Substitution Kinetics of trans-[O2(Py)4Tc]+ in Methanol, DMF, and MeOH/DMF Mixtures (Py = Pyridine, Picoline, and Lutidine) (original) (raw)
Related papers
similar situation might apply. presents kinetic data at differing ionic strengths in acidic solution. The observed rate constants do show a clear trend toward smaller values at higher ionic strengths. However, when we apply the revised Davies equation for ionic strength dependence24 to the rate data, the charge on the attacking nucleophile (assuming the cation to be +3) is calculated to be -0.41. Therefore, the data would indicate (24) Davies, C. W. Ion Association; Butterworths: London, 1962; p 41. that even in 0.1 1 M perchloric acid, a hydroxide attack pathway is still operative. However, in order to account for the calculated charge, water must also contribute to the hydrolysis reaction. Since a similar reactivity is not present with the 4-cyanopyridine N-oxide complex, neighboring group participation by the N-oxide function adjacent to the nitrile group is indicated. To answer whether only water is being activated by the N-oxide group or both water and hydroxide requires further kinetic studies, which are under way at this time.
1990
Second order rate constants have been de ermined for the reaction of2,4-dinitrophenyl acetate with 3and 4-substituted pyridines and 4'-substitut d 4-styrylpyridines in acetonitrile-water (SO%,v/v) mixture at 2S•, 30°, 3SOand 40°C. Activation para eters have been evaluated. Electron-releasing substituents increase the rate while electron-withdrawin substituents retard it. In both the reaction series linear free energy relationship is found to exist betw n the logarithms of rate constants and (J constants with p=-3.39±0.38 (r=0.989; s=0.163; n= 12 for 3-and 4-substituted pyridines and p=-0.390±0.080 (r =,0.984; s = 0.028; n = 8) for 4'-substitut d 4-styrylpyridines at 30T. The ratio of effectiveness of transmission of substituent effect in styryl sy tern relative to pyridine system in this reaction is O.IIS. The Br6nsted plot obtained is linear and PN is fo nd to be 0.692±0.OS6(r = 0.990;s= 0.146; n = 16)indicating the extensive bond formation between pyri ine and the reaction centre in the transition state.
Oxidation of substituted pyridines by dimethyldioxirane: kinetics and solvent effects
Tetrahedron Letters, 2004
The oxidation of a series of substituted pyridines by dimethyldioxirane (1) produced the expected N-oxides in quantitative yields. The second order rate constants (k2) for the oxidation of a series of substituted pyridines (2a–g) by dimethyldioxirane were determined in dried acetone at 23 °C. An excellent correlation with Hammett sigma values was found (ρ = −2.91, r = 0.995). Kinetic studies for the oxidation of 4-trifluoromethylpyridine by 1 were carried out in the following dried solvent systems: acetone (k2 = 0.017 M−1 s−1), carbon tetrachloride/acetone (7:3; k2 = 0.014 M−1 s−1), acetonitrile/acetone (7:3; k2 = 0.047 M−1 s−1), and methanol/acetone (7:3; k2 = 0.68 M−1 s−1). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k2 = 0.78 M−1 s−1 (χ = 0) to k2 = 11.1 M−1 s−1 (χ = 0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents.The second order rate constants for the oxidation of substituted pyridines by dimethyldioxirane at 23 °C in dried acetone were found to correlate with sigma values (ρ = −2.91). The reaction was shown to be very sensitive to protic, polar solvents.
Bulletin of the Korean Chemical Society
The reaction rates of 4-X-2,6-dinitrochlorobenzenes (X = NO2, CN, CF3) with Y-substituted pyridines (Y = 3-OCH3, H, 3-CH3, 4-CH3) in methanol-acetonitrile mixtures were measured by conductometry at 25 oC. It was observed that the rate constant increased in the order of X = 4-NO2 > 4-CN > 4-CF3 and the rate constant also increased in the order of Y = 4-CH3 > 3-CH3 > H > 3-OCH3. When the solvent composition was varied, the rate constant increased in order of MeCN > 50% MeOH > MeOH. The electrophilic catalysis by methanol may be ascribed to the formation of hydrogen bonds between alcoholic hydrogen and nitrogen of pyridines in ground state. Based on the transition parameters, ps, pN, P y, Pxy and solvent effects, the reaction seems to proceed via SzAr-Ad.E mechanism. We also estimated the isokinetic solvent mixtures (pxY = 0) based on cross-interaction constants, where the substituent effects of the substrate and nucleophile are compensated.
Canadian Journal of Chemistry, 1975
Second-order rate constants at 25 °C have been measured for the reaction of triphenylmethyl carbonium ion with pyridine and substituted pyridines in nitromethane and 1,2-dichloroethane solvent. The activation parameters are in the range 1 to 3 kcal mol−1 for the enthalpies of activation and −20 to −45 cal mol−1 deg−1 for the entropies of activation. The results are compared to the analogous measurements for Menschutkin reactions and it is concluded that the enthalpy of activation of a Menschutkin reaction results mainly from the bond breaking process and solvation changes associated with it, since the carbon-nitrogen bond making process seems to be entropy controlled.Rate measurements have also been made for 2-methylpyridine reacting with a series of para-substituted triphenylmethyl carbonium ion substrates in 1,2-dichloroethane solvent. A plot of log k2vs ∑σ+ is non-linear and the curvature is interpreted as a saturation effect.
Reaction kinetics and mechanism of N-(2,4-dinitrophenyl)pyridinium chloride with aniline
Collection of Czechoslovak Chemical Communications, 1973
Influence of aniline, pyridine and pH on the title reaction has been studied in 50% aqueous ethanol. , Transformation of III into IV is rate-limiting at low pH values, the reaction rate being inversely proportional to the proton concentration. Catalytic effect of both aniline and lyate ions becomes increasingly significant with increasing pH, and deprotonation of the intermediate II becomes rate-limiting, The kinetic equations have been derived and the constants of the individual reaction steps calculated, .