Dielectric spectroscopy of 2-butoxyethanol/water mixtures in the complete composition range

Nature of Salt Effects and Mechanism of Covalent Bond Heterolysis

Data on the influence of neutral salts on the rates of unimolecular heterolyses of organic substrates, obtained mainly by the verdazyl method, are summarized here. It is assumed that heterolysis takes place with consecutive formation of four ion pairs: contact (CIP), cavity-separated (CSIP), one solvent moleculeseparated (SIP) and solvent-separated (SSIP). [Formula: see text]

In the limiting step, the CIP interacts with a solvent cavity and the CSIP is formed, which converts quickly into the SIP and subsequently to the SSIP, which also quickly gives the reaction products. In the transition state, bonds between the molecules solvating the CIP are broken. In the absence of salt, the return from external ion pairs does not have much importance. The verdazyl indicator quickly and quantitatively reacts with the SSIP.

The normal salt effect takes place due to the action of salt on the covalent substrate, which catalyses CIP formation. The special salt effect is caused by the association of the salt with the CIP, which prolongs the lifetime of the intermediate and increases the probability of its contact with a solvent cavity.

The negative special salt effect is caused by association of the salt with the SIP or SSIP, which prolongs the lifetime of the intermediates and increases the probability of their contact with a solvent cavity to return to the covalent substrate. When the salt reacts with the SIP, the salt effect does not depend on the concentration and nature of verdazyl, but such a dependence takes place when the salt reacts with the SSIP.

The site of the action of the salt is determined by the Hard and Soft Acids and Bases (HSAB) principle.

Dielectric constant of polyhydric alcohol-DMSO mixture solution at the microwave frequency

Dielectric spectrum of polyhydric alcohol (1,2,3)-DMSO (dimethyl sulfoxide) mixtures, at full concentration, have been determined by the dielectric relaxation spectroscopy (DRS) method at frequencies from 20 MHz to 20 GHz at room temperature. The mixture behavior is described according to four Davidson-Cole terms whose evolution with composition is analyzed. The binding energies and hydrogen bond (HB) numbers between solute-solute and solvent-solute pairs are obtained from the permittivity using the Luzar model. The average HBs number associated with DMSO decreases with increasing mole fraction of DMSO. The binding energy of solute-solute (E11) and solvent-solute (E12) interaction decreases with the increased numbers of carbon atoms in the alcohols.

A Marcus treatment of rate constants for protonation of ring-substituted alpha-methoxystyrenes: intrinsic reaction barriers and the shape of the reaction coordinate

Rate and equilibrium constants were determined for protonation of ring-substituted -methoxystyrenes by hydronium ion and by carboxylic acids to form the corresponding ring-substituted alpha-methyl alpha-methoxybenzyl carbocations at 25 degrees C and I = 1.0 (KCl). The thermodynamic barrier to carbocation formation increases by 14.5 kcal/mol as the phenyl ring substituent(s) is changed from 4-MeO- to 3,5-di-NO2-, and as the carboxylic acid is changed from dichloroacetic to acetic acid. The Brønsted coefficient alpha for protonation by carboxylic acids increases from 0.67 to 0.77 over this range of phenyl ring substituents, and the Brønsted coefficient beta for proton transfer increases from 0.63 to 0.69 as the carboxylic acid is changed from dichloroacetic to acetic acid. The change in these Brønsted coefficients with changing reaction driving force, (inverted theta)alpha/ (inverted theta) deltaG(av) degrees=(inverted theta)beta/(inverted theta)delta G(av) degrees= 1/8lambda = 0.011, is used to calculate a Marcus intrinsic reaction barrier of lambda= 11 kcal/mol which is close to the barrier of 13 kcal/mol for thermoneutral proton transfer between this series of acids and bases. The value of alpha= 0.66 for thermoneutral proton transfer is greater than alpha= 0.50 required by a reaction that follows the Marcus equation. This elevated value of beta may be due to an asymmetry in the reaction coordinate that arises from the difference in the intrinsic barriers for proton transfer at the oxygen acid reactant and resonance-stabilized carbon acid product.

