Characterization of solvent mixtures. Part 8 - Preferential solvation of chemical probes in binary solvent systems of a polar aprotic hydrogen-bond acceptor solvent with acetonitrile or nitromethane. Solvent effects on aromatic nucleophilic substitution r

Article abstract of DOI:10.1002/(sici)1099-1395(199903)12:3<207::aid-poc113>3.0.co;2-w

The use of chemical probes for the characterization of chemical properties was explored for completely non-aqueous aprotic binary solvent mixtures. The Dimroth-Reichardt E_T(30) betaine dye, 4-nitrophenol, 4-nitroanisole, 4-nitroaniline and N,N-

Full text of DOI:10.1002/(sici)1099-1395(199903)12:3<207::aid-poc113>3.0.co;2-w

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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 12, 207–220 (1999)
Characterization of solvent mixtures. Part 8 Ð Preferential
solvation of chemical probes in binary solvent systems of a
polar aprotic hydrogen-bond acceptor solvent with
acetonitrile or nitromethane. Solvent effects on aromatic
nucleophilic substitution reactions
P. M. E. Mancini,* A. Terenzani, C. Adam, A. PeÂrez and L. R. Vottero
Departamento de QuõÂmica Orga nica, Facultad de IngenierõÂa QuõÂmica, Universidad Nacional del Litoral, Santiago del Estero 2829, (3000)
Santa Fe, Repu blica Argentina
Received 2 April 1998; revised 29 May 1998; accepted 5 June 1998
ABSTRACT: The use of chemical probes for the characterization of chemical properties was explored for completely
non-aqueous aprotic binary solvent mixtures. The Dimroth–Reichardt ET(30) betaine dye, 4-nitrophenol, 4-
nitroanisole, 4-nitroaniline and N,N-diethyl-4-nitroaniline were used to study preferential solvation in binary mixtures
of a polar aprotic hydrogen bond-acceptor solvent (DMSO, DMF, acetone, butanone and ethyl acetate) with
acetonitrile or nitromethane at 25°C over the whole range of solvent composition. The indicators were employed to
determine ET(30), p*, a and b solvatochromic parameters of the mixtures. Each solvent system was analysed
according to its deviation from ideal behaviour due to the preferential solvation of the indicators and also from the
complicated intermolecular interactions of the mixed solvent. The validity of the concept of an intrinsic, absolute
property of a solvent mixture and whether such a property can be defined by means of chemical probes is discussed.
These results were related to the solvent effects on some aromatic nucleophilic substitution reactions. The kinetics of
the reactions between 1-fluoro-2,4-dinitrobenzene and primary or secondary amines were studied in ethyl acetate–
acetonitrile and N,N-dimethylformamide–acetonitrile mixtures, respectively, taken as representatives of two clearly
different solvation models. Copyright 1999 John Wiley & Sons, Ltd.
KEYWORDS: binary solvents; solvation; aromatic nucleophilic substitution reactions
INTRODUCTION
decades.2–5 More recently, Abraham6 has proposed
scales of solute hydrogen-bond acidity and solute hydro-
gen-bond basicity and has devised a general solvation
equation.
Negatively and positively solvatochromic dyes are
particularly suitable as standard substances for studying
solute–solvent interactions, since the transition energy of
the indicator depends on the solvation’s sphere composi-
tion and properties. This method also provides informa-
tion on some solvent properties such as polarity and
hydrogen-bonding capability.
The chemical characteristics of solvent mixtures are
customarily determined in the same manner as those of
neat solvents by means of solvatochromic indicators.
However, solute–solvent interactions are much more
complex in mixed solvents than in pure solvents owing to
the possibility of preferential solvation by any of the
solvents present in the mixture. Moreover, the solvent–
solvent interactions produced in solvent mixtures can
affect solute–solvent interactions and therefore prefer-
ential solvation. The validity of the concept of an
intrinsic, absolute property of a solvent mixture and
Solvents play a major role in many chemical processes.
Organic liquids are characterized by several properties
that make them suitable for dissolving and for providing
reaction media for various types of solutes.1 These
properties include physical and chemical quantities.
Solvent effects are closely related to the nature and
extent of solute–solvent interactions locally developed in
the immediate vicinity of the solutes. Chemists have
usually attempted to understand this in terms of polarity,
defined as the overall solvation capabilities that depend
on all possible (specific and non-specific) intermolecular
interactions. In this connection, numerous reports on
solvent polarity scales have been published in the last few
*Correspondence to: P. M. E. Mancini, Departmento de Qu´ımica
Orga´nica, Facultad de Ingenierı´a Qu´ımica, Universidad Nacional del
Litoral, Santiago del Estero 2829, (3000) Santa Fe, Repu´blica
Argentina.
E-mail: pmancini@fiqus.unl.edu.ar
Contract/grant sponsor: Science and Technology Secretariat, UNL,
CAI‡D Program; Contract/grant number: 94-0858-007-054.
Copyright 1999 John Wiley & Sons, Ltd.
CCC 0894–3230/99/030207–14 $17.50

