Semi-stable

The ketimine of isophorone diamine is formed by reacting it with methyl isobutylketone, splitting off water in the process. When said ketimine is added to an isocyanate-terminated prepolymer based on IPDI, a semi-stable system is established with a pot life of several hours. The ketimine is a Schiff base and thus can react even in the absence of water. The complexities and advantages of this system are reviewed by Bock and Halpaap [75] ... 

In order to achieve 2-nitration, acetyl nitrate may be used as the reagent, but unlike pyrrole a semi-stable adduct, 5-acetoxy-2,5-dihydro-2-nitrofuran, is formed as an intermediate product (Scheme 6.22). This eliminates acetic acid when treated with pyridine. Furan also undergoes initial bromination or chlorination (X = Br or Cl) in ethanol at -40 °C, but then addition of two ethoxyl units occurs with the expulsion of halide ion (Scheme 6.23). 

The fracture of the magnesia-spinel composites is either semi-stable or stable, but never catastrophic.105 It may be concluded that crack propagation is a much greater energy-consuming process than crack initiation in these materials. For many industrial applications, the initiation of fracture is less... 

The use of semi-stable foams, which have a short half-life (i.e., time for half of the contained liquid to drain from the foam) of the order of a few minutes, has made possible a high speed process for uniformly impregnating fabrics with controlled,... 

These increases in volume and viscosity make cleanup operations more difficult. Emulsified oil is difficult or impossible to disperse, to recover with skimmers, or to burn. Emulsions can be broken down with special chemicals to recover the oil with skimmers or to burn it. It is thought that emulsions break down into oil and water by further weathering, oxidation, and freeze-thaw action. Meso- or semi-stable emulsions are relatively easy to break down, whereas stable emulsions may take months or years to break down naturally. 

Intermediate between the reactive and the stable ylides, there is the group of so-called moderate or semi-stable ylides. In this case, the ylide carbon atom bears a vinyl, aryl, alkynyl or halogen substituent. The nucleophilicity of the semi-stabilized ylide is further decreased or increased by electron acceptors or donors on the vinyl or aryl group. 

The phosphorus ylides used in carotenoid synthesis generally bear, on the ylide carbon atom, a vinyl group which frequently is conjugated with one or more double bonds. The reactivity of these semi-stable ylides is further decreased in specific cases by conjugation of the diene or triene system with a carbonyl group (see Section F). 

Semi-stable ylides generally exhibit no great steric preference, although there is a slight tendency towards f )-selectivity. In apolar solvents the presence of soluble lithium salts seems to give a trend towards a slight increase in the proportion of (Z)-isomer [20]. 

In carotenoid synthesis, semi-stable ylides are generally employed, frequently in polar protic or aprotic solvents or solvent systems. 

Oxaphosphetanes from semi-stable ylides have been detected spectroscopically only in the special case of the dibenzophosphole group [43]. Naked betaines, i.e. those that are not complexed with metal ions, have not been found because they cyclize too rapidly for detection by NMR [6]. 

Aromatic and sterically restricting aliphatic aldehydes in marginal cases lead to retro-oxaphosphetane ring opening [53]. Reversibility has also been excluded as a significant process for semi-stable and stable ylides [43,54]. Therefore, the stereochemistry of the salt-free Wittig reaction would be established in virtually all cases by the formation of cis- or rraw5-oxaphosphetanes under kinetic control. 

Semi-stable ratholing] outlet diameter of hopper slightly larger than RI and HI < steepest hopper angle and AI < conical hopper outlet diameter. 

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