Kinetics and mechanism of the base hydrolysis of thecis-chlorobis(ethylenediamme)(benzimidazole)cobalt(III) cation

Summary The hydrolysis ofcis-[CoCl(en)₂(bzmH)]²⁺ (en=ethylenediamine, bzmH=benzimidazole) has been studied over the pH range 8.31–11.58 at I=0.1 mol dm^–3 and 25°. Potentiometric titration of aqueous solutions of the [Co(en)₂(bzmH)OH₂]³⁺ complex obtained by silver(I) catalysed aquation of the chloro-complex give pK₁=5.81 and pK₂ = 8.84 for Equilibria (1) and (2) at 25° and I=0.1 mol dm^–3. Spectrophotometric titration of the hydroxy complex also gives a value of pK₂=8.88 for the ionisation of the coordinated benzimidazole. The kinetic data can be interpreted in terms of base hydrolysis ofcis-[CoCl(en)₂(bzmH)]²⁺ (k_OH=220 dm³ mol^–1s^–1) andcis-[CoCl(en)₂(bzm)]⁺ (k_OH=14.9 dm³ mol^–1s^–1). Comparisons with the corresponding imidazole and pyridine complexes are made.     

Small-angle neutron-scattering study and micellar model of the gemini (phenylenedimethylene)bis(n-octylammonium)dibromide surfactant micelles in water

The microstructure of the micelles formed in aqueous solution by gemini surfactants with aromatic spacers, [Br(CH₃)₂N⁺(C _m H_{2 m +1})-(Ph)-(C _m H_{2 m +1})N⁺(CH₃)₂Br, m=8 and Ph = o-, m- or p-phenylenedimethylene] has been examined by small-angle neutron scattering. Aggregation of the gemini surfactants with an o-phenylenedimethylene spacer brings about formation of premicelles and small micelles at concentrations below the second critical micelle concentration, while above this concentration marked micellar growth and variation in shape occurs. It is suggested that the minimum aggregate formed at this critical micelle concentration may be the trimer or tetramer and that this result supports the mechanism of “gemini → submicelle → assembly” for micellar growth. Received: 8 September 1998 Accepted in revised form: 27 November 1998     

Effect of bromide salts on cationic micellar catalysis

 The effect of bromide salts, MBr [M=Na, (CH₃)₄N, (C₂H₅)₄N, (C₄H₉)₄N, C₈H₁₇N(CH₃)₃], on the first-order rate constant, k ₁, of basic hydrolysis of 2,4-dinitrochlorobenzene in micelle solutions of cetyltrimethylammonium bromide has been studied. The main results are as follows. The molar ratio concentrations of OH⁻, m ^S _OH, on the micelle surface in the presence of different concentrations of Br⁻ ions, were calculated on the basis of the pseudophase ion-exchange model, and there is a linear relation between k ₁ and m ^S _OH. The relation between k ₁ and the concentrations of various bromides could be presented with a single curve, and the cations of the bromides have little effect on k ₁. Under the experimental conditions, there is a linear relation between 1/k ₁ and the concentrations of Br⁻; thereby a new method calculating the competition binding constant between OH⁻ and Br⁻ from dynamic data is proposed. The hydrodynamic radii of the micelles increase with the addition of bromide salts. Received: 1 August 2000 Accepted: 31 January 2001     

Mass spectra of ethenol and its deutero analogues

Ethenol, 1-d-ethenol, O-d-ethenol and Z-2-d-ethenol were prepared by pyrolysis of corresponding 5-norbornenols at 800^°C/2 × 10^?6 Torr. The most important fragments in the electron impact mass spectrum of ethenol are [C₂H₃O]⁺ and CHO⁺ and CH₃˙. The hydrogen atom eliminated from the molecular ion comes mainly from the hydroxyl group (68%) and to a lesser extent from C(1) (25%) and C(2) (7%). The loss of the hydroxyl hydrogen is preceded by rate-determining migration of the hydrogen atom from C(1) onto C(2) to yield CH₃C?OH⁺˙ions that decompose to CH₃CO⁺ and H˙. The loss of deuterium from O-d-ethenol shows a very small primary isotope effect (k_H/k_D=1.07), whereas a significant effect is observed for the loss of hydrogen from 1-d-ethenol (k_H/k_D=1.28). The appearance energy of [C₂H₂DO]⁺ from 1-d-ethenol, AE=11.32 eV, gives a critical energy for the hydrogen loss, E=203 kJ mol^?1, which is 90 kJ mol^?1 above the thermochemical threshold for CH₃CO⁺+H˙. The appearance energy of CDO⁺ from 1-d-ethenol was measured as 12.96±0.07 eV, which sets the barrier to isomerization to CH₃CDO⁺˙ at 1121 kJ mol^?1. The ionization energy of ethenol was found to be 9.22±0.03 eV.     

