Download Characterization of Alcohol Solvents by the Empirical Polarity

Characterization of Alcohol Solvents by the Empirical
Polarity Parameters AN, Z, and E x (30)
Horst Elias*, Michael Dreher, Sabine Neitzel, and Harald Volz
Anorganische Chemie III, Eduard-Zintl-Institut, Technische Hochschule Darmstadt,
Hochschulstraße 4, D-6100 Darmstadt
Z. Naturforsch. 37b, 084-687 (1982); received February 1, 1982
Solvent Polarity Scales, Polarity of Alcohols, Acceptor Number AN,
Polarity Parameter Z and Ex(30)
The acceptor number A N and the polarity parameters Z and ET(30) were determined
for a series of alcohols applied as media in the study of kinetic solvent effects. The alcohols
thus characterized are methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, 2-methyl-2butanol, 3-pentanol, 2-chloroethanol, 2-methoxyethanol, 2-phenylethanol, 2-cyanoethanol,
benzyl alcohol, 3-ethyl-3-pentanol, 2,4-dimethyl-3-pentanol, and 3-ethyl-2,4-dimetliyl-3pentanol. In addition, A N was determined for 1,2-dichloroethane and Z for the binary
solvent mixtures methanol/2-methyl-2-butanol, ethanol/2,2,2-trifluoroethanol. and
methanol/pyridine. The data obtained are correlated and the parameters AN, Z, and
ET(30) are critically compared.
Introduction
Studies on the kinetics of ligand substitution in
four-coordinate transition metal complexes in
various organic media [1] led us to attempt the
correlation of the kinetic solvent effects observed
with suitable solvent parameters. Since most of the
solvents applied were protic species there was a
special interest in those physical or empirical solvent
polarity parameters which reflect the ability of the
solvent to form hydrogen bonds and to act as an
electron pair acceptor.
Within the wide spectrum of standard procedures
for the empirical characterization of solvent polarity
the spectroscopic procedures based on solvatochromic organic dyes have become very valuable.
Thus the Z values introduced by Kosower [2] in
1958 and especially the ET (30) values introduced
by Dimroth, Reichardt, Siepmann and Bohlmann
[3] in 1963 represent easily accessible polarity
parameters of great practical importance [4],
Our kinetic studies mentioned above revealed
that the rate constants observed in various alcohols can be properly correlated with the corresponding ET(30) values [5], It is reported [6] that Z
is linearly related to ET(30) and that the acceptor
number AN, as introduced by Gutmann et al. [7] on
the basis of 31P NMR chemical shift values of a
suitable trialkylphosphine oxide, also correlates
linearly with Z and ET(30). It appeared to be useful
* Reprint requests to Prof. Dr. H. Elias.
0340-5087/82/0600-0684/S 01.00/0
and of general interest, therefore, to determine the
hitherto unknown AN, Z, and ET(30) values of a
series of alcohols and to examine the correlations
existing between these parameters.
Experimental
With the exception of 3-ethyl-2,4-dimethyl-3pentanol ("diisopropyl ethyl carbinol") which was
synthesized according to the literature [8] all solvents were commercially available. Those which
were supplied in reagent grade were not further
purified whereas the others were fractionated. The
solvents n-hexane and 1.2-dichloroethane were
treated with conc. H2SO4 [9] prior to distillation.
All solvents were dried with molecular sieves (3 A
or 4 Ä) under nitrogen statically or dynamically.
A sample of the solvatochromic dye 2,6-diphenyl4-(2,4,6-triphenyl-l-pyridinio)-phenolate for the
determination of ET(30) values was kindly provided
by Prof. Reichardt (Universität Marburg). The
solvatochromic compound l-ethyl-4-carbomethoxypyridinium iodide for the determination of Z values
was prepared as described in the literature [10].
The spectrophotometric measurements were done
with a Perkin-Elmer spectrophotometer (PE 554).
The exact position of the maximum of absorption in
the long wavelength range was determined by
repeated recording of the 1st derivative of the
spectra and by averaging the /.max values obtained.
ET(30) and Z were calculated according to ET(30)
or Z (kcal/mol) = 2.859 • 10 4 // max (nm) (the presentation of these parameters in units of kcal instead
of kJ has become standard).
