Kinetics and mechanism of hydrolysis of phenylureas

Article abstract of DOI:10.1039/b205850b

The hydrolysis of phenylureas has been found to be affected by temperature, pH and buffer concentration. Kinetic evidence suggests that the formation of phenylisocyanate, the initial product in the title reaction, occurs via an intermediate zwitterion. Depending on pH and buffer concentrations, the zwitterion can be produced through three parallel routes: at low pH, specific acid-general base catalysis, followed by slow deprotonation of a nitrogen atom by a general base; at high pH, specific basic-general acid catalysis, followed by slow protonation of a N atom by a general acid; at intermediate pH the reaction proceeds through a proton switch promoted by buffers. Bifunctional acid-base buffers such as HCO₃^-/CO₃^2-, H₂PO₄^2- and CH₃COOH/CH₃COO^- are very efficient catalysts. At high buffer concentration, as well as at pH < 3 or > 12, the breakdown of the zwitterion is rate-determining. The results are discussed in relation to recently published papers reporting different pathways.

Full text of DOI:10.1039/b205850b

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Kinetics and mechanism of hydrolysis of phenylureas
Stefano Salvestrini, Paola Di Cerbo and Sante Capasso*
Dipartimento di Scienze Ambientali, Seconda Università di Napoli, via Vivaldi 43,
I 81100 Caserta, Italy
2
Received (in Cambridge, UK) 25th June 2002, Accepted 6th September 2002
First published as an Advance Article on the web 11th October 2002
The hydrolysis of phenylureas has been found to be affected by temperature, pH and buffer concentration.
Kinetic evidence suggests that the formation of phenylisocyanate, the initial product in the title reaction, occurs
via an intermediate zwitterion. Depending on pH and buffer concentrations, the zwitterion can be produced through
three parallel routes: at low pH, specific acid–general base catalysis, followed by slow deprotonation of a nitrogen
atom by a general base; at high pH, specific basic–general acid catalysis, followed by slow protonation of a N atom by
a general acid; at intermediate pH the reaction proceeds through a proton switch promoted by buffers. Bifunctional
Ϫ
3
2Ϫ
3
Ϫ
4
2Ϫ
4
Ϫ
acid–base buffers such as HCO /CO , H PO /HPO and CH COOH/CH COO are very efficient catalysts.
2
3
3
At high buffer concentration, as well as at pH < 3 or > 12, the breakdown of the zwitterion is rate-determining.
The results are discussed in relation to recently published papers reporting different pathways.
more consistent with the body of information available. The
Introduction
study was carried out on 3-(3,4-dichlorophenyl)-1,1-dimethyl-
urea (I, diuron), 3-(4-isopropylphenyl)-1,1-dimethylurea (II,
isoproturon), 1-butyl-3-(3,4-dichlorophenyl)-1-methylurea (III,
neburon) and 3-(3-chloro-p-tolyl)-1,1-dimethylurea (IV, chloro-
toluron), representatives of the substituted phenylurea
1–4
The hydrolysis of phenylureas is a topic of increasing interest
not only because of distinctive aspects of its reaction mech-
anism, but also for the importance of these molecules as the
active principles of efficient herbicides widely used in agri-
5
culture. It has been postulated that abiotic hydrolysis, at least
9
herbicide class. Preliminary results have been reported.
in some cases, is a multi-stage reaction involving first the form-
ation of isocyanate and ammonia derivatives (i), followed by
hydrolysis to the carbamic acid derivative (ii) and then scission
Experimental
2–4,6,7
to carbon dioxide and the aniline derivative (iii).
Materials
R–NH–CO–N<
R–NHCO ϩ N<
(i)
(ii)
The phenylureas 3-(3,4-dichlorophenyl)-1,1-dimethylurea (I,
diuron), 3-(isopropylphenyl)-1,1-dimethylurea (II, isopro-
turon),
1-butyl-3-(3,4-dichlorophenyl)-1-methylurea
(III,
R–NHCO ϩ H O
RNHCO H
2
2
neburon), 3-(3-chloro-p-tolyl)-1,1-dimethylurea (IV, chloro-
toluron) and 3,4-dichloroaniline, were supplied by Dr.
