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.     

Kinetics and mechanism of the acid-catalysed decarboxylation of thecis-α- andcis-β-carbonato(3,6-dimethyl-1,8-diamino-3,6-diazaoctane)-cobalt(III) ions

Summary The acid-catalysed decarboxylation of thecis-- andcis--[CoL(CO₃)]⁺ complexes (L = 3,6-dimethyl-1,8-diamino-3,6-diazaoctane) have been studied over a range of HClO₄ concentrations and the temperatures 25, 35 and 45° at I = 1.0 mol dm^–3 (NaClO₄). The rate expression takes the form k_obs = k₀ + k₁[H⁺] where k_obs is the observed first order rate constant at constant hydrogen ion concentration. The k₀ term makes only a minor contribution to the overall reaction. Both complexes display solvent deuterium isotope effects ofca. 2.6 for the acid-catalysed decarboxylation, consistent with a rapid proton pre-equilibrium mechanism. Activation parameters have been determined and the mechanism of the reaction discussed. The magnitude of the solvent isotope effect is consistent with an A-1 type mechanism involving formation of a 5-coordinate intermediate.     

Solvent Influence on Propagation Kinetics in Radical Polymerizations Studied by Pulsed Laser Initiated Polymerizations

The influence of the reaction medium (organic solvents, water, ionic liquids, supercritical CO₂) on the propagation rate in radical polymerizations has very different causes, e.g., hindered rotational modes, hydrogen bonding or electron pair donor/acceptor interactions. Depending on the origin of the solvent influence propagation rate coefficients, k_p, may be enhanced by up to an order of magnitude associated with changes in the pre‐exponential or the activation energy of k_p. In contrast, non‐specific interactions, size and steric effects lead to rather small changes in the vicinity of the radical chain end and are reflected by modest variations in k_p.

     

Correlation analysis of reactivity in the reduction of Co(en)2Br2+ by Fe(CN)64– in water–methanol/1,4-dioxane media

The reduction of [Co(en)₂Br₂]⁺ by [Fe(CN)₆]^4– in H₂O–MeOH and H₂O–1,4-dioxane mixtures has been studied over a range of solvent compositions [5–30% (v/v)]. The reduction of [Co(en)₂Br₂]⁺ was monitored under second order conditions and was found to be rapid in the various solvent compositions investigated. The favoured mechanism is an outer-sphere electron-transfer process consisting of elementary steps, ion-pair formation (K _IP), electron-transfer (k _et) and successor dissociation. Therefore, the overall rate constant, k ₂ = K _IP k _et[Co(en)₂- Br₂ ⁺][Fe(CN)₆ ^4–]. The rates increase as the proportion of organic cosolvent increases. The rates correlate with solvent properties, such as relative permittivity (_r) and the Grunwald–Winstein parameter, Y _GW, which are used to explain the non-specific interaction upon solvation of mixture of solvents on the incipient reactants and on the ion-pair. In addition, they are also subjected to multiparametric analysis employing Swain's solvent vectors A and B also with Kamlet–Taft's solvatochromic parameters , and *. The reduction rates show an excellent correlation with multiparametric equations and are susceptible to both specific and non-specific solvation effects. A quantitative estimation of the latter components has been attempted.     

Conductance of iron(III) laurate solutions in binary liquid mixtures

Summary The specific conductivity of iron(III) laurate solutions in binary liquid mixtures shows that the soap aggregates into micelles and CMC is found to be independent of temperature and concentration of benzene, acetone and tetrachloromethane in 1-butanol. The variation of molecular conductance,, with soap concentration, C, is expressed by the equation: = 10^AC^B. Both constants A and B vary with solvent composition but B remains constant with increase in temperature. Several parameters such as molecular conductance at infinite dilution, , dissociation constant, K, heat of dissociation, H, entropy, S and free energy, G of dissociation of soap have been evaluated and the effect of the nature of solvents has been discussed.     

Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid of natural isotopic composition initiated by phosphorus pentoxide

The isotopic composition of the consecutive fractions of carbon monoxide produced in the decarbonylation of liquid formic acid of natural isotopic composition initiated by addition of phosphorus pentoxide has been measured in the temperature interval 19–100°C and the observed gradual decrease of the _PDB values and the increase of thek ₁₂/k ₁₃ ratio of the isotopic specific rate constants (KIE values) for each next fraction of CO have been interpreted in terms of conclusions presented in the first paper from this series¹ concerning the decarbonylation of HCOOH (F.A.) in concentrated and diluted with water phosphoric acid media. The initial fast dehydration of F.A. by phosphoric anhydride, P₂O₅, proceeds at room temperture with about 1% carbon-13 KIE. The (k ₁₂/k ₁₃) values increase with time, as the decarbonylation slows down due to the hydration of phosphorus pentoxide with water generated in dehydration of HCOOH and reach the plateau values characteristic for each reaction temperature. These increasing very slowly with reaction times at intermediate temperatures maximum values of (k ₁₂/k ₁₃) ratios are quite close to values of¹³C KIE observed in the decarbonylation of pure F.A. (k ₁₂/k ₁₃=1.0443 at 81°C). Addition of water to liquid F.A. at 90°C and at 100°C caused the further increase of the¹³C KIE. The detailed discussion of the¹³C KIE in the HCOOH–P₂O₅ system has been given.     

The reaction of trichloromethyl radicals with hydrogen chlorid and a new estimation of the rate of combination of trichloromethyl radicals

The effect of the addition of hydrogen chloride on the photolysis of carbon tetrachloride in the presence of cyclohexane has been investigated in a companion paper. The data enable the rate constant ratio k₈/(k₅)^1/2 to be determined. Since k_?8 is well established, k₅ can be estimated from known thermochemical data. The validity of the thermochemical derivation is checked by applying it to trifluoromethyl radicals. The photolysis of bromotrichloromethane and carbon tetrachloride in the presence of hydrogen chloride has been investigated over a range of temperatures. From these results and assuming reaction (5) has no activation energy, Arrhenius parameters for reaction (8) have been determined: The activation energies for the reaction of methyl, trichloromethyl, and trifluoromethyl radicals with hydrogen chloride are compared, and at first sight surprising results are rationalized in terms of relative electronegativity.     

Kinetics and mechanism of the acid-catalysed decarboxylation ofcis-[Co(cyclen)CO3]+

Summary The acid catalysed decarboxylation ofcis-[Co(cyclen)CO₃] has been studied over a range of nitric acid concentrations, at 25, 35.4 and 45°. The rate expression takes the form: k_obs=k₀+k₁ [H⁺], where k_obs is the observed first order rate constant at constant hydrogen ion concentration. The ko term which represents the spontaneous or water reaction is kinetically unimportant at the acidities used in the study. The activation parameters for the acid-catalysed decarboxylation are H=100.4 kj mol^–1 and S ₂₉₈=+51 JK^–1mol^–1. The acid catalysed reaction is subject to a deuterium solvent isotope effect consistent with a mechanism involving a rapid preequilibrium protonation of the complex followed by a slow ratedetermining ring opening of the carbonate ring.     

Kinetics and mechanism of the displacement of CO from Co4(CO)12 by phosphite ligands

Detailed kinetic data are reported for the monosubstitutions of Co₄(CO)₁₂ with phosphite ligands: P(OMe)₂Ph, P(OMe)Ph₂, P(OPr-i)₃ and P(OPh)₃, studied by conventional methods in CHCl₃ as solvent. The reaction rates suggest parallel pathways of dissociation (k ₁) and association (k ₂) and show predominantly an association pathway, the low values of H and negative S adding further support to the proposed mechanism. It is also confirmed that the reaction rates are retarded due to hydrogen-bonding between the H atom of CHC1₃ and the O atoms of the ligands [J. Wang et al., J. Coord. Chem., 23, 345 (1991)]. The results of the reactions of Co₄(CO)₁₂ with P(OMe)₃, P(OMe)₂Ph and P(OMe)Ph₂ in this paper suggest that no quantitative relation exists between the O atoms in the ligand and the reaction rate.     