When does an intermediate become a transition state? Degenerate isomerization without competing racemization during solvolysis of (S)-1-(3-nitrophenyl)ethyl tosylate

(S)-1-(3-Nitrophenyl)ethyl tosylate [(S)-2-OTs] was prepared in >99% enantiomeric excess and the change in the chiral purity of this compound was monitored during solvolysis in 50:50 trifluoroethanol/water. The barely detectable formation of 0.5% (R)-2-OTs after two half times for the solvolysis reaction was used to calculate a rate constant of k(rac) approximately equal to 4 x 10-6 s-1. This is 80-fold smaller than kiso = 3.2 x 10-4 s-1 for the isomerization that exchanges oxygen-16 and oxygen-18 of 3-NO2C6H413CH(Me)OS(18O)2Tos during solvolysis and 10-fold smaller than the minimum value of k(rac) = 4.6 x 10-5 s-1 predicted if isomerization and racemization products form by partitioning of a common ion-pair intermediate of a stepwise reaction. It is concluded that the isomerization reaction proceeds mainly by a pathway that avoids formation of this putative intermediate. It is suggested that the solvolysis reaction of 2-OTs may proceed by a stepwise preassociation mechanism where solvent "reorganization" precedes substrate ionization to form an ion-pair intermediate.

Hydrogen network fluctuations of associating liquids: Dielectric relaxation of ethylene glycol oligomers and their mixtures with water

Complex dielectric spectra of ethylene glycol and of various derivatives as well as of mixtures of water with an ethylene glycol oligomer and with a poly(ethylene glycol) dimethyl ether oligomer have been measured. The spectra can be well represented by a Cole-Cole [Cole and Cole, J. Chem. Phys. 9, 341 (1941)] spectral function. The extrapolated low frequency (static) permittivity of this function has been evaluated to yield the effective dipole orientation correlation factor of the liquids. The relaxation time of the ethylene glycols displays a characteristic dependence upon the ratio of concentrations of hydrogen bond donating and accepting groups, indicating two opposing effects. With increasing availability of hydrogen bonding sites effects of association and also of dynamical destabilization increase. Both effects exist also in the mixture of water with the oligomers. They are discussed in terms of a wait-and-switch model of dipole reorientation in associating liquids. Another feature in the dependence of the dielectric relaxation time of poly(ethylene glycol)/water mixtures upon mixture composition has been tentatively assigned to precritical demixing behavior of the binary liquids in some temperature range.

Thermosolvatochromism of Betaine Dyes Revisited: Theoretical Calculations of the Concentrations of Alcohol−Water Hydrogen-bonded Species and Application to Solvation in Aqueous Alcohols

Solvatochromic data of 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (RB) in aqueous methanol, 1-propanol, 2-propanol, and 2-methyl-2-propanol at 25 degrees C were recalculated by employing a recently introduced model that explicitly considers the presence of 1:1 alcohol-water hydrogen-bonded species, ROH-W, in bulk solution and their exchange equilibria with water and alcohol in the probe solvation microsphere. The thermosolvatochromic behavior of RB in aqueous ethanol was measured in the temperature range from 10 to 60 degrees C; the results thus obtained were treated according to the same model. All calculations require reliable values of Kdissoc, the dissociation constant of the ROH-W species. This was previously calculated from the dependence of the density of the binary solvent mixture on its composition. Through the use of iteration, the volume of the hydrogen-bonded species, VROH-W, and Kdissoc are obtained simultaneously from the same set of experimental data. This approach may be potentially problematic because Kdissoc and VROH-W are highly correlated. Therefore, we introduced the following approach: (i) VROH-W was obtained from ab initio calculations, (ii) these volumes were corrected for the nonideal behavior of the binary solvent mixtures at different temperatures, (iii) corrected VROH-W values were employed as a constant in the equation used to calculate Kdissoc (from density vs binary solvent mixture composition). VROH-W calculated by the COSMO-RS solvation model fitted the density data better than those calculated by the IEFPCM model. In all aqueous alcohols, solvation by ROH-W is favored over that by the two precursor solvents. In aqueous ethanol, a temperature increase resulted in a gradual desolvation of RB, due to a decrease in the hydrogen-bonding of both components of the mixture. The microscopic polarities of ROH-W are much closer to those of the precursor alcohols.

Relaxation rate in the critical dynamics of the micellar isobutoxyethanol-water system with lower consolute point

For the isobutoxyethanol-water mixture of critical composition acoustical scaling function measurements have been performed at frequencies between 200 kHz and 3 MHz. Using the relaxation rates of concentration fluctuations as resulting from dynamic light scattering and shear viscosity measurements, the acoustical data can be well represented by the Bhattacharjee-Ferrell empirical scaling function. The theoretically predicted scaled half attenuation frequency Omega(BF)(1/2)=2.1 follows from the experiments. However, the adiabatic coupling constant, derived from the amplitude of the Bhattacharjee-Ferrell critical term, displays an unreliably strong temperature dependence. It is shown that this dependence upon temperature likely reflects a hidden low-frequency and low amplitude relaxation term in the acoustical spectra of the mixture of critical composition.

