Synthesis of N‐alkoxybenzimidoyl azides and their reactions in electrophilic media

A new general route to N‐alkoxybenzimidoyl azides [ArC(N₃)=NOR] from a reaction of N‐alkoxybenzimidoyl bromide [ArC(Br)=NOR] with sodium azide in DMSO is described. These reactions result in the Z‐geometric configuration. These compounds show a moderate degree of thermal stability as assessed by differential scanning calorimetry, and lack reactivity in traditional 1,3‐dipolar cycloaddition ‘click’ reactions. Upon exposure to electrophilic compounds (trifluoroacetic acid or acetyl chloride), these azide compounds can react by two pathways: a Schmidt‐type rearrangement to form an N‐alkoxyurea or an isomerization–cyclization reaction pathway to form an N‐alkoxytetrazole. The route of the reaction has no dependence on solvent polarity and appears to depend upon the electrophile (H⁺ vs. CH₃CO⁺): reaction of the azide with trifluoroacetic acid results predominantly in the urea; reaction with acetyl chloride results solely in the tetrazole. Calculations indicate that the urea product is thermodynamically favored over the tetrazole product. They also indicate that both reaction conditions result in an equilibration between the starting azide and the tetrazole with the tetrazole being the major component in this equilibrium mixture. The fact that the azide also undergoes a Schmidt‐type rearrangement to form an N‐alkoxyurea when treated with trifluoroacetic acid appears to indicate that the barrier for aromatic ring migration is lower in the protonated azide produced on reaction with trifluoroacetic acid than in the acetylated azide produced on reaction with acetyl chloride. Copyright © 2009 John Wiley & Sons, Ltd.     

An advanced LIF-PLV system for analysing the hydrodynamics in a laboratory bubble column at higher void fractions

Bubble columns are widely used in the chemical industry and biotechnology. Flow and turbulence in such an apparatus are induced by the bubble rise, and the bubble behaviour is strongly affected by swarm effects (i.e. the interaction between bubbles). For analysing the bubble swarm behaviour and simultaneously evaluating the flow structure and bubble-induced turbulence, a bubble column of 140 mm diameter and a height of 650 mm or 1,400 mm (initial water level) were considered. The bubble column was aerated with relatively fine bubbles having a mean size between about 0.5 and 4.0 mm. The gas hold-up was varied in the range between 0.5 and 19%. A two-phase pulsed-light velocimetry (PLV) system was developed to evaluate instantaneous flow fields of both rising bubbles and the continuous phase. The measurement of the liquid velocities in the bubble swarm was achieved by adding fluorescing seed particles. Images of bubbles and fluorescing tracer particles were acquired by two CCD cameras. Hence, the images from tracers and bubbles were easily separated by optical interference filters with a bandwidth corresponding to the emitting wavelength of the fluorescing tracer particles and the wavelength of the applied Nd-YAG pulsed laser, respectively. To improve the phase separation of the system, the CCD cameras were additionally placed in a non-perpendicular arrangement with respect to the light sheet. The acquired images were evaluated with the minimum-quadratic-difference algorithm. The potential of this technique for the analysis of bubbly flows with higher void fraction was explored. In order to obtain averaged velocity maps of bubble and fluid within the entire column, about 1,000 image pairs were recorded and evaluated for each phase. In addition, turbulence intensities of the fluid were deduced from the measurements. The turbulence properties were used to characterise bubble-induced turbulence for various bubble mean diameters and gas hold-ups. Moreover, the determination of the average bubble slip velocity within the bubble swarm was possible.     

Nachweis und Bestimmung von Nitraten in der Milch

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Mathematische Theorie der Diffraction

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Zur analytischen Theorie der Wärmeleitung

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Die ebene und sphärische Welle im polydimensionalen Raum

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