Kinetics and mechanism of the anilinolysis of aryl 4‐nitrophenyl thionocarbonates in aqueous ethanol

The reactions of bis(4‐nitrophenyl), 3‐chlorophenyl 4‐nitrophenyl, and 3‐methoxyphenyl 4‐nitrophenyl thionocarbonates ( 1 , 2 , and 3 , respectively) with a series of anilines are subjected to a kinetic investigation in 44 wt.% ethanol–water, at 25.0 °C and an ionic strength of 0.2 M. Under aniline excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus aniline concentration are linear, with the slopes (k_N) pH independent, k_N being the rate coefficient for the anilinolysis of the thionocarbonates. The Brønsted plot (log k_N vs. pK_a of anilinium ions) for thionocarbonate 1 is linear, with slope (β) 0.62, which is consistent with a concerted mechanism. The Brønsted plots for thionocarbonates 2 and 3 are curved, with slopes 0.1 at high pK_a for both reaction series and slopes 0.84 and 0.79 at low pK_a for the reactions of 2 and 3 , respectively. The latter plots are in accordance to stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T^±) and its anionic analogue (T^?), the latter being formed by deprotonation of T^± by the basic form of the buffer (HPO). The Brønsted curves are explained by a change in the rate‐limiting step, from deprotonation of T^± at low pK_a, to its formation at high pK_a. The influence of the amine nature and the non‐leaving and electrophilic groups of the substrate on the kinetics and mechanism is also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetics and mechanism of the pyridinolysis of diaryl carbonates

The reactions of 4‐methylphenyl and 4‐chlorophenyl 4‐nitrophenyl carbonates ( 1 and 2 , respectively), phenyl, 4‐methylphenyl, 4‐chlorophenyl, and 4‐nitrophenyl 2,4‐dinitrophenyl carbonates ( 3 , 4 , 5 , and 6 , respectively), and bis(2,4‐dinitrophenyl) carbonate ( 7 ) with a series of pyridines are studied kinetically at 25.0 °C in 44 wt% ethanol–water and an ionic strength of 0.2 M (KCl). The reactions are followed spectrophotometrically and under excess amine pseudo‐first‐order rate coefficients (k_obs) are found. For all these reactions, plots of k_obs versus free amine concentration at constant pH are linear, the slope (k_N) being independent of pH. The Brønsted‐type plots (log k_N vs. pK_a of the conjugate acids of the pyridines) are all biphasic (linear portions at high and low pK_a and a curvature in between). These plots are in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T^±), and a change in the rate‐determining step from formation of T^± to its breakdown to products, as the pyridine basicity decreases. Also studied are the effects of the leaving, non‐leaving, and electrophilic groups of the substrate, and of the amine nature, on the value (value at the center of curvature of the Brønsted‐type plots). Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetics and mechanism of the reactions of aryl chlorodithioformates with pyridines and secondary alicyclic amines

The reactions of pyridines and secondary alicyclic (SA) amines with phenyl and 4‐nitrophenyl chlorodithioformates (PClDTF and NPClDTF, respectively) are subjected to a kinetic study in aqueous ethanol (44 wt% ethanol) solution, at 25.0 °C, and an ionic strength of 0.2 M (KCl). The reactions are studied spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus [amine] are linear and pH independent, with slope k_N. The Brønsted‐type plots (log k_N vs. pK_a of aminium ions) are linear for the reactions of PClDTF with SA amines (slope β of 0.3) and pyridines (β = 0.26) and those of NPClDTF with pyridines (β = 0.30). For the reaction of NPClDTF with SA amines the Brønsted‐type plot is biphasic, with slopes β₁ = 0.2 (at high pKa) and β₂ = 1.1 (at low pKa). The pK_a value at the center of curvature (pK) is 7.7. The magnitude of the slopes indicates that the mechanisms of these reactions are stepwise, with the formation of a zwitterionic tetrahedral intermediate as the rate‐determining step, except for the reaction of NPClDTF with SA amines where there is a change in the rate‐determining step, from formation to breakdown of the tetrahedral intermediate, as the amine basicity decreases. Copyright © 2009 John Wiley & Sons, Ltd.     

