient rates are summarised in Table 1.
As already noted the para-nitrophenols react faster than the
corresponding ortho-isomers. This observation is justified for
different reasons. First, the steric hindrance is not influencing
the mechanism of the reaction in the case of the para-sub-
stituted phenols and consequently the reaction might proceed
faster than phenols, which present a bulky group close to the
reaction centre. Secondly, the inductive effect of the group in
para position exerts a lower effect on the aromatic ring. Finally,
the effect of the resonance stabilisation (mesomeric effect) on
the intermediate of the reaction plays an important role with
para-substituted phenols. In phenols containing substituents
which may conjugate with the aromatic system, electron-
donating groups (alkyl chains, amino group,. . .) induce weak-
ening of the OH bond by a combination of effects, i.e., the
destabilisation of the phenol and stabilisation of the phenoxyl
3
Fig. 4 Measured k1st vs. [reactant] for the reaction of NO with:
(a) 4-bromophenol; (b) 4-chlorophenol; (c) 4-hydroxyphenol;
(d) 4-aminophenol; (e) 4-hydroxybenzene ethyl ester.
14
radical by delocalisation of the unpaired electron.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
5381

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3ꢂ
of nitrate ten times higher ([NO ] ¼ 0.1 M) than used here.
Thus, the corresponding ionic strength differs by a factor three.
It was observed that the ionic strength might influence the
progress of the reaction exerting a positive or negative effect on
1
5,16
the experimental second order rate constant.
the reaction of NO with phenol is shown to proceed at a faster
In particular,
3
rate at higher ionic strengths. In an ionic strength range
between I ¼ 1.14 M and I ¼ 2 M, the reaction rate increases
linearly with increasing ionic strength reaching a plateau at I ¼
8,16
2.2 M.
However, the small number of measurements to
compare with does not allow further considerations. More
experimental efforts are needed in order to better characterise
the influence of the ionic strength on the reactions of NO
phenols in the aqueous phase.
3
with
3
.3. Reactivity comparison with OH radical
Fig. 5 Arrhenius plot for the reaction between NO radicals and: (a)
2-ethylphenol; (b) 2-methylphenol; (c) 2-hydroxyphenol; (d) 2-methoxy-
phenol.
3
Since OH is considered the most important radical for its high
reactivity towards organic compounds in the tropospheric
aqueous phase a comparison between the reactivity of OH
3
and NO towards aromatic compounds in aqueous solution is
presented in Table 2.
The reaction rate constant investigated as a function of the
temperature delivers kinetic data which can be described by the
As can be seen, OH radical reacts one order of magnitude
faster than NO3 with aromatic compounds in most cases.
Exceptions are the rate constants derived for 4-chlorophenol
and 4-bromophenol, which are comparable with the ones
obtained for OH radicals. This is a further confirmation of
the role played by the substituent. In fact the availability of
unshared p orbitals belonging to the halogen-substituent in
para-position allows a stabilization of the reaction intermediate
for resonance effects.
15
Arrhenius equation. The equation is commonly applied in
the linearised form and an Arrhenius plot of log k against
1
/T yields a line with y ¼ mx þ b, where y ¼ ln k, x ¼ 1/T,
m (slope) ¼ ꢂE /R and b (intercept) ¼ ln A.
A
Fig. 5 reports the Arrhenius plots observed for the investi-
gated reactions and Table 3 shows the corresponding activa-
tion parameters.
Just a few data are available from the literature for a
comparison to the kinetic data obtained here. However, the
A
activation energies (E ) are in good agreement with the ones
measured for reactions of NO with aromatic compounds and
3
3
2
2
3
.4. T-dependent studies of reactions of NO with
they are very close to the values determined for phenol and
4
-methylphenol, 2-ethylphenol, 2-hydroxyphenol and
-methoxyphenol
8
-methylcresol by Umschlag et al. (see Table 3).
The Gibbs free energies of activation are all in the range of
ꢂ1
Rate constants for reactions of the NO radical in aqueous
3
solution have been studied mainly at room temperature.
