Reactivity of phenyl N-phenylcarbamates in the alkaline hydrolysis reaction

Article abstract of DOI:10.1002/poc.1841

In the present study, we explore the application of several theoretically estimated indices that characterize the reactivity of a series of phenyl N-phenylcarbamates in the alkaline hydrolysis reaction. The rate constants (at 25°C) for the hydrolysis of several derivatives were spectrophotometrically determined. The obtained kinetic data in this study, combined with literature data for other derivatives, were then correlated with theoretically estimated reactivity indices: Hirshfeld and NBO atomic charges, the Parr electrophilicity index (ω), and the electrostatic potential at the carbon and oxygen atoms of the reaction centre (V_C, V_O). The predictive ability of these quantities is discussed in a comparative context. Copyright

Full text of DOI:10.1002/poc.1841

Research Article
Received: 26 November 2010,
Revised: 5 January 2011,
Accepted: 10 January 2011,
Published online in Wiley Online Library: 23 February 2011
(wileyonlinelibrary.com) DOI 10.1002/poc.1841
Reactivity of phenyl N-phenylcarbamates in
the alkaline hydrolysis reaction
Didi Nalbantovaa, Diana Cheshmedzhievaa, Boriana Hadjievaa,
a
a
*
*
Sonia Ilieva and Boris Galabov
In the present study, we explore the application of several theoretically estimated indices that characterize the
reactivity of a series of phenyl N-phenylcarbamates in the alkaline hydrolysis reaction. The rate constants (at 25 -C) for
the hydrolysis of several derivatives were spectrophotometrically determined. The obtained kinetic data in this study,
combined with literature data for other derivatives, were then correlated with theoretically estimated reactivity
indices: Hirshfeld and NBO atomic charges, the Parr electrophilicity index (v), and the electrostatic potential at the
carbon and oxygen atoms of the reaction centre (VC, VO). The predictive ability of these quantities is discussed in a
comparative context. Copyright ß 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: alkaline hydrolysis; phenyl N-phenyl substituted carbamates; reactivity indices; reactivity of organic compounds
kinetic data for such a representative series of derivatives
provided a solid basis for a reliable assessment of the factors
governing reactivity.
The experimental kinetic constants were rationalized in terms
of global and local reactivity indices derived from density
functional theory. Thus the predictive power of the theoretically
evaluated reactivity descriptors was assessed by direct compari-
son with experimental results.
INTRODUCTION
The principal aim of the present study is to assess the quality
of reactivity predictions coming from alternative theoretical
approaches for the alkaline hydrolysis of substituted
phenyl N-phenylcarbamates. Carbamates are well-known class
of compounds that can be synthesized by various methods.[1–4]
The interest for studying their properties is determined
mostly by the biological activities of carbamate derivatives.[1–11]
Various carbamates find application in the agrochemical
industry[5–9] as herbicides, fungicides and pesticides, in the
pharmaceuticals industry[5,6,10] as drug intermediates, and in the
polymer industry[5,6,11] for the synthesis of polyurethane and
peptides. In the course of studying the biological activity of
carbamates it was established that for some representatives
the mode of decomposition during hydrolysis is a factor
influencing their effects.[3] Therefore, the detailed understanding
of the chemistry of the hydrolysis process, the theoretical
prediction of kinetic parameters as well as the achievement of
quantitative compliance with experimental reactivity trends is of
particular importance. These objectives provided the motivation
for carrying out theoretical evaluation of reactivity descriptors as
well as experimental kinetic studies on the hydrolysis of a series
of phenyl N-phenylcarbamates. The bulk of experimental kinetic
data for the studied reaction was taken from the work of
Williams,[12] who reported carefully determined kinetic constants
for the alkaline hydrolysis of twelve compounds from the
series examined. The series was expanded in the present
study with four more derivatives, following the kinetic procedure
of Williams.[12] We determined experimentally the rate constants
for the following derivatives: m-CH3, p-NO2, m-N(CH3)2, and
p-C(CH3)3 phenyl N-phenylcarbamates substituted in the
O-phenyl ring. The series of compounds studied is presented
in Scheme 1. These derivatives contain both electron donating
and electron withdrawing groups, thus allowing to reflect
alternative polar influence of the substituents. Experimental
KINETIC EXPERIMENTS AND THEORETICAL
COMPUTATIONS
Kinetic experiments
Substituted carbamates were obtained through condensation of
the corresponding phenols with equivalent amount of phenyl
isocyanate in the presence of small amount of triethylamine as
catalyst.[13]
The kinetic measurements on the alkaline hydrolysis of a series
of phenyl N-phenylcarbamates were carried out using UV
spectroscopy. We strictly followed the kinetic procedure used
by Williams[12] in his study on the alkaline hydrolysis of phenyl
N-phenylcarbamates. Thus, it was possible to combine his
experimental data with the kinetic constants determined in the
present study for several additional derivatives. In his work
Williams has provided a detailed justification of the adopted
kinetic procedure. For three of the molecules studied in the
present work the rate of hydrolysis was measured at the
*
Correspondence to: B. Galabov, Department of Chemistry, University of Sofia, 1
James Bourchier Ave., 1164 Sofia, Bulgaria.
