Reactivity of benzohydrazide derivatives towards acetylation reaction. Experimental and theoretical studies

Article abstract of DOI:10.1016/j.cplett.2010.01.052

We herein report an experimental and theoretical study on the acetylation reaction of benzohydrazide derivatives towards p-nitrophenyl acetate (NPA). The kinetic data are consistent with a stepwise mechanism with the nucleophilic attack as the rate determining step. From the theoretical analysis it is found that benzohydrazide derivatives establish intramolecular proton rearrangement. The enol form appears as the active species for nucleophilic attack. A reaction mechanism incorporating keto-enol pre-equilibria is proposed. The study is completed with a local reactivity analysis describing the most reactive centers for nucleophilic attack together with a site activation analysis describing inductive substituent effects.

Full text of DOI:10.1016/j.cplett.2010.01.052

Chemical Physics Letters 488 (2010) 86–89
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Reactivity of benzohydrazide derivatives towards acetylation reaction.
Experimental and theoretical studies
a
b
b
c,
*
Paola R. Campodónico a, , Margarita E. Aliaga , José G. Santos , Enrique A. Castro , Renato Contreras
*
a Instituto de Ciencias, Facultad de Medicina, Clínica Alemana Universidad del Desarrollo, Código Postal 771-0162, Santiago, Chile
b Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 6177, Santiago, Chile
c Departamento de Química, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile
a r t i c l e i n f o
a b s t r a c t
Article history:
We herein report an experimental and theoretical study on the acetylation reaction of benzohydrazide
derivatives towards p-nitrophenyl acetate (NPA). The kinetic data are consistent with a stepwise mech-
anism with the nucleophilic attack as the rate determining step. From the theoretical analysis it is found
that benzohydrazide derivatives establish intramolecular proton rearrangement. The enol form appears
as the active species for nucleophilic attack. A reaction mechanism incorporating keto-enol pre-equilibria
is proposed. The study is completed with a local reactivity analysis describing the most reactive centers
for nucleophilic attack together with a site activation analysis describing inductive substituent effects.
Ó 2010 Elsevier B.V. All rights reserved.
Received 14 December 2009
In final form 21 January 2010
Available online 4 February 2010
1. Introduction
sibility is the interaction of the nucleophile with the electrophilic
carbon that may lead to the formation of a tetrahedral intermedi-
Benzohydrazide and their derivatives are poly functional mole-
cules [1,2] bearing ˜C@O, –NH– and –NH2 functionalities in their
structures. Therefore, we can expect these compounds to behave
as ambident nucleophiles in several reactions [3]. The scarce data
about their kinetics and reaction mechanisms prompted us to per-
form experimental and theoretical studies in order to gain insight
in the corresponding reaction mechanisms towards carbonylic
electrophiles, p-nitrophenyl acetate in the present case. We have
shown that experimental and theoretical integrated research may
become a very useful methodology to describe reaction mecha-
nisms in organic chemistry [4].
Acetylation reactions have been studied earlier by Jencks using
hydroxylamine as nucleophile [5,6]. The process may lead to two
different products of reaction, depending on the nucleophilic site
for electrophilic attack (at O or at N) in hydroxylamine [7]. How-
ever, the kinetics and reaction mechanisms of benzohydrazide
derivatives with several acetates have not been completely docu-
mented up to date. Only few works have evaluated the reactivity
of benzohydrazide derivatives in water toward the formation of
carbinolamides [8]. In acetylation reactions, depending on the nat-
ure of the electrophile/nucleophile pair, two mechanisms are pos-
sible: a concerted pathway [9,10], where the nucleophile attack at
the electrophilic carbon in carbonyl group occurs simultaneously
with the leaving group departure within a single step. Another pos-
ate, T , from which the leaving group detaches. This mechanism
will be referred to as stepwise [9,11].