22–27 kJ mol for the ortho-substituted phenols and in the
ꢂ1
7
,8,16
range of 18–38 kJ mol for the phenols studied so far. These
6
Since temperature might influence strongly the rate coefficients
for a given reaction and no literature data are available
concerning the temperature dependences of the rate constant
for the reaction of nitrate radicals with ortho-substituted
phenols, in the present work the rate constants of the reac-
values are quite similar to the values present in literature.
z
z
The entropies of activation (DS ), where DS ¼ R ꢁ (lnA ꢂ
ln (k
T/h) ꢂ 1), show negative values that indicate that for
B
most of the systems studied the reaction might proceed through
the formation of an activated complex which is more highly
ordered than the initial reactants. However, the obtained
values are three times more positive than the activation en-
tion of NO
3
with (a) 2-methylphenol, (b) 2-ethylphenol, (c)
2-hydroxyphenol and (d) 2-methoxyphenol were studied in the
range of temperatures from 278 K to 328 K.
tropies obtained for the H-abstraction reactions of NO
3
with
Table 2 Comparison of the reactivity between OH and NO
3
for the reactions with aromatic compounds in solution
ꢂ1 ꢂ1
2nd, 298 K/M
ꢂ1 ꢂ1
k
s
k2nd, 298 K/M
s
Compound
OH
Ref.
NO
3
Ref.
9
9
1
1
9
9
1
1
1
1
9
9
1
1
9
8
9
9
8
9
9
8
8
8
8
7
7
8
8
8
Benzene
Toluene
Phenol
7.8 ꢁ 10
5.1 ꢁ 10
1.4 ꢁ 10
1.2 ꢁ 10
7.6 ꢁ 10
5.7 ꢁ 10
1.1 ꢁ 10
1.2 ꢁ 10
2.0 ꢁ 10
2.6 ꢁ 10
9.2 ꢁ 10
3.8 ꢁ 10
1.1 ꢁ 10
2.1 ꢁ 10
9.0 ꢁ 10
15
18
19
20
21
22
23
24
25
25
23
26
23
27
28
(4.0 ꢀ 0.6) ꢁ 10
(1.2 ꢀ 0.3) ꢁ 10
(1.9 ꢀ 0.3) ꢁ 10
(2.9 ꢀ 0.3) ꢁ 10
(1.7 ꢀ 0.2) ꢁ 10
(1.0 ꢀ 0.4) ꢁ 10
(8.5 ꢀ 0.2) ꢁ 10
(8.2 ꢀ 0.3) ꢁ 10
(1.1 ꢀ 0.1) ꢁ 10
(6.6 ꢀ 1.8) ꢁ 10
(2.3 ꢀ 0.4) ꢁ 10
(7.1 ꢀ 0.4) ꢁ 10
(5.6 ꢀ 0.8) ꢁ 10
(8.8 ꢀ 0.5) ꢁ 10
(5.0 ꢀ 1.3) ꢁ 10
7
7
8
0
0 a
a
2
4
4
2
4
2
4
2
4
2
4
4
a
-Chlorophenol
-Chlorophenol
-Bromophenol
-Methylphenol
-Methylphenol
-Methoxyphenol
-Methoxyphenol
-Nitrophenol
This work
This work
This work
This work
8
a
0 a
0
0
This work
This work
This work
8
0
a
-Nitrophenol
0 a
0
-Hydroxyphenol
-Hydroxyphenol
-Hydroxybenzoic acid
This work
This work
8
b
b
Measurement performed at pH 4 9. Measurement performed at pH ¼ 7.
5382
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
5383

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z
aliphatic compounds which are in the range DS ¼ ꢂ40 to ꢂ60
However, Santos and Simoes recognise that their proposal
may be controversial, particularly when large discrepancies
exist between data for the same compound obtained from
different experimental and theoretical methods. Furthermore,
Santos and Simoes pointed out that the present knowledge on
the energetics of the phenolic bond is still unsatisfactory. Even
for the phenol itself and some of the compounds considered
here the published values for the O–H bond dissociation energy
vary over a wide range.
ꢂ1
ꢂ1 7
J mol . Furthermore, the values reported in Table 3
present less negative entropies of activation than the other
aromatics.
