A much-needed mechanism and reaction rate for the oxidation of phenols with ClO2: A joint experimental and computational study

Article abstract of DOI:10.1071/CH13101

The oxidation of phenols with chlorine dioxide, a powerful means to eliminate phenol pollutants from drinking water, is explored. Kinetic experiments reveal that 2,4,6-trichlorophenol exhibits a lower oxidation rate than other phenols because the chlorine atoms (σ≤0.22) at ortho and para-positions decrease the benzene's electron density, in agreement with the Hammett plot. The oxidation of phenol was found to be second order with respect to phenol and first order with respect to ClO2 and a possible mechanism is proposed. The phenol/ClO2 oxidation was found to be pH-dependent since the reaction rate constant increases with increasing pH. The oxidation rate was also significantly enhanced with an increasing methanol ratio in water. The oxidation products, such as benzoquinones, were analysed and confirmed by liquid chromatography and gas chromatography-mass spectrometry. Density functional theory computations at both the B3LYP/6-311+G(d,p) and M06-2X.6-311+G(d,p) levels with the SCRF-PCM solvation model (i.e. with water) further supported the proposed mechanisms in which activation barriers predicted the right reactivity trend as shown by the kinetic experiments. CSIRO 2013.

Full text of DOI:10.1071/CH13101

CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 814–824
Full Paper
http://dx.doi.org/10.1071/CH13101
A Much-Needed Mechanism and Reaction Rate
for the Oxidation of Phenols with ClO : A Joint
2
Experimental and Computational Study
A
B
Carlos Alberto Huerta Aguilar, Jayanthi Narayanan,
C
,
D E
Mariappan Manoharan, Narinder Singh,
,
A E
and Pandiyan Thangarasu
A
Facultad de Qu ´ı mica, Universidad Nacional Aut o´ noma de M e´ xico (UNAM),
Ciudad Universitaria, Coyoac a´ n, 04510 M e´ xico D.F., M e´ xico.
B
Divisi o´ n de Nanotecnolog ´ı a, Universidad Polit e´ cnica del Valle de M e´ xico,
Av. Mexiquense, C.P. 54910 Tultitlan, Estado de M e´ xico, M e´ xico.
C
School of Science, Engineering and Mathematics, Bethune-Cookman University,
Daytona Beach, Florida 32114, USA.
D
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar,
Panjab 140001, India.
E
Corresponding authors. Email: nsingh@iitpr.ac.in; pandiyan@servidor.unam.mx
The oxidation of phenols with chlorine dioxide, a powerful means to eliminate phenol pollutants from drinking water, is
explored. Kinetic experiments reveal that 2,4,6-trichlorophenol exhibits a lower oxidation rate than other phenols because
the chlorine atoms (s ¼ 0.22) at ortho and para-positions decrease the benzene’s electron density, in agreement with the
Hammett plot. The oxidation of phenol was found to be second order with respect to phenol and first order with respect to
ClO and a possible mechanism is proposed. The phenol/ClO oxidation was found to be pH-dependent since the reaction
2
2
rate constant increases with increasing pH. The oxidation rate was also significantly enhanced with an increasing methanol
ratio in water. The oxidation products, such as benzoquinones, were analysed and confirmed by liquid chromatography
and gas chromatography–mass spectrometry. Density functional theory computations at both the B3LYP/6-311þG(d,p)
and M06-2X.6-311þG(d,p) levels with the SCRF-PCM solvation model (i.e. with water) further supported the proposed
mechanisms in which activation barriers predicted the right reactivity trend as shown by the kinetic experiments.
Manuscript received: 16 January 2013.
Manuscript accepted: 5 April 2013.
Published online: 26 April 2013.
Introduction
[14–16]
humans. The unpleasant tastes and odours in water can
be completely eliminated by using chlorine dioxide even in
Phenol and phenolic compounds often appear in wastewaters of
chemical, petrochemical, and oil refining industries; for exam-
ple, monochlorophenols are mainly used as intermediates in
dyestuffs and in the manufacture of higher chlorinated phe-
[
17]
small doses.
concentrations as low as 0.1 ppm over a wide range of pH, in
ClO2 is a highly effective microbiocide at
[
18,19]
particular, around (pH 5.0–9.5),
and disinfection strength are not pH dependent,
Fenton oxidant or other photo-catalysts (TiO /H O ). More-
and its oxidation potential
[
1,2]
[20]
nols;
2
2,4-dichlorophenol (DCP) is used in the synthesis of
3]
unlike the
[
,4-dichlorophenoxyacetic acid. In the trichlorophenol series,
,4,5-trichlorophenol is employed as an intermediate in the
2
2 2
2
production of the herbicide 2,4,5-trichlorophenoxyacetic
over, its preparation requires only low toxic and harmless
[
chemicals. ClO is freely soluble in aqueous solutions where
21]
2
[
4]
acid. Because of their toxicity and poor biodegradability,
[5]
it does not ionise to form a weak acid and also has high
selectivity and reactivity with environmentally objectionable
waste materials, such as phenols, sulfides, thiosulfates, mercap-
tans, and amines, and its bacterial disinfection efficiency has
phenolic wastewaters must be specially treated before being
disposed off. Several treatment techniques that have been
applied to remove phenolic compounds from wastewaters
usually involve complicated procedures, showing that they are
not economically viable.