In this work, we report on the kinetics of the acetylation reac-
tion of benzohydrazide derivatives towards 4-nitrophenyl acetate
(NPA) (see Scheme 1). The kinetic data are then rationalized by
using theoretical tools that include the calculation of the potential
energy surface (PES). Some stationary points located at the PES are
then subjected to theoretical analysis to establish global nucleo-
philicity of benzohydrazides and their local reactivity patterns
(identification of active sites in the molecule towards nucleophilic
attack). The aim of this study is threefold: first of all the kinetic
measurements [12] are used to obtain useful information about
the reaction mechanism using Brönsted type-plot in terms of the
observed rate coefficients and available pKa values [13]. Secondly,
the exploration of the PES adds useful information about the pres-
ence of keto-enol pre-equilibria in the proposed reaction mecha-
nism, and finally,
a local reactivity analysis may assist the
identification of nucleophilic site of reactions.
2. Results and discussion
The rate law obtained for the reactions studied is given by Eqs.
(1) and (2), where P, S and N represent the product (4-nitrophenox-
ide anion), the substrate (NPA) and the nucleophile (benzohydraz-
ide derivatives), respectively.
d½Pꢀ
* Corresponding authors. Fax: +56 2 2999306 (P.R. Campodónico), +56 2 2713888
(R. Contreras).
¼ kobsd½Sꢀ
ð1Þ
dt
E-mail addresses: pcampodonico@udd.cl (P.R. Campodónico), rcontrer@uchile.cl
(R. Contreras).
where
0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.01.052

P.R. Campodónico et al. / Chemical Physics Letters 488 (2010) 86–89
87
kobsd ¼ k0 þ kN½free nucleophileꢀ
ð2Þ
Pseudo-first-order rate constants (kobsd) obtained under nucleo-
phile excess for all the reactions obey Eq. (2), where k0 and kN are
the rate coefficients for solvolysis and aminolysis of the substrate,
respectively. The values of both coefficients show no dependence
on pH within the range employed. These values were obtained as
the slope and intercept, respectively, of linear plots of kobsd against
the free nucleophile concentration ([free nucleophile]) at constant
pH. For the studied reactions, k0 values were much smaller than
the corresponding term related to aminolysis (kN [free nucleophile]
in Eq. (2)). The values of kN for the reactions of NPA with benzohyd-
razide derivatives as well as the available pKa values [13] are
shown in Table 1. Fig. 1 shows the Brönsted-type plot for the reac-
tions of benzohydrazide derivatives with NPA. The plot is linear
with slope b = 0.26, which is consistent with b values associated
with stepwise mechanism that proceed through a zwitterionic tet-
rahedral intermediate (T ) [14]. In this reaction the nucleophilic at-
tack is the rate determining step (see Scheme 2) [15,16]. Based on
the reported pKa values [13] and the kinetic data obtained in this
work it is possible to postulate at first glance that the nucleophilic
site of reaction corresponds to the nitrogen atom of the terminal
amine on the benzohydrazide structure (see Scheme 1). Note that
Fig. 1 shows a fair enhancement in nucleophilicity due to the pres-
ence of the electron releasing groups –CH3 and –OCH3 at p-position
of the aromatic ring. Note also that a fair nucleophilic deactivation
induced by the presence of electron withdrawing groups –Cl and –
CF3 is also observed. In order to generalize this result for other
known substituents, we plotted the values of log kN vs. the Ham-
Scheme 1. General structures of benzohydrazide derivatives and p-nitrophenyl
acetate (NPA) considered in this study.
Table 1
Values of pKa for the conjugate acids, rate nucleophilic attack constant (kN) with NPA,
Hammett substituent constant (
r
) and HOMO energy (eHOMO) of benzohydrazide
derivatives.