E-mail: silieva@chem.uni-sofia.bg; E-mail: galabov@chem.uni-sofia.bg
a
D. Nalbantova, D. Cheshmedzhieva, B. Hadjieva, S. Ilieva, B. Galabov
Department of Chemistry, University of Sofia, 1 James Bourchier Ave., 1164
Sofia, Bulgaria
J. Phys. Org. Chem. 2011, 24 1166–1171
Copyright ß 2011 John Wiley & Sons, Ltd.

REACTIVITY OF PHENYL N-PHENYLCARBAMATES
Scheme 1. Carbamates under consideration
absorption maximum of the respective final products. In the case
of p-tert-butylphenyl N-phenylcarbamate the kinetic of the
reaction was followed through the absorption maximum of
the initial carbamate. The rate constants were measured on a
Shimadzu UV-1800 spectrophotometer using 1 cm path length
quartz cells. The temperature was kept constant at 25 ꢀ 0.1 8C by
thermostated cell compartment. The hydrolysis of diphenyl
carbamates was performed in water solution with different
buffers (pH varied in the 7–9 range, 0.1 ionic strengths). The
final concentration of the respective carbamates was about
1 ꢁ 10ꢂ4 M.
B3LYP[15–17] functional in combination with 6-311þG(2d,2p)[18,19]
basis set. All optimized structures were characterized by analytic
computations of harmonic vibrational frequencies at the same
level/basis sets. When a molecule has more than one stable
conformation, all conformers were examined, and the ones with
the lowest energy were employed in the subsequent analysis.
Electrostatic potentials (VC, VO),[20,21] natural bond orbital
(NBO) charges,[22,23] Hirshfeld charges[24,25,26] at the atoms of the
reaction centre as well as the Parr electrophilicity index (v)[27]
were computed with tight self-consistent field convergence.
The electrostatic potential at a nucleus Y can be expressed as
The reactions exhibited pseudo-first-order kinetics, because
of the constant concentration of hydroxide ions. Kinetic
data were generated to estimate the rate constants, kobs, by
plotting ln (A1 ꢂ At) versus time. The correlation coefficients in
the kinetic plots were in the range 0.9980–0.9997. The slope of
the line yielded kobs. As an illustration, experimental spectral
data and the kinetic chart for the hydrolysis of 3-methylphenyl
N-phenylcarbamate is presented in Fig. 1. The spectral data and
plots for several other derivatives are shown in Figs. S1–S3 in the
Supporting Information. The experimentally measured kobs values
are given in Table 1. Each rate constant was determined by three
independent measurements. Further details for the kinetic
measurements are provided in the Supporting Information.