The 31P NMR spectra were recorded at 27-28 °C
(protons decoupled) with a "Pulse-Eourier-Transform"-NMR spectrometer (Bruker WH-90), the
samples being introduced in 5 mm coaxial NMR
tubes (Wilmad Glass Company, Inc.). The inner
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H. Elias et al. • Characterization of Alcohol Solvents
capillary tube was filled with a solution of the
external standard P(OMe)3 (Merck-Schuchardt) in
C6Ü6 and then sealed, whereas the outer tube
contained (n-Bu) 3 PO (Merck-Schuchardt) dissolved
in the solvent to be studied at 4 different concentrations in the range 0.05-0.2 M.
685
AN
<W(S)—<5Cor(w-hexane)
—42.58
100 = —zl<W • 2.348
(2)
The compound Et 3 PO is hygroscopic and therefore
not very suitable from an experimental point of
view. Following Hormadaly and Marcus [11] we
Results
decided to use (n-Bu) 3 PO instead of Et 3 PO as an
The acceptor number AN as introduced by Gutindicator compound, and P(OMe)3 in CeDö instead
mann et al. [7] for the characterization of the electro- of (C6H5)2P0C1 [7] or conc. H 3 P0 4 [11] as external
philic properties of a solvent was derived from the standard.
relative 31P NMR chemical shift value observed for
Most of the experimentally observed chemical
triethylphosphine oxide dissolved in a given solvent. shift values dexp are slightly dependent on the conThe phosphorus compound (CÖHÖ^POCI served as centration of (>I-BU)3PO and were extrapolated to
an external standard in these measurements.
(3oo on the basis of equation (3):
The AN scale was set up by defining AN(n-hexm[(w-Bu) 3 PO]
<5PXD = dr
(3)
ane) = 0 and AN(Et 3 PO • SbCl5) = 100. The calculation of AN for a given solvent S is therefore
based on the following equation (<5COr = observed
chemical shift b corrected for concentration effects
and for differences in volume magnetic susceptibilities) :
AN
<5Cor(S) — dcor(n-hexane)
<5cor(Et3PO-SbCl5) — <5cor(»-hexane;
100
The data obtained for Öqo and for the slope m by
least square fitting are compiled in Table I. as are
the Scot values, corrected for differences in volume
magnetic susceptibilities, and the values for
zldcor = (5cor(S) — (5COr(w-hexane).
Table TI presents hitherto unknown Z and ET(30)
values for a series of alcohols and Table III comprises the Z values obtained for 3 solvent mixtures.
(1)
The term [<5COr(Et3PO • SbCl5) — <5COr(w-hexane)] was Diskussion
found to be —42.58 ppm [7], Eq. (1) reduces thereThe acceptor numbers listed in Table I demonfore to eq.(2):
strate that with the exception of the tertiary alcohol
Table I. Chemical shift values Ö of tri-(n-butyl)phosphine oxide (31P NMR) in various solvents relative to trimethylphosphite, volume magnetic susceptibilities Xy, and derived AN values.
Solvent
No.
<5oo (ppm)a m b (ppm • M-i) — 10 7 Z vc
(5cor(ppm)e
Methanol
Ethanol
2 -Chloroethanol
2 -Methoxyethanol
2-Phenylethanol
2 -Cy anoethanol
1 -Propanol
1 -Butanol
2-Butanol
2-Methyl-1 -propanol
Benzyl alcohol
2 -Methyl- 2 -butanol
n-Hexane
1,2-Dichloroethane
1
2
3
4
5
6
7
8
9
10
11
12
13
14
85.35
86.97
83.19
87.14
87.07
82.78
87.14
87.34
89.17
87.30
85.60
92.28
100.5
94.44
85.16
86.88
83.41
87.11
87.21
82.63
87.11
87.34
89.19
87.31
85.73
92.34
100.4
94.72
— 2.35
— 0.63
+ 0.90
— 0.44
+ 1.28
+ 1.19
+ 0.71
— 1.38
— 1.36
+ 1.66
— 1.54
— 1.32
— 16.9
+ 0.21
5.286
5.757
7.215d
6.0271
6.848fl
5.459(i
6.042
6.173
6.288
6.252
6.806d
6.482
5.716
7.526'1
f
— A dear (ppm)AN
15.24
13.52
16.99
13.29
13.19
17.77
13.29
13.06
11.21
13.09
14.67
8.06
0.0
5.68
35.8 (41.3)8
31.8 (37.1)«
39.9
31.2
31.0
41.7
31.2
30.7
26.3
30.7
34.5
18.9
0.0
13.3 (16.7)»
a By extrapolation to [(n-Bu)3PO] = 0 according to eq. (3); b slope m; see eq. (3); c data taken from "Handbook
of Chemistry and Physics", 58th Edition, Chemical Rubber Co., Cleveland, 1977-1978, E 127; (1 data calculated
from atom and group diamagnetic increments according to W. Haberditzl, Angew. Chem. 08, 277 (1966);
e (JCOR = dm + (2JT/3) [Xy (e.s.) — Xy], where Xy (e.s.) = —6.1866 • 10-7 is the value for C6Ü6 applied as solvent
for the external standard P(OMe)3; £ zl <5Cor = <5cor(S) — <5cor (n-hexane); see eq. (2); s data taken from ref. [7].
