Equilibria in the system of aquachlorohydroxogold(III) complexes in aqueous solution

AuCl₄⁻ + jOH⁻ + kH₂O = AuCl_{4 − j − k} OH _j (H₂O) _k ^{k − 1} + (j + k)Cl⁻ equilibria at 20°C were studied spectrophotometrically, and the constants β _jk in acid aqueous solutions were determined for I = 2.0 mol/L (HClO₄).     

A new method for the determination of the rate constants for the disproportionation of semiquinone radicals from nonstationary kinetics of Chain reactions between quinoneimines and hydroquinones

A new method is suggested for the determination of the rate constants for the disproportionation of semiquinone radicals k ₆ from the kinetics of chain reactions between quinoneimines and hydroquinones under nonstationary conditions. The k ₆ values were determined in solvents of various natures, including comparatively low-polarity solvents in which hydroquinones are poorly soluble. The k ₆ values were found from the dependence of the degree of quinoneimine concentration lowering during a certain time (∼20 s) after reaction beginning (when the reaction occurred under nonstationary conditions) on the rate of initiation. Techniques for overcoming difficulties caused by the absence of the limiting chain propagation stage in reactions between quinoneimines and hydroquinones were suggested. The developed method of the nonstationary kinetics of chain reactions was applied to selectively determine the rate constant for the disproportionation of 2,5-dichloroseniquinone radicals, which is one of parallel chain termination stages in the chain reaction between N,N’-diphenyl-1,4-benzoquinonediimine and 2,5-dichlorohydroquinone, k ₆ = (5.8−7.1) × 10⁶ l/(mol s) in chlorobenzene at 298 K.     

Kinetics and mechanism of the reactions of hexaaqua rhodium (III) with sulphur (IV) in aqueous medium

An O-bonded sulphito complex, Rh(OH₂)₅(OSO₂H)²⁺, is reversibly formed in the stoppedflow time scale when Rh(OH₂) ₆ ³⁺ and SO₂/HSO ₃ ⁻ buffer (1 <pH< 3) are allowed to react. For Rh(OH₂)₅OH₂₊+ SO₂ □ Rh(OH₂)₅(OSO₂H)²⁺ (k₁/k_-1), k₁ = (2.2 ±0.2) × 10³ dm³ mol⁻¹ s⁻¹, k₋1 = 0.58 ±0.16 s⁻¹ (25°C,I = 0.5 mol dm⁻³). The protonated O-sulphito complex is a moderate acid (K _d = 3 × 10⁻⁴ mol dm⁻³, 25°C, I= 0.5 mol dm⁻³). This complex undergoes (O, O) chelation by the bound bisulphite withk= 1.4 × 10⁻³ s⁻¹ (31°C) to Rh(OH₂)₄(O₂SO)⁺ and the chelated sulphito complex takes up another HSO ₃ ⁻ in a fast equilibrium step to yield Rh(OH₂)₃(O₂SO)(OSO₂H) which further undergoes intramolecular ligand isomerisation to the S-bonded sulphito complex: Rh(OH₂)₃(O₂SO)(OSO₂)^- → Rh(OH₂)₃(O₂SO)(SO₃)⁻ (k _iso = 3 × 10⁻⁴ s⁻¹, 31°C). A dinuclear (μ-O, O) sulphite-bridged complex, Na₄[Rh₂(μ-OH)₂(OH)₂(μ-OS(O)O)(O₂SO)(SO₃) (OH₂)]5H₂O with (O, O) chelated and S-bonded sulphites has been isolated and characterized. This complex is sparingly soluble in water and most organic solvents and very stable to acid-catalysed decomposition     

Mechanistic study of the oxidation of N-phenyldiethanolamine by <Emphasis Type="Italic">bis</Emphasis>(hydrogen periodato)argentate(III) complex anion

