No abstract

The hydrolysis of alkoxy(aryl)(phenyl)-λ6-sulfanenitriles in several buffer solutions was found to follow a good pseudo-first-order kinetic equation, giving the corresponding sulfoximides and alcohols (for the case of the hydrolysis of neopentyloxy-λ6-sulfanenitrile, giving a rearranged product, 2-methyl-2-butanol). The dependence of the rate of hydrolysis on the structure of the alkyl group showed the opposite trend to the usual SN2 character, i.e. Me < Pr < Bu ≈ Et < i–Pr, except for neopentyl. The pH-rate profiles indicated that the rate of hydrolysis is also first order in [H+] at pH more than 6.08, and trends to saturate at low pH. According to these kinetic results, a two-step reaction mechanism was proposed which involves a pre-equilibrium protonation on the nitrogen atom of the alkoxy-λ6-sulfanenitriles, followed by a rate-determining C-O bond cleavage via an SN2 or SN1 mechanism on the alkyl carbon atom depending on the structure of the alkyl group. From a double-reciprocal plot of 1/kobs vs. 1/[H+], the pKa value and the rate constant of the second reaction of neopentyloxy(diphenyl)-λ6-sulfanenitrile were estimated to be 5.02 and 7.02×10-3 s-1, respectively. The substituent effects on the phenyl group of neopentyloxy(diphenyl)-λ6-sulfanenitrile afforded a large negative ρ-value (-1.88) for pKa and positive one (+1.66) for the second reaction at 25.2 °C. The small negative ρ-values observed at pH 6.27 for diphenyl(propoxy)-λ6-sulfanenitrile (-0.42) and neopentyloxy(diphenyl)-λ6-sulfanenitrile (-0.26) were found to be the results of a cancellation of those for the opposite trend of the reactions of the pre-equilibrium and the second step. The activation parameters for both the pre-equilibrium and the subsequent reactions were also estimated based on the parameters for the hydrolysis of neopentyloxy(diphenyl)-λ6-sulfanenitrile at pH 6.22 and 2.99. The buffer effect is due to a nucleophilic attack of the buffer base to the alkyl carbon atom of the protonated alkoxy-λ6-sulfanenitriles. The sulfoximide moiety in the protonated λ6-sulfanenitrile is revealed to be a very good leaving group.

The physical density of a macromolecular model (C390H362N6O38) for Japanese bituminous Akabira coal, which was previously constructed on the basis of the combined data of its Curie-point pyrolysis and CP/MAS 13C NMR, and of four kinds of the systematically modified models in the linkage fashion was estimated by the method using computer-aided molecular design (CAMD) software which we have previously proposed. The results suggest that (a) molecules consisting of a linear structure with branches are more suitable as the models, compared with those involving some cyclized parts due to the cross-linked bonds, and (b) medium-sized molecules yielded by fragmentation of the macromolecular model to some extent may be considered as the representatives for the solvent extractable parts in the coal.

ABSTRACT:

A kinetic investigation on the hydrolysis of aryl(fluoro)(phenyl)-λ6-sulfanenitriles was carried out in some aqueous and mixed aqueous-organic solutions. The pH-rate profiles showed that the hydrolysis consists of pH-independent, acid-catalyzed and base-catalyzed reactions. The neutral hydrolysis of fluoro-λ6-sulfanenitriles was found to proceed via an SN1 or an S-nitrilosulfonium cation-like transition state, which is characterized by a large negative Hammett ρ-value (ρ = −1.76 in water; −1.85 in CH3CN/H2O(1/4, v/v); −2.35 in TFE/H2O (1/1, v/v)), relatively large m-values (ca. 0.83 for fluoro(diphenyl)-λ6-sulfanenitrile; ca. 0.82 for fluoro(p-nitrophenyl)(phenyl)-λ6-sulfanenitrile against the solvent ionizing power YOTs-values in acetonitrile–water), a common ion effect in TFE/H2O, and a small salt effect. The large negative activation entropies (−60 – −101 J K−1 mol−1) were presumed to be due to strong solvation of F− with H2O in the reaction systems. The ease of ion dissociation of the S–F bond was examined by a theoretical calculation in a DFT method, to show that the SN1-like transition state is caused by a facile tendency of dissociation of the S–F bond of fluoro-λ6-sulfanenitriles. The acid-catalyzed hydrolysis was found to proceed via a more cation-like transition state involving a concerted proton transfer to the fluorine atom and breaking of the sulfur-fluorine bond in the λ6-sulfanenitrile. The alkaline hydrolysis probably takes place via an SN2 mechanism.