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686
H. Elias et al. • Characterization of Alcohol Solvents 686
Tab. II. Z values and ET(30) values of various alcohols.
Solvent
Z
(kcal/mol)
2-Methyl-2-butanol
70.0
Benzyl alcohol
78.3
2 -Pheny lethanol
2-Chloroethanol
2-Cy anoethanol
3-Pentanol
77.1
79.7c
87.4
74.8
3-Ethyl-3-pentanol
2,4-Dimethyl-3-pentanol
3-Ethyl-2,4-dimethyl3-pentanol
E T (30)
(kcal/mol)
_rl
_d
(41.9 a
41.1 b
[40.7 e
J50.8 a
\50.4e
49.5 a
55.5
59.6
J45.6
\45.7a
38.5
40.1
_d
37.9
HO-CH2-CH2-X ••• H-0-CH 2 -CH 2 -X
(X = C1,CN,0CH3)
have to be considered in addition to the expected
interactions of the type R s P O - H O - R ' between
the indicator compound and an alcohol R'OH. The
low AX values obtained for the tertiary alcohol 12
and for the secondary alcohol 9 are probably due to
inductive and steric effects of the alkyl groups.
The acceptor numbers of methanol, ethanol, and
1,2-dichloroethane have been determined by Gutmann et al. [7] and also in the present study. It
a Data from ref. [4];
data from ref. [13]; c spectrum
of the indicator compound suffers red shift upon
standing of the solution; d no data available due to
insolubility of the indicator compound; e data from
M. H . Aslam, G. Collier, and J. Shorter, J. Chem. Soc.
Perkin Trans. II 1981, 1572.
2-methyl-2-butanol (AN = 18.6) and the secondary
alcohol 2-butanol (AN = 26) the data obtained for
the other alcohols all lie in the range of 30-40. As
compared to the parent compound ethanol the
acceptor number of ethanol carrying strongly electron withdrawing groups such as —Cl (alcohol 3)
or —-CN (alkohol 6) in 2-position is markedly increased. On the other hand, however, the effect
caused by the methoxy group (alcohol 4) and by
the phenyl group (alcohol 5) is unexpectedly small.
It is conceivable that the substituted alcohols 3-6
are somewhat "biphilic" in nature and that intermolecular interactions such as
Fig. 1. Correlation of the acceptor number A N of the
solvents 1 - 1 4 (see Table I) with the polarity parameter
E T (30). The data for E T (30) were taken from ref. [4]
and from Table II.
Table I I I . Z values of various solvent mixtures.
a) 2-Methyl-2-butanol/MeOH
XMeOH
Z (kcal/mol)
0.000 0.230 0.401 0.535 0.642 0.728 0.801 0.862 0.889 0.915 0.960 1.000
70.0
74.8
77.0
78.2
79.2
80.3
81.2
82.1
81.9
82.8
83.4
83.8
b) Pyridine/MeOH
xMeOH
Z (kcal/mol)
0.000
64.7 (64.0) a
0.333 0.461 0.571 0.666 0.727 0.823 0.889 0.919 0.947 0.974 0.990 1.000
75.0
76.0
77.7
78.6
80.1
81.0
82.0
82.8
83.6
83.6
83.9
83.8
c) EtOH/2,2,2-trifluoroethanol
XCF3CH2OH
Z (kcal/mol)
a
0.000
79.6 (79.6) a
0.016 0.041 0.083 0.169 0.258 0.351 0.448 b
80.2
80.2
81.2
83.1
84.5
85.3
86.5
Data taken from ref. [6];
b
indicator compound decomposes at higher mole fractions.
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H. Elias et al. • Characterization of Alcohol Solvents
follows from Table I that the data obtained clearly
disagree in the sense that the present data are consistently smaller by 4-5 AN units. There is no obvious argument explaining this disagreement. The
fact, however, that AN(acetic acid) = 52.9 as reported by Gutmann et al. [7] is also paralleled by a
considerably smaller value of 49.0 as found by Marcus and Hormadaly [11] points to some systematic
deviation (possibly resulting from the application
of different phosphorus reference compounds and
from different purity standards).
