Reactivity of Water and Alcohols toward Carbocations Generated in the Photolysis of 2,2,4,6-Tetramethyl-1,2-dihydroquinoline

The rate constants of the decay of carbocation generated in the photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline and the composition of reaction products were studied as a function of solvent composition in the mixtures H₂O–ROH and MeOH–ROH (R = Et, n-Pr, and i-Pr). The rate constants of carbocation decay in alcohols are more than 20 times higher than the corresponding rate constants in water. As follows from the composition of the products obtained in the photolysis in the alcohol–water mixtures, MeOH is only 1.4 times more reactive than water, and EtOH and n-PrOH are even less active than water. The inconsistency in the product composition in the mixtures and the values of the observed rate constants in these solvents was explained by the two-step mechanism of the reaction: the reversible formation of an adduct of the carbocation with the solvent components and subsequent proton transfer to the solvent to form the final product, with the first step determining the product composition and the second step determining the rate of carbocation decay. The relative rate constants of alcohols and water were determined for the two steps. The preferred solvation of the carbocation with water also contributes significantly to the reaction kinetics and the product composition in the water–alcohol solutions.     

Reactivity of carbocations from 1,2-dihydroquinolines in binary mixtures of methanol with pentane and acetonitrile

The rate constants and activation parameters of the reactions of the carbocation resulting from 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline photolysis with methanol (k ₁) and the methoxide ion (k ₂) have been measured by flash photolysis in binary mixtures of methanol with inert solvents (nonpolar pentane and polar acetonitrile) in wide composition ranges. The changes in the activation parameters for k ₁ at different solvent compositions show that the increase in the rate constant in the pentane mixtures is mainly deter-mined by the increase in the preexponential factor. The decrease in k ₁ in the acetonitrile mixtures is deter-mined by the decrease in the methanol concentration and by the increase in the activation energy. The different roles of the methoxide ion in the reaction are demonstrated. They depend on the nature of the inert solvent in the mixture. The results of this study are considered in terms of methanol clustering in pentane and acetonitrile, the different solubilities of 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline in the components of the binary mixtures, and the difference in distribution and solvation between the carbocation and the methoxide ion in the mixtures.     

Effect of the solvent composition in methanol-pentane and methanol-acetonitrile mixtures on the quantum yield of proton transfer and reactions of transient species in the photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline

The photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline in binary MeOH-C₅H₁₂ and MeOH-MeCN mixtures was studied in dependence on the solvent composition by steady state and pulse photolysis. The photoinduced proton transfer from the N-H bond to the C(3) carbon atom of the heterocycle does not occur up to the methanol concentration of 0.25 (1 vol %) and 2.5 mol l⁻¹ (10 vol %) in the MeOH-C₅H₁₂ and MeOH-MeCN mixtures, respectively. The trends in the increase in the relative quantum yield of the proton transfer reaction (Q _mix/Q _MeOH) and the decay rate constant for 2,2,4,6-tetramethyl-2,3-dihydroquinoline, the product of the proton transfer reaction, in dependence on the composition differ significantly for these binary mixtures. The results are interpreted in terms of peculiarities in the aggregation of methanol and the distribution of the DHQ molecules and transient species in the mixtures.     

Effect of the solvent composition in methanol-pentane and methanol-acetonitrile mixtures on the spectral and luminescent properties of 1,2-dihydroquinolines

The dependence of absorption and fluorescence spectra, quantum yields, and lifetimes of fluorescence on the solvent composition in the MeOH-C₅H₁₂ and MeOH-MeCN mixtures was studied for 2,2,4,6-tetramethyl-1,2-dihydroquinoline (TMDHQ). The variations in the parameters of deconvolution of the absorption and fluorescence spectra by the Gaussian functions in the MeOH-C₅H₁₂ mixtures of various compositions indicate the specificity of methanol clustering in saturated hydrocarbons and hydrogen bonding between TMDHQ and the methanol clusters of different compositions. At low MeOH concentrations (∼0.2 vol %), TMDHQ molecules are practically completely bound with the MeOH molecules by hydrogen bonds. In the MeOH-MeCN mixtures, the changes in the absorption and fluorescence spectra are observed at a substantially higher MeOH concentration (≥10 vol %) and monotonically change at the further increase in the MeOH concentration that is caused by the peculiarities of MeOH clustering in acetonitrile and the distribution of the TMDHQ molecules between the solvent components. At 50–95 vol % of MeOH in the mixture with MeCN, the fluorescence decay kinetics is described by the biexponential curve with the lifetime of the major component (τ₁) decreasing from 7.5 to 1.1 ns in pure MeCN and MeOH, respectively, and the lifetime of the minor component τ₂ ≈ 4 ns corresponding to the fluorescence lifetime in the solution containing 50 vol % MeOH. This indicates the existence of the free TMDHQ molecules, which are not bound with MeOH molecules or their clusters.     