An interesting result is obtained for the reaction of NO
K
7
3
with
2-hydroxyphenol where a very positive value of entropy of
activation was obtained, which is not common for the reactions
7
of NO with organics in the aqueous phase. The presence of
3
two hydroxyl groups in ortho position might greatly influence
the reactivity of this compound through the formation of a net
of hydrogen bonds with a polar solvent such as water. Hence,
the reactant might be in a very high ordered state before
undergoing reaction with NO . In the literature there are
3
several studies concerning the determination of the absolute
rate constants for hydrogen atom abstraction from phenols by
Evans and Polanyi found that the activation energy of an
exothermic H-abstraction reaction changes linearly with the
4
2
C–H bond energy. According to the Arrhenius equation the
logarithm of the rate constant depends linearly on the activa-
tion energy for a series of reactions which do follow the same
reaction mechanism, such as the H-abstraction reactions of
29–34
peroxyl radicals
and by other oxygen- or nitrogen-centered
NO with aliphatic organic compounds. Therefore, the loga-
3
35
radicals. Many of those rate constants have been measured in
different solvents and found to show strong solvent effects in
hydrogen bond accepting solvents (HBA), such as water.
Furthermore, they do not depend on the nature of the attack-
ing radical. These solvent effects have been attributed to
hydrogen bond formation between hydroxylic hydrogen atom
rithm of the rate constants and the X–H bond energy should
depend on each other linearly.
The correlation between the logarithm of the rate constant
per equivalent abstractable H atom (k ) and the bond dis-
H
sociation energy BDE (Table 3) is shown in Fig. 6 for the
compounds where the observed rate constant is fully explained
by an H-abstraction mechanism according to the more recent
thermochemical data mentioned before.
A correlation between the logarithm of the observed rate
constant for the abstraction of the phenolic hydrogen and the
corresponding bond dissociation energy (BDE) is provided
(Fig. 6 and eqn. (2)) which might be used for the prediction
of rate coefficients for H-abstraction reactions of substituted
3
6,37
of the phenolic moiety and the HBA solvent
and it might
play an important role with compounds such as 2-hydroxy-
phenol.
Finally, it was confirmed with the present study that steric
crowding around the hydroxyl group is important in decreas-
ing the reactivity of phenol and this influence might be more
effective than the electronic effect due to the substituent as
already observed in a study of the reactivity of substituted
3
phenols with NO in aqueous solution:
3
8
phenols towards alkyl radicals by Franchi et al.
ꢂ1 ꢂ1
log k /M
H
s
¼ (23.7 ꢀ 2) ꢂ (0.044 ꢀ 0.01)
ꢂ1
ꢁ
BDE/kJ mol n ¼ 6 r ¼ 0.88 (2)
3
.5. Reactivity correlations
where n is the number of compounds considered in the regres-
sion line and r is the corresponding correlation coefficient.
Chemical kinetics provide insights into the mechanism under-
lying a certain process because kinetics is concerned with
changes in properties between an initial state and the transition
state as shown for the temperature dependency studies re-
ported in the previous section. Just a few kinetic data were
3
3
.5.2. Reaction mechanisms for the reaction of NO with
substituted phenols: Electron transfer. Furthermore, in order to
do a comparison between the possible reaction mechanisms
involved and to explore the contribution of a single electron
transfer (SET) on the observed rate constants, the standard
free energy of the reaction DGR has been calculated for the
reactions investigated in this study according to:
7,8,16
available
3
for the reactions of NO with phenols in the
aqueous phase and no correlations are actually published in
the literature.
3
3.5.1. Reaction mechanisms for the reaction of NO with
0
0
ꢃ
0
DG
R
¼ ꢂz ꢁ F ꢁ DE ¼ ꢂz ꢁ F ꢁ [E (X ) ꢂ E (reactant)] (3)
Where z is the number of exchanged electrons, F is the Faraday
substituted phenols: H-abstraction. The kinetic data obtained in
the present work and the data collected from the literature were
used to seek common reaction patterns and possibly distin-
guish between the different mechanisms involved in the oxida-
tion process. In view of this, the available data were
summarised in Table 3 and with the help of kinetic, thermo-
dynamic data and extrathermodynamic relationship mechan-
istic considerations are undertaken.
0
0
constant and DE is the standard electromotive force. DE is
3
As mentioned earlier, NO might react with a phenolic
compound via different pathways, i.e. addition–elimination,
electron-transfer or H-atom abstraction reactions. The rate
coefficient of the latter reaction type is determined by the bond
strength of the weakest X–H bond.
