Second‐order rate constants (k1) have been measured spectrophotometrically for reactions of 2‐methoxy‐3‐X‐5‐nitrothiophene 1a‐c (X = NO2, CN, and COCH3) with secondary cyclic amines (pyrrolidine 2a , piperidine 2b , and morpholine 2 c ) in CH3CN and 91:9 (v/v) CH3OH/CH3CN at 20°C. The experimental data show that the rate constants (k1) values exhibit good correlation with the parameters of nucphilicity (N) of the amines 2a‐c and are consistent with the Mayr's relationship log k (20°C) = s(E + N). We have shown that the electrophilicity parameters E derived for 1a–c and those reported previously for the thiophenes 1d‐g (X = SO2CH3, CO2CH3, CONH2, and H) are linearly related to the pKa values for their gem‐dimethoxy complexes in methanol. Using this correlation, we successfully evaluated the electrophilicity E values of 12 structurally diverse electrophiles in methanol for the first time. In addition, a satisfactory linear correlation (r2 = 0.9726) between the experimental (log kexp) and the calculated (log kcalcd) values for the σ‐complexation reactions of these 12 electrophiles with methoxide ion in methanol has been observed and discussed. 相似文献
The catalytic effects of two aminocationic micelles on the hydrolysis of substituted phenyldecanoate esters and a positively charched benzoate ester (CPNBA) were determined. The micellaric catalysts were of the general structure [CH3(CH2)3N(CH3)2(CH2)nNH2]Br where n=2 (micelle 1); n=3 (micelle 2). The kinetics followed the expression: kobs =ko+kcat x Ka/(Ka+H+)+koOH[OH-]. From the comparison of the kc OH rates with specific base catalysis rates deduced from reactions in non catalytic micelles, it was concluded that the kc OH term, is compatible mainly with an aminolysis reaction catalyzed by hydorxide ion. The Hammett and Bronsted correlations (p=2.8; β=1.0), in addition to the very small deuterium isotope effect, suggested that kcat corresponded with a nucleophilic mechanism. The Bronsted plot of log kcat vs pKa of the phenolate leaving groups in micelles 1 and 2 showed a biphasic behaviour. The break in the curve occured at pKo=5.89 and pKo=6.78 respectively. The partition ratio k±/k-a of the zwiterionic tetrahedral intermediate was derived from the experimental data and produced the following correlation: log k±/k-a=-0.92pKo+0.43pKN+2.466. The ester CPNBA exhibited a deuterium isotope effect of 2.1. From product analysis it was concluded that the reaction proceeds via a general base catalysis of aminolysis. 相似文献
The kinetics and mechanism of the reaction between iodine and dimethyl ether (DME) have been studied spectrophotometrically from 515–630°K over the pressure ranges, I2 3.8–18.9 torr and DME 39.6–592 torr in a static system. The rate-determining step is, where k1 is given by log (k1/M?1 sec?1) = 11.5 ± 0.3 – 23.2 ± 0.7/θ, with θ = 2.303RT in kcal/mole. The ratio k2/k?1, is given by log (k2/k?1) = ?0.05 ± 0.19 + (0.9 ± 0.45)/θ, whence the carbon-hydrogen bond dissociation energy, DH° (H? CH2OCH3) = 93.3 ± 1 kcal/mole. From this, ΔH°f(CH2OCH3) = ?2.8 kcal and DH°(CH3? OCH2) = 9.1 kcal/mole. Some nmr and uv spectral features of iodomethyl ether are reported. 相似文献
Abstract The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)(3-n)+n-1 + HL- ? M(HL)(2-n)+n(kn, forward rate constant; k-n, reverse rate constant); where M=Cu, Co or Ni, HL? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (Kn=kn/k-n) of the mono and bis complexes of Cu2+, Co2+ and Ni2+ with l-tyrosine, determined by potentiometric pH titration are: Cu2+, log K1=7.90 ± 0.02, log K2=7.27 ± 0.03; Co2+, log K1=4.05 ± 0.02, log K2=3.78 ± 0.04; Ni2+, log K1=5.14 ± 0.02, log K2=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu2+, k1=(1.1 ± 0.1) × 109M?1 sec?1, k-1=(14 ± 3) sec?1, k2=(3.1 ± 0.6) × 108M?1 sec?1, k?2=(16 ± 4) sec?1; Co2+, k1=(1.3 ± 0.2) × 106M?1 sec?1, k-1=(1.1 ± 0.2) × 102 sec?1, k2=(1.5 ± 0.2) × 106M?1 sec?1, k-2=(2.5 ± 0.6) × 102 sec?1; Ni2+, k1=(1.4 ± 0.2) × 104M?1 sec?1, k-1=(0.10 ± 0.02) sec?1, k2=(2.4 ± 0.3) × 104M?1 sec?1, k-2=(0.94 ± 0.17) sec?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus. 相似文献