Formation and stability of organic zwitterions — The carbon acid pKas of the trimethylsulfonium and tetramethylphosphonium cations in water

We report second-order rate constants of kDO = 7.5 × 10–4 and 9.9 × 10–5 (mol/L)–1 s–1 for exchange for deuterium of the first methyl proton of the trimethylsulfonium and tetramethylphosphonium cations, respectively, in D2O at 25 °C and I = 1.0 (KCl). The data were analyzed to give the following carbon acidities for these cationic carbon acids in water: (CH3)3S+, pKa = 28.5; (CH3)4P+, pKa = 29.4. These acidities are close to those of the neutral carbon acids acetonitrile and dimethylacetamide. This provides evidence that a portion of the stabilization of the cyanomethyl carbanion is due to resonance delocalization of negative charge from carbon to cyano nitrogen.Key words: carbon acids, carbanions, ylides, proton transfer.

Kinetic and Thermodynamic Barriers to Carbon and Oxygen Alkylation of Phenol and Phenoxide Ion by the 1-(4-Methoxyphenyl)ethyl Carbocation

Rate constant ratios for addition of the three nucleophilic sites of phenol to the 1-(4-methoxyphenyl)ethyl carbocation (1+) in 50/50 (v/v) trifluoroethanol/water were determined from the relative yields of the three phenol adducts, and absolute rate constants were determined from product rate constant ratios for addition of phenol and azide ion to 1+ using k(az) = 5 x 10(9) M(-1) s(-1) for the diffusion-limited reaction of azide ion. A selectivity of 230:20:1 was determined for alkylation of phenol at oxygen, C-4 and C-2 to form 1-OPh and biphenyls 1-(4-C6H4OH) and 1-(2-C6H4OH), respectively, and of 2:2:1 for alkylation of the corresponding nucleophilic sites of phenoxide ion in diffusion-limited reactions. The Mayr nucleophilicity parameter for C-4 of phenol is N = 2.0. Encounter-limited addition of phenoxide ion to 1+ to form 1-OPh is faster than encounter-limited addition of oxygen anions that are either more or less basic than phenoxide ion. Only the products of solvolysis are observed from acid-catalyzed cleavage of 1-OPh in 50/50 (v/v) trifluoroethanol/water, but a 50% yield of biphenyls 1-(4-C6H4OH) and 1-(2-C6H4OH) are observed from spontaneous cleavage of 1-OPh, where the leaving group is phenoxide ion, because of the very low kinetic barriers to collapse of the ion pair intermediate 1+.PhO-. The 230-fold larger rate constant for O-compared to C-2-alkylation of phenol is due primarily to the larger thermodynamic driving force for oxygen addition. There are similar Marcus intrinsic barriers for these two reactions.

Formation and stability of enolates of acetamide and acetate anion: an Eigen plot for proton transfer at alpha-carbonyl carbon

Second-order rate constants were determined in D(2)O for deprotonation of acetamide, N,N-dimethylacetamide, and acetate anion by deuterioxide ion and for deprotonation of acetamide by quinuclidine. The values of k(B) = 4.8 x 10(-8) M(-1) s(-1) for deprotonation of acetamide by quinuclidine (pK(BH) = 11.5) and k(BH) = 2-5 x 10(9) M(-1) s(-1) for the encounter-limited reverse protonation of the enolate by protonated quinuclidine give pK(a)(C) = 28.4 for ionization of acetamide as a carbon acid. The limiting value of k(HOH) = 1 x 10(11) s(-1) for protonation of the enolate of acetate anion by solvent water and k(HO) = 3.5 x 10(-9) M(-1) s(-1) for deprotonation of acetate anion by HO(-) give pK(a)(C) approximately 33.5 for acetate anion. The change in the rate-limiting step from chemical proton transfer to solvent reorganization results in a downward break in the slope of the plot of log k(HO) against carbon acid pK(a) for deprotonation of a wide range of neutral alpha-carbonyl carbon acids by hydroxide ion, from -0.40 to -1.0. Good estimates are reported for the stabilization of the carbonyl group relative to the enol tautomer by electron donation from alpha-SEt, alpha-OMe, alpha-NH(2), and alpha-O(-) substituents. The alpha-NH(2) and alpha-OMe groups show similar stabilizing interactions with the carbonyl group, while the interaction of alpha-O(-) is only 3.4 kcal/mol more stabilizing than for alpha-OH. We propose that destabilization of the enolate intermediates of enzymatic reactions results in an increasing recruitment of metal ions by the enzyme to provide electrophilic catalysis of enolate formation.