Ehrenstorfer GmbH, 3-isopropylaniline and 3-chloro-p-tolyl-
aniline were obtained from Riedel-Dehaën. 3,4-Dichloro-
phenyl isocyanate was from Fluka. All the other chemicals were
from Carlo Erba.
Ϫ
ϩ
RNHCO2 ϩ H
R–NH ϩ CO2
(iii)
2
The reaction (i) is rate-limiting for the overall process; there-
fore, the isocyanate and carbamic acid derivatives form a small
proportion of the reacting mixtures.
Soil microorganisms mainly promote the demethylation
reaction of NЈ-methyl derivatives of N-phenylureas. As
Kinetic measurements
8
regards the mechanism of the first stage of hydrolysis, despite
the extensive work done, no pathway has been proposed that
fully satisfies the kinetic behavior. Recently, it has been pro-
Water solutions of the selected phenylurea (0.10 mM) at the
desidered pH and buffer concentrations were filtered through a
0.45 µm membrane filter and then stored in a thermostated bath
kept, unless otherwise stated, at 60.1 ± 0.1 ЊC. At preselected
times, an aliquot was removed and analyzed by HPLC on a
Waters system, consisting of a 515 HPLC Pump and a 2487
dual λ Absorbance Detector, equipped with a C18 reversed-
phase column (3.9 × 150 mm). Phenylureas and their degrad-
ation products were eluted by a linear gradient of acetonitrile in
2
posed that in acidic media the reaction proceeds through the
protonation of the substrate followed by a rate-determining
attack by water, giving a tetrahedral intermediate. This inter-
mediate then decomposes to an amine and a phenylcarbamic
acid which quickly decarboxylates under acidic conditions
to form the corresponding aniline. In basic media it has been
3
Ϫ1
proposed that the hydrolysis of phenylureas occurs via an
water in 15 min, flow rate 1 mL min , λ = 248 nm. The buffers
intermediate hydroxide ion complex whose breakdown gives the
final products. On the other hand, earlier works have produced
evidence of the formation of zwitterionic intermediates in the
were used in the concentration range of 10–500 mM and at a
fraction (α) of the free base in the buffer of 0.2, 0.4, 0.6 and
Ϫ
ϩ
0.8 for CH COOH/CH COO , pH 4.0–5.2 and TrisH /Tris,
3
3
7
Ϫ
2Ϫ
conversion of ureas into isocyanates in aqueous solutions. In
pH 6.9–8.1; at α = 0.2, 0.6 and 0.8 for HCO /CO
pH 8.9–10.1 and H PO /HPO
,
3
3
Ϫ
2Ϫ
4
the formation of these intermediates water molecules act
as proton transfer agents to either the unprotonated or the
N-protonated substrate.
The aim of this work was to get a better understanding of the
kinetic behavior of the hydrolysis of phenylureas in aqueous
pH 6.0–7.2 The buffer
2
ϩ
4
4-methylmorpholineH /4-methylmorpholine was used only at
α = 0.6, pH 7.4. Dilute KOH and HCl were used in the pH
ranges 1.0–3.0 and 11.0–13.0, respectively. In all samples, a
constant ionic strength of 1 M was maintained by the addition
of an appropriate volume of a concentrated solution of KCl.
solution in order to work out
a kinetic mechanism
DOI: 10.1039/b205850b
J. Chem. Soc., Perkin Trans. 2, 2002, 1889–1893
This journal is © The Royal Society of Chemistry 2002
1889

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The pH values were measured by using a glass electrode at the
same temperature and ionic strength as those of the rate
measurements. The rate constants were calculated by least-
squares analysis, assuming the reaction to be first-order in the
starting phenylurea. The fitting of the experimental data was
satisfactory for all samples.
