Beobachtungen zum mechanismus der Nenitzescu-reaktion

The structure of the carbmolamine 5 was recognised as an intermediate in the Nenitzescu-reaction but until recently it had been formulated as the quinone derivate 3. The course of the Nenitzescu-reaction in acetic acid would be understood if the last step by reduction with hydroquinone of 5 to the 5-hydroxyindol derivative 6 could be clarified. The 4,4′ and 4,6′-diindole derivates 11 and 12 as well as the acetoxyhydroxy-indolederivative 10 h in the absence of hydroquinone were established.     

Zum mechanismus bei der photoinduzierten nachpolymerisation von polymethylmethacrylat

The photoinitiated polymerization of PMMA-blocks containing residual monomer leads at first to a decrease of the refractive index, the molar mass and the degree of conversion with time. Afterwards pronounced increase of these quantities is found. These phenomena are explained by photoinitiated chain breaks followed by depolymerisation of the polymer chains. This process is accompanied by growth of the radicals created from illumination of the PMMA-blocks. A mathematical model is proposed for calculation of the refractive index changes of PMMA-blocks.     

Zum mechanismus des strahlenchemischen abbaus von polytetrafluorethylen

The present work studies the changes in polymer structure and the mechanism of the decomposition of polytetrafluoroethylene resin (PTFE) exposed to high energy radiation (electron beam). Spectroscopic and kinetic observations are used to interpret the degradation process. For the first time the decomposition of PTFE has been carried out on a preparative scale and new results obtained by analysing the degradation products. The radiation-induced degradation of PTFE is accompanied by thermal degradation under certain irradiation conditions. This is due to an increase in temperature of the polymer caused by retardation of highly accelerated electrons (heat accumulation effect).The kinetics are discussed in terms of the reactions and recombination of radicals produced by high-energy radiation both in the polymer melt and the polymer surface. These are related to the overall rate of decomposition.The primary radicals formed by decomposition of PTFE in an inert atmosphere (N₂, Ar) react to produce perfluorinated alkanes and alkenes. In the presence of reactive gases the decomposition fragments originated will react rapidly; e.g. if oxygen is present in the reactive area the radicals form perflourinated peroxyl and oxyl radicals which finally stabilize themselves by CC-scission to perfluorocarbon acid fluorides and carbonyldifluorides.     

Beobachtungen zum mechanismus der nenitzescu-reaktion—II

The pyrrolo-indoles 9, 10 and 12 have been isolated as by-products. The structure of carbinolamine 13, a potential precurser of pyrrolo [2,3-f] indoles, was examined by chemical and spectroscopic methods. Under Nenitzescu-conditions 13 could not be reduced to the pyrroloindole 11, but the furano-indolenin 16 was isolated. The origin of the pyrrolo [3,2-e] indole 12 from carbinolamin 17 was proved by a corresponding reaction. The course of the reaction via the ring-opening of the indole 19 can be excluded and thus a direct ring-closure of F is made probable. Attempts to reduct 17 show that the carbinolamine can not be considered as a precursor of hydroxyindoles in an apolar, aprotic medium. Thus a reaction corresponding to an oxidation-reduction mechanism is not possible under these conditions. The experiments indicate that another mechanism, namely the direct cyclisation of H in this case leads to indole formation.     

Zum mechanismus der reaktion von alkylierten aromaten mit dichlorcarben

Tropylium halides (type B) are not stable. When trying to prepare these from 4 resp. 7 subsequent products are obtained. Makosza reaction of 4 gives the same product 1 as from 2-methyl naphthaline. Thus, B is an intermediate.     

Tetracarbonylwolfram-komplexe mit zwei unterschiedlichen donorliganden : I. Darstellung und mechanismus der thermischen isomerisierung

cis- and trans-tetracarbonyltungsten complexes (R₃P)(R′₃E)W(CO)₄ (R₃P,R′₃E = i-Pr₃P, Ph₂-i-PrP, Ph₃P, Ph₂HP, (Me₂n)₃P, Ph(i-PrO)₂P, (i-PrO)₃P, (PhO)₃P, Ph₃As, Ph₃Sb) are obtained from the chlortetracarbonyltungstates [(R₃P)W-(CO)₄Cl]^- by several different routes. a cis—trans equilibrium is attained at 20–30°C with the trans isomer being thermodynamically more stable in nearly all cases. The cis-trans isomerisation of the phosphine—arsine and phosphine—stibine complexes is shown by ligand exchange experiments to occur via dissociation of the arsine and stibine ligand, respectively. The bis(phosphine) complexes, however, isomerise intramolecularly without bond cleavage.     

Kinetik und mechanismus der epoxidierung von allylalkohol mit peroxykomplexen des molybdäns (VI)

The kinetics of the allylalcohol epoxidation by peroxycomplexes MoO(O₂₂L₁L₂ (1) and MoO(O₂)₂L₁ (2), where L₁=hexamethylphosphotriamide, L₂ = H₂O, has been studied in 1,2- dichlorethane within the 30–60°C temperature range. The reaction-rate observed by the oxygen content decreased is directly proportional to the concentration of both complexes. The reaction rate dependence on the allylalcohol concentration is described by the Michaelis-Menten equation. The parameters of this equation were determined at 50°C. K_m is equal to 0·78 and 0·81 Mol/l for 1 and 2 respectively. The activation energy values are 16·1 ± 0·5 and 18·3 ± 1·0 Kcal/Mol for 1 and 2, respectively. The glicidol yield at 50°C is varying from 70 to 100% for 2 and from 65 to 75% for 1 depending on the convertion level of the reaction. The epoxidation mechanism of allylalcohol by covalent molybdenum (VI) peroxy-complexes is discussed.     