3
9
The analysis of all the available data led Santos and Simoes
to propose a set of recommended values for the O–H bond
dissociation energies in phenols, which was used in the present
study to derive the expected rate constant for an H-abstraction
mechanism with the help of a correlation described in the
7
literature for the reactions of NO with organics in aqueous
solution (eqn. (1)), as reported in Table 3.
3
ꢂ1 ꢂ1
lg kH-abs/M
s
¼ (38.5 ꢀ 5.6) ꢂ (0.084 ꢀ 0.014)
ꢂ1
ꢁ
with a database of 37 reactions considered and a correlation
BDE/kJ mol
(1)
Fig. 6 Plot of the logarithm of the kH-abs versus the bond dissociation
energy for the reaction of NO with: (a) 4-aminophenol; (b) 4-hydroxy-
3
phenol; (c) 4-methoxyphenol; (d) 2-hydroxyphenol; (e) 2-methoxyphe-
nol; (f) 2-nitrophenol.
coefficient of R ¼ 0.89.
5384
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

View Article Online
the difference between the standard potentials of the redox
ꢂ
couple where the reduction potential for (NO
V. The obtained Gibbs free energies are reported in Table 3.
The reduction potentials listed in Table 3 were calculated
3
/NO
3
) is 2.44
41
4
0
with the correlation proposed by Jonsson et al. Experimen-
tally determined one-electron reduction potentials and esti-
mated values for mono-substituted benzene radical cations are
available from literature as functions of the substituent Brown
1
p
.
43
parameter s
4
0
1
Jonsson et al. derived a conditional so scale for ortho-
substituted phenols which is described by eqn. (4).
1
1
s
o
¼ 0.66 ꢁ s
p
(4)
1
The factor of 0.66 by which s must be multiplied is a result
of a comparison of ortho and para effects on O–H bond
p
44,45
strengths.
substituent on phenols have been reported in the literature.
More recently, the Jonsson et al. exercise was repeated by
Linear correlations between ortho and para
4
5,46
3
9
Santos and Simoes using their recommended selected data
1
for the monosubstituted ortho phenols and s
p
leading to
eqn. (5):
1
1
so ¼ 0.62 ꢁ sp
in good agreement with the first one determined by Jonsson
(5)
40
et al. Moreover, Santos and Simoes compared the two
equations (eqns. (4) and (5)) with the full data set of mono-
substituted phenols recognizing that the factor 0.66 proposed
by Jonsson et al. better describes the available data, hence the
factor of 0.66 was used throughout this work.
However, it should be underlined that due to irregular
interactions between ortho substituents and the OH group, it
1
is somewhat controversial to assign a sp value to them.
The logarithms of the observed rate constant were plotted
3
Fig. 7 Rate constants for the single-electron transfer of NO reaction
in aqueous phase with anions and aromatic compounds. (A) J anions;
D: aromatics from refs. 47 and 7. (B) J: anions; D: aromatics same as
plot A; ’: substituted phenols studied in the present work.
against the obtained DG . According to the reaction scheme of
R
an irreversible electron transfer:
kd
kp
ꢂ
þ
A
þ
B
нA ꢄ ꢄ ꢄ Bꢅ ꢂ! A þ B
kꢂd
in the model calculations only the reactions of NO with anions
3
in aqueous solution have been considered. In Fig. 7B the
database of the model was extended also to aromatic com-
pound and the substituted phenols considered in this work.
which may be applied for NO
tions studied in the present work, the following relation applies
for the observed rate constant:
3
single electron transfer reac-
The kinetic data for the reactions of NO with anions and
3
ꢀ
ꢁ
the aromatic compounds represented in Fig. 7 and used for the
47
modelling calculation are taken from Herrmann and Table 3
kꢂd
ketr
ꢂ1 ꢂ1
kobs=l mol
s
¼ kd= 1 þ
ð6Þ
(
this work).
A comparison between the two correlations shows substan-
The experimental data were modelled using eqn. (7), ob-
7
tained as described in Herrmann and Zellner, in order to
obtain the expected electron transfer rate for a given Gibbs free
energy of reaction.
tial agreement. As can be seen from the experimental data, for
ꢂ1
DGR oꢂ50 kJ mol the rate constants for both reaction with
anions and aromatic compounds including the substituted
phenols become essentially independent of the Gibbs free
energy of reaction and are best described by an observed
kd
kobs ¼
2
4
3
5
!