The kinetics of oxidation of phenyldiethanolamine (PEA) by a silver(III) complex anion, [Ag(HIO₆)₂]⁵⁻, has been studied in an aqueous alkaline medium by conventional spectrophotometry. The main oxidation product of PEA has been identified as formaldehyde. In the temperature range 20.0–40.0 °C , through analyzing influences of [OH⁻] and [IO ₄ ⁻ ]_tot on the reaction, it is pseudo-first-order in Ag(III) disappearance with a rate expression: k _obsd = (k ₁ + k ₂[OH⁻]) K ₁ K ₂[PEA]/{f([OH⁻])[IO ₄ ⁻ ]_tot + K ₁ + K ₁ K ₂ [PEA]}, where k ₁ = (0.61 ± 0.02) × 10⁻² s⁻¹, k₂ = (0.049 ± 0.002) M⁻¹ s⁻¹ at 25.0 °C and ionic strength of 0.30 M. Activation parameters associated with k ₁ and k ₂ have also been derived. A reaction mechanism is proposed involving two pre-equilibria, leading to formation of an Ag(III)-periodato-PEA ternary complex. The ternary complex undergoes a two-electron transfer from the coordination PEA to the metal center via two parallel pathways: one pathway is spontaneous and the other is assisted by a hydroxide ion.     

Base hydrolysis ofcis-(methyl/ethylamine)bis(ethylenediamine) (salicylato)cobalt(III) complexes

The kinetics of the base hydrolysis ofcis-[Co(en)₂(RNH₂)-(SalH)]²⁺ (R=Me or Et; SalH=HOC₆H₄CO ₂ ⁻ ) were investigated in aqueous ClO ₄ ⁻ in the 0.004–0.450 mol dm⁻³ [OH⁻] range, I=0.50 mol dm⁻³ at 30–40°C. The phenoxide species is hydrolysed via [OH⁻]-independent and [OH⁻]-dependent paths, the latter being first order in [OH⁻]. The high rate of alkali-independent hydrolysis of the phenoxide species is associated with high ΔH^≠ and ΔS^≠ values, in keeping with the SNICB mechanism involving an amido conjugate base generated by the phenoxide-assisted NH-deprotonation of the coordinated amine. The [OH⁻]-dependent path also involves the conventional SN₁ CB mechanism. The rate constant, k₁, for the SNICB path exhibits a steric acceleration with the increasing size of the non-labile alkylamine, whereas the rate constant, k₂, for the SN₁CB path shows a reverse trend. TMC 2578     

Oxidation of <Emphasis Type="Italic">β</Emphasis>-alanine by Ag(III) complex in alkaline medium: a kinetic and mechanism study

The kinetics of oxidation of β-alanine by the Ag(III) complex has been studied by spectrophotometry. The reaction is first order with respect to the Ag(III) complex. The values of k _obs decrease with an increase of [OH⁻], and then increase with the increase of [OH⁻]. The concentration of OH⁻ was 0.03 mol · L⁻¹ at the turning point at which the rate was the slowest. A plausible mechanism of reaction is proposed, and the rate law has been obtained, which could be applied to explain all experimental phenomena. The activation parameters of the rate-determining step have been also calculated.     

Reactivity trends in the base hydrolysis of 6-nitro-2H-chromen-2-one and 6-nitro-2H-chromen-2-one-3-carboxylic acid in binary mixtures of water with methanol and acetone at different temperatures

Kinetics of the base hydrolysis of 6-nitro-2H-chromen-2-one (NC) and 6-nitro-2H-chromen-2-one-3-carboxylic acid (NCC) in water-methanol and water-acetone mixtures was studied at temperature range from 283 to 313 K. The activation parameters of the reactions were evaluated and discussed. The change in the activation barrier of the investigated compounds from water to water-methanol and water-acetone mixtures were estimated from the kinetic data. The base hydrolysis of NC and NCC in the water-methanol and water-acetone mixtures follows a rate law with k _obs = k ₂[OH⁻] and k _obs = k ₁ + k ₂[OH⁻], respectively. The decrease in the rate constants of NC and NCC hydrolysis, as the proportion of methanol and acetone increases, is accounted for by the destabilization of the OH⁻ ion. The activation and thermodynamic parameters were determined.     