Time-resolved mass spectrometry has been used to deduce the de-excitation rate (kd) of photolytically produced “hot” methyl radicals relative to the H-atom abstraction rate (Ka) from CH31. The rate-constant ratios kd(M)/ka f Me = He, Ar, N2, and CH3l are 0.22 ± 0.02, 0.36 ± 0.03, 0.86 ± 0.13 and 9.6 ± 0.8, respectively. 相似文献
The homogeneous gas-phase decomposition kinetics of methylsilane and methylsilane-d3 have been investigated by the comparative-rate-single-pulse shock-tube technique at total pressures of 4700 torr in the 1125–1250 K temperature range. Three primary processes occur: CH3SiH3 → CH3SiH + H2 (1), CH3SiH3 → CH4 + SiH2 (2), and CH3SiH3 → CH2 = SiH2 + H2 (3). The high-pressure rate constants for the primary processes in CH3SiH3 obtained by RRKM calculations are log (k1 + k3) (s?1) = 15.2 - 64,780 Cal/θ and log k2 (s?) = 14.50 - 67,600 → 2800 Cal/θ. For CH3SiD3 these same rate constants are log k1 (s?) = 14.99 - 64,700 cal/θ log k2 (s?) = 14.68 – 66,700 → 2000 cal/θ, and log k3 (s?) = 14.3 ? 64,700 cal/θ. 相似文献
In the present work, the chromatographic behavior of eight selenium species, namely selenites (Se(IV)), selenates (Se(VI)), seleno‐DL ‐methionine (Se‐Met), selenocystine (Se‐Cyst), selenocystamine (Se‐CM), selenourea (Se‐U), dimethylselenide ((CH3)2Se) and dimethyldiselenide ((CH3)2Se2), was investigated under different stationary and mobile phase conditions, in an effort to unravel secondary interferences in their underlying elution mechanism. For this purpose, two end‐capped and a polar‐embedded reversed‐phase stationary phases were employed using different mobile phase conditions. Retention factors (log kw) were compared with octanol–water distribution coefficients (log D) as well as with log kw values on two immobilized artificial membrane (IAM) columns and two immobilized artificial plasma proteins stationary phases, obtained in our previous work. The role of electrostatic interactions was confirmed by introducing the net charge of the investigated Se species as an additional term in the log kw/log D interrelation, which in most cases proved to be statistically significant. Principal component analysis of retention factors on all stationary phases and octanol–water log D values, however, showed that the elution of the investigated selenium species is mainly governed by partitioning mechanism under all different chromatographic conditions, while the pH of the mobile phase and the special column characteristics have only a minor effect. 相似文献
New types of polydiacetylene multilayer are presented. The first type is based on electrostatic self-organization of diacetylene bolaamphiphiles and polyelectrolytes on a charged substrate followed by subsequent ultraviolet (UV) polymerization. The second type is prepared by direct adsorption of a water soluble polydiacetylene and a polyelectrolyte in alternating sequence. The monomeric diacetylenes are of general formula X(CH2)9CCC C(CH2)9X, with X being a sulfate (1a), phosphate (2) or pyridinium (3) head group. The polydiacetylene (1b) chosen for the multilayer is obtained by γ irradiation of the corresponding diacetylene monomer 1a. It is found that all diacetylene derivatives are well suited for building up self-assembled multilayers and that two of the monomers (1a, 2) can be polymerized on the substrate, while 3 is photo-inactive. The morphology of the multilayers is studied by scanning force microscopy and discussed. The smoothest surface topology is found for multilayers built up from the polydiacetylene 1b and a cationic polyelectrolyte in alternating sequence, while the largest unevenness is found when the anionic diacetylene 1a is alternatingly adsorbed with the cationic bolaamphiphile 3 followed by subsequent UV polymerization on the substrate. 相似文献