Medium effect in the base hydrolysis of pentaamminebromocobalt(III) ion

Summary The effect of organic cosolvent on the rate constant of the base hydrolysis of pentaamminebromocobalt(III) ion was investigated in mixtures of H₂O with MeOH, EtOH,i-PrOH,t-BuOH, and dioxane. The thermodynamic transfer functions, corresponding to the transfer of reactants and activated complex from water to the solvent mixtures, were evaluated from kinetic measurements and from solubilities of the complex salt. The experimental results are in good agreement with a dissociative mechanismvia the conjugate base.     

Thermal properties of ternary systems: The calculation of the standard enthalpy of solution for compounds in binary mixtures

An equation free of fitting parameters is proposed for calculating the standard heats of solution for compounds in nonaqueous binary mixtures. The parameters of the equation are the standard heats of solution of a compound in the components of the mixed solvent. Nonlinear ΔH ⁰(x) trends are reconstructed for solutions of water in i-PrOH + MeOH and MeCN + MeOH, t-BuOH in MeCN + MeOH, squalane in CHCl₃ + CCl₄ and C₆H₆ + CHCl₃, and hexadecane in MeOH + i-Pr₂O and in mixtures of butyl acetate, ethyl acetate, and 1,4-dioxane with 1-octanol. The standard heats of solutions are calculated for water in alcohol + alcohol, alcohol + aprotic solvent, and aprotic solvent + aprotic solvent mixtures     

Kinetics of aquation ofcis- andtrans-chloro-nitrobis-(ethylene-diamine)cobalt(III) ion in binary aqueous mixtures

Kinetics of aquation ofcis- andtrans-[Co(en)₂NO₂Cl]⁺ were investigated in aqueous mixtures of MeOH, EtOH,i-PrOH andt-BuOH at 298.1 K. The values of transfer functions corresponding to the transfer of reactant and activated complex from H₂O to the solvent mixtures were calculated from kinetic measurements and from solubilities of the complex salt. The results of the analysis of solvent effect are discussed in terms of D and I_d mechanisms. TMC 2676     

Relative rates of nucleophilic reactions of benzyl carbocation formed in photolysis of benzyl chloride and benzyl acetate in aqueous solution

Benzyl chloride and benzyl acetate were photolyzed in 30% methanol–water mixtures (V/V) at 0°C. The photolysis produces benzyl carbocations that react with nucleophiles. The reaction products were analyzed by gas chromatography or liquid chromatography. From the amounts of products the relative values of rate constants of reactions of benzyl carbocation with nucleophiles N and water k(N)/k(H₂O) were calculated. Benzyl carbocation reacts with I^?, Br^?, Cl^?, and Ac^? ions with approximately diffusion-controlled rate. A value of 2.4 × 10⁷ dm³ mol^?1 s^?1 for the rate constant k(H₂O) and a lifetime of 0.7 ns were estimated for benzyl carbocation in the aqueous solution.     

Kinetics of aquation of cis-aquabromo-bis(ethylenediamine) complexes of cobalt(III) and chromium(III) in binary aqueous mixtures

Summary The kinetics of aquation of cis-[Co(en)₂(H₂O)Br]²⁺ and cis-[Cr(en)₂(H₂O)Br]²⁺ (en = ethylenediamine) were investigated in aqueous mixtures of MeOH, EtOH, i-PrOH and t-BuOH. The values of transfer functions corresponding to the transfer of reactants and activated complex from water to the solvent mixtures were evaluated from kinetic measurements and from solubilities of the complex salt. Analysis of the solvent effect confirmed a common I_d mechanism for the aquation of the Co^III and Cr^III complexes.     