Chlorine dioxide has been considered an oxidant for the
disinfection of drinking water or wastewater because it produces
less objectionable chlorinated organic products such as
trichloromethanes, tetrachloromethane, or haloacetic acids;
the chlorinated products are generally cancer promoters in
[
5–12]
[22]
been completely analysed.
ClO has been widely used in various industrial processes for
2
[23–26]
the purification of drinking water,
cyanide removal from
[29]
[27,28]
industrial wastewater,
Furthermore, aqueous ClO has been used to treat waste-
and fouling control in sea water.
2
[
13]
[30–33]
[34,35]
water,
[36–38]
isms,
pharmaceutical wastes,
polycyclic aromatic compounds, phenols,
foodborne microorgan-
[39]
[27,40,41]
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

Theoretical and Experimental Oxidation of Phenols with ClO
2
815
[42–44]
sulfides/thiosulfides,
[45]
[31,46]
amines,
[48]
or pesticides.
antibiotics,
and
Nevertheless, in the litera-
Reaction Kinetics
[47]
micropollutants
In a closed vessel (25 mL), phenols (1.0 mM) were oxidised by
using chlorine dioxide (3.0 mM). The substrate was prepared in
methanol/water (50 : 50) at pH 7.0 for the oxidation. The
ture, there are few reports dealing with phenol oxidation by ClO2;
in addition, no detailed report is found for the analysis of phenol
oxidation by means of a combined experimental and computa-
tional study. Thus, the present paper deals with the oxidation
kinetics of phenols with ClO . The rate law was determined along
substrate-to-ClO ratios were 1 : 4, 1 : 6, 1 : 8, and 1 : 10. The pH
2
of the solutions was maintained by phosphate buffer solution
(0.025 M) (K HPO and KH PO ). Rate constants for the oxi-
2
2
4
2
4
with solvent effects and the influence of pH on the oxidation rate
was studied with a possible mechanism. UV-Visible, liquid
chromatography (LC), and gas chromatography–mass spectrom-
etry (GC-MS) techniques were used to characterise the oxidation
products. To further support our kinetics experiments, we consid-
eredthe mechanismsshown inScheme1 for phenol oxidation and
explored them carefully by density functional theory (DFT)
computations. Herein, we mainly aimed to predict the rate-
limiting step of phenol oxidation while yielding benzoquinone
as a final product and to compare the relative reactivity of phenol
and chlorophenols with experimental data.
dation of all the phenols were determined by plotting the sub-
strate concentrations measured at different intervals against
time. Furthermore, the chlorine dioxide decay (blank) in the
solvent was examined and was then incorporated in the calcu-
lation of the reaction rate.
The phenol concentrations were determined by the colouri-
metric method; the phenol solution (10 mL) was first mixed with
NH OH (0.5 N, 0.25 mL) to adjust the pH of the mixture solution
4
to 7.9 ꢁ 0.1 in the phosphate buffer, and 4-aminoantipyrine
(0.1 mL) was then added, followed by K [Fe(CN) ] (0.1 mL).
3 6
The resulting solution mixture was stirred for 15.0 min; the
colourless solution turned to red and was then used to measure
the concentration of the substrate spectroscopically in the visible
region. All the experiments were carried out three times to
obtain consistent results.
Experimental
ClO Preparation
2
Chlorine dioxide was prepared by the reaction of sodium chlo-
rite with acetic anhydride in an aqueous solution, as reported
[
elsewhere. Although chlorine dioxide can be produced from
49]
LC Analysis
[
50–52]
acidic solutions of either sodium chlorite
[
or sodium
we used sodium chlorite as the starting material.
HPLC equipment having a Nucleosil C18 column was used in
the reversed-phase elution mode to analyse the intermediates
formed from the oxidation of phenols, and then a suitable
retention time for the intermediates in isocratic conditions was
achieved. The appearance of peaks in the chromatogram cor-
responding to the ionised compounds was better resolved at low
pH than at high pH, thus LC was recorded for the phenol solution
53–55]
chlorate,
The ClO concentrations were calculated assuming a molar
2
absorptivity coefficient for its aqueous solution at 360 nm of
ꢀ1
ꢀ1
1
cally varied less than 2.0 % during the experiments. An average
225 cm
M ; the absorbance values for the solutions typi-
ClO concentration was used for the calculation of oxidation
2
ꢀ
1
rate constants. For all the experiments, ClO was freshly pre-
2
pared and then used for the experiments.
at a low pH (pH 3.5). The phosphate buffer (0.01 mol L ) in
CH CN/MeOH (80 : 20, v/v) was used to maintain the pH of the
3
Radical systems
X
Xꢁ
Xꢀ
Cl
O
O
O
X
O
H
H
OH
OH
X
Xꢁ
Xꢀ
H
Xꢁ
Xꢀ
Cl
O
Xꢁ
Xꢀ
k1
k2
ꢂ
O
O
O
ꢂ
HClO2
2
ꢂ
kꢄ1
kꢄ2
Xꢀ
Xꢁ
Xꢀ
Xꢁ
X
O
O
Cl
Step I
X
X
C
A
B
TS1
Step II
k3
O
O
O
1
. X ꢃ Xꢀ ꢃ Xꢁ ꢃ H
O
O
Xꢁ
Xꢁ
Xꢀ
Xꢁ
Xꢀ
Xꢀ
2. X ꢃ Cl; Xꢀꢃ Xꢁ ꢃ H
3. X ꢃ Xꢀ ꢃ Cl; Xꢁ ꢃ H
4. X ꢃ Xꢀ ꢃ Xꢁꢃ Cl
Xꢁ
Xꢀ
ꢄXOCl
X
X
O
O
Step III
O
X
Cl
Cl
O
Cl
O
O
P
TS3
TS2
Non-radical systems
Scheme 1. Phenol oxidations of mono-, di-, and trichlorophenols with a chlorine dioxide radical illustrates three different pathways (I–III) which also have be
used for density functional theory (B3LYP/6-311þG**) computations.

