1.14.9: Extrathermodynamics - Solvent Polarity
A solution comprises at least two chemical substances, solvent and solute. The amount of solvent far exceeds the amount of solute so in this sense a solute is dispersed through a solvent [1,2]. Although the solvent molecules are in vast excess, our interest centres on the minor (solute) component because chemists attempt to understand how interactions between a solute molecule and surrounding solvent molecules control the properties of the solute molecule; e.g. control reactivity, solubility, and colour [3]. Out of this interest in solute - solvent interactions emerges the concept of solvent polarity which attempts to characterise this interaction [4].
The general concept of solvent polarity can be understood by considering developments in two subjects;
- Chemical Kinetics,
- Spectro-photometry.
We review briefly each of these subject areas, indicating how the concept of polarity and/or solvent polarity emerged. We show how the intuitive concept of solvent polarity in these subject areas developed in quantitative terms. We used the word intuitive and this usage can be understood in the following terms. Asked to prepare a solution of sodium chloride (table salt), a first year freshman student would choose water as the solvent rather than (liquid) benzene or ethanol because (the student would argue) water is more "polar" than either ethanol or benzene. Here the term "polar" is little more than laboratory jargon. We seek a quantitative measure of solvent polarity.
Y-values
In 1862 Bertholet and Pean de Saint Gilles noted that the rate of chemical reaction depends on the solvent. In 1890, Menschutkin confirmed that finding in a very detailed study. So for more than 100 years chemists have attempted to describe quantitively these solvent effects. Perhaps not suprisingly the first attempts concentrated on the dependendence of rate constants on the relative permittivity of solvents. Many authors sought correlations using the treatments described by Kirkwood [5-8]. The quantity used in these correlations usually takes the form \(\left(\varepsilon_{\mathrm{r}}-1\right) /\left(2 \, \varepsilon_{\mathrm{r}}+1\right)\). But as many authors point out, this Kirkwood function is little better than the relative permittivity for describing interactions at the molecular level. Nevertheless the challenge remained to describe kinetic solvent effects. A particular important stage was the growth of interest in physical organic chemistry [9]. Probably the ‘father’ of this subject was C.K.Ingold. In his classic monograph [9] , Ingold actually used the pharase ‘solvent polarity’when commenting on the rates of reactions through a series of - 1 - solvents of diminishing polarity; water, ethanol, propanone, benzene. But Ingold did not offer a polarity scale. One of the reactions discussed by Ingold was the hydrolysis of \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCl}\).
\[\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{aq}) \rightarrow\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq}) \nonumber \]
This classic reaction (although the mechanism is still debated) formed the basis of a quantitative description of solvent polarity described by Winstein and coworkers [10,11]. Using the rate of reaction described above they identified a reference solvent, a mixture formed by ethanol (80 vol%) and water(20 vol%). If the rate constant for this reaction is \(\mathrm{k}^{0}\) in this solvent mixture and the rate constant is \(\mathrm{k}\) in a new solvent, the Y-value of this new solvent is given by
\[\mathrm{Y}=\log \left(\mathrm{k} / \mathrm{k}^{0}\right) \nonumber \]
In effect \(\mathrm{Y}\) measures the ionising power of the solvent – the extent to which the solvent favours charge separation within the neutral solute. Hence by measuring the rate constant for the above reaction in a given solvent, the polarity of the solvent is obtained as shown by its Y-value.
This kinetic approach to the determination of solvent polarities has attracted attention, particularly in the context probing reaction mechanisms [9-12]. The Y-value approach can be rationalised using an extrathermodynamic analysis [13]. Nevetheless application of the solvent polarity scale based on Y-values is limited. The range of solvents for which Y-values can be measured is restricted.
Z-values
A feature of many dye molecules is the sensitivity of their colour to the solvent. This fact was exploited by Brooker and coworkers who used two dyes to define \(\chi_{\mathrm{B}}\) and \(\chi_{\mathrm{R}}\) values [14]. These scales have not found wide application. A polarity which has attracted attention was suggested by Kosower [15,16]. The scale is based on the uv/visible spectra of N-methyl pyridinium iodide. The low energy absorption band in the spectra characterises the charge transfer from iodide to the pyridinium ring. Kosower examined correlations between Z – and Y- values and between Z-values and other solvent sensitive partaneters . The consensus is that Z provides a reasonable sastifactory measure of solvent polarity.
