Rationalization of the conflicting effects of hydrogen bond donor solvent on nucleophilic aromatic substitution reactions in non-polar aprotic solvent: reactions of phenyl 2,4,6-trinitrophenyl ether with primary and secondary amines in benzene-methanol mixtures

The kinetics of the reactions of phenyl 2,4,6-trinitrophenyl ether with piperidine and cyclohexylamine respectively were studied at different amine concentrations in benzene. The reaction of cyclohexylamine was not base-catalysed while that of piperidine was catalysed by one molecule of the nucleophilic amine. Addition of small amounts of hydrogen-bond donor solvent, methanol to the benzene medium of the reactions produced different effects—rate diminution followed by rate increase in one and continuous rate diminution in the other. These effects are compared with that of aniline (previously studied) in which a continuous rate increase was observed. The results are rationalized in terms of the effect of amine-solvent interaction on the nucleophilicity of the amines in addition to some other factors operating through cyclic transition states leading to products. It is evident from the rationalization that the idea of ‘dimer nucleophile’ in nucleophilic aromatic substitution reactions is erroneous.     

Kinetics and mechanism of the oxidation of tris(2,2′-bipyridine)iron(II) and tris(1,10-phenanthroline)iron(II) complexes by nitropentacyanocobaltate(III) in acidic aqueous medium

The kinetics of the oxidation of tris(2,2′-bipyridyl)iron(II) and tris(1,10-phenanthroline)iron(II) complexes ([Fe(LL)₃]²⁺, LL = bipy, phen) by nitropentacyanocobaltate(III) complex [Co(CN)₅NO₂]^3? was investigated in acidic aqueous solutions at ionic strength of I = 0.1 mol dm^?3 (HCl/NaCl). The reactions were carried out at fixed acid concentration ([H⁺] = 0.01 mol dm^?3) and the temperature maintained at 35.0 ± 0.1 °C. Spectroscopic evidence is presented for the protonated oxidant. Protonation constants of 360.43 and 563.82 dm³ mol^?1 were obtained for the monoprotonated and diprotonated Co(III) complexes respectively. Electron transfer rates were generally faster for [Fe(bipy)₃]²⁺ than [Fe(phen)₃]²⁺. The redox complexes formed ion-pairs with the oxidant with increasing concentration of the oxidant over that of the reductant. Ion-pair constants for these reaction were 160.31 and 131.9 dm³ mol^?1 for [Fe(bipy)₃]²⁺ and [Fe(phen)₃]^2+, respectively. The activation parameters measured for these systems have values as follows: ?H ^≠ (kJ K^?1 mol^?1) = +113.4 ± 0.4 and +119 ± 0.3; ?S ^≠ (J K^?1) = +107.6 ± 1.3 and 125.0 ± 1.6; ?G ^≠ (kJ K^?1) = +81 ± 0.4 and +82.4 ± 0.4; and E _a (kJ mol^?1) = 115.9 ± 0.5 and 122.3 ± 0.6 for LL = bipy and phen, respectively. Effect of added anions (Cl^?, $ {\text{SO}}_{4}^{2 - } $ and $ {\text{ClO}}_{4}^{ - } $ ) on the systems showed decrease in the electron transfer rate constant. An outer-sphere mechanism is proposed for the reaction.     

Time-Dependent Diffusion and Surface-Enhanced Relaxation in Stochastic Replicas of Porous Rock

Understanding the connection between pore structure and NMR behavior of fluid-saturated porous rock is essential in interpreting the results of NMR measurements in the field or laboratory and in establishing correlations between NMR parameters and petrophysical properties. In this paper we use random-walk simulation to study NMR relaxation and time-dependent diffusion in 3D stochastic replicas of real porous media. The microstructures are generated using low-order statistical information (porosity, void–void autocorrelation function) obtained from 2D images of thepore space. Pore size distributions obtained directly by a 3D pore space partitioning method and indirectly by inversion of NMR relaxation data are compared for the first time. For surface relaxation conditions typical of reservoir rock, diffusional coupling between pores of different size is observed to cause considerable deviations between the two distributions. Nevertheless, the pore space correlation length and the size of surface asperity are mirrored in the NMR relaxation data for the media studied. This observation is used to explain the performance of NMR-based permeability correlations. Additionally, the early time behavior of the time-dependent diffusion coefficient is shown to reflect the average pore surface-to-volume ratio. For sufficiently high values of the self-diffusion coefficient, the tortuosity of the pore space is also recovered from the long-time behavior of the time-dependent diffusion coefficient, even in the presence of surface relaxation. Finally, the simulations expose key limitations of the stochastic reconstruction method, and allow suggestions for future development to be made.     

Kinetic studies on the reactions of some tetrakis(arylisocyanide)cobalt(II) complexes with pyridine in 2,2,2-trifluoroethanol

The interaction of pyridine with four tetrakis(arylisocyanide)cobalt(II) complexes, [Co(CNR)₄(ClO₄)₂] R = 2,6-Me₂C₆H₃ (A), 2,4,6-Me₃C₆H₂ (B), 2,6-Et₂C₆H₃ (C) and 2,6-iPr₂C₆H₃ (D), have been studied in 2,2,2-trifluoroethanol medium. The kinetics of the reactions were investigated over the 293–318 K temperature range. The reaction profile exhibited two distinct processes, proposed to be an initial fast substitution followed by a slow reduction, for each of the reactions. The pseudo first-order rate constants for both processes increased with increasing concentration of pyridine with the reduction processes exhibiting saturation kinetics at high pyridine concentrations. Steric hindrance plays a significant role in the rates of the reactions, as the rates decrease in the order k(A) > k(B) > k(C) > k(D). The activation enthalpies, ΔH^‡, increase from A to D while the activation entropies, ΔS^‡, are relatively similar for the four reactions, indicating similar transition states and hence similar mechanisms. Complex B was first synthesized and characterized in this study.     

Chemical and biological studies on Bridelia ferruginea grown in Nigeria

Phytochemical investigation of the methanolic extract of dried leaves of Bridelia ferruginea led to the isolation and identification of fourteen compounds (1–14): compound 1 [mixture of palmitic, stearic and oleic acids], stearyl monoester of 2-O-β-?-glucosylglycerol (2), 6β-hydroxy-(20R)-24-ethylcholest-4-en-3-one (3a), 6β-hydroxy-(20R)-24-ethylcholest-4,22-dien-3-one (3b), lutein (4), vomifoliol (5), corilagin (6), kaempferide-3-O-β-?-glucoside (7), myricetin (8), isomericitrin (9), isoquercetin (10), myricitrin (11), quercitrin (12), rutin (13), and β-sitosterol glucoside (14). The total extract exhibited moderate activity towards CB2 receptor and 90% inhibition against leishmanial pathogen Trypanosoma brucei. Compound 4 exhibited 73% displacement in CB2 receptor with IC₅₀ 56.47 μM, and 93% inhibition towards T. brucei with IC₅₀ 4.16 μM. Compound 11 showed 99% inhibition towards Escherichia coli with IC₅₀ 1.123 μM.