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     

Antioxidant activities of phenols in different solvents using DPPH assay

Studies on the antioxidant activity of two model phenols containing either an electron withdrawing (p-nitrophenol) or electron donating (p-aminophenol) group and p-hydroxyacetophenone in different solvents are reported using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical assay by spectrophotometry and stopped-flow techniques. The second-order rate constants measured with p-nitrophenol were found to be (1.2–5.5) × 10⁻² dm³ mol⁻¹ s⁻¹ but the DPPH radical reacts much faster with p-aminophenol (k = 0.5–1.1 × 10⁴ dm³ mol⁻¹ s⁻¹). The normal kinetic solvent effect in H atom transfer was seen in the case of p-nitrophenol with the solvent independent rate constant k _o = 0.1 dm³ mol⁻¹ s⁻¹. The IC₅₀ values in p-nitrophenol are similar to those measured in p-hydroxyacetophenone. On the other hand, much lower IC₅₀ values of more than four orders of magnitude with p-aminophenol were observed. This work demonstrates that the phenol with the electron donating –NH₂ substituent is a better antioxidant.     

Kinetics and mechanism of oxidation of <Emphasis Type="Italic">N</Emphasis>-methylethylamine by bis(hydrogenperiodato)argentate(III) complex anion

Oxidation of N-methylethylamine by bis(hydrogenperiodato)argentate(III) ([Ag(HIO₆)₂]⁵⁻) in alkaline medium results in demethylation, giving rise to formaldehyde and ethylamine as the oxidation products. The oxidation kinetics has been followed spectrophotometrically in the temperature range of 20.0–35.0 °C, and shows an overall second-order character: being first-order with respect to both Ag(III) and N-methylethylamine. The observed second-order rate constants k′ increase with increasing [OH⁻] of the reaction medium, but decrease with increasing the total concentration of periodate. An empirical rate expression for k′ has been derived as: k′ = (k _a + k _b[OH⁻])K ₁/{f([OH⁻])[IO₄ ⁻]_tot + K ₁}, where k _a and k _b are rate parameters, and K ₁ is an equilibrium constant. These parameters have been evaluated at all the temperatures studied, enabling calculation of activation parameters. A reaction mechanism is suggested to involve two pre-equilibria, leading to formation of an intermediate Ag(III) complex, namely [Ag(HIO₆)(OH)(MeNHEt)]²⁻. In the subsequent rate-determining steps, this intermediate undergoes inner-sphere electron transfer from the coordinated amine to the metal center via two distinct routes, one of which is spontaneous while the other is mediated by a hydroxide ion.     

Electronic effects in the reaction of diphenylcarbonyl oxide with aldehydes

The reactivity of 14 aldehydes with diphenylcarbonyl oxide Ph₂COO was characterized by thek ₃₃/k ₃₁ ratio. The values ofk ₃₃/k ₃₁ vary from 1.3·10⁻² (C₆F₅CHO) to 1.0 (p-Me₂N-PhCHO), 70 °C, acetonitrile as the solvent. A charge transfer complex (CTC) was suggested to be primarily formed during the reaction. The electronic effects of substituents in the reaction were analyzed using the published data. Carbonyl oxide reacts with aldehydes as a nucleophile (at the carbon atom of the −CHO fragment to form 1,2,4-trioxolane) and also as an electrophile (at the aromatic ring with the intermediate formation of CTC). The latter is transformed into either 1,2,4-trioxolane or the products of oxidation of the phenyl ring. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1090–1096, June, 1999.     

Chromium(III) complexes with lutidinic acid: kinetic studies in HClO<Subscript>4</Subscript> and NaOH solutions

