Multistep adsorption of anionic dyes on silica/chitosan hybrid. 1. Comparative kinetic data from liquid- and solid-phase models

In this work, a hybrid silica/chitosan was synthesized and characterized by nitrogen elemental analysis and thermal analysis (TG, DTG, DTA, and DSC) and BET surface area. The hybrid was used in adsorption studies of two anionic dyes from aqueous solutions. A rise of temperature accelerates mass transfer of dyes into the hybrid. However, the maximum adsorption capacities reach similar values from 25 to 55 degrees C. The kinetic data were first evaluated in relation to the decrease of the time-related residual concentration of the dyes in solution, where the second-order model has presented the best fitting. The solid-phase interaction of dye data presents a rough fitting to the traditional first-order Lagergren kinetic model. However, a modified Avrami kinetic equation was successfully fitted to the kinetic quantities, where from five to seven kinetic regions were found. A pore-diffusion model has also demonstrated that the diffusion is the rate-controlling interaction mechanism. However, the experimental-calculated comparative values are the best way to evaluate a specific aqueous- or solid-phase kinetic model.     

Molecular Recognition of Carbonyl Compounds by Uranyl-salophen Based Neutral Receptors Driven by Van Der Waals Forces

The complexation of the salophen-uranyl metallocleft 2 and of its half-cleft analogue 3 with enones and other carbonyl compounds was assessed in chloroform by UV-Vis titration and, occasionally, by FT-IR measurements. Complexes with receptors 2 and 3 are in all cases more stable than those with the control unsubstituted uranyl-salophen 1 , showing that in addition to the primary binding force provided by coordination of the carbonyl oxygen to the uranium, a significant driving force for complexation, typically in the range of 2-3 kcal/mol, results from van der Waals interactions of the guest with the aromatic walls. Replacement of the phenyl group in 3 with larger aromatic residues to give 4 and 5 , led to enhanced complex stabilities, due to more extended contact surfaces between host and guest.     

Synthesis of porphyrin indolin-2-one conjugates via palladium-catalyzed amination reactions

New porphyrin indolin-2-one conjugates were synthesized via palladium-catalyzed amination reactions of iodinated and dibrominated indolin-2-one derivatives with (2-amino-5,10,15,20-tetraphenylporphyrinato)nickel(II). The combination of palladium catalysts and the phosphine ligand dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos) is an effective methodology for catalyzing the coupling of 5-iodo-, 5,7-dibromo- and 4,6-dibromo-1,3,3-trimethylindolin-2-one with 2-aminoporphyrin to give the corresponding mono-(2-aminoporphyrinyl)- and di-(2-aminoporphyrinyl)-substituted indolin-2-ones in satisfactory yields under mild conditions. The mono brominated porphyrinic derivatives also underwent cross-coupling reactions under similar catalytic conditions. The studies also demonstrated that the course of the coupling process depends on factors, such as the catalytic system, number and position of the halogen substituents and the heating condition. Insights into the reactivity trends of the 5-iodo; 4,6- and 5,7-dibrominated indolin-2-one derivatives was carried out using theoretical calculations performed using density-functional theory with the B3LYP functional.     

A Convenient Method to Obtain 4,5-Dihydro-1H-Methylpyrazoles by A Ring Transformation Reaction

A convenient method for the synthesis of alkyl[aryl]-substituted 5-hydroxy-5-trifluoromethyl-4,5-dihydro-1H-1-methylpyrazole (2) from a new ring transformation reaction of alkyl[aryl]-substituted-5-hydroxy-5-trifluoromethyl-4,5-dihydro-1H-1-pyrazolethiocarboxyamide (1) with methylhydrazine in THF, and the thermal dehydration of 2, are reported.     

A very sensitive flow system for the direct determination of copper in natural waters based on spectrophotometric detection

A very sensitive flow injection method with spectrophotometric detection has been developed for the on-line determination of copper in natural waters. The method exhibits a limit of detection three times lower than the most sensitive direct spectrophotometric method previously described and then allows the direct and simple in situ determination of copper in most natural waters.The method was based on the measurement of the absorbance of the coloured complex formed by copper with the chromogenic reagent di-2-pyridyl ketone benzoylhydrazone (dPKBH) in an alkaline medium. This complex presents stoichiometry 1:2 (Cu:dPKBH), and exhibits maximum absorbance at 370 nm. The manifold used was very simple, and consisted of two channels. The first one contained the sample while the second one contained the colorimetric reagent (3.3×10⁻⁴ M dPKBH in 10% ethanol), in a 1.6×10⁻² M phosphate buffer solution at pH 8. The performance of the system was optimised by using both univariate and modified simplex methodologies. When modified simplex was used, the best signal was obtained for a sample injection volume of 529 μl, a reaction coil length of 1.29 m, and a reagent flow rate of 4.8 ml min⁻¹. Under optimum conditions, the response was linear up to 3 mg l⁻¹ copper, the equation of the straight line being y=0.314x+5.2×10⁻⁴ (r²=0.998). The method allowed a sampling frequency of 40 samples per hour and exhibited a precision of 2.11% (as R.S.D., n=11). The limit of detection was 4.6 μg l⁻¹ (calculated as 3s_b/m, where s_b is the standard deviation of the y-intercept and m represents the slope of the straight line), and was therefore more sensitive than all the direct continuous methods reported previously.The method was successfully applied to the analysis of real water samples, with an average relative error of 5.32%.     