Benzohydrazides
pKaa
kN (sꢁ1 Mꢁ1
a,b
)
r
eHOMO(eV)
Substituent
4-OCH3
4-CH3
H
4-Cl
3.46
3.40
3.00
2.80
2.71
–
3.30 ꢂ 10ꢁ3
3.10 ꢂ 10ꢁ3
2.60 ꢂ 10ꢁ3
2.30 ꢂ 10ꢁ3
2.00 ꢂ 10ꢁ3
2.04 ꢂ 10ꢁ3
1.74 ꢂ 10ꢁ3
1.91 ꢂ 10ꢁ3
ꢁ0.27
ꢁ0.17
0.00
0.23
0.54
0.35
0.71
0.66
ꢁ8.53
ꢁ8.90
ꢁ9.17
ꢁ9.30
ꢁ9.70
ꢁ9.67
4-CF3
(*)4-OCF3
(*)3-NO2
(*)4-CN
–
–
ꢁ10.00
ꢁ9.80
a
pKa and kN values were determined in aqueous solution, at 25.0 °C, ionic
strength 0.2 M (KCl).
b
Predicted values of nucleophilic attack constant marked with (*) were obtained
from Eq. (3).
mett substituent constant (r) [17,18]. The result of such a compar-
ison is depicted in Fig. 2, and the corresponding data are displayed
in Table 1. The Hammett plots for the reactions of benzohydrazide
derivatives with NPA gives a straight lines with slope
q
= ꢁ0.27.
In order to rationalize the experimental results and with the ob-
ject of assisting in a more complete description of the reaction
mechanism involved, we performed a theoretical analysis. We first
calculated the potential energy surface (PES) for the reaction of the
title compounds towards NPA at HF/6-311G(d,p) level of theory
using the GAUSSIAN 03 package of programs [19]. The transition state
structures located for the unsubstituted benzohydrazide strongly
suggested the presence of a keto-enol pre-equilibrium. The energy
barrier separating the keto-enol tautomers was computed to be
34 kcal/mol in the gas phase. This figure may become smaller
when corrected by solvation effects, but in any case is a substantial
energy barrier. This result suggests that prior to the nucleophilic
attack, a tautomeric barrier must be surmounted, and that this bar-
rier may act as an effective bottleneck for the reaction. This argu-
ment may explain in part the low response to substituent effects
reflected in the values of rate coefficients experimentally recorded
for the series of benzohydrazide evaluated in this work. The results
based on the analysis of the PES also suggest that the enol form
would be the active nucleophilic species. Theoretical values of glo-
bal nucleophilicity may be readily obtained by representing this
Fig. 1. Brönsted-type plot for the reactions of benzohydrazide derivatives with NPA
in aqueous solution, at 25.0 °C, ionic strength 0.2 M (KCl).
Scheme 2. Possible reaction mechanism for acetylation of benzohydrazide derivatives.

88
P.R. Campodónico et al. / Chemical Physics Letters 488 (2010) 86–89
sence of experimental values for compounds marked with (*) in Ta-
ble 1, the reliability of the predictions made from Eq. (3) may be
reinforced by using the empirical Hammett’s rules about substitu-
ent effects [17,18]. Note that in Table 1, the kN values for the reac-
tions of benzohydrazide derivatives with NPA also suggests
extremely moderate activation/deactivation patterns induced by
chemical substitution. One of the possible reasons to observe mar-
ginal changes in the rate coefficients for this series of benzohydraz-
ide derivatives may be traced to their approximately similar pKa
values and also to the presence of keto-enol pre-equilibria. Based
on these results the reaction pathway sketched in Scheme 3 may
be proposed. The proposed mechanism is consistent with the fol-
lowing kinetic law:
K
k1k2
kobsd
¼
x
ð4Þ
K þ 1 k1 þ k2
where K is the equilibrium constant for the keto-enol tautomerism.
