Structure-reactivity relationships and substituent effect additivity in the aqueous oxidation of chlorophenols by cerium(iv)

Article abstract of DOI:10.1039/c0nj00529k

The oxidation of all 19 chlorophenols and unsubstituted phenol by cerium(iv) was studied in an acidic aqueous solution in order to carry out a systematic test of chlorine substituent effects on the reactivity. All reactions were found to show 21 cerium(iv) phenol stoichiometry and a simple second-order rate equation. Rate constants did not correlate well with characteristic parameters such as pK values, carbon-13 NMR chemical shifts or Hammett substituent constants. Nevertheless, a strict additivity of chlorine substituent effects was found in both characteristic and reactivity parameters. The data suggest that a proton-coupled electron transfer mechanism could be operative. 2,4,6-Trichlorophenol was found to show exceptionally high reactivity towards cerium(iv). The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011.

Full text of DOI:10.1039/c0nj00529k

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PAPER
Structure–reactivity relationships and substituent effect additivity
in the aqueous oxidation of chlorophenols by cerium(IV)w
´ ´ ´ ´
Adrienn Simon, Csaba Ballai, Gabor Lente* and Istvan Fabian
Received (in Montpellier, France) 6th July 2010, Accepted 1st October 2010
DOI: 10.1039/c0nj00529k
The oxidation of all 19 chlorophenols and unsubstituted phenol by cerium(IV) was studied in an
acidic aqueous solution in order to carry out a systematic test of chlorine substituent effects on
the reactivity. All reactions were found to show 2 : 1 cerium(IV) : phenol stoichiometry and a
simple second-order rate equation. Rate constants did not correlate well with characteristic
parameters such as pK values, carbon-13 NMR chemical shifts or Hammett substituent constants.
Nevertheless, a strict additivity of chlorine substituent effects was found in both characteristic and
reactivity parameters. The data suggest that a proton-coupled electron transfer mechanism could
be operative. 2,4,6-Trichlorophenol was found to show exceptionally high reactivity towards
cerium(IV).
the oxygen in the chlorophenol.10 No similar relationship with
Introduction
the pK values of various phenols was found in the same work.10
The correlation must have far-reaching mechanistic implica-
tions, but the lack of similar examples from the earlier literature
was a serious obstacle to making a convincing point about the
consequences. Based on some independent information on the
homolytic bond dissociation energies of chlorophenols,17 it was
suggested that the observed trend might be indicative of
hydrogen atom transfer in the rate determining step.10 The
interpretation of these results would be much easier in compar-
ison with similar substituent effect studies in stoichiometric
redox reactions of chlorophenols in non-catalytic systems.
The three major one-electron oxidation pathways of
phenols are hydrogen atom transfer (HAT), proton-coupled
electron transfer (PCET), and sequential proton-loss electron
transfer (SPLET). In water, SPLET is equivalent to the acid
dissociation of the phenol followed by electron transfer from
the phenolate anion. This is easily identified by the pH
dependence as this sort of mechanism should show inverse
first-order dependence on the concentration of hydrogen ion in
the acidic region. No such distinction is easily made between
HAT and PCET and recent research has focused a lot of
attention on this question.18–28
Chlorophenols constitute a group of widely used compounds
that are currently listed as priority pollutants by European and
American environmental protection agencies.1,2 Their removal
from industrial wastewater usually requires special techniques
as they are resistant to chemical oxidation or microbial
degradation.3–16 From an environmental point of view, the
stoichiometric oxidant in a chemical treatment method should
be an abundant and cost-effective reagent, the by-product
of which does not cause major environmental concerns.
Hydrogen peroxide and peroxomonosulfate ion (oxone) seem
ideal oxidants for this purpose and a number of catalytic
systems have been reported using these compounds. The
best results were obtained with iron complexes as
catalysts.3,6–8,10–12 Mechanistic details of such catalyzed
processes are not easy to explore. A useful but not very often
used tool in those kinetic studies is a systematic change of the
number of chlorine substituents on the oxidizable substrates.
