Acid-base titrations of substituted benzoic acids in micellar systems

The dissociation equilibria of substituted benzoic acids in cationic and anionic micellar systems, investigated potentiometrically, show pK shifts of less than 1.0 in cationic and 0.5–3 in anionic micelles. Visual titrations of otherwise insoluble acids in cationic micelles give satisfactory results.     

The atmospheric chemistry of hydrogen peroxide: a review

This paper reviews the atmospheric chemistry of hydrogen peroxide, taking into account the formation processes of both gas-phase and aqueous H2O2, and the reactions involving hydrogen peroxide in the gas phase and in atmospheric hydrometeors. Gas-phase hydrogen peroxide mainly forms upon dismutation of the hydroperoxyl radical, a product of the reactions between atmospheric hydrocarbons, hydroxyl radicals, nitric oxide, and oxygen. Aqueous hydrogen peroxide originates from the dissolution of the gaseous one, the reduction of molecular oxygen, a series of reactions involving dissolved ozone, and the irradiation of anthraquinones, aromatic carbonyls, and semiconductor oxides. The reactions involving aqueous H2O2 are very important in the context of the chemistry of the atmosphere. They include oxidation of S(IV) to S(VI), photolysis, the Fenton reaction in the presence of Fe(II), and possibly the formation of peroxynitrous acid. Within this framework, the correlation of hydrogen peroxide with other atmospheric components and the time trends of hydrogen peroxide in the atmosphere are easily accounted for.     

Enhancement of the ETS-10 titanosilicate activity in the shape-selective photocatalytic degradation of large aromatic molecules by controlled defect production

In recent times, it has been shown that the microporous ETS-10 titanosilicate can be used as a shape-selective photocatalyst for the decomposition of aromatic molecules (Chem. Commun. 2001, 2131). Its actual use on practical grounds is however discouraged by its too low activity, when compared with that of TiO(2) photocatalysts. In the present work, we show how an ad hoc mild treatment with HF enhances the activity of ETS-10 toward the photodegradation of large aromatic molecules that are unable to penetrate inside the zeolitic pores, such as 2,5-dichlorophenol, 2,4,5-trichlorophenol, 1,3,5-trihydroxybenzene, and 2,3-dihydroxynaphthalene (DHN). The photoactivity of the acid-treated materials is comparable or even greater than that of the nonselective TiO(2) catalyst. Moreover, the enhancement of the photoactivity is accompanied by a remarkable parallel increase of the shape selectivity, particularly toward DHN (k(DHN)/k(P) = 127, where P = phenol). A complete characterization (by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, ultraviolet-visible spectroscopy, and X-ray aborption spectroscopy techniques) of a set of ETS-10 samples which have undergone a progressively severe HF treatment allows us to propose an explanation of the photocatalytic activity and selectivity of the modified materials.     

Living free‐radical polymerization of styrene under a constant source of γ radiation

Living polymerization of styrene was observed using γ radiation as a source of initiation and 1‐phenylethyl phenyldithioacetate as a reversible addition–fragmentation chain transfer (RAFT) agent. The γ radiation had little or no detrimental effect on the RAFT agent, with the molecular weight of the polymer increasing linearly with conversion (up to the maximum measured conversions of 30%). The polymerization had kinetics (polym.) consistent with those of a living polymerization (first order in monomer) and proportional to the square root of the radiation‐dose rate. This initiation technique may facilitate the grafting of narrow polydispersity, well‐defined polymers onto existing polymer surfaces as well as allow a wealth of kinetic experiments using the constant radical flux generated by γ radiation. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 19–25, 2002     

Partition equilibria of phenols between water and anionic micelles : Correlation with the Octanol/Water System

The partition equilibria of a series of thirty substituted phenols bearing different chemical groups between aqueous solutions and sodium dodecylsulfate micelles was investigated by using micellar high-performance liquid chromatography and by studying the variation of the acidity constant. The contribution of each substituent to the free energy of transfer from water to micelles was estimated. The group contribution approach is shown to be valid for polysubstituted compounds under well defined conditions. A comparison with partition coefficients obtained in classical two-phase system (1-octanol/water), frequently used as biochemical model systems, is discussed     

Kinetic determination of adrenaline,l-dopa and their mixtures with a stopped-flow spectrophotometric technique

Catecholamines (adrenaline and L-Dopa) can be determined by a stopped-flow spectrophotometric technique. For individual determinations, catecholamines are oxidized to the corresponding o-benzoquinones by hexachloroiridate(IV). Concentrations in the range 2·10^-4–2·10^-3 M can be determined with errors of about 2 %. For evaluation of mixtures, aminochromes are formed. The method allows a catecholamine concentration of about 5·10^-6 M to be determined in the presence of a ten-fold amount of another catecholamine, with a maximum error of about 10 %.     