In deriving the rate law written in Eq. (4) we have made the follow-
ing approximations: we have first assumed a keto-enol pre-equilib-
rium described by the constant K; next we considered a second
equilibrium involving reactants and a tetrahedral intermediate
(see Scheme 3). If the enol form is the nucleophilic active species,
then we can assume Ko1, and the first term becomes a constant
Fig. 2. Hammett-type plot for the reactions of benzohydrazide derivatives with
NPA and other benzohydrazides not experimentally evaluated.
a
says:
K
a
¼
ð5Þ
K þ 1
and kN can be expressed as:
kN
¼
ak1k2=ðk1 þ k2Þ
ð6Þ
This rate law is consistent with a stepwise mechanism that pro-
ceed through a zwitterionic tetrahedral intermediate (T ) with the
nucleophilic attack as the rate determining step (see Fig. 1 and
Scheme 3) [15,16]. Finally, in order to reinforce our hypothesis that
the enol form is the proximate nucleophile, and that the nucleo-
philic center is the terminal N3 atom we performed a simple calcu-
lation of the local nucleophilicity at the O1, N2 and N3 sites (see
Scheme 3 for atom numbering). Local nucleophilicity is readily ob-
tained from the global nucleophilicity value by multiplying it by
the nucleophilic Fukui function [20]. The result of this simple cal-
culation yields values of local nucleophilicity at O1, N2 and N3:
ꢁ0.0240 eV; ꢁ0.0064 eV; and ꢁ0.0366 eV for the keto form respec-
tively and the enhanced values of ꢁ0.4649 eV; ꢁ1.9092 eV and
ꢁ2.1210 eV respectively for the enol tautomer. During the process
of review, one reviewer called our attention on the possible addi-
tional sites of reactions in ambident nucleophiles. This problem
is related to the charge/orbital control of chemical reactions [21].
We performed a Mulliken population analysis to assess the atomic
charge at centres O1, N2 and N3 of the keto and enol tautomers. The
results are: q(O1) = ꢁ0.469e, q(N2) = ꢁ0.398e and q(N2) = ꢁ0.373e
for the keto form and q(O1) = ꢁ0.416e, q(N2) = ꢁ0.270e and
q(N2) = ꢁ0.381e for the enol form. Because globally speaking the
enol form is more reactive than the keto form we may conclude
that the reaction is orbital controlled and driven by the nucleo-
Fig. 3. Relationships between kN and eHOMO for benzohydrazide derivatives.
property by the value of the HOMO energy level [20]. In order to
validate this reactivity index, a comparison between the measured
rate coefficients and the corresponding eHOMO values was per-
formed. The result of such a comparison is depicted in Fig. 3. The
resulting regression equation is:
log kN ¼ ꢁ0:74327 þ 0:20162eHOMO
;
R ¼ 0:975
ð3Þ
From this equation, the rate coefficient for the nucleophilic at-
tack kN may be predicted from the knowledge of the energy of
the HOMO orbital. These values are compiled in Table 1. In the ab-
Scheme 3. Postulated reaction mechanism incorporating tautomeric equilibria for acetylation of benzohydrazide derivatives.

P.R. Campodónico et al. / Chemical Physics Letters 488 (2010) 86–89
89
philic Fukui function, namely, fkꢁðO1Þ = 0.003, fkꢁðN2Þ = 0.001 and
fkꢁðN2Þ = 0.004 for the keto form and fkꢁðO1Þ = 0.051,
fkꢁðN2Þ = 0.208 and q(N2) = 0.231 for the enol form. These results
reinforce the hypothesis based on experimental data that the
nucleophilic site is the terminal N3 atom (see Scheme 3 for atom
numbering). This result also explain the low response in the rate
coefficients to inductive substituent effects: the marginal activa-
tion/deactivation patterns induced by electron releasing and elec-
tron withdrawing substituents may be traced to the fact that
being the terminal N3 site the active nucleophilic center for these
reactions, substituent at p-positions in the ring are too much far
from the site of reaction at N3. Substituent effects should be more
important at the carbonyl site but after the results obtained here
they should have relevant effects on the keto-enol tautomerism,
not at the nucleophilic attack stage of the reaction.
The product analysis for the reaction between 4-nitrophenyl
acetate and benzohydrazide in the same experimental conditions
corresponded to 4-nitrophenolate anion and the corresponding
diacylhydrazine, thereby reinforcing the theoretical prediction.