In one of our earlier studies, an interesting structure–reactivity
relationship was revealed in the FeIIITPPS-catalyzed (FeIIITPPS:
iron(III) meso-tetra(4-sulfonatophenyl)porphine) oxidation of
chlorophenols by hydrogen peroxide.10 Using various substi-
tuted chlorophenols, the logarithm of the rate constant of the
reaction between the active form of the catalyst and the
chlorophenols was shown to correlate with the 13C NMR
chemical shift of the carbon atom that is directly connected to
The aim of the present work was a thorough study of the
effect of chlorine substituents on the one-electron oxidation of
phenols using all possible chlorinated phenols. Cerium(IV) was
found to be an ideal oxidant for this purpose as it is a strong
one-electron oxidant which can only be used in highly acidic
medium, ensuring that any rate contributions from pathways
involving the phenolate ion (SPLET) could only be minor.
This paper reports our detailed stoichiometric and kinetic
results and correlation analysis between rate constants and
selected characteristic parameters.
Department of Inorganic and Analytical Chemistry, University of
Debrecen, Debrecen, Hungary. E-mail: lenteg@delfin.unideb.hu;
Fax: +36 52-518-660; Tel: +36 52-512-900 Ext. 22373
w Electronic supplementary information (ESI) available: Additional
figures, tables and mathematical derivations referred to in the text of
the article. See DOI: 10.1039/c0nj00529k
c
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New J. Chem., 2011, 35, 235–241 235

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computer-controlled Metrohm Titrino system. The
calibration of the electrodes was based on relevant IUPAC
recommendations.31 1H and 13C NMR spectra were recorded
on Varian VRX-300, Varian VRX-400, Bruker DRX-360 and
Bruker DRX-400 instruments and were calibrated using the
residual signal of the solvent CDCl3. In addition to the
operating softwares of individual instruments, Scientist32 and
Matlab33 were used in the fitting procedures.
Experimental section
Materials
Chlorinated phenols were purchased from Aldrich, Lancaster
Synthesis and Supelco. When necessary (i.e. when the
purchased material was not completely white), they were
purified by vacuum sublimation before use. The purity of all
compounds has been tested in 1H and 13C NMR measure-
ments. Throughout this paper, different chlorophenols will be
abbreviated by giving only the positions of the chlorine
substituents on the phenol (e.g. 236 means 2,3,6-trichlorophenol).
Solutions of Ce(IV) were prepared by dissolving Ce(SO4)2 in
sulfuric acid. The Ce(IV) concentrations of these solutions were
standardized by iodometry. All other chemicals used in this
study were of analytical reagent grade and purchased from
commercial sources. Doubly deionized and ultrafiltered water
from a Millipore Q system was used to prepare the stock
solutions and samples.
Results
Acid dissociation constants and chemical shifts
The acid dissociation constants of all studied chlorophenols
were determined in this study in order to use strictly
identical conditions and measuring methods. A combined
pH-potentiometric and spectrophotometric method was used,
whereby the UV-vis spectra of chlorophenols were recorded as