Determination of the gas-phase reactivity of hydroxyl with chlorinated methanes at high temperature: Effects of laser/thermal photochemistry

Atmospheric pressure absolute rate coefficients have been determined for the gas phase reaction of OH radicals with methyl chloride (k₁), methylene chloride (k₂), and chloroform (k₃) over an extended temperature range using a laser photolysis/laser-induced fluorescence technique. The rate coefficients are best described by the following modified Arrhenius equations: Measurements were obtained as a function of excimer photolysis intensity and are compared with previous results and extended to higher temperatures. Photolysis intensities in excess of 12 mJ-cm^?2 were found to measurably increase (up to a factor of 2) the rate coefficients for k₃ between 400–775 K, with the effect increasing with increasing temperature. A similar, yet much smaller (ca. 20–35%) increase was observed for k₂ between 675–955 K. No effect was observed for k₁ at any temperature. Relative absorption coefficient measurements at 193.3 nm indicated that chlorinated methane photolysis increases with both increasing temperature and increasing chlorine substitution. These measurements suggest that reactant photolysis may be responsible for the observed dependence of k₂ and k₃ on photolysis intensity at elevated temperatures. The puzzling and disconcerting discrepancy between previously published high temperature measurements of k₃ and transition state model predictions is reconciled with these latest measurements. © 1993 John Wiley & Sons, Inc.     

Kinetics and Mechanism of Izomerization of N-Alkoxycarbonyl-5-aroxytetrazoles

The kinetics and mechanism of the N²-N¹-isomerization of 2-methoxycarbonyl-5-(p-X-phenoxy)- tetrazoles (X = H, CH₃, NHCOCH₃, Cl, Br, NO₂) were studied by ¹H NMR spectroscopy in a DMSO-d ₆-CDCl₃ mixture (25:75). The rate of isomerization of the N²-isomer into N¹-isomer fit the first-order equation (after three half-conversion periods). The isomerization is accompanied by hydrolysis and decarboxylation. The Hammett plot of ln(k _X k _H) for the isomerization showed a good correlation with ^- values (^- = 1.33, r = 0.965). A poor correlation with values was obtained. The kinetic data, the effect of solvent polarity, the substituent effects, and the results of AM1 quantum-chemical calculations suggest an ionic mechanism of the isomerization in polar solvents and a concerted mechanism in nonpolar solvents.     

Effects of diffusion on the self-termination kinetics of isopropylol radicals in solution

Rate constants for the bimolecular self-reaction of isopropylol radicals [(CH₃)₂?OH] in various solvents are determined as functions of temperature by kinetic electron spin resonance. For hydrocarbon solvents they are well described by theoretical equations for reactions controlled by translational diffusion if diffusion coefficients of 2-propanol, a constant reaction distance, and a spin statistical factor of 1/4 are applied. Deviations from 2k_t ～ D at high diffusion constants agree with trends expected from recent theoretical models. For hydrogen-bonding solvents large negative deviations are observed. They are attributed to steric constraints and slower rotational diffusion of radical–solvent aggregates. The disproportionation-to-combination ratio of isopropylol increases with solvent viscosity. As previously for tert-butyl, this is explained by anisotropic reorientation during encounters. Further, rate data are given for the decarbonylation of the 2-hydroxy-2-methylpropanoyl radical and for several hydrogen abstraction reactions of isopropylol.     

The structuredness of solvents. 2. Data for ambient conditions

In view of the inadequacy of Trouton's constant for the expression of the structuredness of solvents at ambient temperatures, alternative measures have been sought. Calculations of the entropy deficit _vapS⁰/R of solvents at ambient conditions relative to their vapors and compared to presumably unstructured alkanes with the same number and arrangement of skeletal atoms have been made. Data for nonpolar, dipolar aprotic, non-hydroxylic protic and hydroxylic solvents are presented, with _vapS⁰/R>2 representing structured solvents. Empirically, the heat capacity density of solvents at ambient temperatures, [C _p (l)-C _p (g)]/V], is another meaningful measure of the structuredness of solvents, compatible with the former one. Values>0.6 J-K^–1-cm^–3 signify a structured solvent.     

