Dual-parameter correlations on rate of dehydration step of a condensation reaction in aqueous solutions of ethanol, propan-2-ol and 2-methylpropan-2-ol

Article abstract of DOI:10.1002/poc.438

The reaction kinetics of the dehydration step of the condensation reaction between phenylhydrazine and benzaldehyde was studied spectrophotometrically in aqueous solutions of ethanol, propan-2-ol and 2-methylpropan-2-ol at pH 11.5 at 25 °C. The apparent second-order rate constants k_2app of the reaction increase with mole fraction of water in all the aqueous solutions. Single-parameter correlations of log k_2app versus π* (dipolarity/polarizability), α (hydrogen-bond donor acidity) and E_T^N (normalized polarity parameter) were obtained in all the aqueous solutions. In all cases, the correlations versus E_T^N and π* are acceptable, but correlations versus α are poor (e.g. in aqueous solutions of ethanol the correlation coefficients are 0.964, 0.967, and 0.751 respectively). The results of dual-parameter correlations of log k_2app versus π* and α in all cases represent improvements with regard to the single-parameter models (in aqueous solutions of ethanol r = 0.995; s.e. = 0.038; n = 13). The apparent second-order rate constants of the reaction increase with α, π* and E_T^N. Increasing the hydrogen-bond donor acidity of the solvent stabilizes the activated complex of the reaction via hydrogen-bond formation with the intermediate. A dual-parameter equation of log k_2app versus π* and α was obtained for the combined data set of aqueous solutions (n = 35, r = 0.989, s.e. = 0.062, F_2.32 = 719.21), in which π* has the major effect on the reaction rate relative to α. Copyright

Full text of DOI:10.1002/poc.438

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 884–889
DOI:10.1002/poc.438
Dual-parameter correlations on rate of dehydration step of
a condensation reaction in aqueous solutions of ethanol,
propan-2-ol and 2-methylpropan-2-ol
A. Habibi Yangjeh,1* M. R. Gholami2 and R. Mostaghim3
1Faculty of Science, University of Mohaghegh Ardebili, PO Box 179, Ardebil, Iran
2Department of Chemistry, Sharif University of Technology, PO Box 11365-9516, Tehran, Iran
3Islamic Azad University, Central Tehran, Department of Science, Tehran, Iran
Received 26 March 2001; revised 12 June 2001; accepted 2 July 2001
ABSTRACT: The reaction kinetics of the dehydration step of the condensation reaction between phenylhydrazine
epoc
and benzaldehyde was studied spectrophotometrically in aqueous solutions of ethanol, propan-2-ol and 2-
methylpropan-2-ol at pH 11.5 at 25°C. The apparent second-order rate constants k2app of the reaction increase with
mole fraction of water in all the aqueous solutions. Single-parameter correlations of log k2app versus p* (dipolarity/
polarizability), a (hydrogen-bond donor acidity) and ETN (normalized polarity parameter) were obtained in all the
aqueous solutions. In all cases, the correlations versus ETN and p* are acceptable, but correlations versus a are poor
(e.g. in aqueous solutions of ethanol the correlation coefficients are 0.964, 0.967, and 0.751 respectively). The results
of dual-parameter correlations of log k2app versus p* and a in all cases represent improvements with regard to the
single-parameter models (in aqueous solutions of ethanol r = 0.995; s.e. = 0.038; n = 13). The apparent second-order
rate constants of the reaction increase with a, p* and ETN. Increasing the hydrogen-bond donor acidity of the solvent
stabilizes the activated complex of the reaction via hydrogen-bond formation with the intermediate. A dual-parameter
equation of log k2app versus p* and a was obtained for the combined data set of aqueous solutions (n = 35, r = 0.989,
s.e. = 0.062, F2,32 = 719.21), in which p* has the major effect on the reaction rate relative to a. Copyright  2001 John
Wiley & Sons, Ltd.
