Hydrolysis kinetic studies of schiff bases derived from pyrrolic aldehydes in buffered aqueous ethanol and sulfuric acid solutions: Structural effects of substitutes

Article abstract of DOI:10.1002/kin.20777

Two series of substituted N-pyrrolyl-2-methylene-aniline were synthesized and characterized to study their stability in a large domain of pH (0-14) and especially in the H₀ domain (-4 to 0). The hydrolysis kinetics of the azomethine group was established in homogeneous media using a thermostated UV-vis spectrophotometer. The hydrolysis mechanism was investigated, and the experimental kinetic constants were calculated. Then, the pH-rate diagram profile was determined and the structural effect of substitutes on the kinetic constants was clarified and discussed.

Full text of DOI:10.1002/kin.20777

Hydrolysis Kinetic Studies
of Schiff Bases Derived
from Pyrrolic Aldehydes in
Buffered Aqueous Ethanol
and Sulfuric Acid Solutions:
Structural Effects of
Substitutes
ZOHRA BENGHAREZ, ZINEB EL BAHRI, ABDERREZZAK MESLI
Laboratory of Macromolecular and Physical Organic Chemistry, Faculty of Sciences, Djillali Liabes University,
Sidi Bel Abbes 22000, Algeria
Received 26 May 2012; revised 25 November 2012; 26 December 2012; accepted 27 December 2012
DOI 10.1002/kin.20777
Published online 23 April 2013 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Two series of substituted N-pyrrolyl-2-methylene-aniline were synthesized and
characterized to study their stability in a large domain of pH (0–14) and especially in the H0
domain (–4 to 0). The hydrolysis kinetics of the azomethine group was established in homo-
geneous media using a thermostated UV–vis spectrophotometer. The hydrolysis mechanism
was investigated, and the experimental kinetic constants were calculated. Then, the pH–rate
diagram profile was determined and the structural effect of substitutes on the kinetic constants
was clarified and discussed. ꢀC 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 404–414, 2013
more, their biological activities as antimicrobial, anti-
fungal, and antitumor agents and herbicides have been
largely proved [1–10]. Also, the stability of azome-
thine function has been discussed in numerous studies.
Some empirical hydrolysis curves of some series of
Schiff bases in the range of pH values from 0 to 14
and different mechanisms have been proposed by re-
searchers [11–17]. Schiff bases were also used as lig-
ands for the metal complex; their hydrolysis assisted
by a metal catalyst was investigated [18,19], and they
INTRODUCTION
A great number of Schiff bases have been synthe-
sized and used as substrates for the preparation of in-
dustrially and biologically active compounds. Further-
Correspondence to: Zohra Bengharez; e-mail: dzbengharez@
yahoo.fr.
C
ꢀ
2013 Wiley Periodicals, Inc.

HYDROLYSIS KINETIC STUDIES OF SCHIFF BASES DERIVED FROM PYRROLIC ALDEHYDES
405
CH
N
X
N
Y
A series: Y = H, X = H: 1, CH3: 2, OCH3 : 3, OC2H5 : 4, Cl: 5,
Br: 6
B series: Y = CH3, X = H: 1′, CH3: 2′, OCH3 : 3′, OC2H5 : 4′, Cl: 5′, Br: 6′
Scheme 1
were also used as acid–base indicators as reported by
Khalil et al. [20].
Starck apparatus. The products were recrystallized in
ethanol, and the melting points were measured using
the Totolli apparatus.
Certain aldimines derived from pyrrole-2-
carbaldehyde and pyrrole derivatives were synthesized
and used for the preparation of compounds with
antimicrobial, antibacterial, and tuberculostatic
activities [21–23].
Characterization of Schiff Bases
IR spectra were registered on a FTIR-8300 Shimadzu
spectrophotometer with samples dispersed in KBr pel-
lets. The 1H and 13C NMR spectra of A and B series of
Schiff bases dissolved in deuteriated chloroform were
recorded on Brucker DRX 500 MHZ and 300 MHz
apparatus, respectively. The UV–vis spectrum (200–
400 nm) of each imine in an absolute ethanol solution
at C = 10−4 mol L−1 was carried out using a UV–vis
Shimadzu UV-2401 PC; it presented four bands (λmax
in nm, ε in L cm−1 mol−1).