Preparation of symmetrical and unsymmetrical monomers towards bridge trifluoromethylated poly(p-phenylenevinylene)

The preparation and identification of a series of trifluoromethylated and bis(trifluoromethylated) precursors and monomers for bridge substituted trifluoromethylated poly(p-phenylenevinylene) (PPV) is reported. These monomers were prepared in several steps from terephthaldicarboxaldehyde and they contain symmetrical and unsymmetrical combinations of leaving groups and/or polarizers, including chlorides, sulfoxides, sulphones, tosylates and S-methyl (and also O-ethyl) xanthates. The electron withdrawing effect of the trifluoromethyl group dominated the regioselectivity of substitution, thus allowing the convenient and selective formation of unsymmetrically substituted products in high yield.     

Deconvolution of Molecular Weight Distributions Using Dynamic Flory‐Schulz Distributions

Summary: The deconvolution of molecular weight distributions (MWDs) may be useful for obtaining information about the polymerization kinetics and properties of catalytic systems. However, deconvolution techniques are normally based on steady‐state assumptions and very little has been reported about the use of non‐stationary approaches for the deconvolution of MWDs. In spite of this, polymerization reactions are often performed in batch or semi‐batch modes. For this reason, dynamic solutions are proposed here for simple kinetic models and are then used for deconvolution of actual MWD data. Deconvolution results obtained with dynamic models are compared to deconvolution results obtained with the standard stationary Flory‐Schulz distributions. For coordination polymerizations, results show that dynamic MWD models are able to describe experimental data with fewer catalytic sites, which indicates that the proper interpretation of the reaction dynamics may be of fundamental importance for kinetic characterization. On the other hand, reaction dynamics induced by modification of chain transfer agent concentration seem to play a minor role in the shape of the MWD in free‐radical polymerizations.

This Figure illustrates that MWDs obtained at unsteady conditions should not be deconvoluted with standard steady‐state Flory‐Schulz distributions.     

Potentialities of polyurethane foams for trace level analysis of triazinic metabolites in water matrices by stir bar sorptive extraction

Polyurethane (PU) foams were applied for stir bar sorptive extraction of five triazinic metabolites (desethyl-2-hydroxyatrazine, desisopropylatrazine, desethylatrazine, 2-hydroxyatrazine and desethylterbuthylazine) in water matrices, followed by liquid desorption and high performance liquid chromatography with diode array detection (SBSE(PU)-LD/HPLC-DAD). The optimum conditions for SBSE(PU)-LD were 5 h of extraction (1000 rpm) and 5% (v/v) of methanol for the analysis of desethyl-2-hydroxyatrazine and 2-hydroxyatrazine, 15% (w/v) of sodium chloride for the remaining compounds and acetonitrile as back-extraction solvent (5 mL) under ultrasonic treatment (60 min). The methodology provided recoveries up to 26.3%, remarkable precision (RSD < 2.4%), excellent linear dynamic ranges between 5.0 and 122.1 μg/L (r² > 0.9993) and convenient detection limits (0.4-1.3 μg/L). The proposed method was applied in the analysis of triazinic metabolites in tap, river and ground waters, with remarkable performance and negligible matrix effects. The comparison of the recoveries obtained by PU and commercial stir bars was also performed, where the yields achieved with the former were up to ten times higher proving that PU is appropriate for analysis at trace level of this type of polar compounds in water matrices.     

Hooker’s ‘lapachol peroxide’ revisited

The structure of ‘lapachol peroxide’ was published by Hooker in 1936; however, we recently discovered a mistake in the originally proposed structure; the correct structure, according to X-ray spectroscopy, is given here.     

NMR analysis of a series of substituted pyrazolo[3,4-d] pyrimidines-4-amines

A series of 21 substituted pyrazolo[3,4-d]pyrimidines-4-amines were studied by (1)H and (13)C NMR. The application of two-dimensional techniques, HMQC and HMBC, allowed the complete assignment of the spectra for all the compounds.