Phase-transfer catalysis reaction rates

The data for the reactions of potassium cyanide with benzyl halides at 85 C and 25 C are summarized in Tables I-III and graphical representations of these data are shown in Figures 1-3. The reactions carried out at 85 C were followed to 70% completion, while those at 25 C were followed to 50% completion. In general, excellent first-order kinetic plots were obtained. Each point on the graphs represents an average of at least three kinetic determinations. It is interesting to note that in the absence of added water (solid-liquid phase transfer catalysis), the rates of benzyl halide disappearance were more accurately described by zero-order kinetics. 

FER CATALYSIS GENERAL ACID CATALYSIS GENERAL BASE CATALYSIS HOMOGENEOUS CATALYST HETEROGENEOUS CATALYST INTRAMOLECULAR CATALYSIS MICELLAR CATALYSIS Mich ALT IS-MeNTEN KINETICS PHASE-TRANSFER CATALYSIS PSEUDOCATALYSIS RATE OF REACTION SPECIFIC CATALYSIS. 

Phase-transfer catalysis (Section 22.5) Method for increasing the rate of a chemical reaction by transporting an ionic reactant from an aqueous phase where it is solvated and less reactive to an organic phase where it is not solvated and is more reactive. Typically, the reactant is an anion that is carried to the organic phase as its quaternary ammonium salt. 

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

This method exemplifies a broad class of processes that proceed via transfer of reacting species between two liquid phases. Such processes may require a catalyst that can combine with species present in one phase and effect their transfer in this form to the second phase where the main reaction occurs. Starks23 has termed such a process phase-transfer catalysis and has demonstrated its utility in reactions involving inorganic anions. For example, he has shown that the rates... 

A difficulty that occasionally arises when carrying out nucleophilic substitution reactions is that the reactants do not mix. For a reaction to take place the reacting molecules must collide. In nucleophilic substitutions the substrate is usually insoluble in water and other polar solvents, while the nucleophile is often an anion, which is soluble in water but not in the substrate or other organic solvents. Consequently, when the two reactants are brought together, their concentrations in the same phase are too low for convenient reaction rates. One way to overcome this difficulty is to use a solvent that will dissolve both species. As we saw on page 450, a dipolar aprotic solvent may serve this purpose. Another way, which is used very often, is phase-transfer catalysis ... 

Although the use of phase-transfer catalysis (PTC) for manufacturing esters has the merits of a mild reaction condition and a relatively low cost [1], PTC has its limitations, such as the low reactivity of carboxylic ion by liquid-liquid PTC [2], a slow reaction rate by solid-liquid PTC, and the difflculty of reusing the catalyst by both techniques. 

Phase-transfer catalysis is a special type of catalysis. It is based on the addition of an ionic (sometimes non-ionic like PEG400) catalyst to a two-phase system consisting of a combination of aqueous and organic phases. The ionic species bind with the reactant in one phase, forcing transfer of this reactant to the second (reactive) phase in which the reactant is only sparingly soluble without the phase-transfer catalyst (PTC). Its concentration increases because of the transfer, which results in an increased reaction rate. Quaternary amines are effective PTCs. Specialists involved in process development should pay special attention to the problem of removal of phase-transfer catalysts from effluents and the recovery of the catalysts. Solid PTCs could diminish environmental problems. The problem of using solid supported PTCs seems not to have been successfully solved so far, due to relatively small activity and/or due to poor stability. 

The phase transfer catalysis not only promotes the reactions between the reagents which are mutually insoluble in immiscible phases, but also offers a number of process advantages such as, increase in rate of reactions, increase in product specificity, lowering of energy requirement, use of inexpensive solvents and catalysts, extraction of cations or even neutral molecules from one phase to another etc. 

Dialkvlaminopyridinium. Salts. Although phase transfer catalysis by polar polymers provided displacement reactions under conditions where conventional catalysts such as Bu NBr decompose, rates of reaction, a preference for potassium over sodium salt reactants, and amounts of catalyst necessary for convenient reaction were... 

The ONSH reaction of the carbanion of 2-phenylpropionitrile (45 c) with nitrobenzene in liquid ammonia at -70 °C involves rate-limiting Carom—H bond breaking, as evidenced by the 9.8 times faster rate than for reaction of the analogous substitution of deuterium in 4-<7-nitrobenzene and perdeuterionitrobenzene. Reactions of the carbanion derived from (45c) with 4-chloro-3-trifluoromethylnitrobenzene and 4-chloronitrobenzene in toluene under phase transfer catalysis has also been studied." ... 

Supercritical fluids are benign alternatives to conventional organic solvents that may offer improvements in reaction rate, product selectivity, and product separation. We reported the first use of SCFs for phase-transfer catalysis (PTC), where these benign alternatives also offer greatly improved transport, product separation, catalyst recycle, and facile solvent removal (26-29). 

Because the cellulose ether alkoxide is present entirely in the aqueous phase, the rate-limiting step may be the partitioning (phase transport) of the hydrophobic electrophile across the interface from the organic to aqueous phase. If the reaction rate is controlled by diffusion of the electrophile across the interface, then one would expect a correlation between water solubility of the hydrophobe and its alkylation efficiency. The fact that the actual alkylation reaction is probably occurring in the aqueous phase (or at the interface) yet the electrophile itself is principally soluble in the organic phase has important mechanistic ramifications. This type of synthetic problem, in which one reactant is water soluble and the other organic soluble, should be amenable to the techniques of phase transfer catalysis (PTC) to yield significant improvements in the alkylation efficiency. 

Scheme 8.1 also illustrates an important feature of asymmetric phase-transfer catalysis, namely that the catalyst is involved in two different steps of the mechanism. Thus, the rate of reaction increases because the catalyst accelerates the substrate deprotonation step, but the asymmetric induction occurs during the subsequent enolate alkylation step. 

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