The present study is focused on the hydrolysis
study of two series of Schiff bases based on substi-
tuted N-pyrrolyl-2-methylene-aniline and their stabil-
ity in a large domain of pH. The hydrolysis of some of
these compounds has been previously described quali-
tatively by Dezelic et al. [24], showing their rapid and
complete destruction in acidic, basic, and neutral me-
dia. This research has been succeeded by Mesli et al.
for the complete hydrolysis from pH 0 to 14 using
polarography and amperometry methods [25,26]. The
present paper reports the systematic hydrolysis study
of substituted N-pyrrolyl-2-methylene-aniline and N-
methyl pyrrolyl-2-methylene-aniline (Scheme 1) in a
large range of pH including the H0 domain of mod-
erately concentrated aqueous acid. To our knowledge,
this is the first hydrolysis kinetic investigation of these
products using the UV–vis spectroscopy method par-
ticularly in the zone H0 (from –4 to 0); obviously this
domain of strong acidity was mainly explored [27,28].
Thus, the aim of the paper is to determine the morphol-
ogy of the pH–rate profile of the hydrolysis reaction of
these pyrrolic Schiff bases series (Scheme 1). So, the
apparent rate constants have been calculated and the
structural effect of substitutes discussed.
A Series.
H4
H3
H2 '
H3 '
4
3
3 '
2 '
5
2
4 '
1 '
H5
Ci
Hi
N
X
N
5 '
6 '
H5 '
H6 '
H1
=
X H (N-pyrrolyl-2-methylene-aniline), mp =
◦
1
=
–
94 C; IR: 1625 (C N), 3220 (N H); H NMR (ppm):
H3 (6.68 m), H4 (6.26 m), H5 (6.79 m), Hi (8.26 s),
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
H2 and H6 (7.16 m), H3 and H5 (7.34 m), H4 (7.16
m), H1 (10.25 s); 13C NMR (ppm): Ci (152.16), C1
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(150.53), C2 and C6 (117.37), C3 and C5 (129.67),
ꢁ
C4 (121.40), C2 (131.12), C3 (123.97), C4 (110.81),
C5 (125.9). UV–vis: band I (325, 19281), band II (295,
11667), band III (225, 6213), band IV (202, 10105).
=
X CH3
(◦N-pyrrolyl-2-methylene-p-methyl-
EXPERIMENTAL
=
–
aniline), mp = 101 C; IR: 1615 (C N), 3246 (N H);
1H NMR (ppm): H3 (6.63 m), H4 (6.22 m), H5 (6.75
Synthesis of Schiff Bases
ꢁ
ꢁ
ꢁ
ꢁ
m), Hi (8.25 s), H2 and H6 (7.08d), H3 and H5 (7.09
ꢁ
Schiff bases were synthesized from mixtures
of N-pyrrole-2-carbaldehyde or N-methylpyrrole-2-
carbaldehyde and p-substituted anilines in a stoichio-
metric proportion by a classical method of conden-
sation [29] in anhydrous benzene as a solvent and
p-toluene sulfonic acid as a catalyst using the Dean–
d), H(CH3) (2.20 s); 13C NMR (ppm): Ci (149.57), C1
ꢁ
ꢁ
ꢁ
ꢁ
(149.92), C2 and C6 (117.07), C3 and C5 (130.24),
C4 (129.80), C2 (131.17), C3 (121.23), C4 (110.65),
C5 (123.89). UV–vis: band I (328, 20,961), band II
(296, 12,118), band III (233, 6261), band IV (203,
12007).
ꢁ
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

406
BENGHAREZ ET AL.
=
X OCH3
(N-pyrrolyl-2-methylene-p-methoxy-
(297,13723), band III (220, 11,157), band IV (203,
18,286).