The temperature effect on the reaction rate was analyzed by
performing kinetic runs in the ranges 30–70 ЊC, 0.01–0.5 M
phosphate buffer, fraction of the free base in the buffer = 0.6,
and calculating the pre-exponential factor and activation
energy from the linear regression of ln(k) versus 1/T by least-
squares methods. No correction was made for the temperature
dependence of the buffer pH value because in phosphate buffer
the rate constant depends very little on pH. The entropy and
enthalpy of activation were calculated by the standard formulae
10
derived from the absolute theory of reaction rates.
Characterization of the reaction products
The compounds of the degradation reactions of I, II, III and
IV in aqueous solutions were identified by comparison with
HPLC traces and UV spectra of pure samples. The compound
obtained from I by spontaneous degradation in water–
methanol mixtures and by the reaction of 3,4-dichloro-
Fig. 1 pH rate profile for the degradation reaction of 3-(3,4-
dichlorophenyl)-1,1-dimethylurea (diuron), at T = 60 ЊC and µ = 1 M
(KCl). k is the first-order rate constant at zero buffer concentration.
0
1
phenylisocyanate with pure ethanol was identified by H- and
pH dependence of the reaction rate
13
C-NMR and mass spectra.
Fig. 1 shows a plot of k versus pH for the phenylurea diuron, 3-
o
(
3,4-dichlorophenyl)-1,1-dimethylurea, where k is the observed
pK values
o
a
first-order rate constant extrapolated to zero buffer concen-
tration. In the pH range 5–9, the rate constant is substantially
pH-independent, while at lower and higher pH values the slope
of the curve markedly increases, suggesting an efficient catalysis
by hydronium ion at low pH and by hydroxide ion at high pH.
The equilibrium constants for the acid–base reactions of the
(
Ϫ)
Aryl-NH–CO–N< group to Aryl-N –CO–N< and Aryl-NH–
ϩ
CO–NH < of I, II, III and IV were calculated by the computer
11
program SPARC developed by Carreira. This program, based
on structure activity relationships and perturbed molecular
At extreme pH values (<2 or >12), the k value is again con-
o
orbital theory, enables the calculation of the ionization pK s for
2
2
a
stant. The value of the pH at the inflection point (d k/dpH = 0)
a large number of organic compounds from the molecular
structures.
observed at basic pH (pH = 11.5 ± 0.3) was close to, although
not coincident with, the pK value computed for the deproton-
a
ation equilibrium of I at the same temperature as for the
kinetic experiments and zero ionic strength (pKa = 10.3).
Similar pH–rate constant plots were observed for the other
phenylureas studied in this work.
Results
Under all the experimental conditions, phenylureas underwent
spontaneous irreversible hydrolysis at an appreciable rate giving
detectable amounts of a single aromatic product, which was
identified as the corresponding aniline derivative by HPLC
co-injection of pure compounds and mass spectrometry tech-
niques. The sum of the area of the phenylurea peak plus that of
the aniline derivative, corrected by the different absorbances
at the wavelength of the spectrometer detector, was constant
during the kinetic runs, indicating the absence of reaction
intermediates in an appreciable amount. Moodie and co-
Temperature dependence of reaction rate
The Arrhenius plot for the limiting rate constants reached at
high buffer concentrations is linear in the temperature range
studied. The pre-exponential factor and the apparent activation
Ϫ1
parameters for I are respectively: ln A = 38 ± 1 s ; E = 127 ± 2
a
Ϫ1
Ϫ1
‡
Ϫ1
‡
Ϫ1
kJ mol ; ∆H =124 ± 2 kJ mol ; ∆S = 14 ± 8 J K mol .
1
2
workers reported a half-life value of 20 s for the non-
catalyzed hydrolysis of phenylisocyanate in aqueous solution at
Buffer catalysis
2
5 ЊC and, moreover, showed that the reaction is buffer-
The observed pseudo-first-order rate constant (kobs) determined
at constant pH reveals a marked dependence on the buffer
concentrations. For all the buffers tested the kinetic constant
first increases rapidly at low buffer tested concentrations and
then gradually levels off at higher concentrations, reaching a
limit value that is independent of the type of buffer and its
fraction of free base. The kobs–buffer concentration plots for I at
the fraction 0.6 of free base in the buffers are shown in Fig. 2.