Additional material for this paper is available from the epoc website at http://www.wiley.com/epoc
KEYWORDS: dual-parameter correlation; solvent effect; condensation reaction; polarity parameter; dipolarity/
polarizability; hydrogen-bond donor acidity; kinetics
INTRODUCTION
of liquid water is dominated by strong intermolecular
hydrogen bonding. The complexity of the chemistry of
life in vivo, which occurs primarily in an aqueous
environment, should encourage chemists to focus more
attention on the reactions in aqueous solutions. There-
fore, there is a continuing challenge to understand the
role of water in these processes. In many cases, these
reactions involve complicated and large organic mol-
ecules. However, we know that our understanding of
these phenomena can be improved by studying in depth
the rates of relatively simple organic reactions in aqueous
solutions.4–8
In addition to the nonspecific Coulombic, inductive
and dispersion interactions, other interactions, such as
specific hydrogen bonding, electron pair donor–acceptor
and solvophobic interactions, may play a part in the
solvent effects.9–18 The problem is to identify and to
assess the relative importance of these various factors on
the reaction rate and other chemical and physical
processes.
The considerable influence of solvents on chemical and
physical processes (reaction rates, selectivity, chemical
equilibria, position and intensity of spectral absorption
bands) has been well established.1 Modeling of solvent
effects is one of the most useful methods to obtain
information about the mechanism of organic reactions.1
The study of physicochemical properties that depend on
solute–solvent interactions is much more complex in
mixed than in pure solvents. On the one hand, the solute
can be preferentially solvated by any of the solvents
present in the mixture. On the other hand, solvent–
solvent interactions can strongly affect solute–solvent
interactions.2 The complexity of even the simplest
reactions in aqueous solutions is significantly greater
than those of reactions in organic solvents.3 The structure
*Correspondence to: A. Habibi Yangjeh, Faculty of Science,
University of Mohaghegh Ardebili, PO Box 179, Ardebil, Iran.
E-mail: gholami@ch.sharif.ac.Ir
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 884–889

DUAL-PARAMETER CORRELATIONS ON RATE OF DEHYDRATION
885
Table 1. Apparent second-order rate constants of the
reaction in aqueous solutions of ethanol at 25°Ca
Xw
104 k2app (dm3 molÀ1 sÀ1
)
ETN
a
p*
0.266
0.449
0.583
0.685
0.765
0.830
0.850
0.884
0.900
0.929
0.967
0.984
1.000
5.01 Æ 0.17
6.91 Æ 0.21
7.80 Æ 0.18
8.49 Æ 0.27
12.8 Æ 0.24
18.2 Æ 0.34
20.0 Æ 0.37
27.8 Æ 0.41
31.6 Æ 0.45
33.0 Æ 0.52
39.5 Æ 0.48
46.4 Æ 0.74
49.4 Æ 0.72
0.673
0.702
0.717
0.731
0.761
0.784
0.800
0.842
0.861
0.907
0.959
0.980
1.000
0.93
0.89
0.89
0.88
0.86
0.87
0.90
0.92
0.97
1.05
1.17
1.21
1.26
0.66
0.75
0.81
0.88
0.96
1.03
1.06
1.10
1.11
1.12
1.13
1.13
1.14
Scheme 1
In continuing our study into solvent effects on organic
reactions,12–15 we were interested in solvent effects on
reaction kinetics of condensation reaction of amines with
aldehydes. Surprisingly, for these reactions little atten-
tion has been paid to the study of solvent effects.19–21
Phenylhydrazone formation from benzaldehyde, like
many condensation reactions, occurs with rate-determin-
ing attack of the nucleophile reagent under acidic
conditions and rate-determining dehydration of the
carbinolamine intermediate under neutral and basic
conditions (Scheme 1).22–26 Each of these steps may be
catalyzed by acids and (or) bases (general and speci-
fic).25,26 At pH >10, dehydration of the intermediate of
the reaction is the rate-determining step. In the reaction
there is no base catalysis.27
a
At least three runs were averaged. Errors are standard deviations.