Kinetics and Mechanism of the Cerium(IV) Oxidation of Arnold's Base and the Protonation and Hydroxylation of Michler's Hydrol Blue

In aqueous H₂SO₄, Ce(IV) ion oxidizes rapidly Arnold's base((p-Me₂NC₆H₄)₂CH₂, Ar₂CH₂) to the protonated species of Michler's hydrol((p-Me₂NC₆H₄)₂CHOH, Ar₂CHOH) and Michler's hydrol blue((p-Me₂NC₆H₄)₂CH⁺, Ar₂CH⁺). With Ar₂CH₂ in excess, the rate law of the Ce(IV)-Ar₂CH₂ reaction in 0.100 M H₂SO₄ is expressed -d[Ce(IV)]/dt = k_app[Ar₂CH₂]₀[Ce(IV)] with k_app = 199 ± 8M^?1s^?1 at25°C. When the consumption of Ce(IV) ion is nearly complete, the characteristic blue color of Ar₂CH⁺ ion starts to appear; later it fades relatively slowly. The electron transfer of this reaction takes place on the nitrogen atom rather than on the methylene carbon atom. The dissociation of the binuclear complex [Ce(III)ArCHAr-Ce(III)] is responsible for the appearance of the Ar₂CH⁺ dye whereas the protonation reaction causes the dye to fade. In highly acidic solution, the rate law of the protonation reaction of Michler's hydrol blue is -d[Ar₂CH⁺]/dt = k_obs[Ar₂CH⁺] where K_obs = ((ac + 1)[H*] + bc[H⁺]²)/(a + b[H⁺]) (in HClO₄) and k_obs= ((ac + 1 + e[HSO₄^?])[H⁺] + bc[H⁺]² + d[HSO₄^?] + q[HSO₄^?]²/[H⁺])/(a + b[H⁺] + f[HSO₄^?] + g[HSO₄^?]/[H⁺]) (in H₂SO₄), and at 25°C and μ = 0.1 M, a = 0.0870 M s, b = 0.655 s, c = 0.202 M^?1s^?1, d = 0.110, e = 0.0070 M^?1, f = 0.156 s, g = 0.156 s, and q = 0.124. In highly basic solution, the rate law of the hydroxylation reaction of Michler's hydrol blue is -d[Ar₂CH⁺]/dt = k_OH[OH^?]₀[Ar₂CH⁺] with k_OH = 174 ± 1 M^?1s^?1 at 25°C and μ = 0.1 M. The protonation reaction of Michler's hydrol blue takes place predominantly via hydrolysis whereas its hydroxylation occurs predominantly via the path of direct OH attack.     

Quelques precisions sur les effets catalytiques des micelles cationiques hydroxylees

The alkaline hydrolysis of p-nitrophenyl acetate and of CH₃CO₂(CH₂)₂N⁺(CH₃)₂C₁₆H₃₃, Br^? was studied in the presence of micelles C₁₆H₃₃N⁺(CH₃)₂CH₂CH₂OH, Br^? and CTAB, C₁₆H₃₃N⁺(CH₃)₃,Br^?. A pathway involving an intermediate is suggested for the hydrolysis of the ester. Hydrolysis rate of the intermediate in the presence of micelles is the same as hydrolysis rate for the ester in the absence of micelles. Consequently, hydrolysis of p-nitrophenyl acetate is not catalysed by one type of micelle, while it is enhanced by another type of micelle.     

Zur theorie der α-ketosa¨uren: Mechanismus der enolisierung einiger brenztraubensa¨ureamide

The mechanism of enolisation of pyruvamide is discussed by the influence of substituents on the kinetic CH₃-acidity, by general base-catalysis of enolisation, by the enthalpy and entropy of activation and primary kinetic and kinetic solvent deuterium isotope effects respectively. A Bro¨nsted coefficient β = 0·71 has been obtained in the general base catalysis of pyruvdiethylamide enolisation. The effect of car☐ylsubstituents on the kinetic CH₃-acidity is produced not only by an inductive mechanism. The importance of solvent structure is demonstrated by a strong negative entropy of activation for the H₂O-catalysed reaction. In the H₂O-catalysed enolisation of pyruvdiethylamide a large kinetic deuterium solvent isotope effect k_o^H₂O/k_o^D₂O = 2·39) was obtained at 25°C. In contrast, when hydroxid is the catalyst, the primary kinetic deuterium isotope effect is unusually low (k_H/k_D = 3·5). Thus, in comparison to other keto compounds, a different mechanism of enolisation for the pyruvic acid derivatives must be postulated. Some aspects of this mechanism are discussed in the paper.     