The dependence of the enthalpy of solution of L-methionine on the composition of water-alcohol binary solvents at 298.15 K

The integral enthalpies of solution of L-methionine in water-methanol, water-ethanol, water-n-propanol, and water-iso-propanol mixtures were measured calorimetrically at alcohol concentrations x ₂ = 0–0.4 mole fractions. The standard enthalpies of solution (Δ_sol H ^o) and transfer of L-methionine (Δ_tr H ^o) from water to a binary solvent were calculated. The influence of the structure and properties of L-methionine and the composition of aqueous-organic mixtures on its enthalpy characteristics was considered. The enthalpic pair interaction coefficients (h _xy ) between L-methionine and alcohol molecules were calculated; they were positive and increased in the series methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), iso-propanol (i-PrOH). The enthalpy characteristics of solution and transfer of L-methionine were compared with those of glycine, L-threonine, L-alanine, and L-valine in similar binary solvents.     

Kinetics and Mechanism of the Photoinduced Addition of Water and Methanol to the Double Bond of 2,2,4-Trimethyl-8-Methoxy-1,2-Dihydroquinoline

The mechanism of the photoinduced Markovnikov addition of water and methanol to the double bond of 2,2,4-trimethyl-8-methoxy-1,2-dihydroquinoline was studied by flash photolysis over a wide range of acid and alkali concentrations. In neutral MeOH solutions, the successive formation of two active intermediates was observed. These intermediates have absorption bands at _max = 420 and 500 nm, respectively, and lifetimes of a few milliseconds. The rate constants of decay for these species (k ₁ and k ₂) were measured over the temperature range from 10 to 45°C, and the activation energies E ₁ (3.7 ± 0.4) and E ₂ (6.9 ± 0.7) kcal/mol were determined in MeOH. The addition of an acid accelerates the conversion of the former intermediate into the latter. The rate constant for the reaction of the former intermediate with the acid is (2.0 ± 0.4) × 10⁸ l mol^–1 s^–1. The latter intermediate was proposed to be a corresponding carbocation. In H₂O at pH < 9, the formation of the carbocation (_max = 500 nm) was observed on a millisecond timescale, with the lifetime being an order of magnitude higher than that in MeOH. In alkaline media, only the former intermediate (_max = 420 nm) was observed with a lifetime of 150 or 400 ms in MeOH or H₂O, respectively, at [NaOH] > 2 × 10^–3 mol/l. The quantum yields of the product and the quantum yields of fluorescence were measured as functions of acidity. Possible mechanisms of the reaction are discussed.     

Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λ_pump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λ_max = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.     

A Polarographic Study of Tl + , Pb 2 and Cd 2+ Complexes with Dicyclohexano-18-Crown-6 in Some Binary Mixed Solvents

The complexes of Tl⁺, Pb²⁺ and Cd²⁺ cations with the macrocyclic ligand, dicyclohexano-18-crown-6\linebreak(DC18C6) were studied in water/methanol (H₂₊O/MeOH), water/1-propanol (H₂₊O/1-PrOH), water/acetonitrile (H₂₊O/AN), water/dimethylformamide (H₂₊O/DMF), dimethylformamide/acetonitrile (DMF/AN), dimethylformamide/methanol (DMF/MeOH), dimethylformamide/1-propanol (DMF/1-PrOH) and dimethylformamide/nitromethane (DMF/NM) mixed solvents at 22 °C using differential pulse polarography (DPP), square wave polarography and conductometry. In general, the stability of the complexes was found to decrease with increasing concentration of water in aqueous/non-aqueous mixed solvents with an inverse relationship between the stability constants of the complexes and the concentration of DMF in non-aqueous mixed solvents. The results show that the change in stability of DC18C6.Tl⁺, vs the composition of solvent in DMF/AN and DMF/NM mixed solvents is apparently different from that in DMF/MeOH and DMF/1-PrOH mixed solvents. While the variation of stability constants of the DC18C6.Tl⁺ and DC18C6.Pb²⁺ complexes vs the composition of H₂₊O/AN mixed solvents is monotonic, an anomalous behavior was observed for variations of log K_f vs the composition of H₂₊O/1-PrOH and H₂₊O/MeOH mixed solvents. The selectivity order of the DC18C6 ligand for the cations was found to be Pb²⁺ > Tl⁺ > Cd²⁺.     