◦
=
–
aniline), mp = 94.5 C; IR: 1615 (C N), 3232 (N H),
1H NMR (ppm): H3 (6.89 m), H4 (6.29 m), H5 (6.91
X CH3
(N-methyl-◦N-pyrrolyl-2-methylene-p-
=
1
ꢁ
ꢁ
6ꢁ
5ꢁ
=
m), Hi (8.23 s), H2 , (7.17 d), H3 , (7.24 d), H
methyl-aniline), mp = 45 C; IR: 1622 (C N); H
(–CH3) (3.80 s); 13C NMR (ppm): Ci (158.22), C1
NMR (ppm): H3 (6.72 m), H4 (6.23 m), H5 (6.81 m),
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(145.12), C2 and C6 (116.41), C3 and C5 (114.86),
Hi (8.33 s), H2 and H6 (7.12 d), H3 and H5 (7.20 d),
ꢁ
–
C4 (148.59), C2 (131.34), C3 (122.34), C4 (110.74),
H( CH3) (4.20 s), H1 (4.07 s). UV–vis: band I (329,
C5 (123.15), C (OCH3) (21.36). UV–vis: band I (337,
19,131), band II (297, 11,266), band III (227, 7205),
band IV (203, 11,993).
14,382), band II (294, 10,217), band III (220, 8939),
band IV (204, 14,487).
=
X OCH3 (N-methyl-N-pyrrolyl-2-methylene-p-
◦
1
=
=
X OC2H5
(N-pyrrolyl-2-methylene-p-ethoxy-
methoxy-aniline), mp = 60 C; IR: 1618 (C N); H
◦
=
aniline), mp = 118.5 C; IR: 1618 (C N), 3232
NMR (ppm): H3 (6.8 m), H4 (6.23 m), H5 (6.8 m), Hi
1
ꢁ
ꢁ
ꢁ
ꢁ
–
(N H); H NMR (ppm): H3 (6.63 m), H4 (6.24 m), H5
(8.34 s), H2 and H6 (6.95 d), H3 and H5 (7.20 d),
ꢁ
ꢁ
ꢁ
–
(6.78 m), Hi (8.28 s), H2 and H6 (6.90 d), H3 and
H( CH3) (3.85 s), H1 (4.07 s). UV–vis: band I (336,
ꢁ
–
–
–
H5 (7.17 d), H( CH2 ) (4.03 q), H ( CH3)(1.41 t);
18,577), band II (297, 12,688), band III (219, 8624),
band IV (202, 12,419).
13C NMR (ppm): Ci (157.61), C1 (145.02), C2 and
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
=
C6 (116.66), C3 and C5 (115.48), C4 (148.85), C2
(131.30), C3 (122.40), C4 (110.64), C5 (123.55), C
X OC2H5 (N-methyl-◦N-pyrrolyl-2-methylene-p-
ethoxy-aniline), mp = 48 C; IR: 1618 (C N); 1H
=
–
–
–
( CH2 ) (64.13), C( CH3) (15.31). UV–vis: band I
(336.4, 23,296), band II (296, 13,393), band III (226,
7819), band IV (203, 13,017).
NMR (ppm): H3 (6.80 m), H4 (6.30 m), H5 (6.90 m), Hi
(8.34 s), H2 , 6 (6.95 d), H3 and H5 (7.20 d), H( CH2
(4.09 q), H( CH3) (1.45 t), H1 (4.07 s). UV–vis: band
I (337, 18,511), band II (295, 12,503), band III (222,
8425), band IV (203, 12,973).
ꢁ
ꢁ
ꢁ
ꢁ
–
–
)
–
=
X Cl (N-pyrrolyl-2-methylene-p-chloro-aniline),
◦
1
=
–
mp = 97.5 C; IR: 1614 (C N), 3232 (N H), H NMR
=
X Cl
(ppm): H3 (6.71 m), H4 (6.28 m), H5 (6.72 m), Hi
(N-methyl-N-pyrrolyl-2-methylene-p-
◦
chloro-aniline), mp = 40 C; IR: 1612 (C N); 1H
ꢁ
ꢁ
ꢁ
ꢁ
=
(8.25 s), H2 and H6 (7.15 d), H3 and H5 (7.32 d), NH
(10.25); 13C NMR (ppm): Ci (150.67), C1 (151.00), C2
NMR (ppm): H3 (6.85 m), H4 (6.24 m), H5 (7.13 m),
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
and C6 (118.14), C3 and C5 (130.91), C4 (129.78),
C2 (131.38), C3 (122.77), C4 (111.05), C5 (124.56).