a function of measured pH. Fig. S1 in the ESIw gives a series of
spectra for 2356 as an example. The data were evaluated based
on the following equation with the use of linear algebraic
transformations detailed in the ESIw:
Instruments and softwares
Shimadzu UV-2501PC (scanning), Shimadzu UV-3101PC
(scanning), Shimadzu MultiSpec-1500 (diode-array), Perkin
Elmer Lambda 25 (scanning), Perkin Elmer Lambda 2S
(scanning), and HP-8543 (diode-array) spectrophotometers
were used in this study using standard quartz cells with
1.000 cm optical path length. All kinetic measurements were
performed with an Applied Photophysics DX-17 MV
sequential stopped-flow apparatus in the absorbance mode
using 1.00 cm optical path length. The dead time of the
instrument was measured to be 1.09 ꢀ 0.02 ms by published
methods.29,30 pH was measured by standard combination
glass electrodes (Hanna Instruments or Metrohm) connected
to either a precision Hanna Instruments pH-meter or a
Al;pH el;HA10ꢁpH þ el;A10ꢁpK
¼
ð1Þ
ctot
‘
10ꢁpH þ 10ꢁpK
In this equation, Al,pH is the absorbance measured at wave-
length l and pH, ctot is the analytical concentration of the
phenol, l is the path length, el,HA and el,A are the molar
absorption coefficients for the protonated and deprotonated
forms of the phenol at wavelength l. All determined pK values
are shown in Table 1 along with some relevant literature data
for comparison.34–36 The standard error for the determined
parameter pK was smaller than 0.01 when calculated by the
usual statistical analysis. This is viewed as unreasonably small
Table 1 pK values, 13C NMR chemical shifts and rate constants in the oxidation by cerium(IV) determined for different chlorophenols used in this
study
Literature pKb
pKa
dC1c/ppm
k/Mꢁ1 sꢁ1
Ref: d,e
Ref: f
Phenol
2-Chlorophenol
3-Chlorophenol
4-Chlorophenol
9.86
8.33
8.85
9.34
7.64
7.83
7.40
6.77
8.56
7.99
7.00
6.68
5.75
6.97
6.15
7.82
6.31
5.32
5.31
4.66
9.79d
8.53d
8.78d
9.20d
7.70d
7.89d
7.51d
6.79d
8.59d
8.18d
—
10.02
8.48
9.02
9.38
7.45
7.75
7.35
6.79
8.39
7.93
7.59
7.23
6.12
7.33
6.42
7.74
6.96
—
155.3
151.5
155.9
153.8
152.8
150.3
152.0
148.1
154.6
156.5
151.4
153.0
149.2
150.8
147.1
154.2
151.0
147.9
150.1
148.3
(3.72 ꢀ 0.03) ꢂ 104
(9.07 ꢀ 0.05) ꢂ 103
(3.48 ꢀ 0.01) ꢂ 104
(4.43 ꢀ 0.02) ꢂ 104
(9.08 ꢀ 0.11) ꢂ 103
(5.72 ꢀ 0.05) ꢂ 103
(9.79 ꢀ 0.03) ꢂ 103
(1.58 ꢀ 0.01) ꢂ 103
(4.54 ꢀ 0.05) ꢂ 104
(3.48 ꢀ 0.04) ꢂ 104
(1.00 ꢀ 0.05) ꢂ 104
(1.06 ꢀ 0.06) ꢂ 104
(1.84 ꢀ 0.01) ꢂ 103
(1.63 ꢀ 0.03) ꢂ 104
(2.27 ꢀ 0.08) ꢂ 104
(5.6 ꢀ 0.3) ꢂ 104
2,3-Dichlorophenol
2,4-Dichlorophenol
2,5-Dichlorophenol
2,6-Dichlorophenol
3,4-Dichlorophenol
3,5-Dichlorophenol
2,3,4-Trichlorophenol
2,3,5-Trichlorophenol
2,3,6-Trichlorophenol
2,4,5-Trichlorophenol
2,4,6-Trichlorophenol
3,4,5-Trichlorophenol
2,3,4,5-Tetrachlorophenol
2,3,4,6-Tetrachlorophenol
2,3,5,6-Tetrachlorophenol
Pentachlorophenol
—
—
6.71e
6.42e
—
—
—
—
5.25e
(2.3 ꢀ 0.2) ꢂ 104
(2.21 ꢀ 0.08) ꢂ 103
5.44
5.26
(2.7 ꢀ 0.1) ꢂ 103
(6.0 ꢀ 0.9) ꢂ 103
a
b
Measured in this work, standard error: ꢀ0.02 (see text for detailed explanation). Literature data typically measured at varying ionic strength
c
d
and medium, sometimes with the addition of non-aqueous solvents up to 50%. Precision: ꢀ0.1 ppm, solvent: CDCl3. NIST Database
e
(ref. 34). SC Database (ref. 35). Ref. 36.