8
16
C. A. H. Aguilar et al.
solution. For the pH adjustment of the solution, phosphoric acid
was employed. Furthermore, although a wide range of mobile
phases, such as acetonitrile, THF, and methanol, was studied, it
was found that an appropriate selectivity was suitable when the
Results and Discussion
Oxidation of Phenols and their Kinetic Studies
Because of experimental considerations, kinetic studies were
carried out at 3.0 mM of ClO under excess ClO to phenol (4 : 1,
6 : 1, 8 : 1, 10 : 1). The concentrations of phenol were determined
as a function of time. A plot of [phenol] v. time yielded a
straight line, and a similar behaviour was observed for
2
2
mobile phase was CH CN/MeOH (80 : 20) at a flow rate of
3
ꢀ
1
ꢀ1
0
.5 mL min
in the phosphate buffer (0.01 mol L ). The
ꢀ
1
solutions were filtered through a 0.45 mm nylon filter unit before
LC analysis.
4
(
-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorephenol
Table 1) so that the reaction is of second order with respect to
GC-MS Analysis
phenol with a pseudo reaction rate constant given by this slope.
The value of this rate constant in turn is directly proportional to
the concentration of ClO (Fig. 1), yielding an overall reaction
GC/MS was used to analyse the formation of products from the
phenol oxidation. GC with a capillary column (30 m, 0.25 mm
internal diameter) having 0.5 mm film thickness Rtx-5ms (5.0 %
phenyl–95 % dimethylarylenesiloxane) was employed. For the
column, the oven temperature was programmed as follows:
2
rate second order with respect to phenol and first order with
respect to ClO . The concentration of phenol as a function of
2
time was determined by UV-vis spectroscopy. In order to
determine the concentration of the phenols in the visible region
of the spectra, a colourimetric reaction between the different
phenols and 4-aminoantipyrine was carried out to prepare their
coloured derivatives which absorb in the visible region. The
absorption of the phenols was measured at different times at
ꢀ
1
1
heated at 158C min to 2108C; finally the temperature was set at
008C for 1.0 min, heated to 1608C at 308C min , and then
ꢀ1
ꢀ1
2
708C by heating at 3.58C min . After injecting the sample
(0.5 mL), a delay time of 150 s was set to avoid the solvent peak
in the GC. A splitless mode was used for the injection with the
purge valve closed for 3.0 min. In the column, the flow rate of
5
10 nm (Table 1). The degradation by ClO of phenol is greater
2
ꢀ1
helium was 1.0 mL min . The sample solution, after ClO2
than for the chlorophenols, where the chlorine at the para-
or/and ortho-position hinders the oxidation rate (see Table 1).
(
(
2.5 mM) oxidation with phenol (10.0 mM) in methanol/water
70 : 30) at pH 7.0, was filtered through a 0.45 mm nylon filter
unit before the GC-MS injection. The sample was separated into
individual components using a temperature-controlled gas
chromatogram and then the mass spectrum for each GC com-
ponent was obtained.
Proposal of Mechanism and the Reaction Rate Law
For the oxidation of different phenols, we propose that the ClO2
ꢀ
radical reacts with the substrate to produce a ClO ion or HClO ,
2
2
and a phenoxyl radical which then reacts with another ClO2
radical to form an adduct that consequently loses HOCl. Two
equivalents of ClO are consumed in the reaction to yield the
2
Table 1. The phenol oxidation data (substrate/ClO , 1 : 4)
See text for data definitions
2
product. Under neutral (pH 7) or alkaline conditions, the adduct
is not seen by HPLC or GC because of a rapid pH-dependent
hydrolysis.
ꢀ
1
ꢀ1
ꢀ1
Compounds
Phenol
Ss
k [M
s
]
ln k
DG [kJ mol
]
0
0.083
0.067
0.05
ꢀ2.488
ꢀ2.703
ꢀ2.995
ꢀ3.194
6.066
6.588
7.301
7.785
It was observed that the increase of chlorite ion concentration
in the reaction mixture reduces the reaction rate, and it shows
4
2
2
-Chlorophenol
0.23
0.46
,4-Dichlorophenol
ꢀ
that k . k (Scheme 1, step II). A plot of 1/k v. [ClO ] is
2
ꢀ
shown in Fig. 2 (inset) and yields good linearity, as expected.
1
2
,4,6-Trichlorophenol 0.69
0.041
1
1
2
0
8
6
4
2
0
y ꢃ 1.4405x ꢄ 5.016
2
R
ꢃ 0.9903
1
1
1
8
6
4
2
.4Eꢂ04
4
:1 (ClO /phenol)
3
5
7
9
2
.2Eꢂ04
.0Eꢂ04
.0Eꢂ03
.0Eꢂ03
.0Eꢂ03
.0Eꢂ03
6:1 (ClO /phenol)
[ClO2] [mM]
2
y ꢃ 9.7309x ꢂ 1123.3
y ꢃ 6.1137x ꢂ 953.7
y ꢃ 3.4001x ꢂ 648.16
8
1
:1 (ClO /phenol)
2
0:1 (ClO /phenol)
2
y ꢃ 1.0376x ꢂ 1038.8
0Eꢂ00
0
500
1000
1500
2000
2500
Time [s]
Fig. 1. Phenol oxidation at different concentrations of ClO
2
.