Laser flash photolysis study of the decay kinetics of carbocations of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in the porous glass in the presence of methanol

The efficiency of formation and the decay kinetics of carbocations formed under the photolysis of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in methanol and in a porous glass filled with methanol or dried in air or in vacuo were studied by the laser flash photolysis techniques. In MeOH, the carbocations recombine via the second-order law in the reaction with the MeO⁻ anion formed in an equimolar amount and decay via the first-order law in the reaction with the solvent with rate constants of 3·10⁸ L mol⁻¹ s^{− 1} and 1.4·10³ s⁻¹, respectively. When the solution is placed into the porous glass, no recombination of the carbocations with MeO⁻ is observed, and the reaction with the solvent is somewhat inhibited (rate constant 8·10² s⁻¹). More than tenfold inhibition of the reaction of the carbocations with methanol is observed on going to a monolayer of MeOH on the surface. The main route of carbocation decay in the porous glass dried in vacuo is the geminate recombination with the SiO groups. The corresponding kinetics is described in terms of the model of freely diffusing reactants. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2239–2243, October, 2005.     

Ion-solvent molecule interaction in nonaqueous solutions: Spin-lattice relaxation time of7Li

The spin-lattice relaxation time (T ₁) of⁷Li⁺ was measured in solutions of LiCl and LiClO₄ in protic (MeOH, EtOH,n-PrOH,i-PrOH,n-BuOH, sec-BuOH, formamide, N-methylformamide) and aprotic (MeCN, acetone, methyl ethyl ketone, propylene carbonate, dimethyl sulfoxide, dimethylformamide, hexamethylphosphotriamide) solvents and in mixtures of H₂O-formamide, H₂O–N-methylformamide, H₂O–N,N-dimethylformamide, H₂O-DMSO, H₂O-hexamethylphosphotriamide, and formamide-N,N-dimethylformamide at 25°C. The values of (1/T ₁)₀ obtained by extrapolation are discussed in terms of current theories of the magnetic relaxation of ionic nuclei. Linear correlations were found between (1/T ₁)₀ and Gutmann's donor numbers and Kosower's Z-values. These correlations indicate that relaxation of⁷Li⁺ is dominated by donor-acceptor interaction of the cation with solvent molecules. Concentration dependences of 1/T ₁ for LiCl and LiClO₄ differ from one another in a given solvent, a fact which is accounted for by a specific cation-anion short-range potential. The quantity 1/T ₁ of⁷Li⁺ atC=1 mole per 55.5 moles of mixed solvent as a function of solvent composition show characteristic features, which are discussed in terms of the relaxation mechanism proposed.     

Electrochemistry on Multi-walled Carbon Nanotubes in Organic Solutions

The oxidation of ferrocene (FeCp₂) to ferrocenium cation (FeCp₂⁺\mbox{FeCp}_{2}^{+}) (where Cp: cyclopentadienyl anion, C₅H₅^-\mbox{C}_{5}\mbox{H}_{5}^{-}) was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on either a glassy carbon (GC) electrode or a film consisted of multi-walled carbon nanotubes (MWCNT) in acetone (ACE), methanol (MeOH) and ACE/MeOH binary mixtures. The half-wave potentials (E _1/2), the diffusion coefficients (D) and the heterogeneous electron transfer rate constants (k _s) of the redox couple FeCp₂^+/0\mbox{FeCp}_{2}^{+/0} were determined. On both electrodes tested, the electron transfer process was found to be quasi-reversible and diffusion controlled in all investigated solvent mixtures. The E _1/2 values were shifted toward less positive potentials with the enrichment of mixtures in MeOH, due probably to the larger Lewis basicity of the MeOH molecules. Furthermore, values of both D and k _s diminish with the increase of MeOH content following the progressive increase of viscosity of the solvent medium. Among the electrodes probed, GC provides faster electrochemical process since it affords less charge-transfer resistance (R _ct), and consequently, an insignificant barrier for interfacial electron transfer.     

