Kinetic studies of condensation of aromatic aldehydes with Meldrum's acid

Article abstract of DOI:10.1515/znb-2002-1119

The condensation reaction of Meldrum's acid with aromatic aldehydes in the presence of a catalyst has been investigated spectrophotometrically at 25-50 °C. The reaction follows overall second order kinetics, first order in each of the reactants. Electron-

Full text of DOI:10.1515/znb-2002-1119

Kinetic Studies of Condensation of Aromatic Aldehydes with Meldrum’s Acid
H. A. A. Medien
Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt.
Fax: 202-4831836. E-mail: hmadian59@vahoo.com
Z. Naturforsch. 57b, 1320Ð1326 (2002); received April 9, 2002
Aromatic Aldehydes, Meldrum’s Acid, Spectrophotometry, Kinetic
The condensation reaction of Meldrum’s acid with aromatic aldehydes in the presence of
a catalyst has been investigated spectrophotometrically at 25Ð50 ∞C. The reaction follows
overall second order kinetics, first order in each of the reactants. Electron-withdrawing
groups accelerate and electron-releasing groups retard the rate of condensation. From the
dependence of the rate constants on temperature, the activation parameters have been calcu-
lated and also the isokinetic temperature with a value of 403.5 K. Based on this reaction,
determination of eight aromatic aldehydes in a concentration range of 1.49 Ð91 µg/ml is
proposed. On the other hand, determination of some aromatic aldehydes with Meldrum’s
acid is performed in water at 75 ∞C for 2 h, without adding any catalyst.
Introduction
the reactivity. This work also aims to introduce a
spectrophotometric method for determination of
the aromatic aldehydes.
Meldrum’s acid undergoes standard Knoevenagel
condensation with aromatic and heteroaromatic al-
dehydes furnishing the corresponding arylidene de-
rivatives, which are versatile substrates for different
kinds of reactions [1]. They are useful substrates for
cycloaddition reactions and for the synthesis of het-
erocyclic compounds with potential pharmacologi-
cal activity [2]. The 1,4-addition of nucleophiles to
these products has been widely used synthetically
and displays some advantages over the acyclic malo-
nate analogues. The conjugated reduction is the way
most often used to prepare 5-monoalkyl Meldrum’s
acids derivatives or analogues, also applied to ob-
tain deuterated carboxylic acids [3].
Results and Discussion
Meldrum’s acid like other compounds with an
active methylene group [5, 6, 24, 25], interact with
aromatic aldehydes through two steps according
to Scheme 1.
From the data compiled in Table 1, it was shown
that for all aldehydes studied the kinetics is second
order overall, first order with respect to each of
the reactants, and zero order with respect to piper-
idine, according to the simple rate law:
d[product]/dt = kobs [aldehyde][MA]
Methods are cited in the literature for kinetic
studies of reactions of aromatic aldehydes, and
comprise reactions with chlorodipentylborane [4],
From this equation it can not be distinguished if
the rate-limiting step is the nucleophilic attack of
the carbanion of MA on the aldehyde or the dehy-
5-N-Benzoylamino-1,3,4-thiadiazole-2-acetonitrile
[5], 1,3-dimethybarbituric acid [6], 3-methyl-1-phe-
nylpyrazolin-5-one [7], phenylhydroxylamine [8],
pyridinium chlorochromate [9], phenylhydrazine
[10], condensation with o-phenylenediamine in
presence of H O2 [11] and with cysteine [12].
2
Other kinetic methods are available for oxidation
of aromatic aldehydes [13Ð23].
This work aims to study the kinetics of the con-
densation reaction of some aromatic aldehydes
with the active methylene compound, 2,2-di-
methyl-1,3-dioxane-4,6-dione (Meldrum’s acid,
MA), and to study the effect of substituents on Scheme 1.
0
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H. A. A. Medien · Kinetic Studies of Condensation of Aromatic Aldehydes with Meldrum’s Acid
Table 1. Rate constants of the reaction of p-nitrobenz-
1321
These equations lead to rate expressions (iv)
aldehyde with Meldrum’s acid (MA) at 298 K (Pip =
and (v).
piperidine) for different concentrations of the reactants.
Ð
+
rate = k [MA ][AldH ]
rate = kK K [MA][Ald].
