Water-miscible liquids substrate

The next step is to determine the solubility of the substrate (or its salts) in different solvents. This can also be performed by an automated liquid handling system. Depending upon the solubility of the substrate in water-miscible solvent (alcohols, acetone, tetrahydrofuran, etc.) and water-immiscible solvents (ethyl acetate, methyl-tert-butyl ether, heptane, etc.) the process chemist can identify one or many solvent systems from which the substrate (or its salts) could be ciystallized using the antisolvent addition strategy. 

Hong ES, Kwon OY, Ryu K (2008) Strong substrate-stabilizing effect of a water-miscible ionic liquid [BMIM][BF4] in the catalysis of horseradish peroxidase. Biotechnol Lett 30 529-533... 

Building on earlier work in these laboratories (8) we have overcome the typical mass transfer limitations of phase transfer catalysis for propylene oxidation by the use of 3-component liquid phases based on CO2 expanded liquids (CXLs). For the application to oxidations by aqueous H2O2, the organic component of the CXL is chosen because it is miscible with both dense CO2 and water. In this way, homogeneous systems are produced which decrease mass-transfer limitations and intensify chemical reactions. Previous reports using CXL systems have shown that they enhance the oxidation of the substrate and improve the selectivity at moderate reaction temperatures and pressures (3, 8, 9). 

There are several different methods to separate PNIPAM-supported catalysts from the reaction mixtures. Both liquid-solid separations and liquid-liquid separations can be used. The most frequently used liquid-solid separation method takes advantage of the varying solubility of polymers in different solvents. For example, PNIP AM can be precipitated from THF into hexanes. PNI-PAM copolymers also exhibit lower critical solution temperature (LCST) behavior. Specifically, PNIPAM and its copolymers can be prepared such that these polymers are soluble in water at low temperature but precipitate when heated up. This property may be used as either a purification method or a separation tech-nique.[l 1] A thermomorphic system is a liquid-liquid biphasic system developed in our group. It uses various solvent mixtures with temperature-dependent miscibility to effect separation of catalysts from substrates and products, as shown in Figure 2. 

Water (-fco-solvent) Organic liquid Easy product separation and catalyst recycling Lower cost of chemical processes Lack of toxicity of water Low reaction rate for water poorly miscible substrates Mass transfer limits rate of reaction Treatment of spent water... 

The virtues of water are its availability, cheapness, lack of smell, nontoxicity and non-flammability. However, it is not an ideal paint liquid, because of its limited miscibility with other liquids and because the film-formers designed to be dissolved or dispersed in it usually remain permanently sensitive to it. In fact, its abundance in nature makes it any paint film s worst enemy, since it is always around to cause swelling of the film, hydrolysis and substrate corrosion. 

Solvents and Miscibility of the Liquid Phases. In a biphasic process, one may want to keep the catalyst exclusively in one of the phases, say phase A (Fig. 1). This requires its complete insolubility in the other solvent moreover, no modification of solvent properties of phase B under the actual reaction conditions can be tolerated, leading to dissolution of the catalyst in that phase. In contrast, components of phase B (the solvent, reactants, and products) can have limited solubilities in the catalyst-containing phase, and in fact, in most cases such solubility properties lead to faster reactions due to the better catalyst-substrate contact in the bulk of phase A. For example, the solubility of CH3COOC2H5 in water at 20°C is 6.1 wt% whereas that of water in CH3COOC2H5 is 3.3 wt%. Very similar values were determined for (C2H5)20 in H2O (6.9 wt%) and H2O in (C2H5)20 (3.3 wt%). For other mutual miscibility data with water as one of the phases, see References 49 and 50. 

In solid-liquid phase transfer processes, i.e., those reactions in which a solid reagent is phase transferred by a crown [46] or occasionally by a tertiary amine, a cosolvent is ordinarily used, regardless of whether or not the substrate is a solid. In principle, any solvent which does not itself undergo reaction (unless this is the desired end) is acceptable. The most commonly used solvents for solid-liquid phase transfer processes have been benzene (and other hydrocarbons), dichloromethane and chloroform (and other chlorocarbons) and acetonitrile. The latter solvent can be successfully utilized in solid-liquid systems whereas it should be unacceptable in liquid-liquid systems because of its miscibility with water. Chloroform and dichloromethane are commonly and successfully used, although both undergo reactions the former being readily deprotonated to yield either trichloromethide anion or carbene [38], and the latter suffering nucleophilic displacement [19b, 53, 54]. 

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