f
c
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236 New J. Chem., 2011, 35, 235–241

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as the actual accuracy of the pH measurements was 0.01 in all
cases. Therefore, the conservative estimate of 0.02 will be used
for the standard errors for all pK values determined in this
product of the reaction between 246 and Ce(IV) was
indistinguishable from the independently known spectrum
of an authentic sample of 2,6-dichloro-1,4-benzoquione.37
Furthermore, two characteristic reactions (a photoreaction37
and oxidation by H2O2 solution10) of this quinone were also
confirmed to occur in test experiments after masking
cerium(III) with EDTA (Fig. S3 in the ESIw). In the case of
26 (Fig. S4 in the ESIw), a similar analysis showed that the
product was different from 2,6-dichloro-1,4-benzoquinone,
which was formed in the FeTPPS-catalyzed oxidation of 26.10
The confirmed stoichiometry also rules out the formation
1
work. H NMR and 13C chemical shifts were determined for
all chlorophenols in CDCl3 by routine measurements. The
results are listed in Table 1 and Table S1 in the ESI.w
Reaction stoichiometry
The stoichiometry of the reaction between cerium(IV) and each
chlorophenol was studied by spectrophotometric titration in
0.10 M H2SO4. A solution of the studied phenol was titrated in
a cell and the spectra were recorded after the addition of each
increment. As an example, the titration of 246 is shown in
Fig. 1, which also displays the UV-vis spectra of Ce(IV) and
Ce(III). In all cases, the reaction was shown to feature
Ce(IV) : phenol stoichiometry of 2 : 1 corresponding to an
overall two-electron oxidation of the phenols. This is valid for
conditions under which the phenol is in excess and the product
of this initial reaction was shown to be oxidized further by
Ce(IV) on a time scale longer (from several minutes to hours)
than that of the direct reaction between the original reagents.
This clearly demonstrated the need for titrating a phenol
solution with small increments of Ce(IV) solution in the
spectrophotometric titrations, results from a reverse titration
would have been difficult to interpret. The kinetics of the
process was always studied at phenol excess, where the 2 : 1
stoichiometry clearly prevails.
of 2,6-dichloro-1,4-benzoquinone as it would require
a
four-electron oxidation of 26. The most logical two-electron
oxidation product of 26 would be 3-chloro-1,2-benzoquinone.
This is in agreement with the intense peak observed at 434 nm
in the product spectrum, which is usually characteristic of
ortho-quinones, and is also in agreement with the literature
information.41–44 However, these observations by no means
imply a positive product identification similar to the case
of 246.
Reaction kinetics
The kinetics of the reactions was studied at chlorophenol
excess by the stopped-flow method. In all cases, the following
simple second-order rate equation was found to be valid:
d½CeðIVÞꢃ
¼ ꢁ2k½CeðIVÞꢃ½Cl ꢁ Phꢃ
ð2Þ
dt
No efforts were made to positively identify the two-electron
oxidation products formed from the phenols beyond some
observations in the reactions of 246 and 26. In the case of 246,
the confirmed 2 : 1 stoichiometry corresponds to a two-
electron oxidation of the organic molecule, which usually
leads to 2,6-dichloro-1,4-benzoquione according to well
established literature information.8,10,12,37–40 Further experi-
mental confirmation of this product was made possible by the
fact that neither 246 nor the product Ce(III) has significant
absorption above 320 nm. The UV-vis spectrum of the
In most cases, the chlorophenol could be used in large excess
over Ce(IV). The absorption of the limiting reagent Ce(IV) was
followed at 320 nm, where only Ce(IV) absorbs. An expo-
nential decay was observed in these cases, which proved that
the reaction was first-order with respect to Ce(IV). The
concentrations of chlorophenols were varied by about an
order of magnitude in different experiments, and the direct
proportionality of the observed rate constants with the
concentration of the excess reagent (Fig. 2) proved the first-
order nature of the process with respect to the chlorophenol. It
should be kept in mind that the slopes of these straight lines
give an estimate of 2k because of the stoichiometry.45
Fig. 1 Spectrophotometric titration of an aqueous solution of
2,4,6-trichlorophenol with cerium(IV) sulfate. Initial solution: V =
2.00 cm3, [246] = 0.690 mM. Titrant: [Ce(IV)] = 4.75 mM. Increments:
0.100 cm3. Optical path length: 1.000 cm, T = 25.0 1C. Inset: molar
absorbance values of Ce(IV) and Ce(III).