In situ analysis and imaging of aromatic amidine at varying ligand densities in solid phase

We report the development of a fast and accurate fluorescence-based assay for amidine linked to cellulose membranes and Sepharose gel. The assay is founded on the glyoxal reaction, which involves reaction of an amidine group with glyoxal and an aromatic aldehyde, leading to the formation of a fluorophore that can be analyzed and quantified by fluorescence spectroscopy and imaging. While the assay has been reported previously for aromatic amidine estimation in solution phase, here we describe its adaptation and application to amidine linked to diverse forms of solid matrices, particularly benzamidine Sepharose and benzamidine-linked cellulose membranes. These functionalized porous matrices find important application in purification of serine proteases. The efficacy of a protein separation device is determined by, among other factors, the ligand (amidine) density. Hence, a sensitive and reproducible method for amidine quantitation in solid phase is needed. The glyoxal reaction was carried out on microbead-sized Sepharose gel and cellulose membranes. Calibration curves were developed for each phase, which established linearity in the range of 0–0.45 μmol per mL amidine for free amidine in solution, 0–0.45 μmol amidine per mL Sepharose gel, and 0–0.48 μmol per mL cellulose membrane. The assay showed high accuracy (~ 3.4% error), precision (RSD < 2%), and reproducibility. Finally, we show how this fluorescent labeling (glyoxal) method can provide a tool for imaging membranes and ligand distribution through confocal laser scanning microscopy.

Graphical abstract

     

Diastereoselective 14beta-hydroxylation of baccatin III derivatives

14beta-Hydroxybaccatin III, a compound with limited availability by natural sources, is the starting material for the synthesis of the second-generation anticancer taxoid ortataxel. The 7-tert-butoxycarbonyl (1a) and 7-triethylsilyl (1b) derivatives of 14beta-hydroxybaccatin III 1,14-carbonate were synthesized from 10-deacetylbaccatin III (3). The crucial steps were (a) the C(14)beta hydroxylation of the corresponding 13-oxobaccatin III derivatives by oxaziridine-mediated electrophilic oxidation and (b) the reduction of the C(13) carbonyl group with sodium or alkylammonium borohydrides. This protocol provides a practical way for the semisynthesis of ortataxel from 10-deacetylbaccatin III, a compound readily available from various yews.     

Phenol Photonitration and Photonitrosation upon Nitrite Photolysis in basic solution

Nitrophenols have been detected in some Antarctic lakes, the water of which is basic and rich in nitrate, nitrite and other nutrients. Nitrate or nitrite photolysis could be a possible reaction to explain the presence of these compounds. This work presents evidence for the formation of 2-nitrophenol (2NP), 4-nitrophenol (4NP) and 4-nitrosophenol (4NOP) upon UV irradiation of phenol and nitrite in aerated basic solutions.

The pH dependence of the 2NP initial formation rate is different from those of 4NP and 4NOP. The dependence of the first mainly reflects the phenol/phenolate equilibrium, with phenol yielding 2NP at a higher rate than phenolate. In the case of 4NOP, the initial formation rate vs pH has a maximum at pH 9.5. The pH dependence of 4NOP formation rate suggests that three pathways are likely to operate: nitrosation of undissociated phenol by N₂O₃, prevailing at pH<8.7, nitrosation of phenolate by N₂O₃, prevailing in the pH interval 8.7–10.8, and reaction between phenoxyl radical and ^?NO, prevailing at pH>10.8. Phenol nitrosation by N₂O₃ is favoured when phenol is negatively charged (phenolate), but it is also disfavoured at alkaline pH values, owing to the depletion of N₂O₃ (the nitrosating agent) by basic hydrolysis. Differently from 2NP, the initial formation rate vs pH of 4NP is very similar to that of 4NOP, suggesting that 4NP may originate from the oxidation of 4NOP. Moreover, while in neutral and acidic solutions the formation rate of 2NP is slightly higher than that of 4NP, in the pH interval 8–12 the formation of 4NP is much more rapid than that of 2NP. This indicates that the pH of natural waters influences the ratio of nitroisomers.     

Thiocarbonylthio end group removal from RAFT‐synthesized polymers by a radical‐induced process

The removal of thiocarbonylthio end groups by radical‐addition‐fragmentation‐ coupling from polymers synthesized by RAFT polymerization has been studied. We found that a method, which involves heating the polymer with a large excess (20 molar equivalents) of azobis(isobutyronitrile) (AIBN), while successful with methacrylic polymers, is less effective with styrenic or acrylic polymers and provides only partial end group removal. This is attributed to the propagating radicals generated from the latter polymers being poor radical leaving groups relative to the cyanoisopropyl radical. Similar use of lauroyl peroxide (LPO) completely removes the thiocarbonylthio groups from styrenic or acrylic polymers but, even with LPO in large excess, produces a polymer with a bimodal molecular weight distribution. The formation of a peak of double molecular weight is indicative of the occurrence of self‐termination and ineffective radical trapping. We now report that by use of a combination of LPO (2 molar equivalents) and AIBN (20 molar equivalents) we are able to completely remove thiocarbonylthio end groups of styrenic or acrylic polymers and minimize the occurrence of self termination. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6704–6714, 2009