(iv)
(v)
4
[Aldehyde], M
[MA], M
[Pip], M
10 kobs
,
Ð1
sec
1
2
Ð4
2
5
7
.5¥10
.0¥10
.5¥10
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
4.5
From equation (v), one can conclude that
kobs = kK K [MA].
Ð4
Ð4
Ð4
9.78
14.5
(vi)
1
2
1
0.0¥10
19.58
2.24
4.48
6.74
9.01
Ð4
Equation (v) shows that the reaction follows se-
cond order kinetics, first order each in aldehyde
and MA. The observed pseudo-first order rate
constant (kobs) is therefore a product of the equi-
librium constants of the first two steps and the rate
constant (k) of the third step. The rate constant k
could not be evaluated since no data are available
for the equilibrium constants K and K .The sec-
0
0
0
0
.01
.01
.01
.01
2.5¥10
Ð4
Ð4
Ð4
5.0¥10
7.5¥10
10.0¥10
dration of compound I, both steps being consistent
with a second order equation. The study of the
stoichiometry of the reaction indicates that the
products have a maximum absorption at a molar
ratio of 1:1 (aldehyde:MA).
The kinetic results in Table 1 show that the rate
of the reaction between different substituted benz-
aldehydes and (MA) decreases in the following or-
der:
1
2
ond order rate constants were calculated using the
equation, k = kobs/[MA].
2
The rate constants of the reaction of different
substituted aldehydes at 298 K correlate well with
Hammett’s σ values (Hammett’s polar substituent
constants) for aldehyde, as shown in Fig. 1. The
positive ρ value (1.226) (r = 0.9449) means that
electron-withdrawing groups enhance the rate of
the reaction.
p-NO > p-Cl > H > p-OCH > p-OH > p-N(CH )
3 2
2
3
It can be noted that electron-withdrawing
groups accelerate and electron- releasing groups
retard the rate of reaction. In fact, electron-with-
Activation parameters
The rates of the reaction of six different benz-
drawing groups favor the addition of MA to sub- aldehydes were determined at 298, 303, 313 and
#
stituted benzaldehydes (first step), and electron- 323 K. The activation energies ∆E were deter-
releasing ones favor the dehydration step (second mined from the slopes of the Arrhenius plots of
step). The results for the reaction obtained can
then be rationalized considering the first step is
the rate-limiting one and the second step is the
faster.
Therefore, the reaction process in the presence
of piperidine (Pip) as a base, can be represented
by equations (i)Ð(iii) where Ald, MA and I refer
to the aldehyde, Meldrum’s acid and the interme-
diate I in Scheme 1 respectively:
K1
MAÐ + H Pip
+
(i)
MA + Pip
?
fast
K2
+
+
Ald + H Pip
?
AldH + Pip
(ii)
fast
k
k
Ð
+
Fig. 1. Hammett plot for the reaction of substituted
benzaldehydes with Meldrum’s acid.
MA + AldH 5 5 product (iii)
I
slow
fast
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322
H. A. A. Medien · Kinetic Studies of Condensation of Aromatic Aldehydes with Meldrum’s Acid
Table 2. Rate constants and the activation parameters of the reaction of substituted benzaldehydes with Meldrum’s
acid (MA) using 0.1 M piperidine in chloroform.
2
Ð1 Ð1
∆E#
∆H#
∆G#
∆S#
Substituent
10 k
2
, l·mol
s
cal mol KÐ1
Ð1
2
98 K
308 K
313 K
323 K
kcal·molÐ1
p-NO
p-Cl
H
2
9.79
7.6
14.2
13.2
12.4
11.7
7.50
6.50
16.9
16.0
15.6
15.4
12.0
11.0
23.6
22.9
21.6
20.8
16.2
14.2
8.34
8.42
8.66
8.83
9.06.
9.70
7.74
7.83
8.07
8.25
8.46
9.09
19.07
18.90
18.95
19.19
19.13
19.46
Ð38.02
Ð38.57
Ð36.52
Ð36.72
Ð35.77
Ð34.80
7.0
p-OCH
p-OH
3
6.64
5.2
4.24
p-N(CH
3
)
2
log k versus TÐ1. The activation parameters ∆H#,
2
#
#
∆
S and ∆G were calculated at 298 K and are
given in Table 2.