Transition from concerted to stepwise processes as a function of leaving group ability: density functional theory and experimental study of lyoxide‐promoted cleavages of phosphorothioate and phosphate triesters in water and methanol

The base‐promoted solvolysis of a series of O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphorothioates (1) as well as O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphates (2) was studied computationally by density functional theory methods in methanol and water continuum media to determine the transition between concerted and stepwise processes. In addition, an experimental study was undertaken on the solvolysis of these series in basic methanol and water. The computations indicate that the solvolytic mechanism for series 1 involves lyoxide attack anti to the leaving group in a concerted manner with good leaving groups having pK_a^Lg values < 12.3 in methanol and in a stepwise fashion with the formation of a 5‐coordinate thiophosphorane intermediate when the pK_a^Lg > 12.3. A similar transition from concerted to stepwise mechanism occurs with series 2 in methanol as well as with series 1 and 2 in water, although for the aqueous solvolyses with hydroxide nucleophile, the transitions between concerted and stepwise mechanisms occur with better leaving groups than in the case in methanol. The computational data allow the construction of Brønsted plots of log k₂^?OS versus pK_a^Lg in methanol and water, which are compared with the experimental Brønsted plots determined with these series previously and with new data determined in this work. Both the computational and experimental Brønsted data reveal discontinuities in the plots between substrates bearing O‐aryl and O‐alkyl leaving groups, with the gradients of the plots being far steeper than, and non‐collinear with, the O‐aryl leaving groups for solvolysis of the O‐alkyl‐containing substrates. These discontinuities signify that care should be exercised in interpreting breaks in Brønsted plots in terms of changes in rate‐limiting steps that signify the formation of an intermediate during a solvolytic process. Copyright © 2011 John Wiley & Sons, Ltd.     

Reactions of aryl chlorothionoformates with quinuclidines. A kinetic study

The reactions of quinuclidines with phenyl, 4‐chlorophenyl, 4‐cyanophenyl, and 4‐nitrophenyl chlorothionoformates ( 1 , 2 , 3 , and 4 , respectively) are subjected to a kinetic study in aqueous solution, at 25.0°C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically the release of the corresponding phenoxide anion/phenol generated in the parallel hydrolysis of the substrates. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus [amine] are linear, with slope k_N. The Brønsted‐type plots (log k_N vs. pK_a of aminium ions) are linear, with slopes β = 0.26, 0.22, 0.19, and 0.28 for the reactions with 1 , 2 , 3 , and 4 , respectively. The magnitudes of the slopes indicate that these mechanisms are stepwise, with rate‐determining formation of a zwitterionic tetrahedral intermediate (T^±). A dual parametric equation with the pK_a of the nucleophiles and non‐leaving groups show β_N = 0.26 and β _nlg = ?0.16, also in accordance with the proposed mechanism. On the other hand, the reactivity of these thiocarbonyl substrates and their carbonyl derivatives was studied using their hardness index and compared with their experimental parameters, confirming the proposed mechanisms. By comparison of the title reactions with similar aminolyses, the following conclusions arise: (i) The mechanism of the reactions under investigation is stepwise with rate‐determining formation of T^±. (ii) The reactivity of the substrates toward quinuclidines follows the order 4 > 3 > 2 > 1 . (iii) Quinuclidines are more reactive than isobasic pyridines toward chlorothionoformates. (iv) Chlorothionoformates are less reactive than chloroformates towards quinuclidines in accordance with the HSAB principle. (v) The k_N values for phenyl chloroformate and 4 can be correlated with the pK_a of quinuclidines and also with the hardness values calculated by the HF/3‐21G level of theory. Copyright © 2007 John Wiley & Sons, Ltd.     