Fig. 3 shows the dependence of kobs on the phosphate concen-
tration for the hydrolysis of I, II, III and IV. The shape of the
curves is the same for all the phenylureas tested. The depend-
ence of the slopes of the kobs–buffer concentration curves for I
catalysed. Fast degradation to aniline derivatives may explain
why we could detect no phenylisocyanate derivatives in water
solution by HPLC. However, an indication that the reaction
proceeds through phenylisocyanates was given by the analysis
of the reaction products of I obtained in a water–ethanol
mixture (50% ethanol). In this medium HPLC analysis
revealed, besides the aniline derivative, another product, in 60%
1
relative yield after 15 days, which was identified by H- and
13
C-NMR, and by mass spectra, as ethyl 3,4-dichlorophenyl-
1
carbamate: H-NMR δ 1.30 (3 H, t, CH ), 4.22 (2 H, q, CH ),
3
2
7
.19 (1 H, dd, 6-H), 7.46 (1 H, d, 6-H), 7.59 (1 H, d, 6-H), 7.93
13
o
(
s, 1 H, NH); C-NMR δ 14 (CH ), 61 (CH ), 119 (ortho CH),
at zero buffer concentrations (k cat), which measure the catalytic
3
2
1
20 (ortho CH), 124.5 (para CCl), 130 (meta CH), 132 (meta
efficiency of the buffers, is shown in Fig. 4. Despite the large
CCl), 138.5 (CN); 153 (CO). An electron ionization mass
spectrum, recorded on the quadrupole instrument Trio 2000,
Fisons, gave m/z 233 (Mϩ, 100%).
errors, due to the hyperbolic shape of the k –buffer concen-
cat
tration curves, it is evident that, at least for some buffers, the
curves do not pass through the origin of the axes at α = 0 and at
α = 1, suggesting that both the acidic and basic forms of the
buffers are catalysts of the reaction. It is interesting to note that
This compound was the exclusive product of the reaction of
3
,4-dichlorophenylisocyanate with pure ethanol.
1
890 J. Chem. Soc., Perkin Trans. 2, 2002, 1889–1893

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Fig. 4 Plot of the catalytic constant of the buffers at zero buffer
Fig. 2 Dependence of the observed first-order rate constant (kobs) for
the degradation reaction of 3-(3,4-dichlorophenyl)-1,1-dimethylurea
on the total buffer concentration at the fraction 0.6 of free base, T = 60
0
concentration (k cat) against the fraction (α) of the free base in the
buffers for the degradation reaction of 3-(3,4-dichlorophenyl)-1,1-
dimethylurea, T = 60 ЊC and µ = 1 M.
Ϫ
2Ϫ
ϩ
ЊC and µ = 1 M: (᭹) = HCO /CO3 , (ꢀ) = TrisH /Tris, (᭢) = 4-
3
ϩ
Ϫ
2Ϫ
methylmorpholineH /4-methylmorpholine; (᭿)
= H PO /HPO4 ,
2 4
Ϫ
(
᭜) = CH COOH/CH COO .
3
3
Fig. 5 Brønsted plot for the degradation reaction of 3-phenyl-1,1-
dimethylureas to phenylisocyanates and dimethylamine: closed and
open symbols show the second-order rate constants for the acidic and
basic catalysis, respectively. The letters A, P and C mark the points for
Fig. 3 Dependence of the observed first-order rate constant (kobs) for
Ϫ
Ϫ
2Ϫ
the acid catalysis of CH COOH/CH COO , H PO /HPO4 and
3
3
2
4
the degradation reaction of 3-(3,4-dichlorophenyl)-1,1-dimethylurea
Ϫ
2Ϫ
HCO3 /CO3
.