Fig. 2 Pseudo first-order rate constants as a function of chlorophenol
concentration in the oxidation of various chlorophenols by cerium(IV)
ion. Medium: 0.10 M H2SO4. T = 25.0 1C.
c
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245, 345, 2345, 2346, 2356 and 23456 were not soluble
enough in water to cover a sufficiently large concentration
range in kinetic experiments while still using a large excess of
the chlorophenol over Ce(IV). In these cases, most measure-
ments were carried out under second-order conditions. The
absorbance-time traces were fitted to the following equation:45
appt
Dgeꢁ2k
AðtÞ ¼ e
ð3Þ
appt
1 ꢁ 0:5geꢁ2k
where
D = [Cl–Ph]0 ꢁ 0.5[Ce(IV)]0, g = [Ce(IV)]0/[Cl–Ph]0, kapp = kD
(4)
In eqn (3), e represents the molar absorption coefficient of
Ce(IV) in 0.10 M H2SO4. The successful fit to this equation
alone proves adherence to eqn (2). Nevertheless, kinetic curves
measured at 4–6 different initial concentrations of the
chlorophenols were used in each case to estimate the rate
constant.
Fig. 4 Rate constants of the oxidation chlorophenols by cerium(IV) as
a function of the 13C chemical shift of the phenolic carbon. Grey
triangles: similar plot obtained in the reaction of chlorophenols with
hydrogen peroxide catalyzed by FeIII(TPPS).10
All the determined rate constants are listed in Table 1. The
values refer to measurements done in 0.10 M H2SO4 without
any additional salts. Some detailed kinetic information for
chlorophenols not covered in Fig. 2 are shown in Fig. S5–S8 in
the ESI.w The rate did not depend on the acidity in the H2SO4
concentration range between 0.10 M (pH = 0.86) and 0.50 M
(pH = 0.25).
similar data from an earlier work obtained in the
FeIIITPPS-catalyzed oxidation of chlorophenols where a
strong correlation was observed for comparison.10 There is
no spectacular correlation between pK values and rate
constants, either (Fig. S9 in the ESIw). Average chemical shifts
of aromatic carbon atoms were also used as site-independent
molecular descriptors, but failed to give any reasonable
correlation with the rate constants (Fig. S10 in the ESIw).
Structure–reactivity correlations
Correlations were sought between two characteristic para-
meters, pK and the 13C NMR chemical shift of the carbon
atom connected directly to the oxygen in each chlorinated
phenol (d13C1) and the logarithm of the second-order rate
constant of the oxidation of the phenol by Ce(IV). Fig. 3 shows
that the two characteristic parameters are significantly
different as they do not correlate with each other. On closer
inspection, the graph reveals that the chlorophenols can be
divided into three groups depending on the number of chlorine
substituents in the meta position. There is strong correlation
between the two characteristic parameters within each group.
Fig. 4 shows that the correlation between the logarithm of
the rate constants and d13C1 is weak. The figure also displays
Additivity of substituent effects
The additivity of substituent effects was tested for both
characteristic parameters and for the kinetic data as well by
seeking substituent constants that best adhere to Hammett-
type relationships.46 The first equation (3s equation) assumed
total additivity of substituent effects:
Y = Y0 + noso + nmsm + npsp
(5)
where so, sm and sp are characteristic of the effect of ortho,
meta and para substituents, and Y0 is the reference value of the
property characteristic for unsubstituted phenol. It must be
noted that Y0 was also optimized to give the best fit for
all points rather than simply assigned the value of unsubsti-
tuted phenol. This was necessary in order to avoid assigning
a special role to any particular compound in the fitting
procedure.