Reactions of O‐aryl S‐aryl dithiocarbonates with secondary alicyclic amines in aqueous ethanol. Kinetics and mechanism

The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl), O‐(4‐chlorophenyl) (4‐nitrophenyl), O‐(4‐chlorophenyl) S‐phenyl, and O‐(4‐methylphenyl) S‐phenyl dithiocarbonates ( 1 , 2 , 3 , and 4 , respectively) with a series of secondary alicyclic (SA) amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0 °C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. For some of the reactions, plots of k_obs vs. free amine concentration at constant pH are linear but others are nonlinear upwards. This kinetic behavior is in accordance with a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T^±) and the other anionic (T^?). In some cases, there is a kinetically significant proton transfer from T^± to an amine to yield T^?. Values of the rate micro constants k₁ (amine attack to form T^±), k_?1 (its back step), k₂ (nucleofuge expulsion from T^±), and k₃ (proton transfer from T^± to the amine) are determined for some reactions. The Brønsted plots for k₁ are linear with slopes β₁ = 0.2–0.4 in accordance with the slope values found when T^± formation is the rate‐determining step. The sensitivity of log k₁ and log k_?1 to the pK_a of the amine, leaving and non‐leaving groups are determined by a multiparametric equation. For the reactions of 1 – 4 with 1‐formylpiperazine and those of 3 and 4 with morpholine the k₂ and k₃ steps are rate determining. Copyright © 2010 John Wiley & Sons, Ltd.     

Nucleophilicity of metal carbonyl anions in vinylic substitution reactions

Second order rate constants are reported for the reactions of metal carbonyl anions ([M(CO)_nL]^?) with several vinyl halides: PhCCl?C(CN)₂, Z‐ and E‐Ph(CN)C?CHHal (Hal = Cl, Br) which follow the addition–elimination (Ad_NE) substitution mechanism. The obtained data show that the nucleophilic reactivity of [M(CO)_nL]^? anions towards vinyl halides increases in the same order as in aliphatic S_N2 reactions, but much more steeply, by 14 orders of magnitude in the row log{ }: [CpFe(CO)₂]^? (～14), [Re(CO)₅]^? (7.8), [Mn(CO)₅]^? 2.1, [CpW(CO)₃]^? (0.7) > [CpMo(CO)₃]^? (0). A good correlation exists between nucleophilicities of [M(CO)_nL]^? anions towards vinyl (sp²‐carbon) and alkyl halides (sp³‐carbon) with slope 2.7. The reactivity of [M(CO)_nL]^? in a halogen–metal exchange process (with Z‐PhC(CN)?CHI) follows a similar ‘large’ scale as in the Ad_NE process. The nucleophilicity of [M(CO)_nL]^? anions correlates better with their one‐electron oxidation potentials (E_ox) than with their basicity (pK_a of [M(CO)_nL]H). Copyright © 2008 John Wiley & Sons, Ltd.     

Reactions of O‐aryl S‐aryl dithiocarbonates with pyridines in aqueous ethanol: kinetics and mechanism

The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl) dithiocarbonate ( 1 ), O‐(4‐chlorophenyl) S‐(4‐nitrophenyl) dithiocarbonate ( 2 ), and O‐(4‐chlorophenyl) S‐phenyl) dithiocarbonate ( 3 ) with a series of pyridines were subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0 °C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) were determined. For the studied reactions, plots of k_obs versus free pyridine concentration at constant pH were linear, with the slope (k_N) independent of pH. The Brønsted‐type plots for ( 1 ) and ( 2 ) were biphasic, suggesting a stepwise mechanism with a change in the rate‐determining step, from breakdown to the formation of a tetrahedral intermediate (T^±), as the basicity of the pyridines increases. For the reactions of ( 3 ), at the pK_a range of the pyridines studied, only the breakdown to products of T^± was observed. Copyright © 2009 John Wiley & Sons, Ltd.     