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208
P. M. E. MANCINI et al
Table 1. Solvatochromic parameters of pure solvents at 25°C
Parameter
EAc
BUT
AC
DMF
DMSO
NM
46.3
AcN
ET(30)
p*
38.1
41.3
42.2
43.8
45.1
45.6
0.55a
0.45c
0.49d
(0.56)
0.00a
(0.00)
0.67a
0.60c
0.71a
0.88a
1.00b
0.85a
0.75a
0.66c
0.71d
(0.76)
0.19a
(0.32)
0.62c
0.66d
(0.70)
0.08a
(0.13)
0.88c
0.75c
0.87d
(0.66)
0.06a
(0.09)
(0.89)
(0.88)
0.22a
(0.29)
a
0.00a
0.00a
(0.00)IV
(0.06)V
0.76a
(0.77)IV
(0.43)V
(0.04)IV
(0.05)V
0.69a
(0.71)IV
(0.67)V
b
0.45a
(0.45)
0.48a
(0.58)
0.48b
0.43a
(0.53)
0.06a
0.37e
(0.20)
0.31b
0.40a
(0.44)
a
e
Data from Ref. 11, b Ref. 14, c Ref. 15, d Ref. 16 and Ref. 17. Values in parentheses were determined in this work.
whether such a property can be defined by means of
chemical probes has been discussed recently.7
use as a ‘predictor’ or ‘descriptor’ of the solvent effect on
the behaviour of diverse kinds of solutes and transition
states; and (iv) relating the extent of the preferential
solvation of different chemical probes to the kinetic
properties of ANS reactions development in the same
binary solvent systems.
We have previously studied the preferential solvation
of ET(30) dye for several completely non-aqueous binary
solvent mixtures of the type polar aprotic hydrogen-bond
acceptor (PAHBA) solvent–chloroform or dichloro-
methane, both taken as hydrogen-bond donor solvents.8
Most of these mixtures presented synergetic effects for
the ET(30) polarity parameter, which is revealed in the
PAHBA solvent zone. We have also discussed the
influence on the kinetics of some aromatic nucleophilic
substitution reactions (ANS) of those solvent mixtures in
which the two solvents interact to form a hydrogen-
bonded complex with ET(30) polarity higher than those
of the two pure solvents. This ‘improved polarity’ was
not ‘reflected’ by the critical states of the ANS reaction
analysed. Among the PAHBA solvents, acetonitrile
(AcN) and nitromethane (NM) exhibit a lower HBA
ability and also exhibit a potential ability to donate a
hydrogen atom towards the formation of a hydrogen
bond. In order to contribute to a more comprehensive
analysis of the microscopic properties of binary aprotic
solvent mixtures, it was of interest to discuss the
behaviour of solvent mixtures of the type PAHBA
solvent AcN or NM.
RESULTS AND DISCUSSION
Microscopic properties of binary solvent mixtures
Different single and multiparametric empirical scales of
molecular microscopic properties of solvents have been
developed from reference solutes that behave as a probe
reflecting changes in the solvation shell through varia-
tions in their UV–visible absorption spectra.
The ET(30) scale of Dimroth and Reichardt9 and the
p*, b and a scales constructed by Kamlet and co-workers
by the solvatochromic comparison method10 are the
parameters most used in the uniparametric and multi-
parametric approaches, respectively. The ET(30) ‘polar-
ity’ reflects a combination of polarity and hydrogen-bond
donor capability of solvents and is defined as the molar
transition energy (in kcal mol 1) of a betaine dye
dissolved in the solvent under study. The p*, b and a
parameters (resorting to more specific probes) reflect the
dipolarity/polarizability, the hydrogen-bond acceptor
(HBA) basicity and the hydrogen-bond donor (HBD)
acidity of the solvents, respectively. Different procedures
for the calculation of p*, b and a values have been
collected by Marcus.11 Recently, new probe molecules
(structurally different) have been proposed.12
In the present study, the solvatochromic parameters
[ET(30), p*, b, and a] of completely non-aqueous binary
mixtures of several dipolar aprotic hydrogen-bond
acceptor solvents with acetonitrile and nitromethane
were determined and interpreted. Additionally, it was of
interest to evaluate the influence of these solvent
mixtures on the kinetics of some aromatic nucleophilic
substitution reactions.
This work was aimed at (i) studying the preferential
solvation of different chemical probes in binary solvent
mixtures, analysing the solute–solvent and solvent–
solvent interactions; (ii) exploring whether different
indicators produce convergent values for a property in a
given solvent mixture; (iii) measuring the microscopic
properties of binary solvent mixtures with the purpose of
obtaining numerical values for solvent properties for their
ET(30) values are available for several binary solvent
mixtures, but data for p*, b and a parameters are still
scarce.
Bosch and co-workers13 proposed different theoretical
equations which take into account the solute–solvent and
solvent–solvent interactions in binary mixtures and relate
the ET(30), p*, b and a solvatochromic parameters with
the solvent composition.
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 207–220 (1999)