(
᭺), 3-(4-isopropylphenyl)-1,1-dimethylurea (ꢀ), 1-butyl-3-(3,4-
dichlorophenyl)-1-methylurea (ᮀ) and 3-(3-chloro-p-tolyl)-1,1-
dimethylurea (᭿) on the total phosphate buffer concentration at the
fraction 0.6 of free base, at T = 60 ЊC and µ = 1 M.
ϩ
ϩ kH3O
3 0
[H O ] to the k –pH data obtained in the pH range
ϩ
3.0–6.0. The logarithm of the rate constants for the general acid
catalysis, seen as a whole, does not fit a linear correlation. The
data appear to fit two straight lines, one for the polyfunctional
o
Ϫ
3
2Ϫ
k
attains higher values with the buffers HCO /CO
,
cat
3
Ϫ
2Ϫ
Ϫ
ϩ
Ϫ
2Ϫ
3
Ϫ
4
2Ϫ
4
H PO /HPO
2
and CH COOH/CH COO than for TrisH /
acid–base catalysts, ϪHCO /CO
,
H PO /HPO
and
4
4
3
3
3
2
Tris, suggesting that bifunctional acid–base buffers are particu-
larly efficient catalysts. In line with this conclusion are the kobs
CH COOH/CH COO , and the other for others.
3 3
–
Table 1 reports the kobs limiting values obtained at high phos-
phate concentration and, for comparison, the values obtained
in 0.1 M HCl and 0.1 M NaOH. For every compound the
rate constant in these media had the same value within the
experimental error.
buffer concentration plots reported in Fig. 2. Also worthy of
note is the marked difference in the slope at zero buffer concen-
tration between the bifunctional acid–base buffers CH COOH/
3
Ϫ
Ϫ
3
2Ϫ
3
ϩ
CH COO and HCO /CO and the monofunctional TrisH /
Tris and 4-methylmorpholineH /4-methylmorpholine (the two
3
ϩ
lower curves in Fig. 2), although the pK values of the last two
buffers are intermediate between those of the two bifunctional
acid–base buffers. A more quantitative correlation of the
a
Discussion
The salient features of the kinetic behavior of phenylureas
hydrolysis that are informative for the reaction mechanism can
be summarized as follows:
a) hydronium and hydroxide ions and buffers are catalysts of
the reaction;
b) both the acidic and basic forms of the buffers are catalysts
of the reaction;
c) the nonlinear dependence of the rate constant on buffer
concentrations suggests the existence of an intermediate, the
catalytic rate constant with the pK of the buffers is shown in
a
Fig. 5, the Brønsted plot. A linear relationship is usually
expected in this kind of plot, which describes the relationship
between the logarithm of the acid and basic ionization con-
stants and of the second-order rate constant for the reactions
1
3
subject to general acid or general base catalysis.
ϩ
We computed the values of kHA for H O and H O, pK =
3
2
a
Ϫ1.7 and 15.7 respectively, by fitting the equation k = k [H O]
0
W
2
J. Chem. Soc., Perkin Trans. 2, 2002, 1889–1893
1891

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Table 1 Rate constants (k × 10 /s ) for the hydrolysis of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (I), 3-(4-isopropylphenyl)-1,1-dimethylurea (II),
6
Ϫ1
1
-butyl-3-(3,4-dichlorophenyl)-1-methylurea (III), and 3-(3-chloro-p-tolyl)-1,1-dimethylurea (IV) in acidic, basic and high phosphate buffer
concentration media: T = 60 ЊC, µ = 1 M (KCl)
Phenylurea
HCl 0.1 M
NaOH 0.1 M
High phosphate concentration
I
1.3 ± 0.1
1.8 ± 0.1
1.9 ± 0.1
1.8 ± 0.1
1.4 ± 0.1
2.0 ± 0.1
1.7 ± 0.1
1.7 ± 0.1
1.3 ± 0.1
2.3 ± 0.1
2.2 ± 0.1
1.6 ± 0.1
II
III
IV
Fig. 6 Proposed mechanism for the degradation reaction of 3-phenyl-1,1-dialkylureas to phenylisocyanates and dialkylamine.