Prediction of ortho substituent effect in alkaline hydrolysis of substituted phenyl benzoates in aqueous acetonitrile

The second‐order rate constants k (dm³mol^?1s^?1) for alkaline hydrolysis of meta‐, para‐ and ortho‐substituted phenyl esters of benzoic acid, C₆H₅CO₂C₆H₄‐X, in aqueous 50.9% (v/v) acetonitrile have been measured spectrophotometrically at 25 °C. In substituted phenyl benzoates, C₆H₅CO₂C₆H₄‐X, the substituent effects log k_X ? log k_H in aqueous 50.9% acetonitrile at 25 °C for para, meta and ortho derivatives showed good correlations with the Taft and Charton equations, respectively. Using the log k values for various media at 25 °C, the variation of the ortho substituent effect with solvent was found to be precisely described with the following equation: Δlog k_ortho = log k_ortho ? log k_H = 1.57σ_I + 0.93σ°_R + 1.08E_s^B ? 0.030ΔEσ_I ? 0.069ΔEσ°_R, where ΔE is the solvent electrophilicity, ΔE = E_S ? E_H20, characterizing the hydrogen‐bond donating power of the solvent. We found that the experimental log k values for ortho‐, para‐ and meta‐substituted phenyl benzoates in aqueous 50.9% acetonitrile at 25 °C, determined in the present work, precisely coincided with the log k values predicted with the equation (log k^X)_calc = (log k^H_AN)_exp + (Δlog k^X)_calc where the substituent effect (Δlog k^X)_calc was calculated from equation describing the variation of the substituent effect with the solvent electrophilicity parameter, using for aqueous 50.9% CH₃CN the solvent electrophilicity parameter, ΔE = ?5.84. In going from water to aqueous 50.9% CH₃CN, the ortho inductive term grows twice less as compared with the para polar effect. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane

Extending our earlier findings for [3.3]paracyclophane, NMR line shape studies of the conformational dynamics in [3.2] and [4.3]paracyclophanes are reported, of which the former is conformationally homogeneous and the latter occurs in two enantiomeric forms. For [3.2]paracyclophane, the Arrhenius activation energy E_a = 11.6 ± 0.1 kcal/mol and preexponential factor log (A/s^?1) = 12.92 ± 0.07 were found. In [4.3]paracyclophane, the conformational dynamics are quite complicated because, apart from interconversions of each enantiomer into itself proceeding via inversion of the propano bridge with rate constant k₁, the enantiomers mutually rearrange with rate constant k₂ due to inversion of the butano bridge. The determination of Arrhenius parameters from dynamic ¹H spectra of the aromatic protons for these two conformational processes (E_a = 11.2 ± 0.5 kcal/mol and log (A/s^?1) = 13.6 ± 0.5 for the former, and E_a = 9.7 ± 0.4 kcal/mol and log (A/s^?1) = 13.2 ± 0.4 for the latter) is the highlight of this work. In the investigated temperature range, in [4.3]paracyclophane, the occurrence of other conformational processes beyond those mentioned above can be excluded, because they would produce different line shape patterns than those actually observed. Copyright © 2013 John Wiley & Sons, Ltd.     

How does the s(E + N) equation work? Comparisons with a modified Swain–Scott equation (E + sN) and revision of the N1 scale of solvent nucleophilicity