The entropy of activation values are negative as
expected for bimolecular or ter-molecular reac-
#
tions. In this study the entropy of activation (∆S )
is substituent dependent. Since all the substituents
are located in para position, direct steric interac-
tion is unlikely, so that resonance and/or inductive
effects are the operating factors. Within the series,
electron-withdrawing substituents in the aromatic
aldehyde, through their resonance and/or induc-
tive effects, localize the formal charge in the tran-
sition state. This favours the formation of more
ordered transition states reflecting high and nega-
tive entropies of activation. On the other hand, the
presence of electron releasing substituents in the
#
#
aldehyde alters their nature, leading to much more Fig. 2. The linear relationship between ∆H and ∆S for
delocalization of the charge in the transition state,
and hence reflecting low and negative entropies
the reaction of Meldrum’s acid with different aromatic
aldehydes.
of activation.
The linear correlation between ∆H# and ∆S#
(Fig. 2) for the reaction of different substituents of
The mechanism for the title reaction is common
benzaldehyde with MA shows that all the com- for all members in the series as indicated by
#
#
pounds react by the same mechanism [26]. The (a) the good straight-line plot of ∆H versus ∆S ,
value of the slope was 403.5 K (r = 0.989), which (b) the linear plots of log k at 50 ∞C against log
2
is the value of the isokinetic temperature ꢀ (the k2 at 25 ∞C (gradient = 0.658, r = 0.95).
temperature at which the substituent effects are
At the studied temperatures, an electron-with-
supposed to be reversed). This value is not the real drawing substituent in the para position of the al-
temperature used in the kinetic runs. dehyde increases the reaction rates, while a para
#
The constancy of ∆G may be explained on the positioned electron releasing or donating substitu-
basis of an isokinetic relationship that exists for a ent decreases it. This behaviour could be ex-
series of compounds of slightly different structures plained by the observation that the ratio kp-NO2/
but undergoing reaction essentially by the same kp-N(CH3)2 which was 2.31, 2.18, 1.53, and 1.66 at 25,
#
mechanism. The ∆G may be more or less con- 35, 40, and 50 ∞C, respectively (Table 2). Conse-
stant with relative changes in ∆H# and ∆S as quently, the variation of the rate constant depends
#
pointed out by Leffler [26].
on the nature of the substituent in the aldehyde. The
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H. A. A. Medien · Kinetic Studies of Condensation of Aromatic Aldehydes with Meldrum’s Acid
1323
order of decreasing reactivity of substituted alde-
hyde in chloroform is p-NO > p-Cl > H > p-OCH3
2
>
p-OH > p-N(CH3)2. This order of decrease in
magnitude of the substituent effect is expected on
the basis of both inductive and resonance effects.
Quantitative Determination of the Aldehydes
Tested
The absorbance of the color due to reaction of
the aldehydes tested with excess MA in the pres-
ence of 0.1 M piperidine in chloroform had
reached a maximum intensity after 15 min at
50 ∞C.
Table 3 shows a linear regression analysis of the
data obtained from calibration graphs of the alde-
hydes tested from the relation A = a + bc, where Fig. 3. The calibration graph of p-dimethylaminobenzal-
dehyde through its reaction with Meldrum’s acid in chlo-
A is the absorbance at the relevant peak in a
roform in the presence of piperidine as a
1.0 cm quartz cell, a and b are the intercept and
catalyst.
the slope of the calibration graphs, respectively,
and c is the concentration of the aldehyde (µg/ml)
in the final solution. The points on standard cali-
bration curves each represent the outcome of
three determinations. The relative standard devia-
Determination of aromatic aldehydes without
catalyst in water
tions (r.s.d.) vary between 0.69 and 1.0% for
The use of water as a solvent in organic chemis-
Ð1
10 µg ml of each aldehyde used (n = 3), and the try was rediscovered in the 1980s by Breslow [27,
reproducibility varies between 95.8 and 101% for 28] who showed that hydrophobic effects could
each aldehyde. Fig. 3 shows the relationship be- strongly enhance the rate of several organic reac-
tween absorbance vs. concentration (calibration tions. Previously the scant solubility of the reac-
graph) for p-dimethylaminobenzaldehyde as an tants was the main reason that ruled this solvent
example.
out from studies. Bigi et al [29] have investigated
No interference is caused by the presence of the condensation reaction of Meldrum’s acid and
formaldehyde (10 mM), acetone (5.17 mM), ben- aldehydes as a clean synthesis in water.
zoic acid (2.46 mM), acetaldehyde (6.82 mM) and
In the present investigation, the concentrations
aniline (3.2 mM). In summary, thirteen aromatic of three aromatic aldehydes (N,N-dimethylamino-
aldehydes have been shown to be assessable with benzaldehyde, 3,5-dimethoxy-4-hydroxybenzalde-
a new simple analytical method.
hyde and p-chlorobenzaldehyde) were determined
Table 3. Statistical analysis of the calibration graphs for the determination of some aromatic aldehydes by reaction
with Meldrum’s acid (M) in chloroform at 50 ∞C in the presence of piperidine.