formation of which is rate determining at some catalyst concen-
trations, while, at others, the rate-limiting step is the conversion
the reaction starts by three parallel routes, each leading to the
zwitterion Z . At low pH and low buffer concentration the reac-
tion proceeds through the pathway shown on the left: proton-
±
14
of the intermediate to products. Moreover, the enhanced
reactivity observed with the polyfunctional acid–base catalysts
ation of the dialkyl nitrogen atom N (fast equilibrium between
2
Ϫ
3
2Ϫ
Ϫ
4
2Ϫ
4
Ϫ
(
HCO /CO , H PO /HPO
and CH COOH/CH COO )
the substrate and hydrogen ion, for I pK = Ϫ2.5), followed by
3
2
3
3
a
suggests that they take part in a proton switch step, as it is often
observed in cases where such processes are involved;
slow deprotonation of the nitrogen atom N by the general base
1
1
5
B. This is specific acid–general base catalysis, kinetically equiv-
alent to general acid catalysis. At high pH the reaction proceeds
(pathway on the right) prevalently through specific basic–
general acid catalysis, kinetically equivalent to general base
catalysis: fast reversible deprotonation of N (for I pK = 10.3),
d) for each of the phenylureas studied we observed that the
rate constant in the pH extreme regions and at high polyfunc-
tional buffer concentration has, within the experimental errors,
the same value. This is a clear indication that the hydronium
and hydroxide ions, and the buffers lead to the same inter-
mediate, whose decomposition at high catalyst concentrations
1
a
followed by slow protonation of N by the general acid HA.
2
At high concentration of bifunctional acid–base buffers the
reaction proceeds through a proton switch promoted by the
buffers. At low catalyst concentrations (pH range 3–11 and low
2,3
is rate-limiting. Laudien and Mitzner reported the values of
the kinetic constants for about thirty phenylureas (including
compounds I and II), recorded in 0.1 M H SO and 0.1 M
±
buffer concentrations) the formation of Z is rate-limiting,
2
4
NaOH. Coherently with our results, the rate constant for every
compound had an identical value, within the experimental
errors, both in acidic and basic media;
whereas at high catalyst concentrations, its degradation to the
final product becomes rate-limiting, consequently the reaction
rate no longer depends on pH and buffer concentration. Coher-
‡
e) the pH value at the inflection point observed at basic pH is
ently with this mechanism, the |∆S | value recorded at high
not markedly different from the pK value computed for the
bifunctional buffer concentrations is very low. Fig. 7 reports the
a
deprotonation equilibrium of the substrate;
‡
f ) Finally, the low |∆S | value observed for the reaction in
‡
Ϫ1
high polyfunctional buffer concentrations (∆S = 14 ± 8 J K
Ϫ1
mol for I in phosphate buffer) suggests that, from the starting
substrate to the transition state of the rate-limiting step, there is
no major change in the disorder or motional modes. It is worth
noting that, for a multistep reaction in which the rate-limiting
‡
step is preceded by pre-equilibrium steps, the ∆S observed
value is the sum of the activation entropy of the rate-limiting
step and the entropy changes of the equilibrium steps.
16
Fig. 7 Proton switch assisted by acetic acid.
proposed role for the bifunctional acetate catalysts. The acidic
and basic sites act as concerted catalysts promoting the proton
switch.
In contrast to the mechanism reported here, Laudien and
Mitzner ascribed the rate limiting value at high hydroxide ion
Reaction mechanism
A pathway for the formation of isocyanate and ammonia
derivatives from phenylureas that fits the experimental data is
shown in Fig. 6. Depending on pH and buffer concentrations
1
892
J. Chem. Soc., Perkin Trans. 2, 2002, 1889–1893

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2 R. Laudien and R. Mitzner, J. Chem. Soc., Perkin Trans. 2, 2001,
230–2232.