Theoretical and Experimental Oxidation of Phenols with ClO
2
817
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
0
y ꢃ ꢄ0.2967x ꢂ 0.9397
2
R
ꢃ 0.9717
4
3
3
2
2
1
1
5
.0E
.5E
.0E
.5E
.0E
.5E
.0E
.0E
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
03
03
03
03
03
03
03
02
00
0
1
2
3
4
ꢄ
[
ClO ] [mM]
2
y ꢃ 1.0246x ꢂ 1047.8
0
mM
0.5 mM
y ꢃ 0.7467x ꢂ 1001.1
y ꢃ 0.6109x ꢂ 1004.8
1
2
3
.0 mM
.0 mM
.0 mM
y ꢃ 0.3017x ꢂ 949.16
y ꢃ 0.1081x ꢂ 971.75
0E
0
500
1000
1500
2000
2500
3000
Time [s]
2 2
Fig. 2. Effect of chlorite ion on the phenol oxidation by ClO (substrate/ClO , 1 : 4).
Phenol forms a short lived dimer held together by hydrogen
bonding, which is then stepwise oxidised by ClO2:
Finally, 2) if k .. k it is much more likely for the dimer to
1 2
ꢀ
break apart than to react with ClO , the result is the observed
kinetics:
2
k1
Ð
2
A
B
d½Pꢂ
dt
k1k2
kꢀ1
kꢀ1
2
¼
k½Aꢂ ½ClO2ꢂ; k ¼
ð5Þ
k2
Ð
B þ ClO2
C
P
kꢀ2
Confirmatory evidence for the mechanism are: 1) the reaction
rate was observed to decrease with an increment in temperature;
this unusual observation, is presumably because the complex B
dissociates more readily and so is shorted lived and then unlikely
k3
!
C þ ClO2
ð1Þ
to react with ClO , 2) the reaction rate also decreases with
2
where A is phenol, B is the dimer of phenol, and P are the
oxidation products. Applying the steady-state approximation to
decreasing pH, apparently because in acidic solution the phenols
form analogues of hydronium ions which are then blocked from
forming the complex (phenol dimer) by hydrogen bonding.
[B] yields
2
k1½Aꢂ þ kꢀ2½Cꢂ
½Bꢂ ¼
ð2Þ
Studies of pH versus k
kꢀ1 þ k2½ClO2ꢂ
The effect of increasing the solvent medium pH from 4.0 to 10.0
causes k to increase, yielding a linear relationship between log k
and pH (Fig. 3). This is consistent with the previous reports that
and then also to [C], substituting the above value for [B], yields
the rate constant for organic degradation by ClO is significantly
2
2
k1k2½Aꢂ ꢃ ½ClO2ꢂ
[56–59]
pH dependent if the organic compound is ionisable.
Phe-
½
Cꢂ ¼
ð3Þ
2
kꢀ1kꢀ2 þ kꢀ2k3½ClO2ꢂ þ k2k3½ClO2ꢂ
nol is easily ionisable in aqueous solution, thus the increase of
rate constant with pH was observed as expected. It is known that
as the solution pH increases, the oxidation reduction potential of
d½Pꢂ
dt
Thus
¼ k3½Cꢂ ꢃ ½ClO2ꢂ. Substituting the above value of
[
C] and further assuming that 1) the second reaction is irrevers-
ClO increases while the hydrogen bond between the oxygen
2
ible, k ¼ 0, once the dimer has decomposed by reacting with
and water molecules is weakened.
ꢀ
2
ClO it cannot be reformed by the reverse reaction, the result is
2
Solvent Effect on k
2
d½Pꢂ
dt
k1k2k3½Aꢂ ½ClO2ꢂ
The oxidation rate constant of phenol by ClO in methanol–
2
water media was found to increase considerably with
¼
ð4Þ
kꢀ1k3 þ k2k3½ClO2ꢂ

8
18
C. A. H. Aguilar et al.
2
1
1
0
.0
.5
.0
.5
0
y ꢃ 0.1579x ꢄ 0.1956
2
R
ꢃ 0.9288
5.0Eꢂ03
4.5Eꢂ03
4.0Eꢂ03
3.5Eꢂ03
3.0Eꢂ03
2.5Eꢂ03
2.0Eꢂ03
1.5Eꢂ03
1.0Eꢂ03
5.0Eꢂ02
2
4
6
8
10
12
pH
y = 1.5321x + 984.89
pH ꢃ 3
pH ꢃ 5
pH ꢃ 7
pH ꢃ 9
pH ꢃ 12
y ꢃ 1.4303x ꢂ 997.59
y ꢃ 1.0246x ꢂ 1047.8
y ꢃ 0.5245x ꢂ 978.23
y ꢃ 0.206x ꢂ 952.99
0Eꢂ00
0
500
1000
1500
2000
2500
3000
Time [s]
Fig. 3. pH effect on phenol oxidation by ClO
2
2
(substrate/ClO , 1 : 4).