Nucleophilic and electrophilic promotion of ligand substitution reactions in oxo‐bridged,dinuclear chromium(III) complexes

Base hydrolysis reactions of [Cr(tmpa)(NCSe)]₂O²⁺, [Cr(tmpa)(N₃)]₂O²⁺, [Cr₂(tmpa)₂(μ−O)(μ−PhPO₄)]⁴⁺ and [Cr₂(tmpa)₂(μ−O)(μ−CO₃)]²⁺ follow the pseudo‐first‐order relationship (excess OH⁻): k_obsd=k_o+k_bQ_p[OH⁻]/(1+Q_p[OH⁻]). For the CO₃²⁻ complex, k_b(60°C)=(1.50±0.03)×10⁻² s⁻¹; ΔH‡=61±2 kJ/mol, ΔS‡=−99±7 J/mol K; Q_p(60°C)=(3.8±0.3)×10¹ M⁻¹; ΔH°=67±2 kJ/mol, ΔS°=230±7 J/mol K (I=1.0 M). An isokinetic relationship among k_OH(=k_bQ_p) activation parameters for five (tmpa)CrOCr(tmpa) complexes shows that all follow essentially the same pathway. Activated complex formation is thought to require nucleophilic attack of coordinated OH⁻ at the chromium‐leaving group bond in the k_b step, accompanied by reattachment of a tmpa pyridyl arm displaced by OH⁻ in the Q_p preequilibrium. Abstraction of both thiocyanate ligands was observed upon mixing [Cr(tmpa)(NCS)]₂O²⁺ with [Pd(CH₃CN)₄]²⁺ in CH₃CN solution. The proposed mechanism requires rapid complexation of both reactant thiocyanate ligands by Pd(II) (K_p(25°C)=(4.5±0.2)×10⁸ M⁻²; ΔH°=−32±6 kJ/mol, ΔS°=59±19 J/mol K) prior to rate‐limiting Cr NCS bond‐breaking (k₂(25°C)=(1.17±0.02)×10⁻³ s⁻¹; ΔH‡=98±2 kJ/mol, ΔS‡=27±5 J/mol K). Pd(II)‐assisted NCS⁻ abstraction is not driven by weakening of the Cr( )NCS⁻ bond through ligation of the sulfur atom to palladium, but rather by a favorable ΔS‡ resulting from the release of Pd(NCS)⁺ fragments and weak solvation of the activated complex in CH₃CN solution. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 351–356, 1999     

Relative reactivities of heteroaromatic cations towards reduction by 1,4-dihydronicotinamides

Kinetic data are reported for the equilibration of the 1-methyl-3-nitropyridinium cation with its pseudobase (hydroxide adduct) and for the reduction of this cation by 1-benzyl-1, 4-dihydronicotinamide. The C-2 hydroxide adduct is the kinetically controlled product (pK_R+ = 11.6) when this pyridinium cation is mixed with aqueous base, however, this species rearranges to the C-4 adduct as the themodynamically more stable product (pK_R+ = 9.42). The pH-dependence of this equilibration may be analysed to give k_OH = 1600 M^-1s^-1 for hydroxide ion attack at C-4 of this cation. Reduction of this pyridinium cation by 1-benzyl-1, 4-dihydronicotinamide appears to occur exclusively at C-4 with second-order rate constant k₂ = 0.72 M^-1s^-1 and k₂ ^H/k₂^D = 2.0 in 20% CH₃CN - 80% H₂O, ionic strength 1.0, 25°C.The reactivities of pyridinium, quinolinium, isoquinolinium, acridinium and phenanthridinium cations of pK_R+ = 10.0 towards both hydroxide ion and 1-benzyl-1, 4-dihydronicotinamide are evaluated. Relative reactivities (K₂/K_OH) for these two processes are shown to be acridinium : quinolinium (C-4) : pyridinium (C-4) : quinolinium (C-2) : isoquinolinium : phenanthridinium = 1.6×10⁵ : 3400 : 80 : (4 : 1.0 : 0.7 for predominantly aqueous reaction media. These data support the hypothesis that formation of 1, 2-dihydropyridine systems upon reduction of heteroaromatic cations by 1, 4-dihydronicotinamides occurs via direct one step hydride transfer, while formation of 1,4-dihydro-pyridines in such processes occurs preferentially by a mechanistica11y more complex process involving electron transfer.     