Table 2. Apparent second-order rate constants of the
reaction in aqueous solutions of propan-2-ol at 25°Ca
Xw
104 k2app (dm3 molÀ1 sÀ1
)
ETN
a
p*
The purpose of our work is to identify the medium
effects on the dehydration step of the condensation
reaction, so our study was carried out at pH 11.5. In these
conditions we have:
0.321
0.516
0.646
0.740
0.810
0.825
0.865
0.909
0.925
0.945
0.975
1.000
2.89 Æ 0.08
3.72 Æ 0.10
3.84 Æ 0.12
5.06 Æ 0.16
7.14 Æ 0.19
8.91 Æ 0.18
11.3 Æ 0.24
18.1 Æ 0.25
22.4 Æ 0.35
31.5 Æ 0.32
37.1 Æ 0.48
49.4 Æ 0.72
0.583
0.639
0.667
0.685
0.714
0.719
0.740
0.790
0.833
0.901
0.957
1.000
0.78
0.81
0.81
0.82
0.82
0.82
0.82
0.85
0.88
0.96
1.12
1.26
0.65
0.72
0.77
0.81
0.87
0.89
0.95
1.06
1.12
1.14
1.15
1.14
rate ˆ k2‰IntermŠ
ꢀ1†
ꢀ2†
‰IntermŠ ˆ k1‰AldŠ‰HydŠ=ꢀk ‡ k2†
À1
Because k2 ꢁ kÀ1, then:
rate ˆ k2k1=k ‰AldŠ‰HydŠ ˆ k2K1‰AldŠ‰HydŠ ꢀ3†
À1
a
At least three runs were averaged. Errors are standard deviations.
ˆ k2app‰AldŠ‰HydŠ
ꢀ4†
in which K1 and k2app are equilibrium constant of the first
step and the apparent second-order rate constant of the
reaction, respectively. Solvent effects on the kinetics of
the dehydration step of the condensation reaction of
phenylhydrazine with benzaldehyde were studied in
aqueous solutions of ethanol, propan-2-ol and 2-methyl-
propan-2-ol at pH 11.5 at 25°C.
Table 3. Apparent second-order rate constants of the
reaction in aqueous solutions of 2-methylpropan-2-ol at
25°Ca
Xw
104 k2app (dm3 molÀ1 sÀ1
)
ETN
a
p*
0.369
0.568
0.692
0.778
0.800
0.840
0.888
0.900
0.925
0.955
0.979
1.000
1.51 Æ 0.05
3.09 Æ 0.10
3.63 Æ 0.09
4.27 Æ 0.08
5.01 Æ 0.11
5.80 Æ 0.10
8.34 Æ 0.12
10.0 Æ 0.15
13.4 Æ 0.17
27.8 Æ 0.24
36.6 Æ 0.27
49.4 Æ 0.72
0.546
0.600
0.631
0.653
0.660
0.672
0.701
0.710
0.741
0.846
0.861
1.000
0.69
0.74
0.78
0.80
0.81
0.81
0.82
0.83
0.85
0.90
1.09
1.26
0.63
0.69
0.73
0.77
0.78
0.81
0.86
0.88
0.96
1.10
1.16
1.14
RESULTS AND DISCUSSION
The apparent second-order rate constants for the reaction
were obtained in aqueous solutions of ethanol, propan-2-
ol and 2-methylpropan-2-ol in pH 11.5 at 25°C (Tables
1–3).
As can be seen, the rate constants of the reaction
increase rapidly with the mole fraction of water. The
results show that the increase of the reaction rate in
a
At least three runs were averaged. Errors are standard deviations.
J. Phys. Org. Chem. 2001; 14: 884–889
Copyright  2001 John Wiley & Sons, Ltd.

886
A. HABIBI YANGJEH, M. R. GHOLAMI AND R. MOSTAGHIM
Figure 1. Plot of apparent second-order rate constants of the
reaction versus mole fraction of water in aqueous solutions
of ethanol at 25°C
Figure 2. Plot of apparent second-order rate constants of the
reaction versus mole fraction of water in aqueous solutions
of propan-2-ol at 25°C
aqueous solutions of ethanol is higher than those of
propan-2-ol and 2-methylpropan-2-ol (Figs 1–3).
The normalized polarity parameter ETN of the aqueous
solutions increases with mole fraction of water in
aqueous solutions of the alcohols. At the same mole
fractions of water, ETN of aqueous solutions follows the
sequence ethanol > propan-2-ol > 2-methylpropan-2-
ol.28
dipolarity/polarizability and hydrogen-bond donor acid-
ity.29–31 Because in the intermediate of the reaction the
OH group will be eliminated, it might be expected that
formation of a hydrogen bond to the leaving group would
increase the propensity for elimination. However, single-
parameter correlations of log k2app with a do not give
satisfactory results in all aqueous solutions (Table 4).