From enzyme kinetics, 4‐nitrophenyl‐N‐substituted carbamates 1 are characterized as pseudo‐substrate inhibitors of acetylcholinesterase. However, the activity of the carbamyl enzyme does not recover in the presence of a competitive inhibitor, edrophonium. Therefore, carbamates 1 should be called as the “pseudo‐pseudo‐substrate” inhibitors of the enzyme. Moreover, the ‐logKi, logkc, and logki values are linearly correlated with Taft‐Ingold equation, log (k/ko) = ρ*σ* + δ Es. A three‐step AChE inhibition mechanism by carbamates 1 is proposed. The first step is the pre‐equilibrium protonations of carbamates 1 with ρ* value of ?1.4 from pKa‐σ*‐correlation. The second step is the enzyme‐carbamates 1 tetrahedral intermediate formation from nucleophilic attack of the active site Ser200 on the protonated carbamates 1 . The ρ* value for the ‐logKi‐σ*‐Es‐correlation indicates that the true ρ* value for the second step is 0.5 [= ?0.9 ‐ (‐1.4)]. The δ value of 0.56 for the ‐logKi‐σ*‐Es‐correlation indicates that carbamates 1 with bulky substituents retarded the formation of enzyme‐inhibitor tetrahedral intermediates. The third step (kc step) is the carbamylation step and is the carbamyl enzyme conjugate formation from the enzyme‐carbamates 1 tetrahedral intermediate. The ρ* value of 0.21 for the logkc‐correlation indicates that the transition state for the carbamylation step is more negative charge than the enzyme‐carbamates 1 tetrahedral intermediate. Moreover, the kc step is insensitive to substituent effects since there is a cancellation of electronic demands for bond‐making and bond‐breaking components, like SN2 reactions. The δ value of 0.00 for the logkc‐correlation indicates that the kc step is independent of substituent steric effect. Therefore, the product of this step carbamyl enzyme conjugate is as crowded as the enzyme‐carbamates 1 tetrahedral intermediate and is likely bound to the leaving group, p‐nitrophenol. 相似文献
Substituted phenyl‐N‐butyl carbamates ( 1 ) and p‐nitrophenyl‐N‐substituted carbamates ( 2 ) are characterized as “pseudo‐pseudo‐substrate” inhibitors of acetylcholinesterase. Since the inhibitors protonate in pH 7.0 buffer solution, the virtual inhibition constants (Ki's) of the protonated inhibitors can be calculated from the equation, ‐logKi' = ‐logKi ‐ pKa + 14. The ‐logKi' and logkc values for acetylcholinesterase inhibitions by carbamates 1 correlate with the Hammett equation (log(k/k0) = ρσ); moreover, those by carbamates 2 correlate with the Taft equation (log(k/k0) = ρ* σ*). With modified Hammett‐Taft cross‐interaction variations, multiple linear regressions of the ‐logKi' and logkc values of carbamates 1 and 2 give good correlations, and the cross‐interaction constants (ρXR) are 0.5 and 0.0, respectively. The ρXR value of 0.5 indicates that the carbamate O‐C(O)‐N‐R geometries for the transition states that lead to enzyme‐carbamate tetrahedral intermediates are all pseudo‐trans conformations. Therefore, the carbamate moiety of the inhibitors stretches along the active site gorge of the enzyme but does not bind in the acyl binding site pocket of the enzyme. Overall, the carbamate O‐C(O)‐N‐R geometries for carbamates 1 and 2 , protonated carbamates 1 and 2 , and the tetrahedral intermediate are all retained in pseudo‐trans conformations. The ρXR value of 0.0 suggests that the transition states that lead to the carbamyl enzymes are breaking C‐O bonds and are excluding the leaving groups, substituted phenols. 相似文献
Gamma radiation-induced free radical chain reactions in liquid mixtures of BrCH2CN, eyelohexane (RH), and haloalkanes (XCCl3) were studied. The kinetics of hydrogen and chlorine atom abstraction from CHCl3, CH3CCl3, CH2ClCCl3, CHCl2CCl3, CF3CCl3, C2Cl6, CCl3CN, and CCl4 by CH2CN radicals were investigated by a competitive method. The reactions investigated were Rate constant ratios k3/k2, k5/k6, k7/k2, and k3/k7 were determined at 180°C. In the CCl4? RH? BrCH2CN system k3/k2 was determined in the temperature range of 100–180°C, yielding log k2k3 = ?0.11 ± 0.2 ?(3.34 ± 0.39/θ): where θ = 2.3RT in kcal/mol. The value E2? E3 was combined with existing data on E3 to yield E2(CCl4) = 17.57 kcal/mol. The reactivity trend of CH2CN is compared with that of R radicals. It is shown that in spite of a difference of about four orders of magnitude in kCl values, the reactive cyclohexyl radical is somewhat more selective than CH2CN. It is proposed that the relative reactivities log[k2(XCCl3)/k2(CH3CCl3)] can be correlated in terms of a dual-parameter Taft equation which takes into account both resonance and inductive substituent effects. 相似文献
