Fischer mechanism

According to Micheel, osazone formation starts from an aldose and requires an Amadori rearrangement, so that his scheme may not account for the reaction in the ketose series. It may be concluded that the Fischer mechanism is not valid, and that further studies are needed in order to solve the problem of interaction between ketoses and substituted phenyl-hydrazines. 

Tertiary amine catalyzed reactions were also studied by Tanaka and Kakiuchi (8), who essentially supported the Fischer mechanism, but disagreed on the kinetic order. The copolymerization kinetic scheme proposed by both Fischer and Tanaka postulate three rates, R, R2, and R, as follows ... 

The mechanism of the Fischer-Tropsch reactions has been the object of much study (note Eqs. XVI11-55-XV111-57) and the subject of much controversy. Fischer and Tropsch proposed one whose essential feature was that of a metal carbide—patents have been issued on this basis. It is currently believed that a particular form of active adsorbed carbon atoms is involved, which is then methanated through a series of steps such as... 

An important general method of preparing indoles, known as the Fischer Indole synthesis, consists in heating the phenylhydrazone of an aldehyde, ketone or keto-acld in the presence of a catalyst such as zinc chloride, hydrochloric acid or glacial acetic acid. Thus acrtophenone phenylhydrazone (I) gives 2-phenyllndole (I V). The synthesis involves an intramolecular condensation with the elimination of ammonia. The following is a plausible mechanism of the reaction ... 

The mechanism of the Fischer cyclization outlined in equation 7.1 has been supported by spectroscopic observation of various intermediates[4] and by isolation of examples of intermediates in specialized structures[5]. In particular, it has been possible to isolate enehydrazines under neutral conditions and to demonstrate their conversion to indoles under the influence of acid cata-lysts[6]. 

The mechanisms of the Fischer esterification and the reactions of alcohols with acyl chlorides and acid anhydrides will be discussed m detail m Chapters 19 and 20 after some fundamental principles of carbonyl group reactivity have been developed For the present it is sufficient to point out that most of the reactions that convert alcohols to esters leave the C—O bond of the alcohol intact... 

We have seen Ihis Iheme before m Seclion 19 14 when we presented Ihe mechanism of Ihe Fischer eslerificalion As was Ihe case Ihen formalion of Ihe lelrahedral mlermedi ate IS rate determining... 

Section 2010 Ester hydrolysis can be catalyzed by acids and its mechanism (Figure 20 4) is the reverse of the mechanism for Fischer esterification The reaction proceeds via a tetrahedral intermediate... 

Fig. 2. Principle mechanisms of formation of a receptor—substrate complex (a) Fischer s rigid "lock-and-key" model (b) "induced fit" model showing...

H. Lyr, in H. Lyr, ed.. Modem Selective Fungicides—Properties, Application and Mechanism of Action, Gustav Fischer Vedag, Jena, Germany, 1987, p. 75. 

Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

We have seen this theme before in Section 19.14 when we presented the mechanism of the Fischer esterification. As was the case then, formation of the tetrahedral intermediate is rate-determining. 

A number of reaction pathways have been proposed for the Fischer indolization reaction. The mechanism proposed by Robinson and Robinson in 1918, which was extended by Allen and Wilson in 1943 and interpreted in light of modem electronic theory by Carlin and Fischer in 1948 is now generally accepted. The mechanism consists of three stages (I) hydrazone-ene-hydrazine equilibrium (II) formation of the new C-C bond via a [3,3]-sigmatropic rearrangement (III) generation of the indole nucleus by loss of... 