Reactivity of the carbocation generated in the photolysis of 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline toward azide ion

The reaction of the azide ion with the carbocation generated in the photolysis of 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline in methanol was studied by pulse (conventional and laser) and steady-state photolysis techniques. The adduct of the azide ion was characterized by ¹H NMR spectrum. Experimental results were interpreted taking into account a competition between the addition of methanol and azide ion to the carbocation. The rate constants for the reaction of the azide ion with the carbocation (k _Az) were measured at 2—48 °C in a wide range of [N₃ ^–]₀ concentrations from 2·10^–7 to 0.1 mol L^–1 at different ionic strengths () of the solution. The resulting k _Az values are more than an order of magnitude lower than those for diffusional-controlled reactions and vary from 3.2·10⁸ ( = 0) to 4.5·10⁶ L mol^–1 s^–1 ( = 0.8 mol L^–1) in the presence of NaClO₄ (18 °C). The activation energy of addition of the azide ion to the carbocation is 21 kJ mol^–1, which is by 12 kJ mol^–1 lower than the activation energy of the reaction of the carbocation with methanol. The features of the reaction under study are discussed from the viewpoint of the structures of carbocations generated in the photolysis of dihydroquinolines.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXII.1 Solvent Effects on Activation Parameters of Heterolysis of 1-Chloro-1-methylcyclopentane. Correlation Analysis of Solvation Effects

Kinetics of heterolysis of 1-chloro-1-methylcyclopentane in MeOH, BuOH, cyclohexane, i-PrOH, t-BuOH, tert-C₅H₁₁OH, -butyrolactone, MeCN, PhCN, PhNO₂, acetone, PhCOMe, cyclohexanone, and 1,2-dichloroethane at 25-50°C were studied by the verdazyl method. Correlation analysis of solvent effects on activation parameters of the reaction in 8 protic (additionally, AcOH and CF₃CH₂OH) and 8 aprotic solvents together and separately in either group of solvents was performed. In all the solvents studied, two H -S compensation effects were revealed.     

Hydrophilic and Hydrophobic interactions in alcohol solutions of high molecular weight poly(n-vinyl-2-pyrrolidone)

Inter- and intra-molecular association in alcohol solutions of poly(n-vinyl-2-pyrrolidone) with molecular weight of 670,000 is investigated by viscometric techniques. While the limiting viscosity number, [], of the polymer was observed to remain almost constant in methanol, ethanol and n-propanol solutions, the Huggins constant,k _H showed an increase with increasing carbon content of the alcohols studied. Addition of denaturing agents, such as urea and guanidine hydrochloride, (GC) caused a decrease in both of the above values. The changes brought about in these parameters by the denaturing agents were found to be a result of the changes taking place in the hydrophilic and hydrophobic interactions among polymer, solvent and additive molecules.     

Solvent effect and temperature dependence in the interaction of singlet oxygen with α-phenyl-N-tert-butyl nitrone

The solvent effect on the quenching of singlet oxygen by -phenyl-N-tert-butyl-nitrone /PBN/ has been investigated by laser flash photolysis technique registrating luminescence kinetics of¹O₂. The values of the rate constant /k_q/ of the quenching were at 293 K: /9.0±0.4/×10⁶, /4.4±0.3/×10⁶ and /18.3±0.5/×10^{6 –}M^–1 s^–1 in toluene, chloroform and acetonitrile, respectively. The rate constant for the chemical interaction between¹O₂ and PBN, was k_r<1×10⁵ M^–1 s^–1k_q independently of the solvent. At temperatures between 223 and 293 K in toluene E_q=0.4±0.4 kJ mol^–1.     