1
1
1
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
0
y
ꢃ
0.0106x
ꢂ 0.063
R2
ꢃ
0.9329
1
1
8
6
4
2
.2E
.0E
.0E
.0E
.0E
.0E
ꢂ04
ꢂ04
ꢂ03
ꢂ03
ꢂ03
ꢂ03
ꢂ00
MeOH/H O MeOH/H O MeOH/H O MeOH/H O MeOH/H O
2
2
2
2
2
y ꢃ 3.756x ꢂ 814.96
0
:100
30:70
50:50
70:30
100:0
1
00/0
0/30
0/50
0/50
/100
7
5
3
0
y ꢃ 1.7416x ꢂ 1024.3
y ꢃ 1.0246x ꢂ 1047.8
y ꢃ 0.5858x ꢂ 998.94
y ꢃ 0.3079x ꢂ 970.3
0E
0
500
1000
1500
2000
2500
3000
Time [s]
2
Fig. 4. Solvent effect on the phenol oxidation (substrate/ClO , 1 : 4).
methanol content, being maximum in pure methanol, implying
that the rate constant decreases with increasing polarity of the
medium (Fig. 4). When the water ratio increases so does the
hydrogen-bond formation of phenol with the more polar water
molecule thus shielding the O–H bond of phenol, so that its
reaction rate is retarded, meaning that the formation of
hydrogen bonds plays a substantial role in the reactivity of
phenol.

Theoretical and Experimental Oxidation of Phenols with ClO
2
819
2
,4,6-
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
oxidation rate (Table 1) and it is seen that the reaction rate
increases with decreasing Gibbs free energy. It was found that a
more negative value resulted for phenol than for the other
compounds, because of the greater rate constant for its reaction
trichlorophenol
y ꢃ ꢄ0.9499x ꢄ 2.3578
2
R
ꢃ 0.9947
2,4-dichlorophenol
with ClO . The introduction of electron-withdrawing groups
2
such as Cl in the aromatic phenol structure decreases the
electron density in the ring, thus retarding the attack of the
electron deficient ClO radical on the phenol; in particular,
p-chlorophenol
Phenol
2
ꢄ0.1
the abstraction of H from phenol by ClO turns out to be harder,
2
yielding a lower rate constant for 2,4,6-trichlorophenol than for
the other compounds.
ꢄ3.3 ꢄ3.2 ꢄ3.1 ꢄ3.0 ꢄ2.9 ꢄ2.8 ꢄ2.7 ꢄ2.6 ꢄ2.5 ꢄ2.4
lnk
On the other hand, since there is no electron-withdrawing
group in the phenolic ring which can enhance its reactivity upon
oxidation due to high electron density from the aromatic ring
influenced by the ring-current, the phenol ring can be oxidised
with great ease, especially by hydrogen abstraction from OH by
Fig. 5. Hammett plot: Correlation between substituent constant (s) v. ln k
for the different phenols (substrate/ClO , 1 : 4).
2
2
,4,6-
8
7
7
6
6
5
5
4
4
.0 trichlorophenol
2
,4-dichlorophenol
.5
.0
.5
.0
ClO . Therefore, one can identify the relative reactivity of
2
p-chlorophenol
phenol with the electron accepting groups. The impact of
chloro-substitution of phenols on the rate of reaction can be
justified from the inductive effect due to the s-acceptor behav-
iour of chlorine, although there is a resonance effect driven by
the chlorine lone-pair (p-donor). Consequently, we can antici-
pate that several chloro-groups coincide with a decrease in the
reactivity. Our experiments thus clearly show the following
trend in reaction rate for these oxidations: phenol .
Phenol
y ꢃ ꢄ2.4372x ꢂ 4E–13
.5
.0
.5
.0
2
R
ꢃ 1
ꢄ3.2 ꢄ3.1 ꢄ3.0 ꢄ2.9 ꢄ2.8 ꢄ2.7 ꢄ2.6 ꢄ2.5
ꢄ2.4
lnk
4
-chlorophenol . 2,4-dichlorophenol . 2,4,6-trichlorophenol.
Fig. 6. Gibb’s free energy plot for different phenol oxidation (substrate/
ClO , 1 : 4).
2
LC and GC-MS Studies
The phenol oxidation was analysed by LC and GC methods. The
LC results indicate that the peak height of phenol decreases with
the increase of new signals at retention time (R ) 7.35 and
Hammett Plot for Phenols/ClO Oxidation
2
The Hammett plot, which describes the linear free-energy
relationship among reaction rates, was used to develop quanti-
tative relationships between structure and activity.
t
5
of the reduction of the phenol peak (R 9.40 min). The signals at
.32 min; in addition, the new signals were grown at the expense
T
RT 9.40 and 9.22 min match the retention time of phenol and
hydroquinone (HQ), respectively. In addition, an attempt to
characterise this product using IR spectroscopy indicates the
presence of a C¼O group.
ln k ¼ ln k0 þ rs
ð6Þ
ð7Þ
X
r ¼ lnðkphenol=kxÞ
s
In the GC chromatogram (Supplementary Material, Fig. S1),
For the compounds, the observed rate constants were applied
in the Hammett equation and a good correlation was found
between log k and substituent s values (Fig. 5), i.e. the electron-
donating or electron-withdrawing groups altered the aryl alco-
hol oxidation. Thus, a higher reaction rate for phenol and a lower
rate for 2,4,6-trichlorophenol were observed. Furthermore, for
the Hammett plot, the correlation reaction constant value (r)
was ꢀ0.9499, where the sign of the slope indicates whether a
reaction rate is accelerated or suppressed by electron-donating
or electron-withdrawing substituents. In the present study, a
negative slope (r ¼ ꢀ0.9499) is observed, which indicates that
there is a positive charge at the reaction centre in the transition
state of the rate-limiting step, confirming that the rate of the
reaction is suppressed by electron-withdrawing substituents; in
contrast, the introduction of electron-releasing substituents at
the para-position of the aryl structure enhances the electron
density on the reaction centre and increases the reactivity. In
addition, the magnitude of r is a measure of the susceptibility of
the reaction to the electronic characteristics of the substituent.