follows (in atomic units, bold font denotes vector quantities)[21]
:
Z
X
ZA
rðrÞ
VY ꢃ VðRYÞ ¼
ꢂ
dr
(1)
Að¼YÞ jRAꢂRYj
jrꢂRYj
In this relation, ZA is the charge on nucleus A with radius vector
RA, r(r) is the electronic density function. Equation (1) contains a
summation over all atomic nuclei, treated as positive point
charges, as well as integration over the continuous distribution of
the electronic charge. Unlike atomic charges, which depend
strongly on their definition and additional approximations,
the EPN values reflect the variations of electron densities
rigorously. The dominant contribution to VY comes from the local
densities around the respective atomic sites. More negative VY
values indicate greater electron densities. The EPN index was
defined by Wilson[20] in 1962. This quantity was first introduced
as a reactivity index for organic compounds in the works of
Galabov et al.[28–34] In previous studies[28–30] it was shown that
Computational methods
All computations were performed using Gaussian 03[14] suite
of programs. Geometries were optimized at DFT level with
Figure 1. (A) The change in the UV spectrum of 3-methylphenyl phenylcarbamate as a function of time (in buffer solution with pH 9). (B) The plot
between ln (A1 ꢂ At) at l ¼ 271 nm and time (r ¼ 0.9996)
J. Phys. Org. Chem. 2011, 24 1166–1171
Copyright ß 2011 John Wiley & Sons, Ltd.
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D. NALBANTOVA ET AL.
Table 1. Experimentally determined rate constants at 25 8C for the alkaline hydrolysis of phenyl N-phenylcarbamates and Hammett
substituent constants
b
R
kobs [sꢂ1
]
ka [sꢂ1 Mꢂ1
]
log k
sꢂ
Hc
3-N(CH3)2
3-CH3
3-CHOc
3-Clc
—
5.02 ꢁ 10ꢂ4
5.48 ꢁ 101
5.02 ꢁ 101
3.71 ꢁ 101
1.64 ꢁ 103
1.83 ꢁ 103
1.26 ꢁ 103
1.32 ꢁ 104
2.47 ꢁ 105
2.52 ꢁ 101
3.62 ꢁ 101
4.58 ꢁ 101
3.07 ꢁ 102
4.23 ꢁ 104
4.11 ꢁ 104
6.60 ꢁ 104
2.71 ꢁ 102
1.74
1.70
1.57
3.22
3.26
3.10
4.12
5.39
1.40
1.56
1.66
2.49
4.63
4.68
4.82
5.43
0
d
ꢂ0.16
ꢂ0.07
0.35
0.37
0.37
d
3.71 ꢁ 10ꢂ4
—
—
—
—
c
3-COOC2H5
c
3-NO2
0.71
d
3-CH3, 4-NO2
2.47 ꢁ 10ꢂ2
1.20e
ꢂ0.26
ꢂ0.17
ꢂ0.13
0.19
c
4-OCH3
c
—
—
4-CH3
4-C(CH3)3
d
7.60 ꢁ 10ꢂ4
4-Clc
—
—
—
—
—
c
4-COCH3
0.84
1.00
1.03
1.27
4-CNc
4-CHOc
c
4-NO2
Correlation coefficient r
0.994
a k ¼ kobs/[OHꢂ].
b From Ref.[49]
c From Ref.[12]
d From the present study.
e Sum of s(3-CH3) and sꢂ(4-NO2).
the electrostatic potential at nuclei (EPN) can be applied as a
highly accurate descriptor of the ability of molecules to form
hydrogen bond complexes either as proton donors or proton
acceptors. EPN proved also a reliable reactivity index for chemical
reactions.[31,32] The electrostatic potential at nuclei found recently
further successful applications as a reactivity descriptor and
provided solutions to various chemical problems.[35–45]
The electrophilicity index v, proposed by Parr et al.,[27] was
considered as well. It is defined by the relationship,
Scheme 2. General scheme for the mechanism of the alkaline hydrolysis
of aromatic carbamates
v ¼ m2=2h
where m is the electronic chemical potential and h is the total
hardness.
There is a continuing interest in applying the Hammett
equation[46–48] in characterizing of the reactivity of organic
system. The Hammett relationships were recently substantiated
theoretically by electronic structure computations on a variety of
systems.[41,49–55]
(2)
supported previous studies and confirmed the second stage of
the process, namely formation of an aryl isocyanate and release
of phenolate, as rate controlling.
In the present study we rationalize the reactivity of
aryl carbamates in the alkaline hydrolysis with the aid of
electronic parameters. We tested the quality of theoretical
reactivity descriptors by comparisons with a set of kinetic
constants, which combines the data reported by Williams[12]
with those obtained in the present study. The extended set of
experimental kinetic data provided a valuable basis for analysing
critically the reactivity trends for the reaction studied and the
predictive power of theoretical indices.