UV–vis: band I (331, 14,658), band II (294.6, 13,967),
band III (241.6, 14,264), band IV (204, 25,345).
Hi (8.30 s), H2 , (7.15 d), H3 and H5 (7.35 d), H1
6
(4.07 s). UV–vis: band I (329, 13,604), band II (297,
11,741), band III (227, 8338), band IV (203, 16,925).
=
X Br
(N-methyl-N-pyrrolyl-2-methylene-p-
◦
bromo-aniline), mp = 37 C; IR: 1612 (C N); 1H
=
=
X Br (N-pyrrolyl-2-methylene-p-bromo-aniline),
◦
1
=
–
mp = 107.5 C; IR: 1614 (C N), 3232 (N H); H
NMR (ppm): H3 (6.82 m), H4 (6.22 m), H5 (7.10 m),
ꢁ
ꢁ
ꢁ
ꢁ
NMR (ppm): H3 (6.70 m), H4 (6.28 m), H5 (6.92 m), Hi
Hi (8.30 s), H2 , (7.10 d), H3 and H5 (7.40 d), H1
6
ꢁ
ꢁ
ꢁ
ꢁ
(8.22 s), H2 and H6 (7.04 d), H3 and H5 (7.40 d), NH
(4.07 s). UV–vis: band I (330, 14,118), band II (297,
10,043), band III (227, 9365), band IV (204, 16,858).
(9.80); 13C NMR (ppm): Ci (150.56), C1 (149.87), C2
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
and C6 (117.24), C3 and C5 (130.50), C4 (118.69), C2
(132.22), C3 (122.56), C4 (110.71), C5 (123.60). UV–
vis: band I (332, 19,281), band II (295, 9827) band III
(225, 6740), band IV (203, 12,159).
UV–Vis Analysis and Hydrolysis Kinetics
The hydrolysis of imines was studied at a tempera-
ture of 37 0.1◦C using a UV–vis spectrophotome-
ter (Shimadzu UV-2401 PC; Tokyo, Japan) equipped
with a thermostatically controlled cell holder, in ho-
mogeneous media, i.e., 33% (V/V) of ethanol buffer
mixtures, where the ionic strength is maintained at
0.15. A Tacussel digital pH meter was used for the pH
measurements.
B Series.
H4
H3
H2 '
H3 '
4
3
3 '
2 '
5
2
4 '
1 '
H5
Ci
Hi
N
X
N
C
5 '
6 '
H5 '
H6 '
•
H1
For pH 4–12, the hydrolysis media were prepared
H1
H1
using the buffered aqueous solution/ethanol, ac-
cording to the method of Michaelis and Mizu-
tani [30].
=
X H (◦N-methyl-N-pyrrolyl-2-methylene-aniline),
1
=
mp = 120 C; IR: 1624 (C N); H NMR (ppm): H3
•
ꢁ
(6.78 m), H4 (6.30 m), H5 (6.85 m), Hi (8.37 s), H2
For pH <4 and >12, hydrochloride and sodium
ꢁ
ꢁ
ꢁ
ꢁ
and H6 (7.26 m), H3 and H5 (7.43 m), H4 (7.26 m),
H1 (4.11 s). UV–vis: band I (325, 13,855), band II
hydroxide solutions were used, respectively, in-
stead of the buffer solution.
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

HYDROLYSIS KINETIC STUDIES OF SCHIFF BASES DERIVED FROM PYRROLIC ALDEHYDES
407
Figure
1
Time-resolved absorption spectra for N-
methylpyrrolyl-2-methylene-aniline in pH solution = 7.36.
Figure 2 Hydrolysis of 1: N-pyrrolyl-2-methylene aniline
at pH 3.35 (λ
= 345 nm).
max
•
For the H0 zone (–4 to 0), aqueous sulfuric acid
solutions (from 2% to 54%)/ethanol were pre-
pared as described by Johnson et al. [31].