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CHARACTERIZATION OF SOLVENT MIXTURES
209
Table 2. ET(30) values (kcal mol 1) for PAHBA solvent±co-solvent systems at 25°C
Co-solvent mole fraction
PAHBA
solvent
Co-solvent
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
EAc
NM
AcN
NM
AcN
NM
AcN
NM
40.8
40.8
42.1
42.4
42.8
42.7
43.8
43.9
45.0
45.0
41.6
41.6
42.8
43.0
43.6
43.3
43.9
44.3
45.1
45.0
42.6
42.6
43.5
43.5
44.0
43.7
44.3
44.5
45.2
45.1
43.2
43.3
44.1
44.0
44.5
44.2
44.6
44.6
45.3
45.1
44.2
44.0
44.4
44.5
44.8
44.6
44.7
45.0
45.4
45.2
44.7
44.5
45.0
45.0
45.4
45.1
45.2
45.2
45.6
45.3
45.2
44.8
45.5
45.2
45.7
45.2
45.3
45.4
45.6
45.3
45.8
45.2
46.0
45.5
46.0
45.4
45.8
45.5
45.8
45.4
46.4
45.5
46.3
45.6
46.3
45.5
46.1
45.6
46.0
45.6
BUT
AC
DMF
DMSO
AcN
NM
AcNa
a
From Ref. 18.
The two selected co-solvents (NM and AcN) have
analogous general properties. Taking into account that
for NM the HBA capability values are controversial in
the literature, and considering the possibility of experi-
mental difficulties in the determination of the solvato-
chromic shifts due to overlapping absorptions, the ET(30)
parameter was determined for PAHBA solvent–AcN or
NM) and p*, b and a only for PAHBA solvent–AcN. The
PAHBA selected were ethyl acetate (EAc), acetone (AC),
butanone (BUT), N,N-dimethylformamide (DMF) and
dimethyl sulphoxide (DMSO). The properties of the pure
solvents used to prepare the binary solvent mixtures
studied are given in Table 1.
Determination of ET(30) values
The ET(30) solvatochromic parameter is defined as the
molar transition energy derived from the longest
wavelength solvatochromic UV–visible absorption band
of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-pheno-
late (I), proposed by Dimroth and Reichardt for
empirically measuring the ‘polarity’ of solvents, and is
calculated according to the equation
1
ETꢀ30†ꢀkcal mol † ˆ hcꢀꢀN
ˆ 2:859  10 3ꢀꢀꢀcm
†
ꢀ1†
1
For each system explored, the solvatochromic para-
meters were systematically determined over the full
solvent composition range (at nine molar fractions of co-
solvent) at 25°C.
The calculated ET(30) values for the PAHBA solvent–
AcN or NM systems are presented in Table 2 (the binary
system DMSO–AcN has been reported previously). The
corresponding plots of ET(30) vs XCoS are shown in Figs 1
and 2, including the pure solvents.
In order to analyse the data obtained, the binary
mixtures were divided into two general groups according
to the solvation models reflected in the plots: (a) mixtures
that exhibit positive deviations from linearity in the
relationship between the solvatochromic parameter
values and the solvent composition; and (b) mixtures
with negative deviations from linearity.
The different solvation models of indicator I can be
interpreted from its solute properties. The Dimroth–
Reichardt betaine dye exhibits a significant permanent
dipole moment (suitable for dipole–dipole and dipole–
induced dipole interactions), a large polarizable p-
electron system (suitable for dispersion interactions), a
substantial negative charge on the phenoxide oxygen
[highly basic electron-pair donor centre suitable for
interactions with weak Brønsted acids (H-bonding) and
Lewis acids (EPD/EPA bonding)] and a positive charge
on the pyridinium nitrogen (sterically shielded). There-
fore, indicator I is sensitive to the dipolarity/polariz-
ability and hydrogen-bond donor capability of the
solvents.
Figure 1. Plot of ET(30) vs co-solvent mole fraction for
PAHBA solvent (EAc, BUT, AC) ±NM or AcN solvent systems
Figure 1 presents the plots for mixtures of EAc, BUT
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 207–220 (1999)