Modifications of the Swain–Scott equation (log k/k₀) = sn) give an equation log k₁ = (E + sN₁′); k₁ is the rate constant, E is an electrophilicity parameter, N₁′ is a solvent nucleophilicity parameter and s is an electrophile‐specific sensitivity parameter. The equation is tested using over 300 published first‐order rate constants (k₁) for decay of a range of benzhydrylium cations in various solvents, on which the published N₁ scale of solvent nucleophilicity is based (S. Minegishi, S. Kobayashi and H. Mayr, J. Am. Chem. Soc. 2004, 126, 5174–5181) using the alternative equation log k = s(E + N₁), in which s is a nucleophile‐specific parameter. The modified (E + sN₁′) equation provides a revised N₁′ scale of solvent nucleophilicity, and a more precise fit, with less than half the number of adjustable parameters. It is found that the sensitivities of the benzhydrylium cations to changes in solvent nucleophilicity decrease slightly as reactivity increases, in contrast to s(E + N) equations, which show no trends in s values. It is proposed that more reliable N scales can be defined using (E + sN), because N is determined directly from definitions, and residual errors (e.g. experimental or due to solvation effects) can be incorporated into the slope and intercept. The complex reasons for the success of equations of the type log k = s(E + N) are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental and computational determination of Brønsted coefficients for equilibrium transfer of the O,O‐dimethyl phosphorothioyl group between oxyanion nucleophiles

An experimental determination of the β^Eq value for equilibrium transfer of the O,O‐dimethyl phosphorothioyl group between oxyanion nucleophiles in water and methanol at 25 °C is presented. The respective β^Eq values in the two solvents are experimentally the same at ?1.45 ± 0.08 and ?1.39 ± 0.12. Based on the observation that the Brønsted correlation for the nucleophilic reaction of phenoxides in water with substrate 1d (dimethyl 4‐nitrophenyl phosphorothioate, pK_a^HOAr of 7.14) is linear over the entire range of phenoxides employed (5.53 ≤ pK_a^Nu ≤ 12.38), the reaction for phenoxide nucleophiles displacing phenoxide leaving groups is probably concerted. The obtained data allow one to calculate, for a symmetrical transition state involving 2,4,5‐trichlorophenoxide as a nucleophile and leaving group, an approximately 60% P–OAr cleavage and about 40% P–Nuc bond formation. A computational method is presented for the rapid prediction of the β^Eq values for such processes in water and methanol, and the results are compared with known values from the literature. Copyright © 2011 John Wiley & Sons, Ltd.     

Steric effects on the mechanism of reaction of nucleophilic substitution of β‐substituted alkoxyvinyl trifluoromethyl ketones with four secondary amines

The kinetics of the reaction of β‐substituted β‐alkoxyvinyl trifluoromethyl ketones R¹O‐CR²?CH‐COCF₃ ( 1a – e ) [( 1a ), R¹?C₂H₅, R²?H; ( 1b ), R¹?R²?CH₃; ( 1c ), R¹?C₂H₅, R²?C₆H₅; ( 1d ), R¹?C₂H₅, R²?V^?pNO₂C₆H₄; ( 1e ), R¹?C₂H₅, R²?C(CH₃)₃] with four aliphatic amines ( 2a – d ) [( 2a ), (C₂H₅)₂NH; ( 2b ), (i‐C₃H₇)₂NH; ( 2c ), (CH₂)₅NH; ( 2d ), O(CH₂CH₂)₂NH] was studied in two aprotic solvents, hexane and acetonitrile. The least reactive stereoisomeric form of ( 1a – d ) was the most populated ( E‐s‐Z‐o‐Z ) form, whereas in ( 1e ), the more reactive form ( Z‐s‐Z‐o‐Z ) dominated. The reactions studied proceeded via common transition state formation whose decomposition occurred by ‘uncatalyzed’ and/or ‘catalyzed’ route. Shielding of the reaction centre by bulky β‐substituents lowered abruptly both k′ (‘uncatalyzed’ rate constant) and k″ (‘catalyzed’ rate constant) of this reaction. Bulky amines reduced k″ to a greater extent than k′ as a result of an additional steric retardation to the approach of the bulky amine to its ammonium ion in the transition state. An increase in the electron‐withdrawing ability of the β‐substituent increased ‘uncatalyzed’ k′ due to the acceleration of the initial nucleophile attack (k₁) and ‘uncatalyzed’ decomposition of transition state (k₂) via promoting electrophilic assistance (through transition state 8 ). The amine basicity determined the route of the reaction: the higher amine basicity, the higher k₃/k₂ ratio (a measure of the ‘catalyzed’ route contribution as compared to the ‘uncatalyzed’ process) was. ‘Uncatalyzed’ route predominated for all reactions; however in polar acetonitrile the contribution of the ‘catalyzed’ route was significant for amines with high pK_a and small bulk. Copyright © 2007 John Wiley & Sons, Ltd.     