The identification of the products was achieved by comparison of
the UV–vis and HPLC spectra after completion of the reactions with
those of authentic samples of 4-nitrophenol and diacylhydrazine in
accord to Scheme 3. The latter was synthesized as reported previ-
ously by Carlsen and Jorgensen protocol [22].
mechanism incorporating keto-enol pre-equilibria is proposed.
The study is completed with a local reactivity analysis describing
the most reactive centers for nucleophilic attack, and the predicted
selectivity was confirmed by products study.
Acknowledgements
This work received financial support from project Clínica Alem-
ana-Universidad del Desarrollo 8011046 and Fondecyt Projects:
1070715 and 11060195.
References
[1] E.V. Titov, A.P. Grekov, V.I. Rybachenko, V.V. Shevchenko, Teoreticheskaya I
´
Eksperimentalnaya Khimiya 4 (1968) 742.
[2] G.D. Byrkit, G.A. Michalek, Ind. Eng. Chem. 42 (1950) 1862.
[3] H. Feuer, R. Harmetz, J. Am. Chem. Soc. 24 (1959) 1501.
[4] E.A. Castro, M. Aliaga, P.R. Campodónico, M. Cepeda, R. Contreras, J.G. Santos, J.
Org. Chem. 74 (2009) 9173.
[5] W.P. Jencks, J. Am. Chem. Soc. 80 (1958) 4585.
[6] W.P. Jencks, J. Am. Chem. Soc. 80 (1958) 4581.
[7] A.J. Kirby, J.E. Davies, T.A.S. Brandao, P.F. da Silva, W.R. Rocha, F. Nome, J. Am.
Chem. Soc. 128 (2006) 12374.
[8] M.L. Bender, R.D. Ginger, J. Am. Chem. Soc. 77 (1954) 348.
[9] A. Williams, Concerted Organic and Bio-Organic Mechanism, CRC Press, Boca
Raton, FL, 2000 (Chapter 4, and references therein).
[10] D. Stefanidis, S. Cho, S. Dhe-Paganon, W.P. Jencks, J. Am. Chem. Soc. 115 (1993)
1650.
[11] M.J. Gresser, W.P. Jencks, J. Am. Chem. Soc. 96 (1974) 7018.
[12] W.P. Jencks, M. Gilchrist, J. Am. Chem. Soc. 90 (1968) 2622.
[13] G. Manoussakis, M. Haristos, C. Youri, Can. J. Chem. 51 (1973) 811.
[14] M.L. Bender, Chem. Rev. 60 (1960) 53.
[15] E.A. Castro, R.B. Moodie, J. Chem. Soc. Commun. (1973) 828.
[16] M.J. Gresser, W.P. Jencks, J. Am. Chem. Soc. 99 (1977) 6963.
[17] C. Hansch, A. Leo, R.W. Taft, Chem. Rev. 97 (1991) 165.
[18] P.R. Campodónico, P. Fuentealba, E.A. Castro, J.G. Santos, R. Contreras, J. Org.
Chem. 70 (2005) 1754.
[19] M.J. Frisch et al., GAUSSIAN 98, Revision A.6, Gaussian, Inc., Pittsburgh, PA, 1998.
[20] R. Contreras, J. Andrés, V.S. Safont, P.R. Campodónico, J.G. Santos, J. Phys. Chem.
A 107 (2003) 5588.
[21] G. Klopman, J. Am. Chem. Soc. 90 (1968) 223.
[22] P.H.J. Carlsen, K.B. Jorgensen, J. Het. Chem. 31 (1994) 805.
3. Concluding remarks
The reaction of a series of benzohydrazide derivatives towards
p-nitrophenyl acetate have been both kinetically and theoretically
studied. The kinetic data are consistent with a stepwise mecha-
nism with the nucleophilic attack as the rate determining step.
From the theoretical analysis it is found that benzohydrazide
derivatives establish intramolecular proton rearrangement. The
enol form appears as the active species for nucleophilic attack
and this result consistently explain the low response in the reac-
tion rate coefficients to inductive substituent effects. A reaction