As a matter of fact, the hydrolysis reaction of the
imine functional group passes inevitably through the
carbinolamine group [34]. So, the presence of isobestic
“i” points as mentioned in Fig. 1 indicated that there
was no accumulation of any intermediate, especially
the α-amino alcohol, during the hydrolysis process.
The present results showed that the hydrolysis ki-
netics of these Schiff bases in ethanol buffer mixtures
is considered as a first-order reaction as indicated by
the linearity of plot of log(Dt,c) versus time (Eq. (1)):
To follow the hydrolysis kinetics, the buffer (2 mL)
and ethanolic Schiff base solution (1 mL) were poured
into the UV–vis cell and shaken; for all experiments,
the initial concentration of imine in the UV–vis cell is
equal to 10−4 mol L−1
.
In general, complete UV spectra of solution were
registered over time intervals consistent with the ad-
vancement of the reaction state. However, when the
kinetics was rapid, only the change in UV absorption
with time at the selected wavelength was recorded.
kexp
log Dt,c = log D0,c
−
t
(1)
2.3
where Dt,c is the corrected optical density at time t,
Dt,c = Dt – D∞; Dt is the optical density at time t;
and D∞ is the optical density at the end of hydrolysis
(infinite time). So, the experimental kinetic constant
(kexp) is calculated from the slope of the log(D) =
f(t) equation. Also, the initial corrected optical density
(D0,c) is determined by extrapolation at t = 0.
RESULTS AND DISCUSSION
Schiff Bases Hydrolysis in pH Media from 0
to 14
First, it should be noted that the UV–vis spectra of
all compounds (A and B series) in absolute ethanol
showed four bands, which are described in the Ex-
perimental section. The same UV behavior was noted
for the substituted biphenyl Schiff bases XC6H4CH =
NC6H4Y [32,33], where the band I corresponding to
π–π* electronic transition is affected by the Y substi-
tute. For the A and B pyrrolic series, the four bands
appeared in the same regions whatever be the substi-
tutes; nevertheless, the molar coefficient of extinction
ε is affected by both X and Y substitutes.
Examples of straight lines of the hydrolysis data of
A series are given in Fig. 3. The imine absorption band,
i.e., the analytical wavelength λa, is selected from the
free imine band at each pH; it disappears completely at
infinite time, suggesting that the reaction is complete
in the present operating conditions.
Schiff Bases Hydrolysis in Sulfuric Acid
Solution (H0 = –4 to 0)
Concerning the hydrolysis kinetics of A and B se-
ries of N-pyrrol-2-methylene aniline (YNC4H3CH =
NC6H4X), examples of UV spectra of slow and fast
kinetics are shown in Figs. 1 and 2, respectively.
In these experimental conditions, we have obtained the
same behaviors. Figure 4 presents the hydrolysis of
N-methylpyrrolyl-2-methylene-aniline at H0 = –1.03
(20% of H2SO4), where λa = 400 nm. As mentioned
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

408
BENGHAREZ ET AL.
Figure 3 Typical first order of the hydrolysis reaction of A series of compounds at pH 2.66.
investigated and calculated. Figures 6 and 7 show hy-
drolysis curves of A and B series, respectively. Some
examples of kinetic constant values are given in Tables
I and II. All H0 and pH–rate profiles, i.e., log kexp
f(H0, pH), are bell shaped, and they can be divided into
four zones (Fig. 6).
=
As reported in the literature [11–13,35–39], the
hydrolysis curve of Schiff bases derived from aro-
matic and aliphatic amines can be divided into differ-
ent domains. In the current study, we have numbered
only four domains: H0, Pꢁ, A, and P as mentioned in
Fig. 6.
The proposed mechanism of imine hydrolysis in
the H0 domain (from H0 = –4 to 0) is presented in
Scheme 2, and the general mechanism of hydrolysis
in Pꢁ, A, and P zones (from pH 0–14) is described in
Scheme 3.