Kinetics of the gas‐phase homogeneous unimolecular elimination of selected ethyl esters of 2‐oxo‐carboxylic acids

The gas‐phase elimination kinetics of selected ethyl esters of 2‐oxo‐carboxylic acid have been studied over the temperature range of 270–415 °C and pressures of 37–114 Torr. The reactions are homogeneous, unimolecular, and follow a first‐order rate law in a seasoned static reaction vessel, with an added free radical suppressor toluene. The observed overall and partial rate coefficients are expressed by the following Arrhenius equations:

• Ethyl oxalyl chloride
• log k_overall (s^?1) = (13.22 ± 0.45) ? (179.4 ± 4.9) kJ mol^?1 (2.303 RT)^?1
• Ethyl piperidineglyoxylate
• log k_(CO2) (s^?1) = (12.00 ± 0.30) ? (191.2 ± 3.9) kJ mol^?1 (2.303 RT)^?1
• log k_(CO) (s^?1) = (12.60 ± 0.09) ? (210.7 ± 1.2) kJ mol^?1 (2.303 RT)^?1
• log k_t(overall) (s^?1) = (12.22 ± 0.26) ? (193.4 ± 3.4) kJ mol^?1 (2.303 RT)^?1
• Ethyl benzoyl formate
• log k_(CO2) (s^?1) = (12.89 ± 0.72) ? (203.8 ± 9.0) kJ mol^?1 (2.303 RT)^?1
• log k_(CO) (s^?1) = (13.39 ± 0.31) ? (213.3 ± 3.9) kJ mol^?1 (2.303 RT)^?1
• log k_t(overall) (s^?1) = (13.24 ± 0.60) ? (205.8 ± 7.6) kJ mol^?1 (2.303 RT)^?1

The kinetic and thermodynamic parameters of these reactions, together with those reported in the literature, lead to consider three different mechanistic pathways of elimination. Copyright © 2010 John Wiley & Sons, Ltd.     

Nucleophilic reactivity of thiolate, hydroxide and phenolate ions towards a model O-arylated diazeniumdiolate prodrug in aqueous and cationic surfactant media

The kinetics of aromatic nucleophilic substitution of the nitric oxide‐generating diazeniumdiolate ion, DEA/NO, by thiols (L ‐glutathione, L ‐cysteine, DL ‐homocysteine, 1‐propanethiol, 2‐mercaptoethanol, and sodium thioglycolate) from the prodrug, DNP‐DEA/NO, has been examined in aqueous solution and in solutions of cationic DOTAP vesicles. Second‐order rate constants in buffered aqueous solutions (k_RS‐ = 3.48–30.9 M^?1 s^?1; 30 °C) gave a linear Brønsted plot (β_nuc = 0.414 ± 0.068) consistent with the rate‐limiting S_NAr nucleophilic attack by thiolate ions. Cationic DOTAP vesicles catalyze the thiolysis reactions with rate enhancements between 11 and 486‐fold in Tris‐HCl buffered solutions at pH 7.4. The maximum rate increase was obtained with thioglycolate ion. Thiolysis data are compared to data for nucleophilic displacement by phenolate (k_PhO‐ = 0.114 M^?1 s^?1) and hydroxide (k_OH‐ = 1.82 × 10^?2 M^?1 s^?1, 37 °C) ions. The base hydrolysis reaction is accelerated by CTAB micelles and DODAC vesicles, with the vesicles being ca 3‐fold more effective as catalysts. Analysis of the data using pseudo‐phase ion‐exchange (PIE) formalism implies that the rate enhancement of the thiolysis and base hydrolysis reactions is primarily due to reactant concentration in the surfactant pseudo‐phase. Copyright © 2009 John Wiley & Sons, Ltd.     

