Titanocene-promoted stereoselective eliminations on epoxy alcohols derived from R-(−)-carvone

The reaction of several stereoisomeric epoxy alcohols, obtained from R-(?)-carvone, and their corresponding formates, acetates, and benzoates, promoted by Cp₂TiCl has been studied. The different outcomes of the reaction of epoxy derivatives are rationalized in terms of mechanistically biased processes. The radicals emerging from oxirane cleavage provide two types of reaction: dehydroxylation (deoxycarbonylation) and dehydrogenation.The results offer considerable support for the radical elimination theory of hydroxyl, formyloxyl, and acetoxyl groups. The inability of tertiary radicals to be reduced by the Ti(III) complex is demonstrated unequivocally.     

Approaches to Sampling and Sample Pretreatments for Metal Speciation in Soils and Sediments

Abstract

The main aspects of sampling and sample pretreatment in metal speciation studies of soils and sediments are discussed. The risks of sample contamination by the use of inappropriate materials, containers and tools as well as the possibilities of losses of analyte during sample handling are pointed out. Field sampling methods are described and minimum sample weight criteria for representative sampling of dry soils are presented. Sampling by traps and continuous flow centrifugation methods for suspended sediments and of bottom sediments by grabs or corers are compared. To avoid significant changes in some metal species drying and storage temperatures may need to be controlled and preservation in an inert atmosphere or by irradiation is discussed. The difficulties of establishing definitive protocols for sampling and sample pretreatment are emphasized as well as the need for selecting the appropriate technique in each particular case.     

Reactions of p‐Substituted Phenols with Nitrous Acid in Aqueous Solution

The reaction of phenols with nitrite (nitrous acid HONO, or its conjugated base, NO₂^?) is of importance in stomach fluids (low pH) and in atmospheric hydrometeors (mild acid and basic pH). The initial reaction associated with the oxidation/nitration of 4‐substitued phenols promoted by HONO/NO₂ depends on the pH of the solution. At low pH, the initial step involves the reaction between HONO and phenol, whereas at basic conditions this involves an electron transfer from the phenoxy anion to nitrogen dioxide (NO₂) producing the nitrite anion. The rate of both processes is determined by the donor capacity of the substituent at the 4‐position of the phenol, and the data obtained at pH 2.3 follow a linear Hammett‐type correlation with a slope equal to –1.23. The partition of the gaseous intermediates (NO and NO₂) makes the rate of HONO‐mediated oxidation dependent on their gas–liquid distribution. At low pH, the main process is phenol oxidation, even in oxygen‐free conditions, and the presence of any 4‐substituted phenol decreases the rate of HONO auto‐oxidation.     

Strain-induced crystallization in cis- and trans-polyisoprene blends: Effect of molecular weight of trans-PI

Strain-induced crystallization (SIC) is studied in blends containing 92 wt% of cis-l, 4-polyisoprene (hevea rubber) and 8% of synthetic trans-l, 4-polyisoprene (t-PI), for different molecular weights of t-PI. The initial crystallinity (in the unstrained state) is analyzed in terms of the effect of both molecular weight and the microgel content. The SIC of the natural rubber was measured using the wide-angle x-ray scattering technique. Uncrosslinked solution blends were uniaxially stretched at room temperature, and the dependence of the induced crystallinity on elongation and molecular weight of the t-PI component was analyzed. Also, the molecular weight dependence of the induced crystallization rate is discussed.     

Factor Analysis for Assigning Sources of Groundwater Pollution

Abstract

Factor Analysis (FA) is applied to tentatively establish either jhe sources of pollution in the groundwater in a delimited zone in Catalonia which has been polluted by permeation with non-controlled wastewaters or the groundwater evolution with time in this area. Three types of chemical analysis were considered: general parameters indicating the chemical quality of water, inorganic micropollutants and organic micropollutants. These analyses were carried out with forty-five samples collected in three different periods, and eleven parameters were considered. From the final solutions of the FA method applied several factors were retained for the different sampling periods and these factors were related to different sources of pollution. FA has proved to be more useful when punctual pollution incidents takes place rather than when groundwater evolution with time is studied.     