9
ꢂ1
2
second-order rate constant in the range of 2 ꢁ 10 l mol
0
R
kꢂd
A0
ZX ZR e2 NA
z
DG
ꢂ1
1
þ
exp
þ DG0 1 þ
= RT
s
in both cases.
Towards the exergonic part of the axis the rate constants
e r12
107
z
4
DG
0
measured for the reaction aromatic compounds do not differ
substantially from the ones obtained for the anions. Quite
differently, the aromatic compounds show a different beha-
viour in the more endoergonic region of the plot. Hence, it is
ð7Þ
where A
0
is the preexponential factor. Z
X
and Z
R
are the
ꢃ
formal charge of the radical (X ) and the reactant (R), respec-
tively. e is the elementary charge in electrostatic units. e is the
dielectric constant of the solvent, r12 is the sum of the reactant
concluded that in the case of aromatic compounds the rate
ꢂ1
constants observed in the regime of DG
not reflect an electron transfer from the aromatic ring.
4 ꢂ50 kJ mol do
R
radii in the activated complex considering a spherical shape for
0
both radical and reactant. N
A
is Avogadro’s number. DG
R
A zoom of the plot presented in Fig. 7B is shown in Fig. 8.
Generally, the substituted phenols (’) considered in this work
are in agreement with the calculated curve and the obtained
second order rate constants are somewhat slower with respect
to the predicted value provided from the fit. Moreover, the
compounds (N) that show a large difference between the
observed and the expected second order rate constant are
suggested to react via the H-abstraction mechanism with
z
is the free enthalpy in the prevailing medium, DG
0
is
the component related to the reorganization of the solvent
molecules surrounding the reactants during the proceeding of
the reaction. R is the gas constant and T the absolute tem-
perature.
In Fig. 7A and 7B, the comparison between the experimental
data (symbols) and the calculated curve (line) is shown. Fig. 7A
4
represents the outcome of a former study of Herrmann where
7
3
NO (see also Fig. 6).
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
5385

View Article Online
Fig. 8 Zoom of the plot in Fig. 7B for single electron transfer
reactions of NO radicals in the aqueous phase. Data and symbols as
in Fig. 7A and 7B. See text for explanation of symbols.
Fig. 10 Hammett-type plot for the reaction of OH with: J para-
substituted benzoate; D para-substituted phenols.
3
where a Hammett-type plot for the reactions of OH with para-
substituted phenol and para-substituted benzoate is presented.
The selected coefficient rates considered in Fig. 8 are sum-
marised in Tables 2 and 4.
3
3
.5.3. Reaction mechanisms for the reaction of NO with
substituted phenols: Hammett-type plot. In the literature the
Hammett treatment includes so far only substituents that are
meta or para to the reaction site. When Hammett-plots are
made with data for ortho-substituted reactants, scatter dia-
grams usually result. This failure is usually attributed to steric
effects. Taft first proposed the use of a correlation similar to
the Hammett correlation trying to separate the electronic effect
from the steric effect introducing a series of substituent con-
As can be seen in Fig. 10, in the case of OH no saturation
behaviour is observed and, moreover, all the second order rate
constants can be interpreted by a single regression line indicat-
ing that OH might react with all the compounds through the
same mechanism. Phenols and substituted benzoates are
known to react with OH-radicals in the aqueous phase mainly
by addition to the aromatic ring, where the observed rate
constants are in the order of magnitude comprised between
48
49
0
15
s
stants (E ) for a small number of substituents. In this work, a
tentative Taft-plot for the reactions of NO with ortho-sub-
3
9
10
ꢂ1 ꢂ1
stituted phenols resulted in a very poor correlation (r ¼ 0.59,
n ¼ 5) between the observed rate constants and the steric
constants. This result is explained mainly because steric effects
can be particularly difficult to define.
Whereas the contribution of steric effects is often unpredict-
able and results in a scatter plot, the para-substituted phenols
allowed a better interpretation of the obtained data and better
correlations are generally obtained. In Fig. 9, a Hammett plot
3.8 ꢁ 10 and 2.6 ꢁ 10
M s for the reaction with
4-nitrophenol and 4-methoxyphenol, respectively.