3 R. Laudien and R. Mitzner, J. Chem. Soc., Perkin Trans. 2, 2001,
226–2229.
3
concentrations to the formation of an unreactive side product.
2
At low pH, according to these authors, the limiting value is due
to the rate determining attack of a water molecule. On the
other hand, Sabaliûnas et al. hypothesized the formation of a
dianion intermediate as the cause for the limiting value of the
2
2
4
D. Sabaliûnas, J. Ellington and R. Lekevicius, Int. J. Environ. Anal.
Chem., 1996, 64, 123–134.
4
rate constant at high pH.
5 C.D.S. Tomlin (ed.), The Pesticide Manual, Eleventh Edition,
British Crop Protection Council, Farnham, 1997,
pp. 443–445.
(a) C. J. Giffney and C. J. O’Connoe, J. Chem. Soc., Perkin Trans. 2,
974, 362–368; (b) C. J. O’Connoe and J. W. Barnett, J. Chem. Soc.,
Conclusion
6
7
1
The study of pH, buffer, and temperature effects on the form-
ation of isocyanate and ammonia derivatives from phenylureas
in aqueous solution has produced a detailed description of the
reaction pathway. Depending on pH and buffers, the reaction
Perkin Trans. 2, 1973, 1457–1461.
(a) A. Williams and W. P. Jencks, J. Chem. Soc., Perkin Trans. 2,
1
974, 1753–1749; (b) A. F. Hegarty and L. N Frost, J. Chem. Soc.,
Perkin Trans. 2, 1973, 1719–1728.
starts along parallel routes, each leading to the same inter-
8 (a) P. A. Ellis and N. D. Camper, J. Environ. Sci. Health, 1982,
B17, 277–289; (b) E. Esposito, S. M. Paulillo and G. P. Manfio,
Chemosphere, 1998, 37, 541–548.
Ϫ
mediate zwitterion. Bifunctional acid–base buffers, HCO /
3
2Ϫ
Ϫ
4
2Ϫ
4
Ϫ
CO3 , H PO /HPO
and CH COOH/CH COO , are par-
2
3 3
9
S. Salvestrini, P. Di Cerbo and S. Capasso, Chemosphere, 2002,
8(1), 69–73.
0 K. A. Connors, Chemical Kinetics: The Study of Reaction Rates
in Solution, VCH Publishers, New York, USA, 1990,
pp. 187–243.
1 (a) L. A. Carreira, M. Rizk, Y. El-Shabrawy, N. A. Zakhari
and S. Hilal, Talanta, 1996, 43, 607–619; (b) S. Hilal, S. W.
Karickhoff and L. A. Carreira, Quant. Struct. Act. Relat., 1995, 14,
ticularly efficient catalysts. At high buffer concentrations, as
well as at pH < 3 or > 12, the breakdown of the zwitterion is
rate determining. As the phenylureas are the active principles of
herbicides widely used in agriculture, our results should be
taken into account for the prediction of the fate of these xeno-
biotic compounds after their dispersal in the environment.
4
1
1
3
48–355.
1
2 E. A. Castro, R. B. Moodie and P. J. Sansom, J. Chem. Soc., Perkin
Trans. 2, 1985, 737–742.
Acknowledgements
This work was supported by grants from the Italian “Ministero
della Ricerca Scientifica e Tecnologica” and “Seconda Univer-
sità di Napoli”. We are greatly indebted to Prof. Carreira for
having made accessible the computer program SPARC.
13 A. A Frost and R. G. Pearson, Kinetics and Mechanism, John Wiley
Sons, Inc., New York, 1953, pp. 191–223.
4 (a) S. A. Hand and W. P. Jencks, J. Am. Chem. Soc., 1975,
7, 6221–6230; (b) R. W. Nagorski, T. Mizerski and J. P.
&
1
9
Richard, J. Am. Chem. Soc., 1995, 120, 4718–4719; (c) S. Capasso,
A. Vergara and L. Mazzarella, J. Am. Chem. Soc., 1998, 117,
1
990–1995.
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