Kinetic Study of the Alkaline Hydrolysis of 1,n‐Bis(4‐cyanopyridinium)alkanes: Charge Density and New Conformational Effects on the Reactivity of 1,3‐Bis(4‐cyanopyridinium)propane

The alkaline hydrolysis reaction rates of 1,n‐bis(4‐cyanopyridinium)alkane derivatives C_nbis(CP)²⁺ with n = 3, 6, and 8 were studied and compared to the reaction rate of the N‐methyl‐4‐cyanopyridinium (MCP⁺). C₆bis(CP)²⁺ and C₈bis(CP)²⁺ obeyed the first‐order kinetic law. However for C₃bis(CP)²⁺ data fitted to a consecutive two‐step model reaction, the observed rate constants (k_obs) of C₈bis(CP)²⁺ and C₆bis(CP)²⁺ are approximately 50% and 100%, respectively, higher than those for MCP⁺, an effect mainly assigned to the higher charge density of these two derivatives. For C₃bis(CP)²⁺, the k_obs of the second (slow) step is almost twofold the value observed for C₆bis(CP)²⁺, whereas the first (fast) step is approximately six times higher. As for MCP, the hydrolysis of C_nbis(CP)²⁺ generates pyridone (P^o) and carbamidopyridinium (A⁺) units. For C₃bis(CP)²⁺, however at pHs above 11.5, one additional product is formed. From the existence of the new product and the kinetic evidence, a “sandwiched‐type” complex with the OH^? inserted between the rings is proposed. This structural effect in the C₃bis(CP)²⁺ due to the conformational effect justifies the (i) two kinetic steps, (ii) high rate constants, (iii) high P^o/A⁺ ratios, (iv) observed temperature and salt effects, and (v) the formation of the new product.     

Microenvironment Effects on the Kinetics of the Alkaline Hydrolysis of Bispyridinium Conformers

The alkaline hydrolysis of a series of homologous reactants constituted by two reactive centers bridged by a methylene spacers chain, the 1,n‐bis(2‐azidepyridinium)alkanes (n = 3, 4, 5, 6, and 8), is investigated. The reaction under pseudo–first‐order condition was followed by ultraviolet–visible spectrophotometry. The presence of clear isosbestic point suggests the absence of stable intermediates. However, the intermediates 1‐(2‐azidepyridinium), n‐(2‐pyridone)alkanes (monocationic compounds), were isolated and characterized as well the reaction end products 1,n‐(2‐pyridone)alkanes (noncharged compounds). The kinetic analysis fitted to a two‐step consecutive reaction, where the k₁/k₂ values demonstrate the larger reactivity of the first step over the second one, especially for shorter bridged reactants. The OH^? reaction order is one for each step. Although Debye–Hückel law was obeyed, the experimental point at ionic strength zero is much higher than the extrapolated one. In addition, the k₁ values substantially decrease as KCl is added especially for shorter homologous whereas the effect on k₂ is almost negligible. Simple charge density effects as a function of the spacer's length do not explain the observations. On the other hand, from the pronounced anion selectivity inhibition effects on k₁ for the shorter derivatives, the existence of an equilibrium involving a conformer, a “sandwich‐type” complex with the OH^? between the two pyridinium rings, with an “open‐stretched” conformer is proposed. For short‐bridged reactants, the complex conformer prevails whereas for long‐bridged compounds the stretched structure prevails. The results in acetonitrile/water seem to reflect changes in the equilibrium. The reaction is controlled by the balance between enthalpy and entropy. The contribution of the activation entropy in a short‐bridged reactant is higher than the long‐bridged one, which suggests an activated complex with high structural organization in the first reactants.     

Frontal polymerization of diurethane diacrylates

This work deals with the preparation of poly(urethane acrylates) by using two different polymerization techniques. Namely, the classical batch procedure has been compared with frontal polymerization (FP). A thorough study on the effect of initiator type, concentration, and on the velocity of the front and its maximum temperature has been carried out. Moreover, two different synthetic ways have been studied: the one step poly(urethane acrylate) preparation starting directly from 1,6 diisocyanato hexane and 2‐hydroxyethyl acrylate, and the two step procedure consisting of the synthesis of the corresponding diurethane diacrylate and of its subsequent polymerization. The first method has the advantage of being faster but some caution is necessary due to the excessive heat that is generated if the reaction conditions are not properly chosen. The second approach requires a further step but has the advantage of being more controlled. DSC analysis did not show any significant difference by comparing the thermal properties of the materials obtained by the two techniques (batch and FP). However, since FP runs are very easy and fast to be performed, FP should be seriously taken into proper account when these materials have to be prepared. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3344–3352, 2008     

Catalytic intermolecular amination of C-H bonds: method development and mechanistic insights

Reaction methodology for intermolecular C-H amination of benzylic and 3 degrees C-H bonds is described. This process uses the starting alkane as the limiting reagent, gives optically pure tetrasubstituted amines through stereospecific insertion into enantiomeric 3 degrees centers, displays high chemoselectivity for benzylic oxidation, and enables the facile preparation of isotopically enriched 15N-labeled compounds. Access to substituted amines, amino alcohols, and diamines is thereby made possible in a single transformation. Important information relevant to understanding the initial steps in the catalytic cycle, reaction chemoselectivity, the nature of the active oxidant, and pathways for catalyst inactivation has been gained through mechanistic analysis; these studies are also presented.     