The direct comparison of the two Hammett-plots in Fig. 9
shows clearly that for the reaction with the substituted benzoic
acid, where the involved mechanism is an electron transfer, all
the points can be interpreted by a single regression line. The
influence of the substituents is proportional to the electronic
effect of the substituents and the benzoic acid bearing an
electronegative group has been shown to react more slowly
with respect to the parental unsubstituted benzoic acid.
In the case of the para-substituted phenols two different
behaviours of the regression line are evident (Fig. 9). For the
phenols having an electron-donating group the slope of the
regression line is close to zero, which indicates that for these
compounds there is no evident electronic effect on the observed
3
for the reaction of NO radical with para-substituted phenols
and benzoic acids is shown.
A saturation behaviour is observed, even if the diffusion
limit for the reaction of NO
3
with the hydroxyphenol is
9
ꢂ1 ꢂ1
s
calculated to be kdiff ¼ 7.5 ꢁ 10 l mol
applying the
This trend is actually difficult to
explain. A possible suggestion might be provided in Fig. 10
5
0,51
Smoluchowski equation.
3
rate constants for the reaction with NO . As shown in Fig. 9,
the observed rate constants interpreted with the flat line can be
explained with an H-abstraction mechanism (see Table 3).
Thus, this indicates that during the reaction no ‘‘formal’’
charge is formed. However, in the H-abstraction reactions a
5
6
small negative slope could be observed, whereas, the electron
Table 4 Rate constants for the reaction of OH with para-substituted
benzoate ion at 298 K in aqueous phase and the substituent Brown
1
parameter, s
p
ꢂ1 ꢂ1
1
Compound
k2nd/l mol
s
Ref.
s
p
9
4
4
4
4
4
4
4
4
-Nitrobenzoate ion
-Bromobenzoate ion
-Iodobenzoate ion
2.6 ꢁ 10
3.2 ꢁ 10
2.5 ꢁ 10
5.0 ꢁ 10
3.6 ꢁ 10
8.0 ꢁ 10
6.0 ꢁ 10
8.2 ꢁ 10
17
52
53
28
52
54
55
52
0.79
0.15
0.14
0.11
9
9
9
9
9
9
9
-Chlorobenzoate ion
-Fluorobenzoate ion
-Methoxybenzoate ion
-Hydroxybenzoate ion
-Aminobenzoate ion
ꢂ0.07
ꢂ0.78
ꢂ0.92
ꢂ1.3
3
Fig. 9 Hammett-plot for the reaction of NO with: (D) para-substi-
tuted benzoic acids and (K) para-substituted phenols. See text for
details, H-abstraction contributions are given in percent.
5386
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

View Article Online
transfer is suggested to be the prevailing mechanism with the
para-substituted phenols considered in the regression line.
both anions as well as aromatic compounds including the
substituted phenols become essentially independent of the
Gibbs free energy of the reaction. This indicates electron
transfer as the dominant mechanism. On the other hand for
ꢂ1
4
. Environmental aspects
reactions with DG
R
4 ꢂ50 kJ mol
the observed rate
The NO radical is recognised as playing an important role in
3
night-time atmospheric chemistry. It has also been established
that, since the industrial revolution, the tropospheric abun-
dances of the precursors of NO
increasing as a result of anthropogenic activities. Thus, the
coefficients are not due to concerted electron transfer only.
Finally, it was observed that in ortho-substituted phenol
steric effects plays a decisive role whereas electronic effects
are responsible for the reactivity of para-substituted phenols.
3 2 3
, i.e. NO and O , have been
abundance of NO
have increased.
Following the suggestion that NO
carbons, the reactivity of NO towards naturally occurring and
3
anthropogenic atmospheric trace gases has been investigated.
Although aromatic compounds such as benzene or toluene
3
and its impact on the troposphere must also
might react with hydro-
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7
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1
6
7
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1
8
9
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2
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8
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3
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4
compared to former studies. The model shows that for
7
ꢂ1
DGR o ꢂ50 kJ mol the rate constants for the reaction with
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 3 7 9 – 5 3 8 8
5387

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