Activity and kinetics studies of yeast alcohol dehydrogenase in a reverse micelle formulated from functional surfactants

Yeast alcohol dehydrogenase (YADH) showed substantial decrease in its catalytic activity due to the strong electrostatic interaction between the head groups of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and YADH in AOT reverse micelles. However, the catalytic activity of YADH in a nonionic reverse micellar interface (GGDE/TX-100) obtained from a functional nonionic surfactant N-gluconyl glutamic acid didecyl ester (GGDE) and Triton X-100 (TX-100) was higher than that in AOT reverse micelle under the respective optimum conditions. A comparison of the kinetic parameters showed that the turnover number k_cat in GGDE/TX-100 reverse micelle was 1.4 times as large as that in AOT reverse micelle, but the Michaelis constants in AOT reverse micelle for ethanol K_m^B was twice and for coenzyme NAD⁺ K_m^A was 5 times higher than their counterparts in GGDE/TX-100 reverse micelle. For the conversion of ethanol, the smaller K_m^B and larger k_cat in GGDE/TX-100 reverse micelle resulted in higher catalytic efficiency k_cat/K_m^B. The stability of YADH in GGDE/TX-100 reverse micelle was also found to be better than that in AOT reverse micelle. They were mainly attributed to the absence of electric charge on the head groups of GGDE and TX-100 in the GGDE/TX-100 reverse micelle.      

Solvent/substrate effects on the micelle-catalyzed aquation of some iron(II)-phenanthroline complexes, II. Influence of urea and sodium benzoate on the aquation of Fe(Ph2Phen)32+ in acetone

The micelle-catalyzed aquation rates of iron(II) phenanthroline complex Fe(Ph₂Phen) ₃ ²⁺ have been measured in aqueous acetone as solvent in the presence of urea and sodium benzoate as substrates. The variation patterns manifested by the estimated maximum catalytic factors and k_o vs [substrate] profiles show that the aquation rates of the complex are inhibited by urea, while they are significantly enhanced by sodium benzoate. These observations have been radionalized by considering the relative micelle bond-breaking capacities of the substrates, the relative strengths of the H-bonds formed by the substrate/OH^– headgroups of the micelle and the H₂O/OH^– headgroups of the micelle pairs.     

Isomeric ion-neutral complexes generated from ionized 2-Methylpropanol and n-Butanol: the effect of the polarity of the neutral partner on complex-mediated reactions

Photoionization was used to characterize the energy dependence of C₃H ₇ ⁺ , C₃H ₆ ⁺ , CH₃OH ₂ ⁺ and CH₂=OH⁺ formation from (CH₃)₂)CHCH₂OH^+? (1) and CH₃CH₂CH₂CH₂OH^+? (2). Decomposition patterns of labeled ions demonstrate that close to threshold these products are primarily formed through [CH ₃ ⁺ CHCH₃ ^?CH₂OH] (bd3) from 1 and through [CH₃CH₂CH₂ ^?CH₂=OH⁺] (9) from 2. The onset energies for forming the above products from 1 are spread over 85 kJ mol^?1, and are all near thermochemical threshold. The corresponding onsets from 2 are in a 19 kJ mol^?1 range, and all except that of CH₂=OH⁺ are well above their thermochemical thresholds. Each decomposition of 3 occurs over a broad energy range (> 214 kJ mol^?1), This demonstrates that ion-permanent dipole complexes can be significant intermediates over a much wider energy range than ion-induced dipole complexes can be. H-exchange between partners in the complexes appears to be much faster than exchange by conventional interconversions of the alcohol molecular ions with their distonic isomers. The onsets for water elimination from 1 and 2 are below the onsets for the complex-mediated processes, demonstrating that the latter are not necessarily the lowest energy decompositions of a given ion when the neutral partner in the complex is polar.     