According to Reichardt [12] the existing collection of 38 AN values can be correlated with ET(30)
on the basis of the equation A N = 1.598 • ET(30)
— 50.69
(correlation
coefficient
R — 0.956).
In
Fig. 1 this relationship is shown for the 14 solvents
studied in the present contribution. The correlation is described b y A N =
1.502 E T (30) — 45.22
687
> Z (ideal). (Z (ideal) corresponds to Z values linearly increasing with the mole fraction of the cosolvent). This type of behaviour is observed for the
polarity parameter ET(30) as well [13] and points
to strongly disturbed structuring of the parent
solvent even at small concentrations of the more
polar co-solvent.
In the system EtOH/2,2,2-trifluoroethanol the increasing acidity of the mixture destroys Kosower's
substituted pyridinium iodide at #CF3CH2OH >0.45.
Conclusions
For a series of alcohols the acceptor number AN
as well as the polarity parameters Z and ET(30) have
been determined. All three parameters appear to be
equally well suited for describing the capacity of a
protic solvent to interact through hydrogen bonding
and to function as an electron pair acceptor. From
an experimental point of view, however, the deter-
(R — 0.978). which is in fair agreement with the
results reported by Reichardt.
mination of E T (30) instead of A N or Z is to be
Table II presents some new Z and ET(30) values
preferred for the following reasons: (i) ET(30) vafor ethanol differently substituted in 2-position and
lues are obtained more easily and more rapidly than
for some alkylated alcohols. It is obvious that Z inAN values, (ii) a greater chemical stability and
creases parallel to ET(30). The number of data is
solvatochromic sensitivity of the corresponding intoo small to propose a reasonably reliable correladicator compound favour ET(30) as compared to Z.
tion of the type Z = a • ET(30) -f b, which is reand consequently, (iii) the experimental precision in
ported by Reichardt [12] to be valid for 54 pure
determining ET(30) is greater than in the case of Z.
organic solvents with a = 1.337 and b = 9.80
(R = 0.978). It should be pointed out that the Z
The authors thank Prof. Dr. C. Reichardt (Univalue given for 2-chloroethanol may be too small versität Marburg) and Dr. K. J. Wannowius (T. H.
due to the instability of the dye solution upon Darmstadt) for reading and critically commenting
standing. Table III summarizes the Z values ob- the manuscript; they appreciate the co-operation
tained for three solvent mixtures. It follows from with Mr. G. Gumbel. Financial support through the
Deutsche Forschungsgemeinschaft and the Verband
the data that the behaviour of these mixtures is der Chemischen Industrie e. V. is gratefully acknowlmore or less non-ideal in the sense that Z (observed) edged.
[1] H . Elias, U. Fröhn, A. von Inner, and K . J.
Wannowius, Inorg. Chem. 19, 869 (1980).
[2] E. M. Kosower, J. Am. Chem. Soc. 80, 3253 (1958).
[3] K . Dimroth, C. Reichardt, T. Siepmann, and F.
Bohlmann, Liebigs Ann. Chem. 661, 1 (1963).
[4] C. Reichardt, Angew. Chem. 91, 119 (1979);
Angew. Chem., Int. Ed. Engl. 18, 98 (1979).
[5] H. Elias and K . J. Wannowius, Inorg. Chim.
Acta 64, L 157 (1982).
[6] C. Reichardt, "Solvent Effects in Organic Chemistry"; Monographs in Modern Chemistry 3, Verlag Chemie, Weinheim 1979.
[7] U. Mayer, V . Gutmann, and W . Gerger, Monatsh.
Chem. 106, 1235 (1975); 108,489 (1977); U. Mayer,
Pure Appl. Chem. 51, 1697 (1979).
[8] F. C. Whitmore and R . S. George, J. Am. Chem.
Soc. 64, 1239 (1942).
[9] Organikum, V E B Deutscher Verlag der Wissenschaften, 15. Auflage, p. 796 and 786, Berlin 1977.
[10] See page 266 in ref. [9].
[11] J. Hormadaly and Y . Marcus, J. Phys. Chem. 83,
2843 (1979).
[12] C. Reichardt; private communication; C. Reichardt, "Empirical Parameters of Solvent Polarity and Chemical Reactivity", in H. Ratajczak
and W . J. Orville-Thomas (eds): Molecular Interactions, Vol. 3, p. 241 ff., Wiley, Chichester 1982,
in press.
[13] H . Elias, G. Gumbel, S. Neitzel, and H. Volz,
Fresenius Z. Anal. Chem. 306, 240 (1981).
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