E T Values
There can be little doubt that chemists find the concept of solvent polarity intuitively attractive . Granted the need there is an associated demand for a convenient, readily available method for measuring solvent polarity. Reichardt synthesised a betaine dye which - 2 - is particularly solvent sensitive as shown by the dependence on solvent of an intramolecular charge transfer band [17]. Reichardt expresses the energy of the energy band maximum of the absorption band in kilocalories per mol which defines the \(\mathrm{E}_{\mathrm{T}}\) value for a given solvent. Solutions of the dye in methanol are red, violet in ethanol and green in propanone. So one has a striking visual indicator of solvent polarity.
Foonotes
[1] In a solution which is defined as ideal in a thermodynamic sense there are no (solute molecule) \(\leftrightarrow\) (solute molecule) interactions. Hence the solute molecules are effectively infinitely far apart.
[2] Some indication of the ratio of solute to solvent molecules is indicated by the following rough calculation. Dilute aqueous solutions used in a study of chemical kinetics have concentrations of approx. \(10^{-3} \mathrm{mol dm}^{-3}\). In \(1 \mathrm{dm}^{3}\) of water there are \(55.5\) moles of water so the ratio of solute to solvent molecules is around \(55000\).
[3] Although the term is not used by chemists it may be helpful to imagine each solute molecule bathed in solvent molecules, implying a limitless expanse of solvent molecules around each solute molecule.
[4] We confine attention to the properties of solvents ( e.g. polarities) at ambient pressure and at \(298.2 \mathrm{~K}\); i.e. 25 Celsius which is just above conventional room temperature.
[5] J.G.Kirkwood, J. Chem. Phys.,1934, 2 ,351.
[6] Amis discusses treatments of kinetic data based on solvent permittivities; E. S. Amis, Solvent Effects on Reaction Rates and mechanisms, Academic Press, New York, 1966.
[7] See comments by N. S. Isaacs, Physical Organic Chemistry, Longmans, London,1987.
[8] See also comments concerning attempts to identify a single solvent property which accounts for solvent effects on rates of chemical reactions; J. B. F. N. Engberts, in Water-A Comprehensive Treatise, ed. F.Franks, Plenum Press, New York, 1979,Volume 6, chapter 4.
[9] C.K.Ingold, Structure and Mechanism in Organic Chemistry, G. Bell, London, 1953; see page 347.
[10]
- E. Grunwald and S. Winstein, J. Am. Chem. Soc.,1948, 70 ,841;846.
- S. Winstein and A.H. Fainberg, J. Am. Chem.Soc.,1957, 79 ,5937.
- H.Langhals, Angew.Chem.Int.Ed.Engl.,1982, 21 ,724.
- O. Pytela, Collect. Czech. Chem.Commun.,1988, 53 ,1333.
[11] See also A. Streitweiser, Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962
[12] Y-values have been used in the context of kinetics of reactions of inorganic solutes; M.J.Blandamer, J. Burgess, and S. Hamshere, Transit. Metals Chem.,1979, 4 , 291.
[13] J. E. Leffler and E. Grunwald, Rates and Equilibria of Organic Reactions,Wiley, New York, 1963; Dover Publications, New York, 1989.
[14] L. G. S. Brooker, A. C. Craig, D.W. Heseltine, P.W.Jenkins and L. L. Lincoln, J. Am. Chem. Soc.,1965, 87 ,2443.
[15] E. Kosower, J.Am. Chem. Soc.1958, 80 ,3253.
[16] E. Kosower, Physical Organic Chemistry, Wiley, New York, 1968.
[17] C.Reichardt, Chem. Rev.,1994, 94 ,2319; Chem. Soc. Rev., 1992,147; Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, 2nd. edn.,1988.