Alcohol-water mixtures dielectric constant

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

An interesting observation reported in Table XLIX is the increase in the hydroquinone/catechol ratio from 1.44 to 1.99 when the dielectric constant of the medium is decreased from 58.9 to 39.2 by addition of methanol to water. A similar increase in the hydroquinone/catechol ratios was also observed in phenol hydroxylation catalyzed by TS-1 (266) in dioxane-water and tert-butyl alcohol-water mixtures. The para/ortho ratio increased nearly 10-fold when 10% dioxane was added to water. Similarly, the para/ortho ratio more than doubled (1.3-3.0) when 10% tert-butyl alcohol was added to water. An opposite trend, namely, a decrease in the para/ortho ratio from 1.4 to 0.6, was observed when 10% formamide (s = 108) was added to water. Because of geometric constraints in the MFI pores, catechol is expected to be formed more easily on the external surface of TS-1 crystallites than in the pores (91). Hydroquinone, less spatially demanding, can form in the TS-1 channels. A greater coverage of the hydrophobic... 

Experimentally, it is observed that divalent ions can trigger condensation in alcohol-water mixtures. In our model, both A and q also depend on the dielectric constant of the solvent, so that the attraction increases and the repulsion decreases with decreasing e this lowers the threshold value of Zmm. We note, however, that the effect of alcohol could be much more subtle and could depend on microscopic details such as the structure of water near the DNA and counterions that are neglected in our model. 

In aquation of the cations [M(NH3)5Br] + (M = Cr or Co) in alcohol-water mixtures, solvent composition variation has much more effect on A.S than on iiH. This is attributed to solvation-shell ordering effects in the transition state. Enhancement of reactivity in the reaction of cobalt(ii) with chlorophyllic acid in methanol on addition of lithium nitrate is attributed to inhibition of transition-state solvation. The effective radius of a transition state can be guessed from the variation of rate constant with dielectric constant. This approach has been used for the bromide-bromate reaction. In contrast to this concentration of attention on the transition state, it may be noted that it is the stabilization of the reactant in relation to water structure that is thought to control the variation of the racemization rate of the ( + )-[Co(phen)3] + cation in t-butyl alcohol-water mixtures. ... 

Aqueous titrations of amines are amply discussed elsewhere d. Nonaqueous titrations. Three main purposes may be served by carrying out titrations in nonaqueous solvents increased solubility, change of the pH scale, and resolution of mixtures. The prediction of a potentiometric titration curve in an arbitrary solvent is a difficult task, in which many factors intervene, such as dielectric constant, definition of acid and base in relation to the solvent, electrodes, actual structure of conjugate acids and bases, etc. Acetic acid, sulphuric acid, acetonitrile, and alcohol-water mixtures have been extensively studied and were reviewed elsewhere Some solvents will be treated here briefly ... 

Neutral salts, when added to almost any solution of a non-elwtrolyte in water, affect the solubility of the solute. If the latter has a dielectric constant lower than that of water, the effect of the salt addition is to increase the activity (vapor pressure) of the solute, and thus to decrease its solubility. The most familiar instance to most chemists is the separation of an alcohol-water mixture into two phases on the addition of a salt such as sodium carbonate. In most such systems the solubility of the alcohol can be described by an equation of the form... 

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

Even though Hildebrand theory should not apply to solvent systems having considerable solvent-solvent or solute-solvent interactions, the solubility of compounds in co-solvent systems have been found to correlate with the Hildebrand parameter and dielectric constant of the solvent mixture. Often the solubility exhibits a maximum when plotting the solubility versus either the mixed solvent Hildebrand parameter or the solvent dielectric constant. When comparing different solvent systems of similar solvents, such as a series of alcohols and water, the maximum solubility occurs at approximately the same dielectric constant or Hildebrand parameter. This does not mean that the solubilities exhibit the same maximum solubility. 

Ionization of an alkyl halide requires formation and separation of positive and negative charges, similar to what happens when sodium chloride dissolves in water. Therefore, SN1 reactions require highly polar solvents that strongly solvate ions. One measure of a solvent s ability to solvate ions is its dielectric constant (e), a measure of the solvent s polarity. Table 6-6 lists the dielectric constants of some common solvents and the relative ionization rates for fm-butyl chloride in these solvents. Note that ionization occurs much faster in highly polar solvents such as water and alcohols. Although most alkyl halides are not soluble in water, they often dissolve in highly polar mixtures of acetone and alcohols with water. 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

The factor A in equation (123) is proportional to 1/(D T), as shown on page 150 hence, a further test of this equation is to determine the slope of the plot of log S/So against Vy from Solubility data at different temperatures and in media of different dielectric constants. Such measurements have been made in water at 75° (D = 63.7), in mixtures of water and ethyl alcohol (D = 33.8 to 78.6), in methyl alcohol (D = 30), in acetone (D = 21), and in ethylene chloride D = 10.4). The results have been found in all cases to be in very fair agreement with the requirements of the Debye-Huckel limiting law as may be expected, appreciable discrepancies occur when the saturating salt is of a high valence type, especially in the presence of added ions of high valence. ... 

Dielectric Constants of Some Solvents and Solutions. The dielectric constant of the solvent, as will be recalled from the discussion in Chapters 7 and 18, is important in the interpretation of the thermodynamic and conductance data on solutions of electrolytes, according to the interionic attraction theory. Up to the present time the data which are useful for tests of that theory have mostly been obtained on aqueous and alcoholic solutions, and on solutions in mixtures of dioxane and water. It is to be hoped that in the near future studies will be made on solutions in other solvents the dielectric constants and other relevant properties of which are now, in many cases at least, accurately... 

Dipole moment 1.83-1.90. Dielectric constant (25 ) 51.7. Latent heal of fusion (mp) 3.025 kcal/mole latent heat of vaporization (bp) 9760 kcal/mole (calc), Crit temp 38Diacidic base. K, (25°) about 9 X 10 7. Forms salts with inorganic acids. Highly polar solvent. Powerful reducing agent. Dissolves many inorganic substances. Misc with water, methyl, ethyl, propyl, isobutyl alcohols. Forms an azeotropic mixture with water, bp 40 120.3°, which contains 55 mole-% (68.5 weight-%) NjH,. LD,g in mice (mg/kg) 57 i.v. 59 orally (Witkin),... 

The crudeness of the modeP should dissuade us from accepting this as gospel. Current disillusionment with the Born equation is understandable, but its minor triumphs should not be denied. It does predict that Ap for molecular acids should be a linear function of the reciprocal of the dielectric constant, and this is verified for water-alcohol mixtures of moderate to high dielectric constant. ... 

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