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Effect of Hydrogen-Bonding Solutes on Dielectric Constant cially the discussion following eq 18. (4) The use of the parameter pZ,app2 in the examination of dglldcp may be avoided by letting dglldcp = (dg,/dc) (dddcz) -k (dgl/dcl)(dcl/dcp), and evaluating the partial derivatives from eq 1-3. (5) These empirical conclusions do not apply generally to hydroxylic solvents. For instance, in octanoic acid (co = 2.46) fiz,app2 p z 2 is close to zero for
-
a wide range of solutes: T.-P. I and E. Grunwald, J. Am. Chem. SOC.,98, 1351 (1976). (6) See, for example, J. H. Hildebrand and R. L. Scott, "The Solubility of Nonelectrolytes", Reinhold, New York, N.Y., 1950. (7) M. R. Crampton and E. Grunwald, J. Am. Chem. SOC.,93,2987 (1971). (8) E. Grunwald and A. Effio. J. Solution Chem., 2, 373 (1973).
Hydrogen Bonding in Polar Liquid Solutions. 4. Effect of Hydrogen-Bonding Solutes on Dielectric Constant and Solvent Structure in 1-Octanolla Ernest Grunwald,*Ib Kee-Chuan Pan, and Adan Effio Department of Chemistry, Brandeis University, Waitham, Massachusetts 02 154 (Received June 23, 1976) Publication costs assisted by the Petroleum Research Fund
Dielectric constant (e) was measured as a functilon of concentration at 24.9 "C for the following hydrogenbonding solutes in 1-octanol: benzaldehyde, acetone, (t-Bu)&O, (i-Bu)COCH3, (CH3)2S0, pyridine (Py), 2,4- and 2-6-(t-Bu)zPy,2-(i-Pr)-6-(t-Bu)Py,CHC13, and (CyH5)3COH.Molar dielectric increments (dt/dc, a t c2 = 0) were generally negative even though t for the majority of the pure liquid solutes is greater than t for 1-octanol. Solute-induced medium effects differed considerably from the relationship established for non-hydrogen-bonding solutes. Adopting a hydrogen-bonded chain model for 1-octanol, the mean chain length was found, by near-infrared spectroscopy, to be 27.7 at 25 "C. Adopting a model for sitewise equilibrium between free OH-donor sites, free 0-acceptor sites, and OH-0 hydrogen-bonded sites, the sitewise association constant K = 117 (M-l) a t 25 "C; AH" = -8.58 kcal, AS" = -19.3 gibbs/mol of hydrogen bonds. The sitewise equilibrium model predicts a marked breakdown of hydrogen-bonded solvent structure in the presence of hydrogen-bonding solutes.
In part 3 we considered the effects of non-hydrogenbonding solutes on dipole correlation in hydroxylic solvents.2 We shall now consider the effects of hydrogen-bonding solutes, which according to our data are even more complicated. For illustration, Figure 1 shows dielectric constant t as a function of c2 for dimethyl sulfoxide (DMSO, p~ = 3.91 D) in 1-octanol (OctOH, p 1 = 1.76 D). If DMSO were a non-hydrogen-bonding solute, with solute-induced medium effects given by eq 3-2, the relation between t and c2 would follow the dashed line in Figure 1,whose slope is positive. If DMSO and OctOH were forming a 1:l complex whose dipole moment, as reported in part 2, is 4.56 D, and if solute-induced medium effects were again given by (3-2), the slope would be approximately zero. By contrast, the experimental slope at low concentrations is negative! Pure DMSO (e0 = 46.7)3 is much more polar than OctOH (to = 10.01). Thlus the slope o f t vs. c2 cannot remain negative indefinitely. As shown in Figure 1,dtldc, changes sign a t c2 F= 0.24 M. Because DMSO is known to be an efficient hydrogen-bond a ~ c e p t o rwe , ~ expect the formation of solvation complexes of the general formula DMSO-(OctOH),. The solvation number m may be an integer or an average for a distribution, and m may vary with c 2 . If this is granted, then the negative initial slope allows of two interpretations: (1)Solvation complexes of DMSO in OctOH are markedly less polar than expected from the structure of the 1:l complex in benzene. (2) Hydrogen bonding between DMSO and OctOH couples the DMSO molecules to the hydrogen-bonded solvent structure and
i,hereby introduces a new kind of solute-induced medium effect that lowers the dielectric constant. Having found in part 2 that dipole moments of OctOH-L complexes, for typical ligands L, are quite insensitive to the solvent medium, we consider the first interpretation to be less probable. In this paper we shall report dielectric constants for a variety of hydrogen-bonding solutes in 1-octanol and show that i,he behavior of DMSO is part of a perplexing general pattern. 'We shall then consider the hydrogen-bonded structure of the solvent and show, by straightforward application of principles of chemical equilibrium, that hydrogen bonding to a solute greatly reduces the average aggregation number. In 1-octanol, such "structure breaking" of the solvent is attended by a decrease in the dielectric constant. In part 5 we shall develop Lhese concepts into a quantitative theory. Experimental Dielectric Constants Typical plots of At = t - €0 vs. c~ for hydrogen-bonding s801utesin 1-octanol are shown in Figures 1-3. Results for all slolutesare listed in Table I. Because the pyridine solutes ionize as bases in water, it is worth noting that ionization according t o Py
+ HOOct + PyH+.OOct-
~t
PyH+
+ OOct-
was found to be negligible. The evidence for this is that the conductivity of the solutions remained in all cases essentially the same as that of the solvent, about 0.4-0.8 nmho/cm. Free-ion concentrations as small as lop5 M could have been detected easily. Ion-pair dissociation constants for hydroThe Journal of Physical Chemistry, Vol. 80, No. 27, 1976
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E. Grunwald, K.-C. Pan, and A. Effio
gen-bonded ion pairs such as PyH+-X- in 1-octanol are in the range 10-6-10-5 M.5 Plots of At vs. cp are of three types: (A) At = S2c2; (B) At = S2c2 J2cz2 (Jzcp2 is relatively small); (C) At vs. e2 shows marked, characteristic curvature and passes through a minias ,listed in Table mum. In cases A and B the initial slopes s ~ I, are accurate to 0.1 M-' or better; in case C the standard errors of Szare greater, but should be within 0.3 M-I. If one may generalize from the results in Table I, the type of relationship between A t and c g depends on the magnitude of V S .Solutes with relatively large V2 (>170 cm3/mol) show linear or nearly linear plots, as illustrated in Figure 3, while solutes with small V2 (