Aldehydes
λ
max
Concentration
range [µg/ml]
ε [l molÐ1 cmÐ1
]
Slope (b) Intercept (a)
[
nm]
p-Dimethylamino-benzaldehyde
445
365
365
370
340
340
325
360
1.49-14.9
9.1-91
21327.27
3030
2199
4015
1648
1000
569
0.1431
0.0167
0.016
-0.03
-0.009
0.0087
0.021
-0.003
0.00
3
,5-Dimethoxy-4-hydroxybenzaldehyde
p-Methoxybenzaldehyde
p-Hydroxybenzaldehyde
p-Nitrobenzaldehyde
m-Nitrobenzaldehyde
p-Chlorobenzaldehyde
Benzaldehyde
6.8-68
6.1-61
0.033
7.55-75.5
7.55-75.5
7.025-70.25
5.3-53
0.0109
0.0066
0.004
0.0033
0.00
1000
0.0063
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H. A. A. Medien · Kinetic Studies of Condensation of Aromatic Aldehydes with Meldrum’s Acid
with Meldrum’s acid in water at 75 ∞C after 2 h
without adding any catalyst. A calibration graph
Ð3
for 5¥10 M each of the aldehydes, was obtained
by transferring (0.1Ð1.0 ml) aliquots in water to
test tubes followed by the addition of 1.0 ml of the
0.1 M solution of Meldrum’s acid (MA) in water.
The mixtures were thermostated at 75 ∞C in a
water bath for 2 h and then diluted to 10 ml with
water, and the absorbance of the resulting yellow
color was measured at the maximum wavelength
of each aldehyde derivative.
The points on standard calibration curves each
represent the outcome of three determinations.
Table 4 shows a linear regression analysis of the
data obtained from calibration graphs of the alde-
hydes tested from the relation A = a + bc. Fig. 4
shows the relationship between absorbance vs.
concentration (calibration graph) for p-dimethyl-
aminobenzaldehyde as an example.
Fig. 4. The calibration graph for p-dimethylaminobenzal-
dehyde through its reaction with Meldrum’s acid in
water without catalyst.
In conclusion it is shown that the determination
of aldehydes with Meldrum’s acid efficiently oc-
curs in water as a solvent without adding any cata-
lyst, by heating at 75 ∞C for 2 h. Thus this reaction
can be used to remove aldehydes from wastewater
produced in the fine chemicals industry.
a solvent. The standard solution of piperidine
(
E. Merck) was directly prepared in chloroform.
Kinetic procedure
The rate of reaction of the aldehydes tested (p-
dimethylamino-benzaldehyde,
p-nitrobenzalde-
Materials and Methods
hyde, p-hydroxybenzaldehyde, p-methoxybenz-
aldehyde, p-chlorobenzaldehyde, and benzalde-
hyde) with MA in the presence of piperidine was
followed by monitoring the increase in the absorb-
ance of the product, at its wavelength of maximum
absorption, with time.
Apparatus
Visible absorption spectra were recorded on a
Unicam SP 1800 recording spectrometer. A SP 6Ð
2
00 Pye-Unicam spectrophotometer was used for
the kinetic spectrophotometry. IR spectra were re-
corded (KBr) on a Pye Unicam SP 1000 spectro-
Ð3
Freshly prepared samples of aldehydes (5¥10
1
M), piperidine (0.1 M) and 0.1 M MA (0.1 M) in
chloroform were allowed to equilibrate to 25 ∞C
for 10 min in a thermostat. Mixing was carried out
by withdrawing 0.5Ð2.0 ml aliquots of the alde-
hyde solution to 10 ml flasks containing 1.0 ml of
photometer. H NMR spectra were taken at 270
MHz using CDCl as a solvent.
