Reaction initial direction

These compounds may be obtained by the Hantszch heterocyciization method (see Chapter II, Section 11.3). A -widely used two-step preparative method (Scheme 195) involves initial reaction of a 2-amiriothiazole -with 339 in pyridine (631-638) in aqueous sodium carbonate (639) or by fusion without solvent (640). The formed 340 is then hydrolyzed in acidic (641, 642, 1593) or alkaline medium (643-646). The direct reaction of 342 (Scheme 196) -with 2-aminothiazoles is less common and takes place in... 

Thus, for a successful fluorination process involving elemental fluorine, the number of coUisions must be drasticaUy reduced in the initial stages the rate of fluorination must be slow enough to aUow relaxation processes to occur and a heat sink must be provided to remove the reaction heat. Most direct fluorination reactions with organic compounds are performed at or near room temperature unless reaction rates are so fast that excessive fragmentation, charring, or decomposition occurs and a much lower temperature is desirable. 

Tubular Reactors. The tubular reactor is exceUent for obtaining data for fast thermal or catalytic reactions, especiaHy for gaseous feeds. With sufficient volume or catalyst, high conversions, as would take place in a large-scale unit, are obtained conversion represents the integral value of reaction over the length of the tube. Short tubes or pancake-shaped beds are used as differential reactors to obtain instantaneous reaction rates, which can be computed directly because composition changes can be treated as differential amounts. Initial reaction rates are obtained with a fresh feed. Reaction rates at... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

The attention of the authors was particularly directed toward the increased activity of the nickel catalyst film when copper was added. This increase is revealed in a change of the initial reaction rate of copper itself and of all the alloys (except those containing 25-35% nickel) they are more active than nickel itself. A respectively similar difference was observed for the activation energy and the preexponential factor. 

Broxton and Roper measured the rate of dissociation (A 3) of the (ii)-diazo ether, A 2, and the rate of the protection reaction (A p), i.e., the transformation of the (Z)-into the (ii)-ether ( protection because the diazo ether is protected against dediazoniation almost completely if present as the ( >isomer). Rate constants kx and k are known from Ritchie and Virtanen s work (1972). The results demonstrate firstly that the initial reaction of the diazonium ion takes place in such a way that almost exclusively the (Z)-ether is formed directly (ki/k3 = 120). The protection rate constant kp is a simple function of the intrinsic rate constants as shown in Scheme 6-4. 

The maintenance of product formation, after loss of direct contact between reactants by the interposition of a layer of product, requires the mobility of at least one component and rates are often controlled by diffusion of one or more reactant across the barrier constituted by the product layer. Reaction rates of such processes are characteristically strongly deceleratory since nucleation is effectively instantaneous and the rate of product formation is determined by bulk diffusion from one interface to another across a product zone of progressively increasing thickness. Rate measurements can be simplified by preparation of the reactant in a controlled geometric shape, such as pressing together flat discs at a common planar surface that then constitutes the initial reaction interface. Control by diffusion in one dimension results in obedience to the... 

The a-substitution in the alkenylcarbene complex seems to be crucial to direct the reaction to the five-membered rings. The mechanism proposed for this transformation supposes an initial 1,2-addition of the enolate to the carbene carbon atom to generate a zwitterionic intermediate. Cyclisation promoted by... 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

The speed with which taste stimulation occurs, coupled with the fact that stimulation with toxic substances does no damage to the receptors, led Beidler to suggest that taste stimulus need not enter the interior of the taste cell in order to initiate excitation. Because a taste cell has been shown to be sensitive to a number of taste qualities, and to a large number of chemical stimuli, he and his coworkers concluded that a number of different sites of adsorption must exist on the surface of the cell. Therefore, they assumed that taste response results from adsorption of chemical stimuli to the surface of the receptor at given receptor sites. This adsorption is described by a monomolecular reaction similar to that assumed by Renqvist, Lasareff, and Hahn, but with a difference. From the fact that each type of chemical-stimulus compound has a unique level of saturation of the taste receptor, it was concluded that the magnitude of the response is dependent on the initial reaction with the receptor, and not on other, subsequent receptor-reactions that are common to all types of receptor stimulation. Therefore, it was assumed that the magnitude of neural response is directly proportional to the number of sites filled, the maximum response occurring when all of the sites are filled. Beidler derived a fundamental... 

Considerable attention has been directed to the formation of nitroarenes that may be formed by several mechanisms (a) initial reaction with hydroxyl radicals followed by reactions with nitrate radicals or NO2 and (b) direct reaction with nitrate radicals. The first is important for arenes in the troposphere, whereas the second is a thermal reaction that occurs during combustion of arenes. The kinetics of formation of nitroarenes by gas-phase reaction with N2O5 has been examined for naphthalene (Pitts et al. 1985a) and methylnaphthalenes (Zielinska et al. 1989) biphenyl (Atkinson et al. 1987b,c) acephenanthrylene (Zielinska et al. 1988) and for adsorbed pyrene (Pitts et al. 1985b). Both... 

Since sulphones 204 are easily available compounds one would expect that they could be used as starting materials for the preparation of sulphoxides via the selective removal of one oxygen atom from the sulphonyl group (equation 112). Up to now, there is only one example reported of a direct reduction of a sulphone to a sulphoxide. The bicyclic dideuterio sulphone 205 after 24 h treatment with three-fold excess of diisobutyl aluminium hydride in boiling dichloromethane gave the corresponding sulphoxide 206 in 36% yield (equation 113). A two-step procedure for the selective reduction of sulphones to sulphoxides, which involves an initial reaction of sulphone 204 with aryldiazonium tetrafluoroborate 207 to form aryloxysulphoxonium salt 208 and its subsequent reduction (equation 114), was alluded to by Shimagaki and coworkers and... 

The nucleophiles participating in reaction with QM3 are typically obvious from the QM linkage identified in the products of all deoxynucleosides except for dA. Direct reaction to form the observed adduct at the exo 6-amino group of dA was still considered surprising due to its weak nucleophilicity, and an alternate pathway involving initial reaction at the most nucleophilic dA N1 followed by a Dimroth rearrangement that interconverts the N1 to the 6-amino groups seemed more reasonable... 

The direct, stereoselective conversion of alkynes to A-sulfonylazetidin-2-imines 16 by the initial reaction of copper(l) acetylides with sulfonyl azides, followed, in situ, by the formal [2+2] cycloaddition of a postulated A-sulfonylketenimine intermediate with a range of imines has been described <06AG(E)3157>. The synthesis of A-alkylated 2-substituted azetidin-3-ones 17 based on a tandem nucleophilic substitution followed by intramolecular Michael reaction of primary amines with alkyl 5-bromo-4-oxopent-2-enoates has been... 

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