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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 Unauthenticated Download Date | 6/16/17 4:50 PM 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]. Unauthenticated Download Date | 6/16/17 4:50 PM 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. Unauthenticated Download Date | 6/16/17 4:50 PM 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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