Figure
4 Time-resolved absorption spectra for N-
methylpyrrolyl-2-methylene-aniline in the H0 solution =
–1.03 (20% of H2SO4).
First, in the H0 domain, the water base catalyzed
decomposition of the cationic form (SH2OH+) of
the carbinolamine intermediate corresponds to the
hydrolysis-limiting step (step c in Scheme 2). Then,
the apparent kinetic constant is given by
previously, we tested Eq. (1) to identify the hydrolysis
mechanism and, consequently, to calculate the appar-
ent kinetic constants (kexp). In this case (H0 zone), λa
is selected from the protonated imine band. Thus, we
present in Fig. 5 plots log Dt,c versus time for A series
hydrolysis at H0 = –1.03.
log kobs − α log(aH O) + log khk
(2)
2
Morphology of Hydrolysis Curves and
Structural Effects of Substitutes
where
The experimental kinetic constants of the hydrolysis
reaction of all products in the pH and H0 ranges were
kh = [SH2OH+]/[SH+][H2O]
(3)
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

HYDROLYSIS KINETIC STUDIES OF SCHIFF BASES DERIVED FROM PYRROLIC ALDEHYDES
409
Figure 5 Typical first order of the hydrolysis reaction of A series of compounds at H0 = –1.03 (20% of H2SO4).
Figure 6 General hydrolysis curve: log kexp (s−1) = f(pH, H0) of A pyrrolic series.
aH O is the water activity, and α is a number of water
The results showed slope values close to 1.27 for
2
molecules involved in the transition state [34].
In this zone, only the hydrolysis of N-p-
chlorobenzylydene-aniline has been previously dis-
cussed by Cordes and Jenks [36]; they demonstrated a
linear relationship between the kinetic constant and H0
value, and the slope of log kobs versus H0 was equal to
1.24.
For A and B series of pyrrolic Schiff bases, lin-
ear relationships have been obtained too and the slope
values are summarized in Tables III and IV.
A series and 1.20 for B series; we concluded that the
pyrrolic and benzylic Schiff bases have practically the
same hydrolysis behavior in the H0 zone.
Also, Cordes and Jenks [36] presented a linear re-
lationship between log kexp and log aH O (Eq. (2)) and
2
the slope (α) was equal to 3.12, indicating the involve-
ment of at least three water molecules in the transition
state.
The results displayed in Tables III and IV show that
the slope value of log kexp = f(log aH O) plots varied
2
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

410
BENGHAREZ ET AL.
H+
NH +
(a)
(b)
C
N
C
+
+
)
(S)
NH +
(SH
kh
+
H2O
CH
OH
(SH 2OH +
NH 2
C
+
)
k
(c)
H3O+
(SH 2OH +
(Products)
)
H2O
+
+
Scheme 2
Regarding the structural effect of the X substitute in
the H0 zone, the Hammett correlation (Fig. 8) gave
positive values of reaction constants equal to 1.53
(r = 0.980) and 1.55 (r = 0.985) for A and B se-
ries, respectively. So, the destruction of protonated α-
aminoalcohol intermediate is favored when X had an
electron-withdrawing effect. In addition, we noted that
the Y substitute (H or CH3) did not modify the reaction
constants of Hammett.
Figure 7 General hydrolysis curve: log kexp (s−1) = f(pH,
H0) of B pyrrolic series.
Second, for Pꢁ, A, and P zones (pH 0–14) and from
Scheme 3, the general observed kinetic constant can
be expressed by Eq. (4) [26]:
practically from 9.3 to 10.4 for A series and from 10.0
to 13.7 for B series. We concluded that for A and
B series, respectively, 10 and 11 water molecules on
average participate in the transition state but only one
of them had a nucleophilic reactive role.
Nevertheless, we must specify that Bunnett [40] did
not observe correlations between kinetic constants and
water activity; he found anomalous and high slope val-
ues. Consequently, he considered that Eq. (2) remained
as an empirical relationship and the water molecule
does not have a significant activity on the hydrolysis
mechanism in concentrated acid media [40].
k1k5[H+] + Kek2k5
kobs
=
(4)
([H+] + KSH ) (k−1[H+] + k−2 + k5)
+
where Ke is the autoprotolysis constant of water.
