Kinetics and mechanism of the benzenethiolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates

The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Br?nsted-type plots for the nucleophilic rate constants are linear with slopes beta = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Br?nsted plot was found for the reactions of DNPC and DNPTC at pK(a) ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle.     

Concerted mechanisms of the reactions of methyl aryl carbonates with substituted phenoxide ions

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC, DNPC, and TNPC, respectively) with substituted phenoxide ions are subjected to a kinetic study in water at 25.0 degrees C, ionic strength 0.2 M (KCl). Production of the leaving groups (the nitro derivatives) is followed spectrophotometrically. Under excess of the phenoxide ions pseudo-first-order rate coefficients (k(obsd)) are found throughout. Plots of k(obsd) vs substituted phenoxide concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the phenols) are linear with slopes beta = 0.67, 0.48, and 0.52 for the phenolysis of NPC, DNPC, and TNPC, respectively. The magnitudes of these Br?nsted slopes are consistent with a concerted mechanism. In the particular case of the phenolysis of NPC the expected hypothetical curvature center of the Br?nsted plot for a stepwise mechanism should be pK(a)(0) = 7.1 (the pK(a) of 4-nitrophenol). This curvature does not appear within the pK(a) range of the substituted phenols studied (5.3--10.3), indicating that these reactions are concerted. The phenolysis of DNPC and TNPC should also be concerted in view of the even more unstable tetrahedral intermediates that would be formed if the reactions were stepwise. The reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate perhaps to the point of nonexistence. The k(N) values for the title reactions are larger than those for the concerted phenolysis of the corresponding ethyl S-aryl thiolcarbonates. The k(N) values found in the present reactions are subjected to a dual regression analysis as a function of the pK(a), of both the nucleophile and leaving group, the coefficients being beta(N) = 0.5 and beta(lg) = -0.3, respectively. These coefficients are consistent with a concerted mechanism.     

Enhanced catalytic activity of Fe phthalocyanines linked to Au(111) via conjugated self-assembled monolayers of aromatic thiols for O2 reduction

We have obtained and characterized self assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) and 1-(4-mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) functionalized with iron phthalocyanine (FePc) adsorbed on gold (111) electrodes. The catalytic activity of these SAMs/FePc was examined for the reduction of O₂ in aqueous media (pH = 4) and compared with that of bare gold and of gold coated directly with FePc molecules. Scanning tunneling microscopy (STM) studies confirm the functionalization of the 4-ATP by FePc. The electrocatalytic studies carried out with Au/FePc, Au/4-ATP/FePc and Au/MDPP/FePc electrodes show that the O₂ reduction takes place by a 4-electron transfer to give water in contrast to a 2-electron-transfer process observed on the bare gold. The activity of the electrodes increases as follows: Au < Au/FePc < Au/4-ATP/FePc < Au/MDPP/FePc.     

Adsorption behavior of hydrophobically modified polyelectrolytes onto amino- or methyl-terminated surfaces

The adsorption of hydrophobically modified polyelectrolytes derived from poly(maleic anhydride-alt-styrene) (P(MA-alt-St)) containing in their side chain aryl-alkyl groups onto amino- or methyl-terminated silicon wafers was investigated. The effect of the spacer group, the chemical nature of the side chain, molecular weight of polyelectrolyte, and ionic strength of solution on the polyelectrolyte adsorbed amount was studied by null ellipsometry. The adsorbed amount of polyelectrolyte increased with increasing ionic strength, in agreement with the screening-enhanced adsorption regime, indicating that hydrophobic interactions with the surface play an important role in the adsorption process. At constant ionic strength, the adsorbed amount was slightly higher for polyelectrolytes with larger alkyl side chain and decreased with the hydrophobicity of aryl group. The adsorption behavior is discussed in terms of the side chain flexibility of the polymer. Characteristics of the adsorbed layer were studied by atomic force microscopy (AFM) and contact angle measurements. AFM images show the presence of aggregates and closed globular structure of polyelectrolyte onto the amino- or methyl-terminated surface, which agrees with a 3D and 2D growth mechanism, respectively. Fluorescence measurements showed that the aggregation of polyelectrolyte containing the hydrophobic naphthyl group occurs already in the solution. However, the aggregation of polyelectrolytes containing the phenyl group in its side chain is not observed in solution but is induced by the amino-terminated surface. This difference can be explained in terms of the higher flexibility of side chain bearing the phenyl group. The polyelectrolyte films showed a high chemical heterogeneity and moderate hydrophobicity.     

Nucleophilic substitution reactions of diethyl 4‐nitrophenyl phosphate triester: Kinetics and mechanism

The reactions of diethyl 4‐nitrophenyl phosphate ( 1 ) with a series of nucleophiles: phenoxides, secondary alicyclic (SA) amines, and pyridines are subjected to a kinetic study. Under excess of nucleophile, all the reactions obey pseudo‐first‐order kinetics and are first order in the nucleophile. The nucleophilic rate constants (k_N) obtained are pH independent for all the reactions studied. The Brønsted‐type plot (log k_N vs. pK_a nucleophile) obtained for the phenolysis is linear with slope β=0.21; no break was found at pK_a 7.5, consistent with a concerted mechanism. The Brønsted‐type plots for the SA aminolysis and pyridinolysis are linear with slopes β=0.39 and 0.43, respectively, also suggesting concerted processes. The concerted mechanisms for the latter reactions are proposed on the basis of the lack of break in the Brønsted‐type plots and the instability of the hypothetical pentacoordinate intermediates formed in these reactions. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 708–714, 2011     

Tuning the Covering on Gold Surfaces by Grafting Amino-Aryl Films Functionalized with Fe(II) Phthalocyanine: Performance on the Electrocatalysis of Oxygen Reduction

Current selective modification methods, coupled with functionalization through organic or inorganic molecules, are crucial for designing and constructing custom-made molecular materials that act as electroactive interfaces. A versatile method for derivatizing surfaces is through an aryl diazonium salt reduction reaction (DSRR). A prominent feature of this strategy is that it can be carried out on various materials. Using the DSRR, we modified gold surface electrodes with 4-aminebenzene from 4-nitrobenzenediazonium tetrafluoroborate (NBTF), regulating the deposited mass of the aryl film to achieve covering control on the electrode surface. We got different degrees of covering: monolayer, intermediate, and multilayer. Afterwards, the ArNO₂ end groups were electrochemically reduced to ArNH₂ and functionalized with Fe(II)-Phthalocyanine to study the catalytic performance for the oxygen reduction reaction (ORR). The thickness of the electrode covering determines its response in front of ORR. Interestingly, the experimental results showed that an intermediate covering film presents a better electrocatalytic response for ORR, driving the reaction by a four-electron pathway.