Electrophilic Nitration of Electron-Rich Acetophenones

Summary. It is demonstrated that electron-rich disubstituted acetophenones react according to various electrophilic nitration conditions that generally lead to ipso substitution accompanying the conventional reaction. The hydroxy substituent does not seem prone to favor such behaviour.     

Acid Catalyzed Cyclization of Bis Silyl Diacylhydrazines: A New 1, 3, 4-Oxadiazole Synthesis

Diacylhydrazines are converted in good yields into 1,3,4-oxadiazoles with dichlorodimethyl-silane and trifluoromethane sulfonic acid.     

Disilylated Compounds as Precursors of Heterocycles: A New and Easy Oxadiazole Synthesis

Silyl diacylhydtazines have been synthesized. When they are treated with 3 slight amount of a uariety of catalyst (nucleophiles, F, true or Lewis acids, Pd, Pt.), 1,3,4-oxadiazoles are obtained. This cyclization can be performed in one step by using hexamethyldisilazane or dimethyldichlorosilane as the silylation agent.     

Determination of Addition and Fragmentation Rate Coefficients in RAFT Polymerization via Time‐Resolved ESR Spectroscopy after Laser Pulse Initiation

Summary: A method for simultaneous determination of both the addition and fragmentation rate coefficients of the RAFT equilibrium reactions is presented, which is based on laser single pulse initiation in conjunction with microsecond time‐resolved ESR spectroscopy. The build‐up and subsequent decay in concentration of the intermediate radical are measured and k_ad and k_β values are deduced from fitting the concentration versus time profiles to simple kinetic models.

Normalized ESR signal intensity vs. time after firing an initiating laser pulse in BMPT‐mediated butyl acrylate polymerization.     

An evaluation of analytical techniques for determination of lead, cadmium, chromium, and mercury in food-packaging materials

Closed microwave digestion and a high-pressure asher have been evaluated for wet-oxidation and extraction of lead, cadmium, chromium, and mercury from a range of typical packaging materials used for food products. For the high-pressure asher a combination of nitric and sulfuric acids was efficient for destruction of a range of packaging materials; for polystyrene, however, nitric acid alone was more efficient. For microwave digestion, a reagent containing nitric acid, sulfuric acid, and hydrogen peroxide was used for all materials except polystyrene. Use of the high-pressure asher resulted in the highest recoveries of spiked lead (median 92%), cadmium (median 92%), chromium (median 97%), and mercury (median 83%). All samples were spiked before digestion with 40 μg L^–1 Cd, Cr, and Pb and 8 μg L^–1 Hg in solution. The use of indium as internal standard improved the accuracy of results from both ICP–MS and ICP–AES. Average recovery of the four elements from spiked packaging materials was 92 ± 14% by ICP–MS and 87 ± 15% (except for mercury) by ICP– AES. For mercury analysis by CVAAS, use of tin(II) chloride as reducing agent resulted in considerably better accuracy than use of sodium borohydride reagent.     

Nonhydraulic Effects in Particle-Driven Gravity Currents in Deep Surroundings

In this article, we present an approach to modeling the flow of particle-driven gravity currents produced by the sudden release of well-mixed, fixed-volume suspensions into deep surroundings. Our model accounts for the initial turbulent energy of mixing in the release volume, characteristic of the classical lock–release experiments, as well as the spatiotemporal variability in the driving buoyancy forces attributable to particle settling. We show that, in contrast to compositionally driven flows, particle-driven flows cannot be described consistently in terms of shallow water theory. Specifically, we show that the presence of particles in the flow dynamics produces significant horizontal velocity shear, thereby changing the flow configuration in important ways from flows assumed to be governed by the shallow water equations. These new flow properties are calculated and contrasted with flow properties derived on the basis of the shallow water equations to show that the shallow water analysis misses dynamical features of the flow. We also show that our model provides significant improvement over the previous shallow water-based models in predicting the experimentally determined deposition patterns associated with the lock–release experiments.