Measurement of rheological and ultrasonic properties of food and synthetic polymer melts

A slit die viscometer has been used in conjunction with a co-rotating twin screw extruder to study the rheological behaviour of maize grits, potato powder and low density polyethylene, as a function of feed rate, screw speed and temperature. The shear viscosity of both maize and potato decreased with increasing feed rate. Increasing the temperature or screw speed at any given feed rate also reduced the viscosity. The ultrasonic velocity through the material has also been shown to be sensitive to the extruder operating conditions. Overall, the ultrasonic velocity decreased as screw speed and temperature increased and feed rate decreased.     

Effects of artificial cracks and poisson's ratio upon photoelastic stress-intensity determination

A series of stress-freezing photoelastic experiments were performed with multiple replications upon edge-cracked strips for three types of “cracks” in current use:

1. Rectangular slots 0.152 mm wide,
2. 1.59-mm-wide slots terminating in a 30-deg vee notch of approximately 0.025-mm root radius, and
3. Natural cracks (approximately 0.0025-mm root radius).

Stress-intensity results were compared with the Gross-Srawley analysis; in addition (1) was compared with Savin's solution. It was concluded that (2) and (3) yield the same results but (1) was slightly higher. Both (2) and (3) were about 12 percent higher than the Gross-Srawley results. This is shown to be related to a Poisson's ratio effect.     

Asymmetric synthesis of novel spirocycles via a chiral phosphoric acid catalyzed desymmetrization

A straightforward method for the asymmetric preparation of novel lactone and lactam spirocycles is described. An initial desymmetrization via a chiral Brønsted acid yields enantioenriched lactones which readily undergo a second cyclization to give the desired spirocycle.     

A novel single particle study of steam gasification kinetics of a coal-derived char at high temperatures and pressures

A novel single particle experiment was developed to allow for detailed char gasification measurements in pure steam at temperatures from 1000 °C to 1400  °C and pressures from 1  bar to 15  bar. A coal-derived activated carbon was thoroughly characterized with respect to its composition, physical structure, and reactivity revealing properties consistent with chars reported in the literature. The single particle approach allowed for the boundary and initial conditions to be well known and for the mass of the particle to be accurately measured before and after testing to provide high-quality conversion data. The resulting conversion data were analyzed using the random pore model and the shrinking core model, of which the random pore model provided the best fit. Apparent activation energies were calculated using the random pore model which provided values ranging from 57.1 kJ/mol to 129 kJ/mol which are nominally half of the magnitude of the values reported in the literature under kinetically controlled conditions, thus demonstrating that regime II conditions were present. Additionally, the activation energies decreased with increasing temperature further demonstrating the presence of regime II conditions. The calculated reaction order ranged from 0 to 0.5 and decreased with increasing pressure agreeing well with literature values and trends.     

A modular and divergent approach to spirocyclic pyrrolidines

An efficient three-step sequence to afford a valuable class of spirocyclic pyrrolidines is reported. A reductive cleavage/Horner–Wadsworth–Emmons cascade facilitates the spirocyclisation of a range of isoxazolines bearing a distal β-ketophosphonate. The spirocyclisation precursors are elaborated in a facile and modular fashion, via a [3 + 2]-cycloaddition followed by the condensation of a phosphonate ester, introducing multiple points of divergence. The synthetic utility of this protocol has been demonstrated in the synthesis of a broad family of 1-azaspiro[4,4]nonanes and in a concise formal synthesis of the natural product (±)-cephalotaxine.

A three-step, modular and divergent sequence accessing challenging spirocyclic pyrrolidines has been developed, featuring a novel reductive spirocyclization cascade.     

Konzertierte nukleophile aromatische Substitutionen

Jüngste Entwicklungen in der experimentellen und theoretischen Chemie haben zur Identifizierung einer schnell wachsenden Klasse von nukleophilen aromatischen Substitutionsreaktionen geführt, die einem konzertierten Mechanismus (cS_NAr) folgen, und nicht dem klassischen, zweistufigen S_NAr‐Mechanismus. Während klassische S_NAr‐Reaktionen auf die substantielle Aktivierung des aromatischen Ringes durch elektronenziehende Substituenten angewiesen sind, ist eine solche Aktivierung für den konzertierten Reaktionsverlauf nicht zwingend nötig.