The acidic plateau Pꢁ or the bell summit in the hy-
drolysis curve (pH 0–4) corresponded to the transition
in the limiting step of the hydrolysis reaction. In this
case, the kinetic constant of hydrolysis is independent
of pH as indicated by Eq. (5). The rate-limiting step
Table I Some Values of Experimental Hydrolysis Kinetic Constants of A series (Y = H)
kobs (s−1
)
Zone, pH
X = OCH3
X = OC2H5
X = CH3
X = H
X = Cl
X = Br
H0, –0.76
Pꢁ, 2.25
A, 5.04
1.43 × 10−3
6.47 × 10−3
4.90 × 10−3
0.69 × 10−4
1.31 × 10−3
7.19 × 10−3
4.76 × 10−3
0.77 × 10−4
2.21 × 10−3
9.57 × 10−3
5.88 × 10−3
0.91 × 10−4
3.68 × 10−3
2.96 × 10−2
1.45 × 10−2
0.99 × 10−4
6.98 × 10−3
6.75 × 10−2
1.65 × 10−2
0.48 × 10−4
8.75 × 10−3
6.60 × 10−2
2.34 × 10−2
0.50 × 10−4
P, 12.38
Table II Some Values of Experimental Hydrolysis Kinetic Constants of B Series (Y = CH3)
kobs (s−1
)
Zone, pH
X = OCH3
X = OC2H5
X = CH3
X = H
X = Cl
X = Br
H0, –0.66
Pꢁ, 2.28
A, 5.00
1.03 × 10−3
1.44 × 10−2
1.14 × 10−2
0.23 × 10−3
1.63 × 10−3
1.80 × 10−2
1.11 × 10−2
0.22 × 10−3
2.74 × 10−3
2.15 × 10−2
1.49 × 10−2
0.22 × 10−3
4.60 × 10−3
4.39 × 10−2
2.32 × 10−2
0.26 × 10−3
7.49 × 10−3
1.19 × 10−1
3.71 × 10−2
0.13 × 10−3
1.06 × 10−2
1.02 × 10−1
3.94 × 10−2
0.13 × 10−3
P, 12.10
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

HYDROLYSIS KINETIC STUDIES OF SCHIFF BASES DERIVED FROM PYRROLIC ALDEHYDES
411
KSH+
H+
+
(a′)
C
N
C
NH
+
+
)
(S)
NH
(SH
k1
H+
(b′)
+
+
H2 O
C
C
CH
NH
+
k
−
1
OH
(SHOH)
k2
−
NH +
OH
CH
OH
NH
(c′)
(d′)
+
k
−
2
k3
N −
−
C
N
(S)
OH
CH
+
k
−3
OH
(SHO−
)
k4
N−
(e′)
(f′)
CH
OH
NH
CH
H2 O
OH
+
+
k
−
4
OH
(SHO−
(SHOH)
)
k5
CH
OH
NH
H2 N
C
+
O
(Products)
(SHOH)
Scheme 3
corresponds to step bꢁ, i.e., the addition of water
molecule on the protonated imine:
Concerning the substitute effect, good Hammett
correlations, i.e., log kobs = f(σx), were obtained for
both the A and B series (Fig. 9). The reaction con-
stants (ρ) were equal to 1.75 (where the coefficient of
kobs = k1
(5)
Table III Values of Slope and Coefficient of Correlation (r) for Equations log kexp = f(H0) and log kexp = f(log aH O) for
2
A Series
A Series
Parameter
1
2
3
4
5
6
SD
Slope of log kexp = f(H0)
1.305
0.993
9.322
0.976
1.284
0.993
9.742
0.986
1.274
0.991
10.418
0.977
1.251
0.995
10.252
0.982
1.278
0.993
10.279
0.988
1.274
0.991
10.364
0.988
0.0159
r
–
0.398
–
Slope of log kexp = f(log aH O
)
2
r
r: Coefficient of correlation.
SD: Standard deviation.