An approach containing more parameters has also been
tried. In this approach, it was not assumed that two ortho
substituents, for example, have the same effect on the property
together as two independent ortho substituents would have.
The corresponding equation (5s equation) is given as:
Y ¼ Y00 þ no1so0 2 þ no2so0 þ nm1sm0 1 þ nm2sm0 2 þ nps0p ð6Þ
The additivity tests were done based on the 20-element data set
by seeking the best-fitting s values with linear least-squares
fitting for each of the three properties pK, d13C1, and log k.
The best fitting so, sm and sp values for eqn (5), together with
the correlation coefficients, are displayed in Table 2. The
goodness of the fits are illustrated in Fig. 5–7, where the
Fig. 3 13C chemical shifts of the phenolic carbon as a function of the
pK values for the studied chlorophenols.
c
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Table 2 Substituent constant values and correlation coefficients from
the additivity studies of pK, chemical shift and rate constant values
2
r3s
2
r5s
so
sm
sp
pK
d13
ꢁ1.54 ꢀ 0.03 ꢁ0.79 ꢀ 0.03 ꢁ0.45 ꢀ 0.06 0.9938 0.9949
C
ꢁ3.3 ꢀ 0.1 0.58 ꢀ 0.10 ꢁ1.50 ꢀ 0.16 0.9865 0.9893
0.24 ꢀ 0.11 0.9519 0.9543
log k ꢁ0.53 ꢀ 0.07 0.03 ꢀ 0.07
Fig. 7 Rate constant measured in the oxidation of the studied
chlorophenols by cerium(IV) as a function of the sum of substituent
constants excluding 2,4,6-trichlorophenol. 3s: with the use of eqn (5).
5s: with the use of eqn (6).
Discussion
The rate equation of the oxidation of chlorophenols by
cerium(IV) shows that the phenolate anion cannot be a reactive
species in these processes. Very clearly, inverse first-order
acidity-dependence would be expected in this case under the
highly acidic conditions used here. In addition, the true second
order rate constant of the process would be higher than the
diffusion controlled limit of 1010 Mꢁ1 sꢁ1 for some of the
studied compounds (phenol, 2, 3, 4, 23, 24, 25, 34, 35, 234,
245, 345) if electron transfer was initiated form the phenolate
ion. This observation rules out the SPLET mechanism of
one-electron transfer. It is also notable that none of the kinetic
studies showed saturation of rate with increasing concentra-
tion of chlorophenol, which indicates that there is no
substantial association (complex formation) between the
reactants. The most notable deviation from the expected
straight lines in the pseudo first-order rate constant-
chlorophenol concentration plots occurs for 246 (shown in
Fig. 2), where a careful look at the plot might hint of some
curvature. However, this is statistically insignificant. Actually,
it would have been very useful to do experiments at higher
concentrations of 246 because any saturation character would
have been more pronounced in that region. Unfortunately,
this was not possible because the experiments already
approached the solubility limit of 246. Therefore, there is no
experimental reason to believe that there is a significant
saturation character for this plot.
Fig. 5 pK values for the studied chlorophenols as a function of the
sum of substituent constants. 3s: with the use of eqn (5). 5s: with the
use of eqn (6).
Fig. 6 13C NMR chemical shifts for the studied chlorophenols as a
function of the sum of substituent constants. 3s: with the use of
eqn (5). 5s: with the use of eqn (6).
analyzed property is displayed as a function of the appropriate
sum of substituent constants S and S0. The definitions of these
quantities follow from eqn (5) and (6):
Fig. 3 shows that the two characteristic parameters (pK and
d13C1) analyzed here are under different influences. A closer
look at this figure reveals that the points actually give linear
correlation in three distinct series depending on the number of
meta chloro substituents. The fact that the correlation is quite
good within each of the three series (m-H2, m-HCl, and m-Cl2)
indicates that the para and the ortho substituents have very
similar effects on the pK and on d13C1. This will be analyzed
further at the discussion of the additivity tests.