Chromium(III)-lutidinato complexes of general formula [Cr(lutH) _n (H₂O)_6−2n ]³⁻ⁿ (where lutH⁻ is N,O-bonded lutidinic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(lutH)₃]⁰ leads to only one ligand dissociation, whereas base hydrolysis produces chromates(III) as a result of subsequent ligand liberation steps. The kinetics of the first ligand dissociation were studied spectrophotometrically, within the 0.1–1.0 M HClO₄ and 0.4–1.0 M NaOH range. In acidic media, two reaction stages, the chelate-ring opening and the ligand dissociation, were characterized. The dependencies of pseudo-first-order rate constants on [H⁺] are as follows: k _obs1 = k ₁ + k ₋₁/K ₁[H⁺] and k _obs2 = k ₂ K ₂[H⁺]/(1 + K ₂[H⁺]), where k ₁ and k ₂ are the rate constants for the chelate-ring opening and the ligand dissociation, respectively, k ₋₁ is the rate constant for the chelate-ring closure, and K ₁ and K ₂ are the protonation constants of the pyridine nitrogen atom and coordinated 2-carboxylate group in the one-end bonded intermediate, respectively. In alkaline media, the rate constant for the first ligand dissociation depends on [OH⁻]: k _obs1 = k _OH(1) + k _O[OH⁻], where k _OH(1) and k _O are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and K ₂ is an equilibrium constant between these two protolytic forms. Kinetic parameters were determined and a mechanism for the first ligand dissociation is proposed. The kinetics of the ligand liberation from [Cr(lut)(OH)₄]³⁻ were also studied and the values of the pseudo-first-order rate constants are [OH⁻] independent.     

Investigation of the Electrochemical Reduction of Benzophenone in Aprotic Solvents Using the Method of Cyclic Voltammetry

The reduction of benzophenone (Bzph) in 3-pentanone (PEN), acetone (ACE), N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile (ACN) and dimethyl sulfoxide (DMSO) with n-tetrabutylammonium hexafluorophosphate (TBAPF₆) as background electrolyte was studied using the technique of cyclic voltammetry at the temperature of 263.15 K. The half-wave potentials (E _1/2) were extracted. The reduction of Bzph occurs in two successive one-electron steps to produce first the free radical anion Bzph⁻ and then the dianion Bzph²⁻. The results indicated that the radical anion Bzph⁻ is reoxidized to Bzph in all investigated solvent media whereas the dianion Bzph²⁻ is reoxidized to Bzph⁻ only in THF. The heterogeneous electron-transfer rate constants (k _s ) were evaluated by employing the electrochemical rate equation proposed by Nicholson. The rate of electron transfer for the Bzph/Bzph⁻ couple was found to be relatively slow in all investigated solvent media. Consequently, the electron-transfer processes can be recognized as quasi-reversible. The diffusion coefficients (D) of Bzph in the investigated solvent media have been calculated using the modified Randles-Sevcik equation. The effect of the physical and chemical properties of the solvent medium on the electrochemical behavior of Bzph has been examined.     

Dramatic solvent effect on the volume of the Diels-Alder reaction between tetracyanoethylene and cyclopentadiene

The partial molar volumes (V) and the enthalpies of dissolution (Δ_dis H) for tetracyanoethylene, cyclopentadiene, and their Diels—Alder adduct were determined at 25°C. Eleven solvents of the π- and n-donor type were used. The use of alkylbenzenes as solvents for tetracyanoethylene induces pronounced changes in the enthalpy of dissolution (up to 26 kJ mol⁻¹) and in the partial molar volume (up to 11 cm³ mol⁻¹), whereas these parameters for the adduct change slightly. TheV and Δ_dis H values for cyclopentadiene virtually do not depend on the nature of the solvent. In the case of tetracyanoethylene and the adduct in n-donor solvents, considerable variations of theV and Δ_dis H values are observed; they are not linear functions of the change in the partial molar volume of the adduct. Therefore, the reaction volumes in acetonitrile (−40.69) and ethyl acetate (−45.56) differ sharply from those ino-xylene (−24.28) and mesitylene (−21.76 cm³ mol⁻¹). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1046–1050, June, 2000.     