Smog chamber/Fourier transform infrared (FTIR) techniques were used to measure the kinetics of the reaction of n‐CH3(CH2)xCN (x = 0–3) with Cl atoms and OH radicals: k(CH3CN + Cl) = (1.04 ± 0.25) × 10−14, k(CH3CH2CN + Cl)= (9.20 ± 3.95) × 10−13, k(CH3(CH2)2CN + Cl)= (2.03 ± 0.23) × 10−11, k(CH3(CH2)3CN + Cl)= (6.70 ± 0.67) × 10−11, k(CH3CN + OH)= (4.07 ± 1.21) × 10−14, k(CH3CH2CN + OH)= (1.24 ± 0.27) × 10−13, k(CH3(CH2)2CN + OH)= (4.63 ± 0.99) × 10−13, and k(CH3(CH2)3CN + OH) = (1.58 ± 0.38) × 10−12 cm3 molecule−1 s−1 at a total pressure of 700 Torr of air or N2 diluents at 296 ± 2 K. The atmospheric oxidation of alkyl nitriles proceeds through hydrogen abstraction leading to several carbonyl containing primary oxidation products. HC(O)CN, NCC(O)OONO2, ClC(O)OONO2, and HCN were identified as the main oxidation products from CH3CN, whereas CH3CH2CN gives the products HC(O)CN, CH3C(O)CN, NCC(O)OONO2, and HCN. The oxidation of n‐CH3(CH2)xCN (x = 2–3) leads to a range of oxygenated primary products. Based on the measured OH radical rate constants, the atmospheric lifetimes of n‐CH3(CH2)xCN (x = 0–3) were estimated to be 284, 93, 25, and 7 days for x = 0,1, 2, and 3, respectively. 相似文献
The kinetics of ternary complex formation involving Cu(5-X-1, 10-phen) and threonine (CuAL, A=5-X-1, 10-phen; L=threonine or represented by O-N; X=NO2, Cl, H, CH3) has been studied by temperature-jump and stopped-flow methods. The formation rate constants, kf(M?1·s?1), for the complexation reaction, CuA + L CuAL, are as follows; X=NO2, 8.68×108; X = Cl, 7.13×108; X=H, 6.12×108; X=CH3, 5.42×108. The rate constants for zwitterion attack are nil within experimental error. It has been found that a linear free energy relationship exists between the stability (logKCuACuAL) of the complexes CuAL and log kf as follows: logKCuACuAL = 0.13+0.83 logkf, r = 0.99. It suggested that the formation rate governed the stability of the ternary complexes. The rates of formation of the ternary complexes increased with decreasing electron-donating property of the substituents. A linear relationship was found to exist as expressed by the following equation: log(kRf/KOf = 0.097σ, r = 0.96. A mechanism involves a rapid equilibrium between CuA and L followed by a slow ring closure of L. 相似文献
The compositions, stability constants, and rate constants of intramolecular redox decomposition of cerium(IV) complexes with anions of aminoacetic (H2NCH2COOH), iminodiacetic [HN(CH2COOH)2], nitrilotriacetic [N(CH2COOH)3], ethylenediaminetetraacetic [(CH2COOH)2N(CH2)2N(CH2COOH)2], and hexamethylenediaminetetraacetic [(CH2COOH)2N(CH2)6N(CH2COOH)2] acids were determined by potentiometric, spectrophotometric, and kinetic methods at pH in the range 1.3?2.0 in perchlorate and nitrate media at an ionic strength I = 0.1 and a temperature of 298.15 K. Direct linear correlation between the logarithms of the stability constants of the complexes, log β101, and logarithms of the cumulative protonation constants, log Вm+k (k = 1–2), of aminopolyacetic acid anions Lm–, and inverse linear correlation between log β101 and logarithms of the rate constants of intramolecular redox decomposition of the complexonates [CeL]4–m (m = 1–4), log kn=1, were found. 相似文献
On the Base-Catalysed H/D-Exchange of the Acetylenic Hydrogen Atom in Aromatic Alkynyl Compounds H/D-exchange rates for a number of compounds of the general type 1 (X = p-CH3O, m-CH3O, p-CF3, m-CF3, p-CH3, p-Cl, H; Z ? O, NH, CH2) were determined in N-methyl-pyrrolidine (NMP)/D2O mixtures at 25° (see Table 1). It is shown that the log k values of the H/D-exchange correlate nicely (r = 0.995) with the chemical shift of the acetylenic proton in 1 . Thus, the H/D-exchange rate is given by log k (min?1) = 2.91 · δ (ppm) - 7.79 for the NMP/D2O mixture at 25°. 相似文献