S = noso + nmsm + npsp
(7)
S0 ¼ no1s0o2 þ no2so0 þ nm1sm0 1 þ nm2sm0 2 þ nps0p
ð8Þ
As the values of so, sm and sp have been determined to give the
best fit, points in this representation should be scattered
around a straight line that has a slope of unity. Deviations
from these straight lines represent the deviations from
additivity. The correlation coefficients given in Table 2 give
a numerical idea about the goodness of fit.
Fig. 4 shows very poor correlation between the logarithm of
rate constants and the d13C1 values. An even worse correlation
c
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is found between log k and pK values (Fig. S9 in the ESIw).
The points in Fig. 4 can be broken down to three clusters
depending on the number of ortho chloro substituents.
Basically, o-Cl2 phenols (except 246) are oxidized relatively
slowly and o-H2 phenols relatively fast, with o-HCl phenols
occupying a middle position in this respect. This is quite
different from the earlier case when we studied the
FeIIITPPS/H2O2 system as shown by the grey points in
Fig. 4.10 Even if one assumes a weak correlation for the rate
constants of the cerium(IV) oxidation, this correlation is in the
opposite direction (log k is smaller with larger d13C1) than in
the case of FeIIITPPS/H2O2 system.10 In addition, the product
in one of the reactions was proved to be quite different: 26 is
oxidized to 2,6-dichloro-1,4-benzoquinone in the case of
FeIIITPPS/H2O2,10 whereas this is clearly not the case in the
present study of cerium(IV) oxidation. On these grounds, it is
very safe to conclude that these two reactions have quite
different mechanisms. Since there is some reason to argue that
the FeIIITPPS/H2O2 system oxidizes phenols by HAT,10 the
cerium(IV) oxidation could follow the PCET pathway.
19 points. A version of this graph with all 20 points is shown as
Fig. S11 in the ESIw, and illustrates that the effect of even a
single point can hide an otherwise significant correlation in the
statistical analysis. The largest substituent effect, similarly to
the previous two cases, is exerted by the ortho group, and the
negative value of so indicates that ortho substitution decreases
the reactivity in the cerium(IV) oxidation. Earlier, there have
been numerous attempts to characterize ortho substituents
with Hammett constants.47–52 These efforts did not yield
widely accepted so values, which is classically interpreted as
a consequence of steric effects. However, this interpretation
would not be without contradictions in our special case. First
of all, chlorine is a single atom, which is much closer in steric
bulk to a hydrogen atom than any other chemical groups with
several connected atoms. In addition, the effect compared to
the effect of the para substituents is actually weaker than
similar ratios for characteristic parameters (pK and d13C1).
Therefore, there is not much experimental ground for implying
steric effects in this process.
Meta substitution has no significant effect on the reactivity
as shown by the sm value, which is not significantly different
from 0. Para substitution, on the other hand, clearly increases
the reactivity: sp is positive and its value is about a half of ꢁso.
Based on these observations, 4, 34 and 345 are expected to
have the largest rate constants, which is in full agreement with
the data shown in Table 1. It is also observed that the effects
are strongly position-dependent: there is a 50-fold difference in
rate constants and a pK change of more than 2.0 determined
just within the series of six trichlorophenols. The chlorine
substituents are not directly involved either in acid dissocia-
tion or in the rate determining step of the cerium(IV) reaction.
Therefore, their effect must be exerted solely by modifying the
electronic structure of the aromatic system. As pointed out
previously, this can be often understood in terms of electron
withdrawing and inductive effects.
Fig. 5–7 show that the additivity of substituent effects is
excellent for pK values, but the d13C1 and log k data (excluding
246 in this last case) also show fits that are better than in most
published examples of substituent effect relationships. It
should be noted that the correlation coefficients only show a
very modest improvement when eqn (6) is used. This, again,
proves that the additivity of substituent effects is quite good
for each of the studied properties and the fit cannot be
improved by including further parameters.