We examined the effects of structural changes on several
theoretical quantities that characterize the variations of electron
densities at particular atomic sites of the functional group
involved in the hydrolysis process. As already discussed, three
types of local reactivity descriptors were evaluated: NBO atomic
charges[22,23]; Hirshfeld atomic charges[24–26]; electrostatic poten-
tial at nuclei values (EPN).[20,21] In addition we also evaluated the
RESULTS AND DISCUSSION
Literature data[12,56–58] revealed that the favoured mechanism of
the alkaline hydrolysis of phenylcarbamates follows an E1cB
elimination–addition stepwise pathway (Scheme 2). Studies on
the effect of substituents indicated[56] that the reaction rate
is controlled by the second stage of the process associated
with the release of a phenolate. As discussed, Williams[12] has
studied the kinetics of the alkaline hydrolysis of a series of twelve
substituted phenyl N-phenylcarbamates. He established good
correlations between the logarithm of the rate constants and
the Hammett sꢂ constants. The conclusions from this work
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J. Phys. Org. Chem. 2011, 24 1166–1171

REACTIVITY OF PHENYL N-PHENYLCARBAMATES
N(CH3)2, hamper the reaction with the electronegative hydroxide
ion. The final column contains the sꢂ constants of the respective
substituents. An excellent correlation (Fig. 2) between log k and
sꢂconstants is established (r ¼ 0.994), in accord with the earlier
findings of Williams.[12] This result has been interpreted as a clear
indication that a transition state with well expressed phenolate
character controls the rate of the alkaline hydrolysis of
phenylcarbamates.[12]
We conducted correlation analysis for the dependences
between log k values and the four theoretical reactivity indices
evaluated in the present work. Table 2 compares the derived
electronic parameters with the experimental log k. The last row
of the table contains the correlation coefficients for the
dependences between log k and the reactivity indices con-
sidered. Since the hydrolysis of aryl carbamates involves breaking
of the ester –CO–O– bond we focused our analysis on the
variation of electronic structure around the carbonyl carbons and
ester oxygens. Highest correlation coefficients are obtained with
the set of Hirshfeld atomic charges at the carbonyl carbon atom
(r ¼ 0.980). Somewhat unexpectedly, the electrostatic potentials
VC and VO did not perform quite well. The global electrophilicity
index did not correlate with the experimental rate constants
(r ¼ 0.888). All correlation coefficients obtained are, however,
distinctly lower than correlation with the sꢂ constants.
These results should not be regarded as too surprising. As
already discussed, the first stage of the reaction involves the
elimination of hydrogen from the NH group (Scheme 2). The
rate-determining second stage involves the formation of an
isocyanate grouping and the release of phenolates, which
contain the substituents. The structure of the rate-controlling
transition state as obtained from B3LYP/6-31þG(d,p) compu-
tations is shown in Fig. 3. The leaving group has well expressed
phenolate ion character. Thus, the electronic structure of the
Figure 2. Dependence between experimental rate constants (log k) for
the alkaline hydrolysis of substituted phenyl N-phenylcarbamates and sꢂ
substituent constants (r ¼ 0.994)
Parr electrophilicity index v. This quantity is expected to
characterize the global electrophilicity of the molecule con-
sidered.
The kinetic data employed in the analysis are summarized in
Table 1. The second column contains the four rate constants
determined in this study. The influence of polar groups in the
aromatic ring is reflected by the variations of the electrostatic
potential values as well as the NBO and Hirshfeld charges at the
atoms of the electrophilic centre. The electron withdrawing
substituents favour the interaction between the two reactants.
Inversely, electron donating substituents, such as CH3, OCH3 and
Table 2. Experimental rate constants and theoretical reactivity indices at B3LYP/6-311þG(2d,2p) for reactant phenyl
N-phenylcarbamates
Hirshfeld charges [e]
NBO charges [e]
EPN [a.u.]