Linear correlations of log k2app with p* (dipolarity/
polarizability) give good results for all three types of
aqueous solution:
Single-parameter correlations of log k2app versus ETN in
aqueous solutions of ethanol, propan-2-ol and 2-methyl-
propan-2-ol give acceptable results:
EtOH--H2O
log k2app ˆ À4:718ꢀÆ0:159† ‡ 2:002ꢀÆ0:158†ꢀÃ
ꢀ9†
EtOH--H2O
log k2app ˆ À5:182ꢀÆ0:206† ‡ 2:968ꢀÆ0:248†ETN
ꢀ5†
ꢀn ˆ 13; r ˆ 0:967; s.e. ˆ 0:090; F1;11 ˆ 160:37†
ꢀn ˆ 13; r ˆ 0:964; s.e. ˆ 0:095; F1;11 ˆ 144:79†
2-PrOH--H2O
log k2app ˆ À5:108ꢀÆ0:144† ‡ 2:288ꢀÆ0:150†ꢀà ꢀ10†
ꢀn ˆ 12; r ˆ 0:979; s.e. ˆ 0:090; F1;10 ˆ 231:45†
2-PrOH--H2O
log k2app ˆ À5:396ꢀÆ0:162† ‡ 3:169ꢀÆ0:208†ETN
ꢀ6†
ꢀn ˆ 12; r ˆ 0:979; s.e. ˆ 0:090; F1;10 ˆ 231:61†
t-BuOH--H2O
log k2app ˆ À5:619ꢀÆ0:197† ‡ 3:537ꢀÆ0:270†ETN
ꢀ7†
ꢀn ˆ 12; r ˆ 0:972; s.e. ˆ 0:114; F1;10 ˆ 171:72†
By combining all the data for the aqueous solutions the
following correlation of log k2app with ETN was obtained:
log k2app ˆ À5:518ꢀÆ0:109† ‡ 3:381ꢀÆ0:142†ETN
ꢀ8†
ꢀn ˆ 35; r ˆ 0:972; s.e. ˆ 0:097; F1;33 ˆ 568:08†
All of the above correlations show that the rate of the
reaction increases with increasing normalized polarity
parameter of the media. We concluded that the polarity of
the activated complex of the reaction for the dehydration
step is higher than that of the intermediate of the reaction.
The normalized polarity parameter is a blend of
Figure 3. Plot of apparent second-order rate constants of the
reaction versus mole fraction of water in aqueous solutions
of 2-methylpropan-2-ol at 25°C
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 884–889

DUAL-PARAMETER CORRELATIONS ON RATE OF DEHYDRATION
887
Table 4. Regression coef®cients and slopes of solvent parameters in aqueous solutions of ethanol, propan-2-ol, 2-
methylpropan-2-ol and for all the aqueous solutions at 25°C
Aqueous solution
Ethanol
ETN
p*
a
Intercept
r
2.968
–
2.002
–
1.659
–
2.288
–
1.872
–
2.576
–
2.327
–
2.298
–
1.870
–
–
À5.182
À4.718
À4.520
À5.056
À5.396
À5.108
À5.055
À5.351
À5.619
À5.333
À5.330
À5.393
À5.518
À5.078
À5.245
À5.362
0.964
0.967
0.751
0.995
0.979
0.979
0.822
0.994
0.972
0.991
0.890
0.993
0.972
0.975
0.831
0.989
–
–
1.813
0.688
–
–
Propan-2-ol
3.169
–
–
–
2.340
0.708
–
–
2-methylpropan-2-ol
All aqueous solutions
3.537
–
–
–
2.604
0.321
–
–
3.381
–
–
–
–
2.555
0.758
t-BuOH--H2O
t-BuOH--H2O
log k2app ˆ À5:393ꢀÆ0:104† ‡ 2:327ꢀÆ0:213†ꢀÃ
log k2app ˆ À5:333ꢀÆ0:097† ‡ 2:576ꢀÆ0:109†ꢀà ꢀ11†
‡ 0:321ꢀÆ0:240†ꢁ
ꢀ15†
ꢀn ˆ 12; r ˆ 0:991; s.e. ˆ 0:064; F1;10 ˆ 560:18†
ꢀn ˆ 12; r ˆ 0:993; s.e. ˆ 0:062; F2;9 ˆ 303:07†
As anticipated, a good correlation between log k2app and
p* exists for the aqueous solutions’ combined data set:
A dual-parameter correlation of log k2app versus p* and a
was obtained for the aqueous solutions’ combined data
set:
log k2app ˆ À5:078ꢀÆ0:086† ‡ 2:298ꢀÆ0:091†ꢀà ꢀ12†
ꢀn ˆ 35; r ˆ 0:975; s.e. ˆ 0:092; F1;33 ˆ 632:78†
log k2app ˆ À5:362ꢀÆ0:073† ‡ 1:870ꢀÆ0:091†ꢀÃ
‡ 0:758ꢀÆ0:118†ꢁ
ꢀ16†
These equations show that with increasing dipolarity/
polarizability of the medium the reaction rate increases,
because the activated complex of the reaction is
stabilized with increasing dipolarity/polarizability of
media. In order to show the relative effects of
dipolarity/polarizability p* and hydrogen-bond donor