EPR study on the charge-transfer photochemistry of the bromide mixed-ligand dithiocarbamate complex of copper(II)

The EPR technique has been used to study the photolysis of the mixed-ligand complex Cu^II(Et₂dtc)Br in a 1:1 solvent mixture of chloroalkane and alcohol, where the chloroalkane is CCl₄, CHCl₃ or CH₂Cl₂ and the alcohol is MeOH, EtOH, i-PrOH or i-BuOH, in comparison with Cu^II(Et₂dtc)Cl photolysis in CHBr₃:ROH. It was found that while Cu^II(Et₂dtc)Br photolysis in chloroalkane:ROH yielded Cu^II(Et₂dtc)Cl as an intermediate, the opposite conversion of Cu^II(Et₂dtc)Cl to Cu^II(Et₂dtc)Br proceeded via Cu^II(Et₂dtc)Cl photolysis in CHBr₃:ROH. The final photolytic products in both cases were tetraethylthiuramdisulphide and the corresponding copper(II) salt (CuCl₂ or CuBr₂, respectively). The results obtained by EPR allowed to get some insight into the behaviour of the primary photolytic products towards both components of the mixed solvent.     

Effects of water-alcohol binary solvents on the thermochemical characteristics of L-tryptophane dissolution at 298.15 K

The enthalpies of L-tryptophane solution in water-methanol, water-ethanol, water-1-propanol, and water-2-propanol mixtures at alcohol concentrations of x ₂ = 0–0.4 mole fractions were measured by calorimetry. The standard enthalpies of L-tryptophane solution (Δ_sol H ^°) and transfer (Δ_tr H ^°) from water to the binary solvent were calculated. The influence of the composition of the water-alcohol mixture and the structure and properties of L-tryptophane on the enthalpy characteristics of the latter was considered. The enthalpy coefficients of pair interactions (h _xy ) of L-tryptophane with alcohol molecules were calculated. The coefficients were positive and increased in the series: methanol (MeOH), ethanol (EtOH), 1-propanol (1-PrOH), and 2-propanol (2-PrOH). The solution and transfer enthalpies of L-tryptophane were compared with those of aliphatic amino acids (glycine, L-threonine, DL-alanine, L-valine, and L-phenylalanine) in similar binary solvents.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XLI. Solvent Effect on the Rate of 3-Methyl-3-chloro-1-butene Solvolysis. Correlation Analysis of Solvation Effects and Role of Solvent Nucleophilicity

The kinetics of 3-methyl-3-chloro-1-butene solvolysis at 25°C in MeOH, EtOH, BuOH, i-BuOH, PentOH, 2-PrOH, 2-BuOH, HexOH, OctOH, t-BuOH, t-PentOH, cyclohexanol, and allyl alcohol was studied by the verdazyl method; v = k[C₅H₉Cl], SN1 + E1 mechanism. The reaction rate shows a satisfactory correlation with the parameter of the solvent ionizing power E _T and is independent of the solvent nucleophilicity.     

Effects of solvent on the reactions of coordination complexes: Part 17. Kinetics and mechanism of base hydrolysis of (αβS) (salicylato) (tetraethylenepentamine)cobalt (III) in methanol + water and dimethylsulphoxide + water media

The base hydrolysis of (αβS) (salicylato) (tetraethylenepentamine)cobalt(III) has been investigated in MeOH + water and DMSO + water media (0–70% (v/v) cosolvents) at 20.0 ? t°C ? 35.0 and I = 0.10 mol dm^?3 (ClO₄^?). The phenoxide species [(tetren)CoO₂CC₆H₄O]⁺ undergoes both OH^?-independent and OH^?-catalyzed hydrolysis via SN₁ICB and SN₁CB mechanism, respectively. The OH^?-independent hydrolysis of the phenoxide species is catalyzed by both DMSO + water and MeOH + water media, the former exerting a much stronger rate accelerating effect than the latter. The OH^?-catalyzed reaction is strongly accelerated by DMSO + water medium but insensitive to the composition of MeOH + water medium up to 40% (v/v) MeOH beyond which it was not detectable under the experimental conditions. Data analysis has been attempted on the basis of the solvent stabilizing and destabilizing effects on the initial state and transition state of the concerned reactions. The nonlinear variation of the activation parameters, ΔH^≠ and ΔS^≠, with solvent compositions presumably indicates that the solvent structural effects mediate the energetics of solvation of the initial state and transition state of the concerned reactions. The linearity in ΔH^≠ vs. ΔS^≠ plot accomodating all data for k₁ and k₂ paths in DMSO + water and MeOH + water further suggests that the solvent effects on these parameters are mutually compensatory.