摘要:

AbstractThe past decade of organic chemistry may be charcterized as a period of violent development of new synthetic methods. This was accompained by a systematization of the analysis of synthetic problems (synthetic strategy). The planning of the synthesis of an organic target molecule is greatly facilitated by distinguishing between reagents X(C)n… with normal reactivity (acceptor properties at C1,3,5…, donor properties at X, C2,4…) and with reactivity umpolung (acceptor properties at X, C2,4…, donor properties at C1,3,5…). In this context, reactivity umpolung turned out to be useful as a heuristic principle, as a classification scheme, and as an aid for locating so‐called strategic bonds (synthon, transform, and antithesis according toE. J. Corey). There are six principal methods of umpolung: 1 2n‐oxidation, heteroatom exchange and modification, homologation and its reversal, the cyclopropane “trick”, use of acetylenes, and redox reactions; under certain circumstances none of these techniques is necessary in cases where direct umpolung is possible. Throughout the article, normal reactivity is indicated by green print; reactivity umpo

ISSN:0570-0833    

DOI:10.1002/anie.197902393

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

摘要:

AbstractWhereas the protonation of α‐diazo‐β‐diketones and α‐diazo‐β‐oxo esters with hydrogen chloride and antimony pentachloride yields the corresponding hydroxyalkenediazonium hexachloroantimonates, the “acid‐sensitive” α‐diazomonocarbonyl compounds can be transformed into resonance‐stabilized alkenediazonium salts by an analogousO‐alkylation with triethyloxonium hexachloroantimonate. A general route for synthesizing these compounds consists in the transformation of thep‐toluenesulfonylhydrazones of α‐halogenated aldehydes and ketones with Lewis acids. Alkenediazonium ions which are not resonance stabilized can be isolated at room temperature only if their decomposition at higher temperatures leads,viaevolution of nitrogen, to vinyl cations in particularly high energy levels. Facile substitution reactions at the CC double bond with anisole and methanol demonstrate the strongly electrophilic nature of alkenediazonium salts. When electron‐donating substituments are attached to C2 of an ethylenediazonium ion, the bond order of the NN triple bond can be reduced to a level compar

ISSN:0570-0833    

DOI:10.1002/anie.197902591

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

摘要:

AbstractSorption and desorption in zeolites (molecular sieves) have to be considered as complex processes, involving simultaneous diffusion in zeolite crystals, mass transfer in the intercrystalline void of a pellet, and heat transfer between the zeolitic sorbent and its surroundings. The kinetics of sorption and desorption, respectively, ofn‐C4H10in zeolite X and of CO2in zeolite A have been investigated: only the initial rates of uptake or release of the sorbet are controlled by mass transfer alone whereas ultimately they also depend on the rate of heat transfer from the sorbent to its surroundings orvice versa. Diffusivities of the sorbate in the zeolite crystals can be obtained from the kinetics of mass transfer, provided the resistance due to viscous or Knudsen flow between the crystals of the zeolitic sorbent can be eliminated. A sample consisting of a monolayer of single crystals had to be used for this purpose in then‐butane/zeolite X system; the intracrystalline diffusivity obtained in this way is not in conflict with data obtained by NMR spectroscopy. The intracrystalline diffusivities obtained in this way—taking into account the coupling of several processes during sorption — are higher than values reported in the lit

ISSN:0570-0833    

DOI:10.1002/anie.197902661

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

摘要:

AbstractPolymeric materials with novel properties for new technological applications are increasingly obtained by combining existing polymers, while the synthesis of new monomers has receded into the background. These polymer combinations or “alloys” (polyblends) are characterized by their chemical composition, the conformation of the chain molecules, and the morphology,i.e.the state of order at supramolecular level. Multiphase constitution is a typical characteristic of these substances, with a decisive influence on their macroscopic properties. The morphology of multiphase polymer alloys can be controlled to a limited extentviathe chemical composition of their components when homopolymers are mixed in the melt or as dispersions. Graft copolymerization, on the other hand, makes it possible to achieve the desired morphology at a given chemical composition. Furthermore, transprent two‐phase polymer alloys can be obtained under certain conditions. In multiphase polymers the reduction of stress without fracture, caused by mechanical loading will be treated using models. Certain combinations of properties such as hardness and toughness are connected with the coexistence of disperse and continuous phases. Equilibrium thermodynamical criteria for liquid mixtures wil be used to explain demixing phenomena in polymers. In the last few years it has become possible to determine the chain conformation experimentally using neutron scatt

ISSN:0570-0833    

DOI:10.1002/anie.197902731

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

摘要:

AbstractPure silicon carbide and silicon nitride have valuable properties in bulk pore‐free form; however, their industrial exploitation has hardly been possible so far. Neither compound can be melted or sintered in pure form; hot pressing or sintering at normal pressure requires the presence of additives; and the reaction‐sintering process in which only Si and C or Si and N are employed as additives affords porous materials.–The novel process of chemical vapor deposition has partly overcome the drawbacks of the previous methods. In the new process SiC is produced,e.g., by pyrolysis of CH3SiCl3, and Si3N4by reaction of SiCl4with NH3. This technique can also be used for pore filling in objects made of SiC and Si3N4(gas phase impregnation) and for producing extremely fine SiC and Si3N4(gas phase impregnation) and for producing extremely fine SiC and Si3N4powder and SiC monofilaments suitable as components for SiC composites. Moreover, gas phase impregnation can also give fiber compo

ISSN:0570-0833    

DOI:10.1002/anie.197902951

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197903041

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197903051

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197903061

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197903071

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197903082

出版商:Hüthig&Wepf Verlag    

年代:1979

数据来源: WILEY