acidity a on the reaction rate, dual-parameter correlations
of log k2app versus a and p* were obtained in aqueous
solutions of ethanol, propan-2-ol and 2-methylpropan-2-
ol. These correlations gave interesting results:
ꢀn ˆ 35; r ˆ 0:989; s.e. ˆ 0:062; F2;32 ˆ 719:22†
The regression coefficients and standard deviations of the
equations are satisfactory. As can be seen, dipolarity/
polarizability has the major effect on the reaction rate. In
fact, in the dehydration step of the reaction, the polarity
of the intermediate increases with progressing of the
reaction; as a result, the activated complex leading to the
product of the reaction is expected to be favored by an
increasing dipolarity/polarizability of the media. The
solvent also stabilizes the activated complex of the
dehydration step of the reaction by hydrogen-bond
interactions.
In order to show the efficiency of the suggested dual-
parameter equation, the experimental values of log k2app
versus the calculated values from Eqn. (16) are plotted
for all the aqueous solutions of alcohols (Fig. 4).
As can be seen, the experimental and calculated values
of log k2app are in good agreement for all aqueous
solutions. A plot of %residual versus experimental values
of log k2app shows that the suggested dual-parameter
equation is statistically acceptable, because the distribu-
EtOH--H2O
log k2app ˆ À5:056ꢀÆ0:082† ‡ 1:659ꢀÆ0:082†ꢀÃ
‡ 0:688ꢀÆ0:096†ꢁ
ꢀ13†
ꢀn ˆ 13; r ˆ 0:995; s.e. ˆ 0:038; F2;10 ˆ 472:87†
2-PrOH--H2O
log k2app ˆ À5:351ꢀÆ0:093† ‡ 1:872ꢀÆ0:119†ꢀÃ
‡ 0:708ꢀÆ0:144†ꢁ
ꢀ14†
ꢀn ˆ 12; r ˆ 0:994; s.e. ˆ 0:049; F2;9 ˆ 393:88†
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 884–889

888
A. HABIBI YANGJEH, M. R. GHOLAMI AND R. MOSTAGHIM
Figure 4. Plot of the experimental values of log k2app versus
the expected values obtained from Eqn. 416) for all the
aqueous solutions at 25°C
Figure 5. Plot of the %residual versus the experimental
values of log k2app for all the aqueous solutions at 25°C
at 25°C. Pseudo-first-order (kobs) conditions were used in
all cases. The rate constants were obtained by the initial
rate method. All the kinetic runs were carried out at least
in triplicate. Concentrations of phenylhydrazine were
4 Â 10À3 to 20 Â 10À3 M and those of benzaldehyde were
5 Â 10À5 to 4 Â 10À4 M. Water was circulated around the
cell of the spectrophotometer and the water temperature
was thermostatically maintained at 25 Æ 0.1°C.
tion of points is random (Fig. 5):
%residual ˆ 100  ꢀlog k2 exp
À log k2expec†= log k2 exp
ꢀ17†
in which log k2 exp and log k2expec are the experimental
and expected values of log k2app respectively.
The rates of some organic reactions in water and
several aqueous solutions are higher than those in organic
solvents. Hydrophobic interactions often play an im-
portant role in these cases.32–36 In this study, the effect of
hydrophobic interactions is negligible, because the rate-
determining step of the reaction is dehydration step. As a
result, there is no reduction in the surface accessible in
the activation process and hydrophobic interactions do
not play a role. This is fully consistent with the dual-
parameter correlations of log k2app with p* and a in all the
aqueous solutions discussed here.
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