On the Influence of Pore-Scale Dispersion in Nonergodic Transport in Heterogeneous Formations

Flow of an inert solute in an heterogeneous aquifer is usually considered as dominated by large-scale advection. As a consequence, the pore-scale dispersion, i.e. the pore scale mechanism acting at scales lower than that characteristic of the heterogeneous field, is usually neglected in the computation of global quantities like the solute plume spatial moments. Here the effect of pore-scale dispersion is taken into account in order to find its influence on the longitudinal asymptotic dispersivity D₁₁we examine both the two-dimensional and the three-dimensional flow cases. In the calculations, we consider the finite size of the solute initial plume, i.e. we analyze both the ergodic and the nonergodic cases. With Pe the Péclat number, defined as Pe=U/D, where U, , D are the mean fluid velocity, the heterogeneity characteristic length and the pore-scale dispersion coefficient respectively, we show that the infinite Péclat approximation is in most cases quite adequate, at least in the range of Péclat number usually encountered in practice (Pe > 10²). A noteworthy exception is when the formation log-conductivity field is highly anisotropic. In this case, pore-scale may have a significant impact on D₁₁, especially when the solute plume initial dimensions are not much larger than the heterogeneities' lengthscale. In all cases, D₁₁ appears to be more sensitive to the pore-scale dispersive mechanisms under nonergodic conditions, i.e. for plume initial size less than about 10 log-conductivity integral scales.     

Compatibilization of Poly(Lactic Acid) (PLA)/Plasticized Cellulose Acetate Extruded Blends through the Addition of Reactively Extruded Comb Copolymers

In the perspective of producing a rigid renewable and environmentally friendly rigid packaging material, two comb-like copolymers of cellulose acetate (AC) and oligo(lactic acid) OLA, feeding different percentages of oligo(lactic acid) segments, were prepared by chemical synthesis in solvent or reactive extrusion in the melt, using a diepoxide as the coupling agent and were used as compatibilizers for poly(lactic acid)/plasticized cellulose acetate PLA/pAC blends. The blends were extruded at 230 °C or 197 °C and a similar compatibilizing behavior was observed for the different compatibilizers. The compatibilizer C1 containing 80 wt% of AC and 14 wt% of OLA resulted effective in compatibilization and it was easily obtained by reactive extrusion. Considering these results, different PLAX/pAC(100-X) compounds containing C1 as the compatibilizer were prepared by extrusion at 197 °C and tested in terms of their tensile and impact properties. Reference materials were the uncompatibilized corresponding blend (PLAX/pAC(100-X)) and the blend of PLA, at the same wt%, with C1. Significant increase in Young’s modulus and tensile strength were observed in the compatibilized blends, in dependence of their morphologic features, suggesting the achievement of an improved interfacial adhesion thanks to the occurred compatibilization.     

On the initial phase of native oxide formation on Si〈1 1 1〉

An Auger Electron Spectroscopy study of the initial phase of native oxide formation on Si〈1 1 1〉 is reported. Observed Auger peaks for low levels of oxygen exposure are explained in terms of a model of monoatomic and peroxyl surface bonding and the position of associated bands deduced with respect to the SiO₂ valence band edge. For high levels of oxygen exposure, only monatomic bonding is found whilst the full SiO₂ valence band structure appears.     

Selective electrodehalogenation of bromophenols on Ag electrocatalyst

Among the innovative technologies for the detoxification and disposal of industrial effluents, our recently described electroreductive dehalogenation of organic halides on silver electrocatalyst provides a promising route, to be easily "plugged-in" into integrated waste treatments. Although electroreductions do not lead to total substrate mineralization, complete and selective dehalogenation may be achieved by the appropriate choice of the cathode material, thus decreasing the biotoxicity of the waste. In particular the use of silver as electrocatalyst greatly reduces the energy demand, and ensures higher yields and no by-products also when treating highly toxic aromatic halides. In the present work the electrocatalytic role of silver in the electroreductive dehalogenation process has been studied on mono- and polyhalogenated bromophenols, taken as model substrates, both in terms of selectivity of the reaction pathway and of reactivity of the position on the aromatic ring of the bromo leaving group. The results point to the possibility of effectively extending the methodology also to the more common and less reactive chlorinated derivatives.     

Detection of enzymatic activity by means of a diffraction-based biosensor

We demonstrate here, for the first time, the capability of a diffraction-based sensor to detect enzyme activity and provide a simple measure of enzyme kinetics. Patterned samples of mouse IgG were exposed to the enzyme trypsin and the progress of enzymatic degradation of IgG was followed by measuring the decrease in diffraction signal intensity as a function of time. The diffraction signal intensity decreased exponentially as a function of time over a range of trypsin concentrations from 2–100 g mL^–1. Atomic-force microscope images of the samples before and after exposure to trypsin show that the thickness of patterned protein is greatly reduced by the enzyme action, providing useful insight into the mechanism of signal change for the diffraction sensor.