The permittivities of PVDF/PMMA blends at hydrostatic pressure

The complex permittivities of poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends have been measured under variation of temperature T(20°-60°C), frequency v(5 Hz-300 kHz), and hydrostatic pressure p (0-260 MPa). The values can be represented by a master curve with the shift factors △p/△ log (v/Hz) = ?140 MPa at room temperature and △ (1 /T) /△ log (v/Hz) = ?2. 10^?4 K^?1 at atmospheric pressure. The dependence of the activation energy △E _A on pressure p is then given by △E _A=(100 + 0.02 MPa^?1 p) kJ/mol. Furthermore, the results indicate that the β-relaxation of PVDF is due to motions in the crystal-amorphous interphase. The interactions between the two polymers, which are miscible at all compositions, disturb the correlations between the PVDF monomer units at that location as well as the mobility of the PMMA side group.     

Nitrosation of dialkylhydroxylamines,and computational and NMR investigations of the interconversion of conformational isomers of N‐nitroso‐dimethylhydroxylamine

The effect of acidity upon the rate of nitrosation of N‐benzyl,O‐methylhydroxylamine ( 3 ) in 1:1 (v/v) H₂O/MeOH at 25 °C has been investigated. The pseudo‐first‐order rate constant (k_obs) for loss of HNO₂ as the limiting reagent decreases as [H₃O⁺] increases. This is compatible with two parallel reaction channels (Scheme 2 ). One involves the direct reaction of the free hydroxylamine with HNO₂ (k₁ = 1.4 × 10² dm³ mol^?1 s^?1, 25 °C) and the other involves the reaction of the free hydroxylamine with NO⁺ (k₂ = 5.9 × 10⁹ dm³ mol^?1 s^?1). In contrast, there is only a very slight increase in k_obs with increasing [H₃O⁺] for nitrosation of N,O‐dimethylhydroxylamine ( 4 ) in dilute aqueous solution at 25 °C to give N‐nitroso‐dimethylhydroxylamine, 5 . This also fits a two‐channel mechanism (Scheme 3 ). Again, one involves the nitrosation of the free base by NO⁺ (k₂ = 8 × 10⁹ dm³ mol^?1 s^?1, 25 °C) but the other channel now involves catalysis by chloride (k₃ = 1.3 × 10⁸ dm³ mol^?1 s^?1). Arising from these results, we propose an estimate of pK_a ～ ?5 for protonated nitrous acid, (O = N? OH), which is appreciably different from the literature value of +1.7. The interconversion of cis and trans conformational isomers of 5 has been investigated by temperature‐dependent NMR spectroscopy in CDCl₃, methanol‐d₄, toluene‐d₈ and dimethyl sulfoxide‐d₆. Enthalpies and entropies of reaction and of activation have been determined and compared with computational values obtained at the B3LYP/6‐31G* level of theory. The cis form is slightly more stable at normal temperatures and no solvent effects upon the thermodynamics or kinetics of the conformational equilibrium were predicted computationally or detected experimentally. In addition, key geometric parameters and dipole moments have been calculated for the cis and trans forms, and for the lowest energy transition structure for their interconversion, in the gas phase and in chloroform. These results indicate electronic delocalisation in the ground states of 5 which is lost in the transition structure for their interconversion. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetic studies of the reaction of phenacyl bromide derivatives with sulfur nucleophiles