3
Reagents
All aromatic aldehydes used were of AR grade 0.1 M piperidine, 1.0 ml of 0.1 M MA solution, and
(
BDH). The Meldrum’s acid used was of AR the appropriate volume of chloroform. A portion
grade (Aldrich). Chloroform (Analar) was used as of the reaction mixture was transferred to a quartz
Table 4. Statistical analysis of the calibration graphs for the determination of some aromatic aldehydes by reaction
with Meldrum’s acid in water at 75 ∞C for 2 h without catalyst.
Aldehydes
λ
[
max Concentration ε [l molÐ1 cmÐ1
nm] range [µg/ml]
]
Slope (b) Intercept (a)
r
p-Dimethylamino-benzaldehyde
445
7.45Ð74.5
9.1Ð91
7.025Ð70.25
2247.88
1564
413
0.0150
0.0086
0.003
0.0053
0.002
-0.001
0.995
0.992
0.975
3
,5-Dimethoxy-4-hydroxybenzaldehyde 365
p-Chlorobenzaldehyde
325
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H. A. A. Medien · Kinetic Studies of Condensation of Aromatic Aldehydes with Meldrum’s Acid
1325
cuvette placed in a thermostated (25 ∞C) cell com-
The first order rate constants were also calcu-
partment. A blank solution was similarly prepared lated from the plots of log (A Ð A ) versus time
ϱ
t
without aldehyde and the absorbance readings (where A and A are the values of the absorbance
ϱ
t
were also recorded and followed as a function of at the end of the reaction and at time t respec-
time. Other set of runs were carried out in a sim- tively) which are in good agreement with those
Ð3
ilar manner using MA (5¥10 M) and aldehyde values calculated from the initial rate method indi-
(
0.1 M) solution.
cated above. The second order rate constants k2
Initial rates were determined by the method of calculated from the equation k = kobs/[MA].
2
initial rates [30, 31], whereby the absorbance (A)
was plotted against time for each run. The initial
slopes (dA/dt) for each run were determined, and
Analytical Procedure
1.0 ml of Meldrum’s acid solution (0.1 M) and
used in the following equation for calculating the 1.0 ml of a 0.1 M piperidine solution in chloroform
rate of the reaction [30, 31].
were transferred into 10 ml volumetric flasks. 0.1Ð
Ð3
1
.0 ml aliquots of stock solution (5¥10 M) of
dA C Ð C
ϱ
i
the aldehydes (p-nitrobenzaldehyde, p-hydroxy-
benzaldehyde, p-methoxybenzaldehyde, 3,5-di-
rate =
¥
dt A Ð A
ϱ
i
methoxy-4-hydroxybenzaldehyde,
m-nitrobenz-
Ci and C are the initial and final concentra-
ϱ
aldehyde, benzaldehyde, p-chloro-benzaldehyde,
tions of the colored reaction products respectively,
calculated by using the molar extinction coeffi-
cients ε, A and A are the absorbencies of the
Ð3
1
0
M
p-dimethylaminobenzaldehyde)
was
added.
i
ϱ
The mixtures were thermostated at 50 ∞C in a
water bath for 15 min and then diluted to 10 ml
with chloroform. The absorbance of the resulting
yellow color at the maximum wavelength of each
aldehyde derivative was measured against an ap-
propriate blank. The calibration graphs were con-
structed by plotting the final concentration of each
aldehyde against the absorbance values. The
colored species at the initial and the end of the
reaction. Since both C and A are nearly equal to
i
i
zero at the beginning of the reaction, the above
equation reduced to:
dA Cϱ
t rate =
¥
dt Aϱ
The observed rate constants kobs using excess points on standard calibration graphs each repre-
MA, calculated from the initial rate (R ) values by sent the outcome of three determinations.
i
dividing it by the concentrations of the aldehydes.
Preparation and analysis of reaction products
The method of initial rates was employed in or-
der to avoid complications from product decom-
To a solution of MA (0.004 mol) in chloroform,
position at high catalyst concentrations. Duplicate aromatic aldehyde (0.004 mol) was added. To the
runs were made at each concentration. Prelimi- reaction mixture 5 ml of 0.1 M piperidine were
nary reaction orders were determined from slopes added. The reaction mixture was warmed at 50 ∞C
of log (initial rate) versus log (concentration) for 5 min and left at 25 ∞C overnight. The crystals
plots. Because of the difficulty in estimating ionic obtained were filtered off, dried and crystallized
strength in chloroform, the influence of ionic from chloroform. The physical properties, spectra
strength was not investigated.
and elemental analyses are given in Table 5.