The determined s values summarized in Table 2 deserve
more attention. The
s values obtained from different
properties are not directly comparable as the absolute changes
in pK, d13C1 and log k are different. From the s values
obtained based on pK values, it is clear that distance is the
primary factor in determining the acidity of the phenolic OH
groups. All three s constants are negative (any substitution
with chlorine decreases the pK), which is probably due to the
electron withdrawing effect of a chlorine substituent. The
s values decrease in the so 4 sm 4 sp order. The two other
properties show different behavior. The d13C1 values reflect a
behavior similar to the textbook direction rules in aromatic
electrophilic substitution, which is based on a balance between
electron withdrawing and inductive effects. Ortho and
(to a smaller degree) para substituents decrease the chemical
shift, which means that they increase the electron density on
the phenolic carbon. The so/sp ratio is 2.2 for d13C1, which is
not very far away from the similar ratio calculated based on
pK values (3.4). This is the primary origin of the good
correlation between pK and d13C1 within each of the three
series shown in Fig. 3. The sm value obtained from d13C1 is
positive, which means that meta substitution removes some
electron density from the phenolic carbon. The magnitude of
the effect, however, is much smaller than para or ortho effects.
This is, again, in excellent agreement with the classic inter-
pretation of the inductive effect.
As mentioned above, 246 did not fit the trend of rate
constants defined by the rest of the chlorophenols. This is a
quite unexpected finding and a warning for the future design
of experiments because 246 has been used as a typical
chlorophenol in many earlier studies.3,7,11 246 is not
exceptional in its characteristic parameters, but the fact that
it is the chlorophenol with the lowest d13C1 should not go
unnoticed. Therefore, it is the chlorophenol with the most
electron rich phenolic carbon atom and it is not unfeasible that
the oxidation mechanism for 246 is different from the
remaining group of 19 compounds. It is also to be recalled
that 246 is the case when some hints of rate saturation are seen
in the rate equation which might be attributable to the
complexation of reactants. No similar phenomena were
observed experimentally for any of the 19 other compounds.
The generally accepted Hammett substituent constants45,46,48,52
for chlorine are sp = 0.23 and sm = 0.37. This fits the values
obtained from the pK data in this work (sp/sm = 1.61 from
the accepted values and 1.75 from the pK data of Table 2),
which is perhaps not surprising as the Hammett substituent
constants were originally defined based on the pK values of
substituted benzoic acids.46 The Hammett constants are
primarily used to interpret kinetic data. In the series of
reactions studied in this work, they are unsuitable for this
Sigma values obtained from rate constants show a third
trend. First of all, the rate constant of 246 did not follow the
pattern set by the remaining 19 compounds. Therefore, the
statistical analysis was done and Fig. 6 is shown based on
c
240 New J. Chem., 2011, 35, 235–241
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

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purpose, which is illustrated by a Hammett plot for the
six chlorophenols not containing any ortho substituents
(Fig. S12, ESIw). This plot does not show any correlation.
15 K. C. Christoforidis, M. Louloudi and Y. Deligiannakis, Appl.
Catal., B, 2010, 95, 297.
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Conclusion
In this work, we have shown that the oxidation of chloro-
phenols by cerium(IV) follows 1 : 2 stoichiometry and a simple
second-order rate equation in the acidic region. pK values
and 13C NMR chemical shifts have been determined as
characteristic parameters. The rate constants do not correlate
well with these characteristic parameters or with the conven-
tional Hammett substituent constants. However, all three
sets of parameters follow well defined additivity rules. 2,4,6-
Trichlorophenol shows a substantially different behavior than
other chlorophenols do, which should be taken into account
when selecting typical chlorophenols as model compounds in
future studies.
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The authors thank the Tempus Foundation and Hungarian
Science Foundation (OTKA) for financial support under grant
No. K68668. The research was supported by the EU and
co-financed by the European Social Fund through the
Social Renewal Operational Programme under the project
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Some experiments were carried out using the facilities of Iowa
Sate University and Ames Laboratory in Ames, Iowa, US.
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