R
log k
qC
qO
qC
qO
VC
VO
v [eV]
H
1.74
1.70
1.57
3.22
3.26
3.10
4.12
5.39
1.40
1.56
1.66
2.49
4.63
4.68
4.82
5.43
0.2443
0.2433
0.2440
0.2452
0.2457
0.2447
0.2468
0.2476
0.2429
0.2438
0.2436
0.2449
0.2469
0.2472
0.2473
0.2481
0.980
ꢂ0.1322
ꢂ0.1324
ꢂ0.1324
ꢂ0.1314
ꢂ0.1296
ꢂ0.1315
ꢂ0.1279
ꢂ0.1255
ꢂ0.1347
ꢂ0.1330
ꢂ0.1335
ꢂ0.1312
ꢂ0.1264
ꢂ0.1267
ꢂ0.1259
ꢂ0.1246
0.973
0.9389
0.9397
0.9394
0.9380
0.9386
0.9385
0.9378
0.9378
0.9393
0.9392
0.9393
0.9382
0.9388
0.9376
0.9383
0.9375
0.832
ꢂ0.5792
ꢂ0.5798
ꢂ0.5792
ꢂ0.5779
ꢂ0.5775
ꢂ0.5778
ꢂ0.5747
ꢂ0.5742
ꢂ0.5792
ꢂ0.5788
ꢂ0.5793
ꢂ0.5779
ꢂ0.5748
ꢂ0.5754
ꢂ0.5748
ꢂ0.5737
0.972
ꢂ14.6087
ꢂ14.6148
ꢂ14.6103
ꢂ14.5988
ꢂ14.6023
ꢂ14.6044
ꢂ14.5936
ꢂ14.5929
ꢂ14.6123
ꢂ14.6109
ꢂ14.6114
ꢂ14.6028
ꢂ14.5994
ꢂ14.5931
ꢂ14.5963
ꢂ14.5905
0.949
ꢂ22.3106
ꢂ22.3187
ꢂ22.3129
ꢂ22.2987
ꢂ22.3025
ꢂ22.3055
ꢂ22.2921
ꢂ22.2913
ꢂ22.3149
ꢂ22.3132
ꢂ22.3139
ꢂ22.3032
ꢂ22.2991
ꢂ22.2914
ꢂ22.2953
ꢂ22.2879
0.947
2.25
1.86
2.19
4.46
2.53
3.45
6.14
5.48
2.09
2.17
2.15
2.52
3.84
4.77
4.26
5.85
0.888
3-N(CH3)2
a
3-CH3
3-CHOa
3-Cla
a
3-COOC2H5
3-NO2
3-CH3, 4-NO2
4-OCH3
4-CH3
4-C(CH3)3
4-Cla
4-COCH3
4-CN
4-CHOa
a
a
a
a
a
4-NO2
Correlation coefficient r
a From Ref.[12]
J. Phys. Org. Chem. 2011, 24 1166–1171
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D. NALBANTOVA ET AL.
Figure 3. B3LYP/6-31þG(d,p) optimized structure of the rate-controlling
transition state in the alkaline hydrolysis of phenyl N-henylcarbamate
Figure 4. Dependence between experimental rate constants (log k) and
EPN values for product phenolates (VO) for the alkaline hydrolysis of
phenyl N-phenylcarbamates
initial reactants, as reflected in the theoretical reactivity indices
included in Table 2, may not be so well related to the electronic
structure of the transition state for the rate-controlling stage. It is,
therefore, reasonable to analyse the variations of the electronic
structure in the phenolate ions, which form at the rate-determining
transition state. We examined the correlations between kinetic
constants and the evaluated reactivity indices for the phenolate
ions. The theoretical data are presented in Table 3 and are com-
pared with the log k values. The last row contains the correlation
coefficients for the dependences between NBO and Hirshfeld
atomic charges as well as the respective EPN values (VO) with logk.
Survey of the data obtained reveals that the best correlation
between theoretical reactivity parameters and experimental
constants is obtained with the EPN values (r¼ 0.984). The plot
between logk and EPN is shown in Fig. 4. The results obtained
illustrate once more the accuracy of theoretical description of
local properties provided by the electrostatic potential at nuclei.