Calculation of activation volumes of thermal conversions of trimethylsilyl(cumyl) peroxide by the spline approximation method

The overall rate constants k_s of thermal conversions of trimethylsilyl(cumyl) peroxide (TMSCP) at pressures P up to 10 kbar are separated into components taking into account the ratios of the corresponding products: the rate constants of radical decomposition k_d and the rate constant of rearrangement k _r. Spline approximation of experimental dependences of lnk on P is used to determine the continuous dependence of the activation volume V^# on P in the solvents cyclohexane, anisole, toluene, n-nonane, isopropylbenzene, and tert-butylbenzene. The dependences of V_s^#, V_d^#, V_r^# on P are synchronous and nonordinary, and their nature significantly depends on the choice of a solvent. Spline approximation of experimental dependences of the TMSCP concentrations on the reaction time in isopropylbenzene at various P is used to find differential constants k_s , which generally increase with the reaction time, especially in the pressure interval 5–10 kbar.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 836–841.Original Russian Text Copyright © 2004 by Zhulin, Rudakov, Antonovskii.     

Complex formation between nickel(II) and 2-(2-aminoethyl) benzimidazole: A kinetic and equilibrium study

Summary The reversible complex formation between 2-(2-aminoethyl) benzimidazole (AEB) and nickel(II) was studied by stopped flow spectrophotometry at I = 0.30 mol dm^–3. Both the neutral and monoprotonated form of AEB reacted to give the NiAEB²⁺ chelate. At 25 °C, the rates and activation parameters for the reactions Ni_II + AEB NiAEB²⁺ and Ni^II + AEBH⁺ NiAEB²⁺ + H⁺ are k _f ^L(dm^–3 mol^–1 s^–1) = (2.17 ± 0.24) × 10³, H (kJ mol^–1) = 40.0 ± 0.8, S (JK^–1 mol^–1) = – 47 ± 3 and k _inff ^pHL (dm³ mol^–1 s^–1) = 33 ± 10, H (kJ mol^–1) = 42.0 ±2.7, S (JK^–1 mol^–1) = – 72 ± 9. The dissociation of NiAEB²⁺ was acid catalysed and k _obs for this process increased linearly with [H⁺] in the 0.01–0.15 mol dm^–3 (10–30 °C) range with k _H(dm³ mol^–1s^–1) (25 °C) = 329 ± 6, H (kJ mol^–1) = 40 ± 2 and S (JK^–1 mol^–1) = – 61 ± 8. The results also indicated that the formation of NiAEB²⁺ involves a chelation-controlled, rate-limiting process. Analysis of the S ° data for the acid ionisation of AEBH _inf2 ^p2+ and the formation of NiAEB²⁺ showed that the bulky AEBH⁺ ion has a solvent structure breaking effect as compared to AEB [s _aqS ° (AEBH⁺) – s _aq ° (AEB) = 69 JK^–1 mol^–1], while AEBH _inf2 ^p2+ is a solvent ordering ion relative to NiAEB²⁺ [s _aq° (NiAEB²⁺) – ovS _aq ° (AEBH _inf2 ^p2+ ) = 11 JK^–1 mol^–1].Author to whom all correspondence should be directed.     

Hydrogen transfer from ketyl radicals to nitroxyl radicals

The transfer of an H atom from a ketyl radical (KR) to a nitroxyl radical is the sole reaction occurring between benzophenone or acetone KR and nitroxyl radicals (NR). The rate constants (k_H) of the reaction of the KR of substituted benzophenones with the NR-4-hydroxy-2,2,6,6-tetramethylpiperidine-1oxyl and 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl in various solvents were measured by the pulse photolysis method. In low-viscosity solvents (up to 1-2 cP), the values of k_H are not limited by the diffusion of the reagents. The k_H values decrease with increase in the Hammet's -constants of the substituents in the KR and with decrease in the reduction potential of the NR, which indicates a charge transfer from the KR to NR in the transition state of the reaction. A cyclic structure was proposed for the transition state. The reaction is characterized by a low isotopic effect (k_H/k_D=1.4–1.5). The dependence of log k_H on the solvation parameter of the solvent E_t(30) is V-shaped in character.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 999–1003, May, 1990.