Verteilung und Einschlußmechanismus der Verunreinigung bei der kriechenden Kristallisation im System NH4Cl−FeCl3

Zusammenfassung Die Verteilung und der Einschlußmechanismus der Verunreinigung bei der kriechenden Kristallisation im Fall der Bildung der sogenannten anormalen Mischkristalle NH₄Cl–FeCl₃ wird behandelt.

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Distribution of impurities and mechanism of inclusion in the creeping crystallization of the system NH₄Cl–FeCl₃

The distribution of impurities, and the mechanism of inclusions in the case of creeping crystallization of the so called abnormal mixed crystal of NH₄Cl–FeCl₃ are investigated.

     

Übergangsmetall–carben-komplexe : LXXIII. Kinetik und mechanismus der isomerisierung von cis- und trans-tetracarbonyl-methoxymethylcarben-phosphin-chrom(0)-komplexen

The kinetics of the isomerisation reactions of tetracarbonyl(methoxymethylcarbene)(trialkylphosphine)chromium(0), (CO)₄Cr[C(OCH₃)CH₃] [PR₃] (R = C₂H₅, C₆H₁₁), according to the equation:

have been studied in methylcyclohexane and toluene. The interconversion of the geometrical isomers follows a first-order rate law: d[(II)]/dt = k₁[(II)]. Excess of free phosphine or carbon monoxide has no significant influence on the rate of the reaction. Phosphine exchange experiments also show that the isomerisation takes place via neither a phosphine nor a C cleavage. The results are discussed on the basis of an intramolecular isomerisation mechanism, which also plausibly explains the very small values of the activation entropies.     

Kinetik und mechanismus der elektrochemischen reduktion von 2,2′-dipyridyl I. Primärprozesse: Vorgelagerte isomerisierung eines komplexes

Polarography and chronopotentiometry are used to study the primary processes of the reduction of 2,2′-bipyridine at mercury electrodes in aqueous solutions (12 < pH < 14). Bipyridine and its first reduction product are strongly adsorbed. The reduction is preceded by a complex equilibrium between bipyridine and the cation of the supporting electrolyte (Li, Na, K, Ba). Due to the great excess of the cations, the equilibrium is shifted completely to the side of the complex, which exists as cis- and trans-isomer. The cis-form is more stable, forming a chelate, the trans-form however is more easily reducible. The kinetic and thermodynamic data of the isomerization are determined. The first reduction product reacts irreversibly giving a substance which can be reoxidized to bipyridine via a radical intermediate.     

Zum mechanismus Massenspektrometrischer Fragmentierungsreaktionen–III: Stereospezifische Reaktionen in den Massenspektren der Methyläther cyclischer Polyalkohole

The mass spectra of stereoisomers of polymethoxy cycloalkanes depend on the geometry of the molecular ions. The magnitude of the stereochemical effect is influenced by the stability of the cyclic molecular ions. Due to energetically favourable ring-fragmentations the effect is cancelled by vicinal methoxy substituents and diminished by a methyl group next to a methoxy substituent. Stereochemically controlled fragmentations are the eliminations of a methoxy group in the form of a methanol or formaldehyde molecule from the molecular ions. By an investigation of di- and trimethoxy cyclohexanes, specifically labelled with deuterium, it is shown that both reactions are initiated by a transfer of an H-atom from a carbinol-C-atom to an O-atom of a methoxy group. Whether or not these energetically favourable reactions will occur depends on how close the H- and O-atoms involved can approach each other in the possible conformations of the molecular ion. The stereochemical control of the fragmentation of 1,3-dimethoxy cyclopentane, containing a more or less fixed five membered ring is small, that of dimethoxy cycloheptanes with a flexible seven membered ring is of comparable magnitude as the steric effect in the mass spectra of cyclohexane derivatives.     

Quantenchemische untersuchungen zum mechanismus der elektrophilen substitution. I. Zur potentialhyperfläche des systems benzol/h+

Results of semiempirical calculations (CNDO/2-FK and MINDO/2 methods) for the σ-π complex problem on protonated benzene are given and compared with previous ones. The semiempirical methods were chosen according to the agreement of their results with new theoretical energy data (E_HF + E_korrel) concerning the classical–nonclassical problem of protonated ethylene. By these methods the corresponding part of the energy surface of the benzene/H⁺ system is simulated. The stationary points of this surface are found by a gradient method with complete optimization of the geometry. On the basis of this method we determined the energy profile of a reaction coordinate between the classical (σ-complex) and nonclassical (π-complex) cation. The so called strong π-complex is a saddle point between two σ-complex minima and can be interpreted as transition state of 1,2-proton shifts. Hypotheses for possible minimum energy paths of electrophilic attacks in the given region of the surface are discussed.