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210
P. M. E. MANCINI et al
solvent property at XNM = 0.9. The binary mixtures with
BUT and AC exhibit continuous non-linear increases in
the ET(30) value, showing that the curvature in the region
rich in these PAHBA solvents is less defined, and the
deviations from linearity are smaller than those for
mixtures with EAc. All these results indicate that the
betaine dye tends to be preferentially solvated by the co-
solvent. The shape of the curves denotes enrichment of
the solvation shell of the chemical probe in the polar
solvent with a potential capability to establish solute–
solvent interactions by hydrogen bonding, leading to an
additional stabilization of the strong negative charge on
the phenoxide oxygen in the electronic ground state of I,
resulting in increased ET(30) values.
Figure 2 shows the plots for binary mixtures of DMF
and DMSO with both co-solvents. The difference
between the ET(30) values of the co-solvents and the
pure solvents is 0.6 ꢁ DET(30) ꢁ 2.5, lower than those
for the mixtures previously analysed. All systems exhibit
a similar behaviour, showing nearly linear curves with
small negative deviations from linearity. The ET(30)
value increases smoothly in the co-solvent-rich region
and flattens out at XCoS < 0.5. In these binary mixtures,
the solvent–solvent interactions can notably affect the
solute–solvent interactions, and therefore preferential
solvation. The highly dipolar solvents with strong
hydrogen-bond acceptor capability (DMF and DMSO)
can interact with AcN and NM (highly dipolar solvents
with potential hydrogen-bonding ability) forming associ-
ates by means of hydrogen bonds and dipole–dipole
interactions. In the solvation shell, DMF and DMSO can
compete with indicator I to establish interactions by
hydrogen bonding with the co-solvent. Some mixtures
with DMSO show an ET(30) value slightly lower than
those for the pure solvents.
Figure 2. Plot of ET(30) vs co-solvent mole fraction for
PAHBA solvent (DMF, DMSO)±NM or AcN solvent systems
and AC with both co-solvents, showing a striking
behaviour with positive deviations from linear relation-
ships between the ET(30) values and the solvent
composition. It should be pointed out that the difference
between the property values of the co-solvents and the
pure solvents is 3.5 ꢁ DET(30) ꢁ 8.2 kcal mol 1 (1 kcal =
4.184 kJ). The binary systems with EAc as one com-
ponent of the mixtures exhibit a large increases in the
ET(30) value with small increases in co-solvent concen-
tration and large deviations from linearity, particularly in
the EAc-rich region. It can be observed that the EAc–NM
system presents a negligible synergistic effect for the
The results obtained indicate that a similar behaviour is
Table 3. p* Values for PAHBA solvent±acetonitrile systems at 25°C
Acetonitrile mole fraction
PAHBA
solvent
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
EAc
p* (II)
0.59
0.57
0.58
0.69
0.66
0.675
0.71
0.68
0.695
0.88
0.88
0.88
0.98
1.00
0.99
0.63
0.59
0.61
0.69
0.66
0.675
0.71
0.68
0.695
0.88
0.84
0.86
0.97
1.00
0.985
0.64
0.61
0.625
0.71
0.68
0.695
0.71
0.68
0.695
0.87
0.82
0.845
0.96
0.65
0.61
0.63
0.72
0.69
0.705
0.73
0.70
0.715
0.87
0.80
0.835
0.94
0.93
0.935
0.66
0.66
0.66
0.74
0.71
0.725
0.75
0.72
0.735
0.87
0.79
0.825
0.92
0.88
0.90
0.69
0.66
0.675
0.74
0.71
0.725
0.75
0.72
0.735
0.87
0.77
0.82
0.89
0.86
0.875
0.71
0.66
0.685
0.74
0.71
0.725
0.75
0.72
0.735
0.86
0.77
0.815
0.87
0.84
0.855
0.74
0.71
0.725
0.74
0.71
0.725
0.75
0.72
0.735
0.85
0.75
0.80
0.86
0.79
0.825
0.75
0.73
0.74
0.75
0.72
0.735
0.75
0.72
0.735
0.83
0.75
0.79
0.83
0.79
0.81
p* (III)
p* (avg)
p* (II)
BUT
AC
p* (III)a
p* (avg)
p* (II)
p* (III)a
p* (avg)
p* (II)
DMF
DMSO
p* (III)
p* (avg)
p* (II)
p* (III)
p* (avg)
0.97
0.965
a
Values estimated from Eqn (4).
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 207–220 (1999)