Hydrophobic N‐Diazeniumdiolates and the Aqueous Interface of Sodium Dodecyl Sulfate (SDS) Micelles

Zwitterionic diazeniumdiolates of the form RN[N(O)NO^?](CH₂)₂NH₂⁺R, where R=CH₃ ( 1 ), (CH₂)₃CH₃ ( 2 ), (CH₂)₅CH₃ ( 3 ), and (CH₂)₇CH₃ ( 4 ) were synthesized by reaction of the corresponding diamines with nitric oxide. Spectrophotometrically determined pK_a(O) values, attributed to protonation at the terminal oxygen of the diazeniumdiolate group, show shifts to higher values in dependence of the chain lengths of R. The pH dependence of the decomposition of NO donors 1 – 3 was studied in buffered solution between pH 5 and 8 at 22 °C, from which pK_a(N) values for protonation at the amino nitrogen, leading to release of NO, were estimated. It is shown that the decomposition of these diazeniumdiolates is markedly catalyzed by anionic SDS micelles. First‐order rate constants for the decay of 1 – 4 were determined in phosphate buffer pH 7.4 at 22 °C as a function of SDS concentration. Micellar binding constants, K_SM, for the association of diazeniumdiolates 1 – 3 with the SDS micelles were also determined, again showing a significant increase with increasing length of the alkyl side chains. The decomposition of 1 – 3 in micellar solution is quantitatively described by using the pseudo‐phase ion‐exchange (PIE) model, in which the degree of micellar catalysis is taken into account through the ratio of the second‐order rate constants (k_2m/k_2w) for decay in the micelles and in the bulk aqueous phase. The decay kinetics of 1 – 3 were further studied in the presence of cosolvents and nonionic surfactants, but no effect on the rate of NO release was observed. The kinetic data are discussed in terms of association to the micelle–aqueous phase interface of the negatively charged micelles. The apparent interfacial pH value of SDS micelles was evaluated from comparison of the pH dependence of the first‐order decay rate constants of 2 and 3 in neat buffer and the rate data obtained for the surfactant‐mediated decay. For a bulk phase of pH 7.4, an interfacial pH of 5.7–5.8 was determined, consistent with the distribution of H⁺ in the vicinity of the negatively charged micelles. The data demonstrate the utility of 2 and 3 as probes for the determination of the apparent pH value in the Stern region of anionic micelles.     

Unusual Rate Enhancement in the Hydroxide-Ion-Catalyzed Cleavage of Acetyl Salicylate Ion (Aspirin Anion) in the Presence of Anionic Micelles

The reaction rate studies on the hydrolytic cleavage of acetyl salicylate ion (AS^-) within the [^-OH] range 0.010-0.025 M reveal AS^- and ^-OH as the reactants. The effects of micelles of sodium dodecyl sulfate (SDS) on observed pseudo-first-order rate constants (k_obs) for the hydrolytic cleavage of AS^- have been studied at different [OH^-]. At a constant [OH^-], the rate constants (k_obs) follow an empirical relationship: k_obs = C + F [SDS]_T where [SDS]_T represents total SDS concentration. The magnitudes of C and F increase with an increase in [OH^-]. These data are explained in terms of the pseudophase model of the micelle.     

Loss of methyl from [H2CC(OH)?CH3]+˙ ions prepared by electron impact ionization of unstable 2-hydroxypropene

Unstable 2-hydroxpropene was prepared by retro-Diels-Alder decomposition of 5-exo-methyl-5-norbornenol at 800°C/2 × 10^?6 Torr. The ionization energy of 2-hydroxypropene was measured as 8.67±0.05 eV. Formation of [C₂H₃O]⁺ and [CH₃]⁺ ions originating from different parts of the parent ion was examined by means of ¹³C and deuterium labelling. Threshold-energy [H₂C?C(OH)? CH₃]^+˙ ions decompose to CH₃CO⁺+CH₃^˙ with appearance energy AE(CH₃CO⁺) = 11.03 ± 0.03 eV. Higher energy ions also form CH₂?C?OH⁺ + CH₃ with appearance energy AE(CH₂?C?OH⁺) = 12.2–12.3 eV. The fragmentation competes with hydrogen migration between C(1) and C(3) in the parent ion. [C₂H₃O]⁺ ions containing the original methyl group and [CH₃]⁺ ions incorporating the former methylene and the hydroxyl hydrogen atom are formed preferentially, compared with their corresponding counterparts. This behaviour is due to rate-determining isomerization [H₂C?C(OH)? CH₃]^+˙ →[CH₃COCH₃]^+˙, followed by asymmetrical fragmentation of the latter ions. Effects of internal energy and isotope substitution are discussed.     