Moreover, the reaction constant was correlated with Gibbs free
energy data (Fig. 6).
the prominent peaks were at R
these were selected for analysis by MS. The first two signals
corresponded to the R of phenol (phenoxide ion) and benzo-
T
3.39, 8.96, and 10.49 min and
T
quinone, respectively. The corresponding molecular masses
were m/z 92.9 (phenoxide ion) and 107 (benzoquinone), and
their fragmentation patterns coincide with that of phenoxide and
benzoquinone, respectively (see Supplementary Material), con-
firming the formation of benzoquinone as a major product of the
phenol oxidation by ClO
nation by HOCl cannot be ruled out by the coupling of different
oxidation. The possibility of chlori-
2
[
60–62]
free radicals.
The mechanism (see Scheme 1) indicates that two equiva-
lents of ClO are consumed in the reaction and step II is the rate
determining step in the ClO consumption. Due to the one-
2
2
electron acceptor characteristic of chlorine dioxide, the
expected reaction mechanism could involve a radical formation
[
63,64]
(Scheme 1). As for the reactions of ClO
with phenols,
the
2
formation of an unstable intermediate can be expected that will
later be converted into hydroquinone and hypochlorous acid. In
acidic solutions (, pH 4), there is another key reaction in which
chlorite is ‘recycled’ into the ClO
ation of its acid form. In the oxidation, the approximate number
pool by the disproportion-
2
In addition, for the degradation rate of phenols, the free
energy data were obtained by applying the following equation,
DG ¼ ꢀRT ln k, in order to relate the free energy data with the
of electron equivalents per ClO
peroxide-based oxidations.
is 5, as compared with 2 in the
2
0

8
20
C. A. H. Aguilar et al.
6
21
Table 2. The activation free energies (kinetics, DG ) and reaction enthalpy (thermodynamics, DH
B3LYP/6-3111G(d,p) level (in water) for a series of different phenol/ClO oxidations producing 1,4-benzoquinone derivatives (see Scheme 1) in
2
r
) in kcal mol , calculated at the SCRF-PCM-
comparison with the barriers calculated at the SCRF-M06-2X/6-31111G(d,p) level
B3LYP/6-311þG(d,p) level
Compound
I
II
III
6
6
6¼
DG
DG
DH
r
DG
DH
r
DH
r
Phenol (1)
6.08
5.58
ꢀ12.94
ꢀ13.45
ꢀ11.78
ꢀ12.68
22.51
24.24
26.76
27.02
ꢀ7.47
ꢀ4.15
ꢀ3.04
ꢀ1.42
2.05
6.96
7.52
7.75
ꢀ95.33
ꢀ47.77
ꢀ48.29
ꢀ49.09
4
2
2
-Chlorophenol (2)
,4-Dichlorophenol (3)
,4,6-Trichlorophenol (4)
8.47
10.10
Compound
M06-2X/6-311þG(d,p) level
DG
6
6
6
DG
DG
Phenol (1)
7.02
8.55
16.32
18.28
21.68
23.74
2.52
4
2
2
-Chlorophenol (2)
8.85
10.93
13.69
,4-Dichlorophenol (3)
,4,6-Trichlorophenol(4)
11.45
14.81
TS1
TS2
TS3
1.050
1.415
1.437
1.892
1
2.606
1.122
2.023
1.043
1.453
3.029
2
2.603
2.505
1.280
2.077
1.061
3.018
1.405
3
2.559
2.491
2.074
1.282
2.999
1.123
2.447
1.288
4
2.553
2.078
1.286
Fig. 7. Optimised transition state geometries (TS1, TS2, and TS3) for the three successive steps of phenol oxidations by ClO
˚
2
(
Scheme 1) computed at the SCRF-PCM-B3LYP/6-311þG(d,p) level along with some selected bond distances (A). Compounds:
phenol (1), 4-chlorophenol (2), 2,4-dichlorophenol (3), and 2,4,6-trichlorophenol (4).

Theoretical and Experimental Oxidation of Phenols with ClO
2
821
1
1
2
0
8
6
4
2
0
TS1
Computational Analysis
4
1
Herein, the progress of phenol/ClO oxidation for four different
2
3
reactions has been analysed based on the proposed mechanism
[
65–67]
in Scheme 1 using DFT computations.