Multiple dual-mode countercurrent chromatography applied to chiral separations using a (S)-naproxen derivative as chiral selector

Countercurrent chromatography (CCC) is a liquid–liquid chromatographic technique without a solid support. Several alternative elution modes can be applied to take advantage of the special nature of the liquid stationary phase. Among these dual-mode (DM) and multiple dual-mode (MDM) consist of switching alternatively between Reversed and Normal Phase operation during the experiment (once for DM and several times for MDM). In this paper, MDM has been applied to the chiral CCC separations of two racemic mixtures, (±)-N-(3,4-cis-3-decyl-1,2,3,4-tetrahydrophenanthren-4-yl)-3,5-dinitrobenzamide and N-(3,5-dinitrobenzoyl)-(±)-leucine, using (S)-naproxen N,N-diethylamide as chiral selector (CS). Although the behaviour of the two analytes differed, improved resolution factors were successfully obtained. Results are rationalized on the basis of the distinct partition behaviour of the CS/enantiomer complexes in the biphasic system.     

Effect of solvent on the gelation properties of a metallo-organic polymer of [Fe(II) (4-octadecyl-1,2,4-triazole)3(ClO4)2]n

In this contribution we report on the preparation of thermally responsive supramolecular gels obtained through self-assembling of metallo-organic polymers of lipophilic Fe(II) complexes of 1,2,4-triazole functionalized with octadecyl chains ([Fe(II) (4-octadecyl-1,2,4-triazole)₃(ClO₄)₂]_n) in three organic solvents: toluene, cis-decalin and trans-decalin. A gel phase is formed in these solvents by cooling the homogeneous complex solutions below a well-defined temperature, the so-called gelation threshold. These gels are reversible as they form homogeneous solutions upon heating above the melting temperature. The systems have been characterized for their thermal and viscoelastic properties through differential scanning calorimetry and rheological experiments, respectively. The effect of the solvent type and concentration on the gelation behaviour of the metallo-organic polymer has been analysed. The results obtained point to structural differences and different gelation mechanisms for the gels prepared in different solvents and they also suggest the possibility to control the spin-crossover transition temperature associated to the sol-gel transition.     

Adaptation of the ORAC assay to the common laboratory equipment and subsequent application to antioxidant plastic films

The oxygen radical absorbance capacity (ORAC) method has been adapted to the instrumental laboratory and optimized for the determination of the antioxidant capacity of a novel active packaging. As the ORAC assay requires the monitorization of a reaction at controlled temperature by means of the fluorescence signal decrease over time, specific instrumental is usually necessary. In this work, a common liquid chromatographic device has been adapted to perform the ORAC assay, leaving it accessible to any laboratory. Using this adaptation, five different essential oils have been determined resulting in the following antioxidant order: clove (2.66 g Trolox per gram of essential oil), oregano (2.25), cinnamon (1.93), rosemary (1.66), and ginger (1.47). After incorporating the essential oils to the film, its antioxidant capacity has also been checked and related to the concentration of essential oil as well as the thickness of the active film. The results point out that for the same amount of essential oil incorporated measured as grams per square meter, thicker films have more antioxidant capacity than the thinner and more concentrated ones. Furthermore, the antioxidant capacity found in the films was always higher than expected taking into account the amount of essential oil incorporated. Some likely explanations have been proposed, leading to the improvement of the antioxidant film under development.     

Generalized and rapid supramolecular solvent-based sample treatment for the determination of annatto in food

A supramolecular solvent (SUPRA) made up of octanoic acid aggregates is proposed for the microextraction of bixin and norbixin, the two major components of the natural food colouring annatto, in food. The procedure involved the extraction of sub-gram quantities (200mg) of homogenized food with 0.8mL of the supramolecular solvent. The overall sample treatment took about 20 min, and several samples could be simultaneously treated using conventional lab equipment. No clean-up or solvent evaporation were required. Extraction efficiencies mainly depended on the major components making up the SUPRAS (i.e. octanoic acid and tetrahydrofuran) and were not affected by the pH or the temperature in the ranges studied (1-4 and 10-80°C, respectively). Bixin and norbixin in the extracts were quantified by liquid chromatography (LC) and diode array detection (DAD). They were separated in a Hypersil C18 column using a mobile phase consisting of 5% acetic acid and methanol (15:85, v/v). The retention times for norbixin and bixin standards were 5.1 and 8.6 min, respectively. Recoveries in samples ranged between about 78% and 113%. The precision of the method, expressed as relative standard deviation, was about 1.5% and the quantitation limits for bixin and norbixin were 0.19 and 0.23 mg kg(-1), respectively, which were far below the maximum limits permitted by the European Union for the level of addition to food. Concentration of norbixin in samples belonging to the five major groups of food commodities defined in the literature, ranged between 3.75 and 43.8 mg kg(-1) whereas bixin was only found in one snack sample (6.6 mg kg(-1)). The method is simple and rapid, while delivering accurate and precise results, and can be used for the routine determination of annatto in food for the control of the compliance of current legislation.