Omega phase

Mechanism of Phase-Transfer Catalysis The Omega Phase... 

It has already been mentioned that in the absence of added water the reaction kinetics follows a pseudo zero-order rate profile the rate-controlling step under these conditions appears to involve the complexation of the crown in the organic phase with the salt in the solid phase. In contrast to this, in the presence of small quantities of water the reaction kinetics follows a pseudo first-order rate profile. Thus it appears that the water facilitates the interaction between the crown and the salt by forming an omega phase since the displacement process now becomes the rate-controlling step. The phase region where the displacement process actually takes place is not certain at this juncture. 

Liotta, C. L. "Omega-Phase Catalysis," this volume. 

Figure 6. Suggested PTC reaction mechanism in an omega phase (16).

Based on the IR data about the nucleation gels and the synthesis data with use of them, one may conclude (1) when the nucleation gel is aged more than about 50 hours, the gradual formation of IR-faujasite in nucleation gel is coincident with the trend that omega phase becomes... 

Microdiffraction.—Perhaps more important than SAD techniques, particularly in the context of catalyst research, microdiffraction allows the user to benefit from the small probe size generated in STEM in the structural analysis of small particles and localized areas in thin foils. If the small probe is stopped on a particle, then clearly a transmission diffraction pattern will be observable after the beam has traversed the sample, provided we have the means available for its display. In CTEM such a pattern will, of course, be formed by the imaging system in a manner identical to SAD, but in STEM the pattern must be scanned across the detector. This is accomplished by means of a set of post-specimen scan coils which once more scan the diffracted beams across the axial bright-field detector. Such a pattern is shown in Figure 13 where a beam of approximately 10 A FWHM was stopped on a small second-phase particle during the omega-phase transformation in a Zr-Nb alloy. The relatively poor definition of the reflection is a consequence of both the convergent nature in the probe (necessary in order to obtain the smallest probe sizes) and a S/N limited by the available current in the probe. Nevertheless, weak reflections with half-order indices are clearly visible between the main alloy reflections and it is therefore possible to attempt structural... 

Figure 13 Microdiffraction pattern recorded with <20 A probe on a second-phase region during an omega-phase transformation in a zirconium-niobium alloy ...

M. Sanati and A. Saxena. Domain walls in [omega]-phase transformations. Physica D, 123 368-379, 1998. 

Pradhan, N.C., and M. M. Sharma, Kinetics of Reactions of Benzyl Chloride/ -Chlorobenzyl Chloride with Sodium Sulfide Phase-Transfer Catalysis and the Role of the Omega Phase, Ind. Eng. Chem. Res., 29, 1103 (1990). 

AUR/FER] Aurelio, G., Fernandez Guillermet, A., Assessment of the structural relations between the bee and omega phases of Ti, Zr, Hf and other transition metals, Z. Metallkd, 91, (2000), 35-42. Cited on page 83. 

In many PTC systems, a third phase can form where reaction occurs and rates are enhanced. This third phase can form in many multiphase systems, and has been termed the omega phase [18]. The first example of an omega phase was discovered in the ITC reaction of cyanide ion on a benzyl halide in the presence of a crown ether as phase transfer catalyst. This liquid-solid PTC system was found to have rates dependent on the amount of water present. It was determined that in the absence of either water or the... 

The effects of added water on the rates of displacement of benzyl bromide and benzyl chloride with KCN salt in toluene catalyzed by 18-crown-6 were reported [145], It was observed that a small amount of water considerably increased the reaction rates compared to the anhydrous conditions and that the rate increased sharply to a maximum value in the presence of an optimum amount of added water. An important observation was that under anhydrous conditions, the reaction followed zero-order kinetics while in the presence of added water it followed first-order kinetics. It was suggested that the initial small amounts of added water coated the surface of the salt particle, which extracted the crown ether from the organic phase to form a new interfacial region called the omega (cd) phase. It was believed that the catalytic reaction took place mainly in the omega phase, since the quantity of added water corresponding to the maximum quantity of crown ether on the surface of the salt particles correlated well with the optimum quantity of added water. 

Furthermore, the results of a study on the distribution of 18-crown-6 between the toluene phase and the omega phase indicated that the amount of crown ether in the toluene phase remained low and relatively constant even though 3.50-11.45 mmol of crown ether were added [146], It appeared that the omega phase acted like a sponge that was capable of adsorbing the added crown ether. For the 18-crown-6-catalyzed displacement of benzyl bromide with KCN salt in toluene [146], it was found that the observed first-order rate constant was quite independent of the amounts (5.0 12.0 mmol) of 18-crown-6, which implied that the displacement reaction probably took place mainly in the organic phase. Mechanistic rationalization of this crown ether-catalyzed two-phase reaction is described as follows ... 

It was also required that KCN and KBr salts, crown ether (L), and water were stirred together in toluene for about 1 h to allow the equilibria [Eqs (65)-(68)] to be established before the addition of benzyl bromide. It is reasonable to assume that in the omega phase (ft)) the solution is always saturated with (KL CN - H20) and (KL Br -wH20), i.e.,... 

When the omega phase is formed, the overall reaction rate can be described by pseudo-first-order kinetics with respect to the organic reactant. While the reaction follows pseudo-zero-order kinetics as the substitution reaction is conducted in the presence of crown ether and in the absence of water, it is independent of the benzyl halide concentration. Crown ether directly dissociates the cation of the reacting salt. A reaction mechanism was proposed for the esterification reaction of solid potassium 4-nitrobenzoate and benzyl bromide by using crown ether [197], The overall reaction is... 

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