Table IV Values of Slope and Coefficient of Correlation (r) for Equations log kexp = f(H0) and log kexp = f(log aH O) for
2
B Series
B Series
Parameter
1ꢁ
2ꢁ
3ꢁ
4ꢁ
5ꢁ
6ꢁ
SD
Slope of log kexp = f(H0)
1.024
0.993
10.707
0.998
1.241
0.987
10.832
0.995
1.274
0.991
11.176
0.990
1.222
0.990
10.002
0.998
1.194
0.993
11.510
0.983
1.215
0.988
13.777
0.996
0.080
–
1.186
–
r
Slope of log kexp = f(log aH O
)
2
r
r: Coefficient of correlation.
SD: Standard deviation.
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

412
BENGHAREZ ET AL.
Figure 8 Hammett correlation for A and B series of pyrrolic Schiff bases in the H0 zone.
Figure 9 Hammett correlation for A and B series in the Pꢁ zone.
correlation, r = 0.997) and 1.92 (r = 0.976) for the
A and B series, respectively. Then, the positive val-
ues of ρ confirmed the water nucleophilic attack on
the protonated imine, which is favored by the electron-
withdrawing effect of X substitutes [26,36].
But, for these substituted pyrrolic series, the ex-
perimental slopes of the linear plots of log kexp
=
f(pH) varied from 0.5 to 0.8. So, Eq. (6) is not suf-
ficient to describe the hydrolysis behavior of A and B
series of N-Y-pyrrolyl-2-methylene-X-aniline and in
this case the step cꢁ (Scheme 3) cannot be neglected.
Consequently, Eq. (4) becomes Eq. (7), where the hy-
drolysis reaction is simultaneously specific acid and
base-catalyzed:
In zone A, i.e., the pH range of 4–8, on the one
hand, the hydrolysis reaction is catalyzed by acidity
since the observed rate constants decrease markedly
with an increase in pH whatever the substitute X. On
the other hand, the kinetic constant value increased
slightly when X is an electron-withdrawing substituent
(Tables I and II).
k1[H+] + Kek2
kobs
=
(7)
([H+] + KSH
)
+
If we considered the protonated imine (SH+) at-
tacked only by water molecules (step bꢁ), the theoreti-
cal equation (4) can be simplified to Eq. (6):
The basic plateau P appeared from pH 8–13, and in
this domain Reeves [41] demonstrated that the rate-
limiting step corresponded to the nucleophile attack of
hydroxide anion on the protonated imine. The hydrol-
ysis constant is then given by Eq. (8), where the kinetic
constant is independent from pH:
k1[H+]
kobs
=
(6)
+
KSH
Kek2
where the slope of the plot log(kobs) = f(pH) is equal
kobs
=
(8)
to 1.
+
KSH
International Journal of Chemical Kinetics DOI 10.1002/kin.20777

HYDROLYSIS KINETIC STUDIES OF SCHIFF BASES DERIVED FROM PYRROLIC ALDEHYDES
413
methylene-aniline derivatives. The experimental ki-
netic constants were calculated, and the results showed
bell-shaped hydrolysis curves of log kexp = f (pH, H0).
Good Hammett correlations were obtained in the zone
Pꢁ of pH from 0 to 4 and in the H0 zone (from –4
to 0). In the zone of H0 of interest, linear relation-
ships were obtained both between log kexp and H0 and
between log kexp and log aH O. Finally, the hydrol-
2
ysis mechanism was elucidated in each zone of pH
and H0.
The authors are particularly grateful to Professor Roger
Guilard, Institute of Molecular Chemistry, University of Bur-
gundy (ICMUB, UMR CNRS 5260), Dijon, France, and Pro-
fessor Hassen Amri, Director of the Laboratory of Organic
and Organometallic Chemistry, Tunis, Tunisia, for their con-
tribution and characterization of the products.
Figure 10 Hydrolysis curves of
bases (N-2-pyrrolylmethylene-aniline and N-methyl-N-2-
pyrrolylmethylene-aniline, respectively).
1
and 1ꢀ Schiff
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International Journal of Chemical Kinetics DOI 10.1002/kin.20777