The reaction of the substituted phenacyl bromides 1a–e and 2a–e with thioglycolic acid 3 and thiophenol 6 in methanol underwent nucleophilic substitution S_N2 mechanism to give the corresponding 2‐sulfanylacetic acid derivatives 4a–e, 5a–e and benzenethiol derivatives 9a–e, 10a–e. The reactants and products were identified by mass spectra, infrared and nuclear magnetic resonance. We measured the kinetics of these reactions conductometrically in methanol at a range of temperatures. The rates of the reactions were found to fit the Hammett equation and correlated with σ‐Hammett values. The ρ values for thioglycolic acid were 1.22–1.21 in the case of 4‐substituted phenacyl bromide 1a–e, while in the case of the nitro derivatives 2a–e they were 0.39–0.35. The ρ values for thiophenol were 0.97–0.83 in the case of 4‐substituted phenacyl bromide 1a–e, while in the case of the nitro derivatives 2a–e they were 0.79–0.74. The Brønsted‐type plot was linear with a α = ?0.41 ± 0.03. The kinetic data and structure‐reactivity relationships indicate that the reaction of 1a–e and 2a–e with thiol nucleophiles proceeds by a concerted mechanism. The plot of log k₄₅ versus log k₃₀, the plot log(k_x,3‐NO2/k_H) versus log(k_x/k_H), and the Brønsted‐type correlation indicate that the reactions of the thiol nucleophiles with the substituted phenacyl bromides 1a–e and 2a–e are attributed to the electronic nature of the substituents. Copyright © 2011 John Wiley & Sons, Ltd.     

Higher reactivity of 3‐pyridinium boronic acid compared with 3‐pyridinium boronate ion toward 4‐isopropyltropolone in acidic aqueous solution: fundamental reaction analyses for an effective organoboron‐based chemosensor

The pK_as of 3‐pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two pK_as assignable to the boron center and pyridine moiety. The pK_a assignment performed by ¹¹B nuclear magnetic resonance spectroscopy revealed that both boron centers in 3‐pyridylboronic acid [3‐PyB(OH)₂] and the N‐methylated derivative [3‐(N‐Me)Py⁺B(OH)₂] have strong acidities (pK_a = 4.4 for both). It was found that introduction of a substituent to pyridine‐C atom in 3‐pyridylboronic acid drastically increased the acidity of the pyridinium moiety, but decreased the acidity of the boron center, whereas the introduction to pyridine‐N atom had no influence on the acidity of the boron center. Kinetic studies on the complexation reactions of 3‐pyridinium boronic acid [3‐HPy⁺B(OH)₂] with 4‐isopropyltropolone (Hipt) carried out in strongly acidic aqueous solution indicated that the positive charge on the boronic acid influenced little on its reactivity; 3‐HPy⁺B(OH)₂ reacts with Hipt and protonated H₂ipt⁺, and its reactivity was in line with those of a series of boronic acids. Kinetics in weakly acidic aqueous solution revealed that 3‐HPy⁺B(OH)₂ reacts with Hipt faster than its conjugate boronate [3‐HPy⁺B(OH)₃^–], which is consistent with our recent results. The reactivity of 3‐(N‐Me)Py⁺B(OH)₂ towards Hipt was also examined kinetically; the reactivities of 3‐(N‐Me)Py⁺B(OH)₂ and 3‐(N‐Me)Py⁺B(OH)₃^– are almost the same as those of their original 3‐HPy⁺B(OH)₂ and 3‐HPy⁺B(OH)₃^–, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of substitution of oxygen by sulfur in the nonleaving group of a carbonate: kinetics of the phenolysis and benzenethiolysis of S‐methyl aryl thiocarbonates

The phenolysis and benzenethiolysis of S‐methyl 4‐nitrophenyl thiocarbonate ( 1 ) and S‐methyl 2,4‐dinitrophenyl thiocarbonate ( 2 ) in water are studied kinetically. The Brønsted plots (log k _N versus nucleophile basicity) are linear for all reactions. The Brønsted slopes for 1 and 2 are, 0.51 and 0.66 (phenolysis) and 0.55 and 0.70 (benzenethiolysis), respectively. These values suggest a concerted mechanism for these reactions, as found in the corresponding carbonates. Namely, substitution of OMe by SMe in the nonleaving group does not change the mechanism. Copyright © 2007 John Wiley & Sons, Ltd.