A large number of Brpnsted and Lewis acid catalysts have been employed in the Fischer indole synthesis. Only a few have been found to be sufficiently useful for general use. It is worth noting that some Fischer indolizations are unsuccessful simply due to the sensitivity of the reaction intermediates or products under acidic conditions. In many such cases the thermal indolization process may be of use if the reaction intermediates or products are thermally stable (vide infra). If the products (intermediates) are labile to either thermal or acidic conditions, the use of pyridine chloride in pyridine or biphasic conditions are employed. The general mechanism for the acid catalyzed reaction is believed to be facilitated by the equilibrium between the aryl-hydrazone 13 (R = FF or Lewis acid) and the ene-hydrazine tautomer 14, presumably stabilizing the latter intermediate 14 by either protonation or complex formation (i.e. Lewis acid) at the more basic nitrogen atom (i.e. the 2-nitrogen atom in the arylhydrazone) is important. 

Figure 3-31C. Double mechanical seal, two rotary elements against common stationary. (By permission, Fischer, E. E., Chem Processing, Oct. 1983 [24].)...

The mechanisms of corrosion inhibition will be described separately for acid and neutral solutions, since there are considerable differences in mechanisms between these two media. Definitions and classifications of inhibitors are given in Section 17.2 and by Fischer. ... 

Mechanism of Fischer esterification. The reaction is an acid-catalyzed, nucleophilic acyl substitution of a carboxylic acid. 

Acid-catalyzed ester hydrolysis can occur by more than one mechanism, depending on the structure of the ester. The usual pathway, however, is just the reverse of a Fischer esterification reaction (Section 21.3). The ester is first activated toward nucleophilic attack by protonation of the carboxyl oxygen atom, and nucleophilic addition of water then occurs. Transfer of a proton and elimination of alcohol yields the carboxylic acid (Figure 21.8). Because this hydrolysis reaction is the reverse of a Fischer esterification reaction, Figure 21.8 is the reverse of Figure 21.4. 

Active Figure 21.8 MECHANISM Mechanism of acid-catalyzed ester hydrolysis. The forward reaction is a hydrolysis the back-reaction is a Fischer esterification and is thus the reverse of Figure 21.4. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Instinctively it would seem that Step C would be rate controlling and the slowest. In the case of the Fischer-Tropsch reaction, one would postulate that the surface would offer more methylene groups for chain polymerization. This mechanism differs from that of Vlasenko and Uzefo-vich (4) essentially in the concept that the whole molecule interacts with the surface. Furthermore, the HCOH intermediate is wholly horizontal to the surface rather than perpendicular. 

The kinetics and mechanism of living radical polymerization have been reviewed by Fischer,21 Fukuda et at.,22 and Goto and Fuktida.23 In conventional radical polymerization, new chains are continually formed through initiation w hile existing chains are destroyed by radical-radical termination. The steady state concentration of propagating radicals is 10"7 M and an individual chain will have a lifetime of only 1-10 s before termination within a total reaction lime that is... 

In addition to protons, other electrofugic leaving groups such as SO3 (i. e., anions of sulfonic acids), Cl, Br, I, C02, and others can also be displaced in azo coupling reactions with aromatic substrates. The mechanism of such substitutions is in principle the same as that of dehydrogenation (see Fischer and Zollinger, 1972). 

The kinetic and thermodynamic properties of Fischer-type carbene complexes have also been addressed by Bernasconi, who relates the strength of the 7r-donor substituent to the thermodynamic acidity [95-101] and the kinetics and mechanism of hydrolysis and reversible cyclization to differences in the ligand X [96,102]. 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

Fischer alkenylcarbene complexes undergo cyclopentannulation to alkenyl AT,AT-dimethylhydrazones (1-amino-1-azadienes) to furnish [3C+2S] substituted cyclopentenes in a regio- and diastereoselective way along with minor amounts of [4S+1C] pyrrole derivatives. Enantiopure carbene complexes derived from (-)-8-(2-naphthyl)menthol afford mixtures of trans,trans-cycloipentenes and ds,ds-cyclopentenes with excellent face selectivity [75]. The mechanism proposed for the formation of these cyclopentene derivatives is outlined in Scheme 28. The process is initiated by nucleophilic 1,2-attack of the carbon... 

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