All stationary point
geometries in the mechanism were optimised by using the
2
[
68–71]
[72]
B3LYP functional
implemented in the Gaussian09 package.
with the 6-311þG(d,p) basis set
as
In order to attain
[
73]
more authentic structures and energies, we re-optimised all gas-
phase geometries at the SCRF-B3LYP/6-311þG(d,p) level
using the PCM model where water was used as a solvent. The
unrestricted B3LYP calculations were carried out wherever
radical systems (step I) were involved in the mechanism. As far
as the relative reactivity trend is concerned, the performance of
UB3LYP/6-311þG(d,p) is acceptable for radical reactions
ꢄ2
ꢄ4
Phenols ꢂ ClO2
ꢄ6
ꢄ8
ꢄ10
ꢄ12
ꢄ14
ꢄ16
Phenol radical
ꢂ HClO2
STEP – I
[
74–79]
(I and II) as shown by recent studies
but at the same time
some of the studies on radicals have shown the weakness of this
[
80–83]
DFT method.
B3LYP analysis, we also performed the computation at the
In view of this and to further assist our
3
0.0
27.5
25.0
2.5
20.0
7.5
5.0
12.5
10.0
TS2
3
& 4
[
84–86]
SCRF-M06-2X/6-311þG(d,p) level
since this new gen-
eration DFT method has been proven to be as accurate as high
2
2
[
80–82]
1
level theories
for radical systems. Although the absolute
1
1
quantities of activation free energies show some variation
between these two DFTs, the relative trends in the barriers
(
Table 2) are quite remarkable. In addition, to test the reliability
7
5
2
.5
.0
.5
0
of both DFT methods, we carried out a set of higher level cal-
culations for the formation of a phenoxide radical from phenol
Phenol radical
ꢂ ClO
and ClO (i.e. step I) at both CCSD(T)/6-31þG(d,p) and CBS-
2
QB3 levels on B3LYP/6-311þG(d,p) geometries (see Table S3
in the Supplementary Material). The activation barrier for this
pathway at the B3LYP level was in reasonable agreement with
the higher level barrier but it was quite comparable with the
M06-2X barrier. Therefore, one can consider either B3LYP or
M06-2X quantities while discussing the kinetic aspect of the
reactions. Frequency calculations confirmed that equilibrium
geometries were found to have all real frequencies whereas
transition state (TS) geometries had only one imaginary fre-
quency (see Supplementary Material).
ꢄ2.5
ꢄ5.0
ꢄ7.5
2
STEP – II
Cyclohexadienone
analog
ꢄ10.0
2
0
0
0
TS3
1
ꢄ10 Cyclohexadienone
analog
ꢄ20
The frontier molecular orbitals (FMOs) calculated at the
DFT level reveal that the energy gap between the HOMO and
LUMO decreases (see Supplementary Material) from phenol (1)
to 4-chlorophenol (2) to 2,4-dichlorophenol (3) to 2,4,6-
trichlorophenol (4) i.e. as the number of chloro-substituents
increases. In view of this, the reactivity of phenol towards
oxidation should increase with the increase in the number of
chloro groups. This qualitative analysis clearly indicates the
reactivity order as expected, but the quantitative energies
obtained from the reaction profile geometries in comparison
with experimental data will provide more conclusive remarks
ꢄ30
ꢄ40
ꢄ50
ꢄ60
ꢄ70
ꢄ80
ꢄ90
2
–4
1
STEP – III
ꢄ100
1,4-benzoquinone
ꢂ Cl O
ꢄ110
2
Fig. 8. The potential energy diagram (kinetic and thermodynamic ener-
gies) for a series of different phenol/ClO reactions (phenol (1), 4-chlor-
ophenol (2), 2,4-dichlorophenol (3), and 2,4,6-trichlorophenol (4)) forming
,4-benzoquinone derivatives (Scheme 1) obtained from SCRF-PCM-
for phenol/ClO oxidations.
2
The oxidation of phenols 1–4 by ClO proceeds by three
2
2
successive steps (Scheme 1) and these mechanisms were ana-
lysed by concerted TSs at the B3LYP/6-311þG(d,p) level (see
Supplementary Material for M06-2X/6-311þG(d,p) geome-
tries). Computations clearly illustrate the nature of these TSs
1
B3LYP/6-311þG(d,p) computations. See Table 1 for a clear trend in the
reaction barriers and energies.
(
follows: (I) The addition of phenol to ClO involves a hydrogen
Fig. 7) by formed and cleaved bonds in these pathways as
between ClO and the para-carbon of phenol is seen to be
2
2
abstraction in which a weakening of the O–H bond (1.04–
˚
formed faster but it disrupts the p-delocalization of the benzene
ring; thus the energy of the TSs (TS2s) should be higher than
the expected value. A gradual decrease in the C?O distance
while going from 1 to 4 reflects the lateness of the TS in the
series of reactions and this would provide a subsequent increase
in the activation barrier. (III) The benzophenones have been
1
.12 A) due to bond cleavage along with the formation of a
˚
H?O bond (1.40–1.45 A) in TS1s leads to the formation of a
phenoxide radical. (II) The formation of a cyclohexadienone
derivative occurs upon further addition of ClO to a phenoxide
2
˚
radical. Herein, a new O?C bond (2.5 – 2.6 A) occurring

8
22
C. A. H. Aguilar et al.
2
2
2
2
2
2
2
8
7
6
5
4
3
2
Moreover, the effect of chloro substituents on phenol upon
oxidation by chlorine dioxide can be understood from the
calculated activation free energies shown in Table 2 and
Fig. 8. Herein, as the number of chloro groups increases from
phenol to 4-chlorophenol to 2,4-dichlorophenol to 2,4,6-
trichlorophenol (1–4), the activation barrier found in all three
pathways increases in this order. Remarkably, our DFT predic-
tion (Table 2) indicates the same trend in the reactivity of these
phenols as we have shown by kinetics experiments (Table 1),
although a trend in the exothermicity of these reactions is
slightly random. Computations again prove that the s-accepting
ability of the chloro group actually decreases the reactivity of
phenol towards the oxidation process. The main emphasis of this
study can be demonstrated from a correlation between ln k
values and predicted activation free energies (,98 %) in
Fig. 9, which exhibits an excellent agreement between comput-
ed and experimental quantities. Thus, our combined DFT and
experimental study provides strong evidence to support the
proposed mechanism and relative reactivity trend in phenol
and chlorophenols for the oxidation reaction by ClO2.