Kinetic Study of the Oxidation of N,N-Dimethylethanolamine by Bis(Hydrogenperiodato)Argentate(III) in Aqueous Solution

The oxidation of N,N-dimethylethanolamine (DMEA) by bis(hydrogenperiodato) argentate(III) ([Ag(HIO₆)₂]⁵⁻) was studied in aqueous alkaline medium. Formaldehyde and dimethylamine were identified as the major oxidation products after the oxidation of DMEA. The oxidation kinetics was followed spectrophotometrically in the temperature range of 25.0 °C–40.0 °C. It was found that the reaction was first order in [Ag(III)]; the oberved first-order rate constants k _obsd as functions of [DMEA], [OH⁻] and total concentration of periodate ([IO₄^-]_tot[\mathrm{IO}_{4}^{-}]_{\mathrm{tot}}) were analyzed and were revealed to follow a rate expression: k_obsd = (k₁ +k₂[OH^-])K₁K₂[DMEA]/{f([OH^-])[IO₄^-]_tot+ K₁ + K₁K₂[DMEA]}k_{\mathrm{obsd}} = (k_{1} +k_{2}[\mathrm{OH}^{-}])K_{1}K_{2}[\mathrm{DMEA}]/\{f([\mathrm{OH}^{-}])[\mathrm{IO}_{4}^{-}]_{\mathrm{tot}}+ K_{1} + K_{1}K_{2}[\mathrm{DMEA}]\}. Rate constants k ₁ and k ₂ and equilibrium constant K ₂ were derived; activation parameters corresponding to k ₁ and k ₂ were computed. In the proposed reaction mechanism, a peridato-Ag(III)-DMEA ternary complex is formed indirectly through a reactive intermediate species [Ag(HIO₆)(OH)(H₂O)]²⁻. In subsequent rate-determining steps as described by k ₁ and k ₂, the ternary complex decays to Ag(I) through two reaction pathways: one of which is spontaneous and the other is prompted by an OH⁻.     

Chromium(III)-isocinchomeronato and quinolinato complexes: kinetic studies in NaOH solutions and effect on 3T3 fibroblast proliferation

The aquation of chromium(III)-isocinchomeronato and quinolinato complexes, mer-[Cr(icaH)₃]⁰ and mer-[Cr(quinH)₃]⁰ (where icaH⁻ and quinH⁻ are N,O-bonded isocinchomeronic and quinolinic acid anion, respectively) was studied in NaOH solutions. The process leads to successive ligand liberation in the fully deprotonated species. The kinetics of the first ligand liberation were studied spectrophotometrically in the visible region. A mechanism is proposed in which the rate of the chelate-ring opening at the Cr–N bond is much faster than the rate of the Cr–O bond breaking. The rate-determining step is described by the rate law: k _obs1 = k _OH(1) + k _O Q ₂ [OH⁻], where k _OH(1) and k _O are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and Q ₂ is an equilibrium constant between these two protolytic forms. The first pseudo-first-order rate constants (k _obs1) were calculated using SPECFIT software for an A → B → C reaction pattern. The results are compared with those determined in acidic medium. Kinetics of the second and third ligand liberation were also studied and values of successive pseudo-first-order rate constants (k _obs2, k _obs3) are [OH⁻] independent. Effect of chromium(III)-quinolinato and isocinchomeronato complexes on 3T3 fibroblast proliferation was evaluated. Cytotoxicity of these complexes is low, suggesting they may be promising candidates as novel dietary supplements.     

Rapid determination of 226Ra in drinking water samples using dispersive liquid-liquid microextraction coupled with liquid scintillation counting

A new radioanalytical method was developed for rapid determination of ²²⁶Ra in drinking water samples. The method is based on extraction and preconcentration of ²²⁶Ra from a water sample to an organic solvent using a dispersive liquid-liquid microextraction (DLLME) technique followed by radiometric measurement using liquid scintillation counting. In DLLME for ²²⁶Ra, a mixture of an organic extractant (toluene doped with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone) and a disperser solvent (acetonitrile) is rapidly injected into the water sample resulting in the formation of an emulsion. Within the emulsion, ²²⁶Ra reacts with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone and partitions into the fine droplets of toluene. The water/toluene phases were separated by addition of acetonitrile as a de-emulsifier solvent. The toluene phase containing ²²⁶Ra was then measured by liquid scintillation counting. Several parameters were studied to optimize the extraction efficiency of ²²⁶Ra, including water immiscible organic solvent, disperser and de-emulsifier solvent type and their volume, chelating ligands for ²²⁶Ra and their concentrations, inorganic salt additive and its concentration, and equilibrium pH. With the optimized DLLME conditions, the accuracy (expressed as relative bias, B _r ) and method repeatability (expressed as relative precision, S _B ) were determined by spiking ²²⁶Ra at the maximum acceptable concentration level (0.5 Bq L⁻¹) according to the Guidelines for Canadian Drinking Water Quality. Accuracy and repeatability were found to be less than −5% (B _r ) and less than 6% (S _B ), respectively, for both tap water and bottled natural spring water samples. The minimum detectable activity and sample turnaround time for determination of ²²⁶Ra was 33 mBq L⁻¹ and less than 3 h, respectively. The DLLME technique is selective for extraction of ²²⁶Ra from its decay progenies.     

