It is shown how to apply homogeneous nucleation theory to predict the limits of superheat of multicomponent liquid mixtures. Measurements of such limits for n-pentane-n-hexane, n-hexane-cyclohexane, benzene-cyclohexane, and benzene-n-hexane mixtures as a function of composition are presented and compared with theory. SCOPEBarriers to the nucleation of bubbles may lead to a superheating of liquids which is so large that when nucleation does occur, they boil with explosive violence. Such explosive boiling tends to occur whenever a low boiling nucleating liquid comes in contact with a much higher boiling host liquid if their temperature difference is sufficiently large. This limit of superheat is therefore readily attained and represents quite a significant hazard to industry.I n a recent review in this journal (Blander and Katz, 1975), it was shown that homogeneous nucleation theory was quite successful in predicting the measured limits of superheat of pure (that is, one component) substances.However, the low boiling liquid is often a mixture of two or more components. It is the purpose of this paper to show how to apply nucleation theory to multicomponent mixtures, to describe an improved version of the rising droplet column used to measure the limits of superheat, and to present results on an ideal and on several somewhat nonideal binary mixtures which were studied to test the validity of the theory. CONCLUSIONS AND SIGNIFICANCEThe theory for the homogeneous nucleation of a superheated liquid was successfully generalized to multicompo-Correspondence concerning this paper should be addressed to Joseph L. Katz. Bruce S. Holden is with The Dow Chemical Company, Midland, Michigan 48640. nent mixtures by employing the chemical equilibrium approximation; that is, the pressure and composition of the gas in the embryonic bubbles is determined by its being in equilibrium with the surrounding superheated liquid for bubbles of every size. With this approximation, the equaical Engineers, 1978. tion for the rate of nucleation of multicomponent mixtures
Abstract:The introduction and removal of protecting groups is ubiquitous in multi-step synthetic schemes. From a green chemistry standpoint, however, alternative strategies that employ in situ and reversible protection and deprotection sequences would be attractive. The reversible reactions of CO2 with amines could provide a possible vehicle for realizing this strategy. Herein, we present (1) the products of reaction of benzylamines with CO2 in a variety of solvents with and without the presence of basic additives; (2) 498(4-aminomethyl)phenyl) methanol with isopropenyl acetate in acetonitrile containing DBU in the absence and presence of CO2.
The heterogeneous palladium-catalyzed Suzuki reactions between model aryl bromides (4-bromoanisole, 4-bromoaniline, 4-amino-2-bromopyridine, and 2-bromopyridine) and phenylboronic acid have been successfully conducted in water with no added ligand at the 100-mL scale using 20-40 millimoles of aryl bromide. The product yields associated with these substrates were optimized and key reaction parameters affecting the yields were identified. The results clearly indicate that the reaction parameters necessary to achieve high yields are substrate dependent. In addition, it is demonstrated that aqueous Suzuki reactions of substrates containing basic nitrogen centers can produce quantitative yields of desired products in the absence of added ligand.KEYWORDS: palladium catalysis without added ligand, Suzuki reactions in water, heterogeneous catalysis, scale-up, green and sustainable Suzuki reactions.
The Suzuki coupling reaction of basic nitrogen containing substrates (2-bromo- and 2-chloro-4-aminopyridine, and 2-bromo and 2-chloropyridine) with phenylboronic acid using Pd(TPP)2Cl2/K3PO4 in acetonitrile-water biphasic solvent systems under a CO2 or a N2 atmosphere is discussed. It was observed that 2-halo-4-aminopyridine produced quantitative yields of coupled products under a CO2 atmosphere while the yields for the 2-halopyridines were poor. In contrast, the yields of coupled products for the 2-halopyridines substrates were quantitative under a N2 atmosphere while only poor yields were realized for the 2-halo-4-aminopyridines under the same conditions. Evidence is presented which suggests that the presence of CO2 alters the pH of the aqueous phase of the reaction system and the accompanying efficiency of the coupling process. Using a series of buffers to adjust the pH of the aqueous phase, the pH dependence associated with the efficiency of the coupling process is illustrated.
A series of aqueous heterogeneous Suzuki coupling reactions of substrates containing basic nitrogen centers with phenylboronic acid in the absence of added base and ligand is presented. High yields of products were obtained by employing aryl bromides containing aliphatic 1°, 2°, and 3° amine substituents, and good to high yields were obtained by employing a variety of substituted bromopyridines. In the former series, the pH of the aqueous phase changed from basic to acidic during the course of the reaction, while in the latter series the aqueous phase was on the acidic side of the pH scale throughout the entire course of reaction. A mechanistic interpretation for these observations, which generally preserves the oxo palladium catalytic cycle widely accepted in the literature, is presented.
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