y ꢃ ꢄ6.7842x ꢂ 5.8484
2
R
ꢃ 0.9516
ꢄ3.3 ꢄ3.2 ꢄ3.1 ꢄ3.0 ꢄ2.9 ꢄ2.8 ꢄ2.7 ꢄ2.6 ꢄ2.5 ꢄ2.4
lnk
Fig. 9. A correlation of ln k (experimental) with the predicted activation
energies (B3LYP/6-311þG**) of the rate-limiting step obtained for the
2
oxidation of phenol and chlorophenols by ClO .
formed from cylohexadienone derivatives by via TS3s where
carbonyl group (C¼O) formation occurs to a lesser extent
Summary and Conclusion
(
Fig. 8) in chloro-derivatives compared with the typical system
Chlorine dioxide has been shown to be an effective oxidant for
the degradation of phenols. Among the studied compounds,
phenol exhibits a greater oxidation rate constant than the other
compounds, agreeing with the Hammett plot (log k v. s). In
contrast, for 2,4,6-trichlorophenol, a lower reaction rate was
observed because of the electron withdrawing chlorine groups at
the para and ortho-positions, thus producing a more positive DG
˚
as revealed by the C¼O distance (,1.42 v. ,1.28 A) and this is
an indication of a slower reaction of all the chlorophenols.
The progress of the oxidation of phenol and the chlorophe-
nols with ClO has been analysed by three consecutive reaction
2
energy pathways (Fig. 8) based on the proposed mechanisms.
It is to be noted that the difference in activation barriers between
the B3LYP and M06-2X levels is found to be roughly
ꢀ1
value than the other compounds. The phenol/ClO oxidation is
2
1
–4 kcal mol throughout the reaction paths, but we have
found to be of second order with respect to phenol and first order
with respect to ClO , in agreement with our proposed mecha-
shown below only B3LYP data for further discussion. Compu-
tations indicate that the first step of the reaction starting from
phenol to form a phenoxide radical proceeds much faster than
the second step that yields a cyclohexadienone analogue as
shown by the activation free energies in Table 2 and Fig. 2,
whereas the third pathway which forms ‘benzophenone’ is
found to be the fastest one. In particular, the formation of
cyclohexadienones occurs by a loss of aromaticity in the
phenol ring which leads to a significantly high barrier (22–
2
nism for the oxidation. Furthermore, the oxidation of phenol by
ClO is pH dependent since the reaction rate constant increases
2
with increasing pH. The oxidation rate is also considerably
enhanced with the increasing methanol ratio in the water
medium. In the oxidation products analysed by LC and GC/MS,
glycine and benzoquinone are found to be the major products.
Predictions based on DFT computations (B3LYP/6-311þ
G(d,p)) provided strong support for the proposed mechanisms
because the reactivity trend shown by kinetics is in excellent
agreement with the predicted activation barriers.
ꢀ1
2
7 kcal mol ). It is also to be noted that the hydrogen abstrac-
tion process (I) is only slightly slower than the benzophenone
formation (III) because the activation free energies are found to
ꢀ
1
be 6–10 and 2–7 kcal mol , respectively. Reaction enthalpies
in Table 2 (see also Fig. 8) reveal that the phenoxide radical
formation is quite exothermic as shown by enthalpy of reactions
of approx. ꢀ12 to ꢀ13 kcal molꢀ in all four reactions but such
Supplementary Material
Supplementary Material containing the experimental and theo-
retical data can be found on the Journal’s website.
1
ꢀ1
exothermicity drops off (E ꢀ1 to ꢀ7 kcal mol ) while forming
the cyclohexedienone/p-benzoquinone derivatives by dearoma-
tisation as occurs in the kinetic pathway. Concurrently, the
formation of benzophenone in step III shows a relatively much
Acknowledgements
The authors acknowledge the Direcc ´ı on General de Asuntos del Personal
Acad e´ mico (Project PAPIIT No IN226310 and IN217813; PASPA) and
Mexico-India-Joint Research Project [CONACYT/MEX -DST/INT] for
economic support. The authors also thank Dr Ernest Zeller for his valuable
comments and suggestions on the paper.
ꢀ
1
higher exothermicity (ꢀ48 to ꢀ95 kcal mol ) due to a strong
p-conjugation in the product. Thus, DFT computations predict
that the final step of our mechanism is much more favourable
kinetically as well as thermodynamically and the formation of
cyclohexedienone/p-benzoquinone (step II), illustrating a very
high barrier, could be the rate-limiting step (Table 2 and Fig. 8).
Interestingly, both kinetic and thermodynamic energies shown
in the reaction pathways are in good accordance with the
Hammond’s postulate because the reaction with greater exo-
thermicity arises from a lower barrier although the ultimate
product provides an unusually high exothermic energy.
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