The present results provide an opportunity to compare the
predictive power of experimental (sꢂ) and theoretically derived
(atomic charges, EPN, v) reactivity indices. Quite clearly the best
correlation is achieved between the experimental rate constants
and the sꢂ constants (Fig. 2). Studies on other reactions have
shown, however, that theoretically derived reactivity parameters
can provide competitive, and in a number of cases, superior than
Table 3. Experimental rate constants for the alkaline hydrolysis of phenyl N-phenylcarbamates and theoretical reactivity indices for
product phenolates at B3LYP/6-311þG(2d,2p)
Hirshfeld charges [e]
NBO charges [e]
EPN [a.u.]
R
log k
qO
qO
VO
Ha
3-N(CH3)2
1.74
1.70
1.57
3.22
3.26
3.10
4.12
5.39
1.40
1.56
1.66
2.49
4.63
4.68
4.82
5.43
ꢂ0.4749
ꢂ0.4707
ꢂ0.4730
ꢂ0.4618
ꢂ0.4566
ꢂ0.4663
ꢂ0.4520
ꢂ0.4075
ꢂ0.4862
ꢂ0.4783
ꢂ0.4739
ꢂ0.4659
ꢂ0.4265
ꢂ0.4324
ꢂ0.4218
ꢂ0.4065
0.960
ꢂ0.7947
ꢂ0.7903
ꢂ0.7922
ꢂ0.7798
ꢂ0.7733
ꢂ0.7861
ꢂ0.7697
ꢂ0.7117
ꢂ0.8080
ꢂ0.7980
ꢂ0.7929
ꢂ0.7844
ꢂ0.7354
ꢂ0.7438
ꢂ0.7299
ꢂ0.7109
0.952
ꢂ22.6269
ꢂ22.6237
ꢂ22.6258
ꢂ22.6065
ꢂ22.6096
ꢂ22.6135
ꢂ22.5989
ꢂ22.5694
ꢂ22.6305
ꢂ22.6279
ꢂ22.6233
ꢂ22.6138
ꢂ22.5873
ꢂ22.5872
ꢂ22.5831
ꢂ22.5680
0.984
a
3-CH3
3-CHOa
3-Cla
a
3-COOC2H5
3-NO2
3-CH3, 4-NO2
4-OCH3
4-CH3
4-C(CH3)3
4-Cla
4-COCH3
4-CN
4-CHOa
a
a
a
a
a
4-NO2
Correlation coefficient r
a From Ref.[12]
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J. Phys. Org. Chem. 2011, 24 1166–1171

REACTIVITY OF PHENYL N-PHENYLCARBAMATES
experimental indices description of chemical reactivity.[31–33,55]
The current results revealed that the theoretical quantities can
be most closely associated with the actual mechanism of
the reaction (in our case with properties of the leaving group).
In aromatic systems, appropriate reactivity constants for
polysubstituted derivatives are not available, while there are
no limitations in evaluating theoretical indices for such systems.
In general, the application of both experimental and theoretical
quantities in describing reactivity is desirable wherever possible.
Chemical reactivity is usually discussed in terms of properties
of the initial reactants. The example of alkaline hydrolysis of
aryl carbamates, however, shows that such an approach may not
always be correct. For the systems studied, the electronic
properties of the leaving group are better descriptors of reactivity
because of their closeness to the structure of the rate-controlling
transition state. Thus, knowledge on the mechanism of the
process is essential for the correct analysis of reactivity and
the comparisons between theoretical descriptors and exper-
imental kinetic data.
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CONCLUSIONS
A number of reactivity indexes were explored in describing
the reactivity of 16 substituted in the aromatic ring phenyl N-
phenylcarbamates in the alkaline hydrolysis reaction. An
extended set of experimental kinetic constants were correlated
with Hirshfeld and NBO charges at the atoms of the reaction
centre, the electrostatic potential at nuclei at the same atoms,
and the Parr electrophilicity index. The theoretical indices were
evaluated for both reactants and the leaving groups. The best
correlation between theory and experiment was obtained with
the EPN index (r ¼ 0.984) for the leaving group (phenolate ions).
This result is in accord with literature findings, indicating that the
rate of the hydrolysis process is governed by the second
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This research was supported by the National Science Fund
(Bulgaria), Grant DO 02-124/08 and DCVP 02/2-2009 (Project
UNION).
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