Base hydrolysis of <Emphasis Type="Italic">mer</Emphasis>-trispicolinatoruthenium(III): kinetics and mechanism

The mer-[Ru(pic)₃] isomer, where pic is 2-pyridinecarboxylic acid, undergoes base hydrolysis at pH > 12. The reaction was monitored spectrophotometrically within the UV–Vis spectral range. The product of the reaction, the [Ru(pic)₂(OH)₂]⁻ ion, is formed via a consecutive two-stage process. The chelate ring opening is proceeded by the nucleophilic attack of OH⁻ ion at the carbon atom of the carboxylic group and the deprotonation of the attached hydroxo group. In the second stage, the fast deprotonation of the coordinated OH⁻ ligand leads to liberation of the monodentato bonded picolinate. The dependence of the observed pseudo-first-order rate constant on [OH⁻] is given by k_\textobs1 = \frack ₊ k₁ [\textOH ^- ] + k ₊ k₂ K₁ [\textOH ^- ]² k _- + k₁ + ( k ₊ + k₂ K₁ )[\textOH ^- ] + k ₊ K₁ [\textOH ^- ]² k_{{{\text{obs}}1}} = \frac{{k_{ + } k_{1} [{\text{OH}}^{ - } ] + k_{ + } k_{2} K_{1} [{\text{OH}}^{ - } ]^{2} }}{{k_{ - } + k_{1} + \left( {k_{ + } + k_{2} K_{1} } \right)[{\text{OH}}^{ - } ] + k{}_{ + }K_{1} [{\text{OH}}^{ - } ]^{2} }} and ( k_\textobs2 = \frack_ca + k_cb K₂ [\textOH ^- ]1 + K₂ [\textOH ^- ] ) \left( {k_{{{\text{obs}}2}} = \frac{{k_{ca} + k_{cb} K_{2} [{\text{OH}}^{ - } ]}}{{1 + K_{2} [{\text{OH}}^{ - } ]}}} \right) for the first and the second stage, respectively, where k ₁, k ₂, k _-, k _ca , k _cb are the first-order rate constants and k ₊ is the second-order one, K ₁ and K ₂ are the protolytic equilibria constants.     

OH−-induced β-elimination reactions with 1-(2-Xethyl)-4-nitrobenzene substrates in solvent mixture H2O/CH3CN and H2O/DMSO: Reactivity and mechanism

The behaviour of 1-(2-bromoethyl) 4-nitrobenzene (1), N,N,N-triethyl-2-(4-nitrophenyl)ethanaminium bromide (2) and N,N-diethyl-N-[2-(4-nitrophenyl)ethyl]octan-1-aminium bromide (3) in the OH⁻-induced elimination reactions with formation of 1-nitro-4-vinylbenzene in mixtures of DMSO/H₂O or CH₃CN/H₂O has been investigated. With all three substrates an increase in dipolar aprotic solvent content implies a limited increase of the second-order rate constant k _OH up to ≅605, and then an exponential increase is observed. The variation of activation parameters ΔH ^# and dGS ^#, measured in DMSO/H₂O mixtures, is parallel for 1 and 2. This similar behaviour of 1 and 2 with respect to variation in solvent composition is evidence that it is not possible to use this technique of solvent effect for the mechanistic diagnosis of elimination reactions.