Micelle hydration by1H-NMR

¹H-NMR studies were carried out for solution of amphiphilic betaine ester derivatives (of the general formula (CH₃)₃N⁺CH₂COOC _n H_2n+1Cl^– (V-n), wheren=10, 12, 14, and 16) andn-dodecyltrimethylammonium chloride (I-12). The spectra were taken at concentrations above and below critical micelle concentrations and chemical shifts were analyzed. It was stated that micelles are hydrated at the depth of the two CH₂ groups in the case ofV-n and the CH₂COO group in the case ofI-12. Therefore, the CH₂COO group during the micellization behaves as if it were CH₂CH₂ group.     

Steady-State Kinetics of Aminopeptidase Catalysis: A Stopped-Flow Radiationless Energy Transfer Study

Stopped-flow radiationless energy transfer experiments have been carried out to investigate the hydrolysis of some dansyl peptide substrates (S) catalyzed by aminopeptidase (E). RET between enzyme tryptophanyl residues and the dansyl group in the substrate allowed direct observation and quantitation of the enzyme-substrate (ES) complexes. Analysis of the stopped-flow RET traces gives k_cat = 1.32 s^?1 and K_M = 47 μM for Leu-Ala-NH(CH₂)₂NH-Dns (Leu-Ala-DED) and k_cat = 4.80 s^?1 and K_M = 196 μM for Leu-Gly-NH(CH₂)₂NH-Dns (Leu-Gly-DED). The activation energies of the enzymatic reactions were determined from the Arrhenius plots to be 57 and 38 kJ mol^?1 for Leu-Ala-DED and Leu-Gly-DED, respectively. The kinetic results indicate that the enzyme binds Leu-Ala-DED more tightly than Leu-Gly-DED as revealed by a small value of K_M. That this enzyme catalyzes the turnover of Leu-Gly-DED more efficiently than Leu-Ala-DED is reflected in a large value of k_cat and a small activation energy. The RET signals during the hydrolysis of Leu-Val-NH(CH₂)₂NH-Dns were extremely weak probably because of the inefficient energy transfer in the ES complex or the retention of the product in the enzyme after completion of the reaction. Aminopeptidase was inactive towards the dansyl compounds of the single amino acid studied. This fact may be due to an unfavorable conformation of these compounds in the ES complexes (small k_cat) or a weak binding of the substrates to the enzyme (large K_M) or both.     

First and second dissociation constants of deuterio-o-phthalic acid in D2O from 5 to 50°C

The first and second dissociation constants of deuterio-o-phthalic acid in deuterium oxide have been determined by the emf method over the temperature range of 5 to 50°C. The pD values for potassium deuterium phthalate have been calculated from these two constants and experimentally verified. The thermodynamic properties for the dissociation of deuterio-o-phthalic acid have been evaluated. At 25°C, these values in the molality scale are: pK _1A =3.505, pK _2A =5.890, and pD=4.518. From K _1A and K _2A , respectively: G ^o =20.003, 33.582 kJ-mol^–1; H ^o =2.851, 2.208 kJ-mol^–1; S ^o =–76.7, –105.2 J-mol^–1-K^–1; and C _p ^o =–52.7, –315.6 J-mol^–1-K^–1. The isotope effect is discussed.     

Conductance of the hydroxide ion intert-butyl alcohol-water and 1,4-dioxane-water mixtures

Limiting molar conductances λ_o of potassium hydroxide in 2 to 25 mol%tert-butyl alcohol (TBA)-water mixtures were determined at 25°C as a function of pressure up to 196 MPa. λ_o’s of KOH in (2.5 to 15 mol%) 1,4-dioxane-water mixtures at 25°C and 1 atm were also determined. The excess conductance λ _o ^e of the OH^- ion estimated as [λ _o ^e (OH^-) = λ_o(KOH) - λ_o(KCl)] decreased with an increase in the TBA or dioxane content, as did the excess proton conductance λ _o ^e (H⁺) [λ _o ^e (H⁺) = λ_O(HC1) - λ_o(KCl)]. Although λ _o ^e (OH^-) is smaller than λ _o ^e (H⁺) at all solvent compositions studied, the rate of decrease in λ _o ^e with organic content is larger for the OH^- ion than for the H₃O⁺ ion in both solvent mixtures except in the water-rich region of TBA-water mixtures. λ _o ^e (OH^-) increases with pressure more strongly in TBA-water mixtures than in pure water, and the rate of increase in λ _o ^e (OH^-) with pressure has a maximum at 5 mol% of TBA. These results are discussed in terms of the difference in stability of hydrogen bonds between the OH^- or the H₃O⁺ ion and water molecules and the increase in repulsive forces due to the orientation [H-O O-H] of water molecules in the mixtures.