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H. A. A. Medien · Kinetic Studies of Condensation of Aromatic Aldehydes with Meldrum’s Acid
[
1] B. C. Chen, Heterocycles 32, 529 (1991).
2] B.Pita, E. Sotelo, M. Suarez, E. Ravina, E. Ochoa,
[16] T. R. Reddy, G. S. S. Murthy, V. Jagannadham, Oxid.
[
Commun. 9, 83 (1986).
Y. Verdecia, H. Novoa, N. Blaton, C. de Ranter, [17] S. Venkateshwarlu, V. Jagannadham, Oxid. Com-
O. M.Peeters, Tetrahedron 56, 2473 (2000).
3] A. J. Kadam, U. V. Desai, R. B. Mane, J. Labelled [18] S. Anandan, R. Gopalan, J. Indian Chem. Soc. 62,
Comp. Radiopharm. 42, (1999) 835. 216 (1985).
mun. 8, 149 (1986).
[
[
[
[
[
[
[
4] R. Bolton, N. P. Gates, A. W. S. Jones, Aust. J. [19] K. K. Banerji, J. Org. Chem. 51, 4767 (1986).
Chem. 43, 293 (1990).
[20] S. C. Pati, A. K. Sahu, Y. Sriramulu, Curr. Sci. 57,
325 (1988).
[21] S. C. Pati, A. K. Sahu, Y. Sriramulu, Indian J. Chem.
26, 693 (1987).
[22] K. Ganapathy, R. Gurmurthy, N. Moham, G. Siva-
gnanam, Acta Science Indica Chem. 12, 11 (1986).
[23] P. Oswal, S. L. Oswal, H. B. Naik, Proc. Indian Natl.
Sci. Acad. 53, 200 (1987).
5] H. A. A. Medien, A. W. Erian, Microchem. J. 65,
3
1 (2000).
6] H. A. A. Medien, A. W. Erian, Microchem. J. 65,
37 (2000).
7] H. A. A. Medien, M. M. H. Khalil, Anal. Lett. 31,
51 (1998).
2
4
8] B. I. M. Costa, R. Budal, R. A. Yunes, J. Chem. Soc.,
Perkin Trans. 2 11, 1861 (1991).
[24] A. W. Erian, S. M. Sherif, A. A. Alassar, Y. M. Elk-
holy, Tetrahedron 50, 1877 (1994).
[25] A. W. Erian, Chem. Rev. 93, 1991 (1993).
9] M. K. Pillay, A. Abdoul Jameel, Indian J. Chem.,
Sect. A 31, 46 (1992).
[
[
[
[
10] L. A. P. Rodriquez , F. M. Gutierrez, F. P. Mata, Z. [26] J. E. Leffler, J. Org. Chem. 20, 1202 (1955).
Phys. Chem. 267, 689 (1986).
[27] R. Breslow, D. C. Rideout, J. Am. Chem. Soc. 102,
7816 (1980).
11] T. A. Gorshkova, Y. Amadu, M. A. Volodina, A. N.
Kashin, Zh. Anal. Khim. 41, 1253 (1986).
12] P. Ruiperez, J. Vera, F. O. Martinez, An. Quin. 81,
[28] L. Chao-Jun, C. Tak-Hang, Organic Reactions in
Aqueous Media, J. Wiley & Sons, New York (1997).
[29] F. Bigi, S. Carloni, L. Ferrari, R. Maggi, A. Mazza-
cani, G. Sartori, Tetrahedron Lett. 42, 5203 (2001).
[30] P. W.Atkins, Physical Chemistry, Oxford University
Press, Oxford (1979).
2
9 (1985).
13] K. K. Gupta, D. Sanghamitra, S. Gupta, M. Adhi-
kari, A. Banerjee, Tetrahedron 46, 2431 (1990).
14] H. A. A. Medien, Anal. Lett. 27, 2727 (1994).
15] A. L. Baurnstark, M. Beeson, P. C. Vasqruez, Tetra- [31] P. Stevens, M. A. D. Phil, Chemical Kinetics, Chap-
hedron Lett. 30, 5567 (1989). man & Hall, London (1970).
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