Organic cosolvents

One of the more efficient and versatile methods of increasing solubility and one that does not require modification of the solute is to use an organic cosolvent. The cosolvent reduces the hydrogen bond density of aqueous systems, so that it is less effective in squeezing out non-polar solutes from solution. Cosolvents 

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Some hquid defoamers are preemulsified relatives of paste defoamers. In addition to the fatty components mentioned above, kerosene [8008-20-6] or an organic cosolvent such as 2-propanol have been used to enhance stabiUty of the oil—water emulsion and the solubiUty of the defoamer s active ingredients. These cosolvents are used less frequently as concerns increase about volatile organic emissions (VOCs) from the paper machine. Additionally, the use of ultrapure mineral oil in defoamers has become commonplace. Concern about the creation of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) in the pulping process has led to the discovery of unchlorinated precursor molecules, especially in recycled mineral oil and other organic cosolvents used in defoamer formulations (28). In 1995 the mineral oil that is used is essentially free of dibenzodioxin and dibenzofuran. In addition, owing to both the concern about these oils and the fluctuating cost of raw materials, the trend in paper machine defoamers is toward water-based defoamers (29). 

Dica.rboxyIic AcidMonoesters. Enzymatic synthesis of monoesters of dicarboxyUc acids by hydrolysis of the corresponding diesters is a widely used and thoroughly studied reaction. It is catalyzed by a number of esterases. Upases, and proteases and is usually carried out in an aqueous buffer, pH 6—8 at room temperature. Organic cosolvents may be added to increase solubiUty of the substrates. The pH is maintained at a constant level by the addition of aqueous hydroxide. After one equivalent of base is consumed the monoesters are isolated by conventional means. 

Although equihbrium-controUed peptide synthesis has been successfully used on a number of occasions, including thermolysin-catalyzed synthesis of aspartame (126) and semisynthesis of insulin (127), the method has a significant drawback a water-miscible organic cosolvent added to the reaction medium to suppress the ionization of unactivated carboxy components significantly reduces the reaction rate. 

TSK-GEL SW columns allow use of elution buffers comprised completely of water-soluble organic solvents, whereas the TSK-GEL PW packings limit organic cosolvent use to a maximum of 50%. 

The charged group introduced into products by the aldol donors (phosphate, carboxylate) facilitates product isolation and purification by salt precipitation and ion exchange techniques. Although many aldehydic substrates of interest for organic synthesis have low water solubility, at present only limited data is available on the stability of aldolases in organic cosolvents, thus in individual cases the optimal conditions must be chosen carefully. 

In the following text, examples of solvent effects on enzyme selectivity, referred either to systems based (i) on water-miscible organic cosolvents added to aqueous buffers or (ii) on organic media with low water activity, are discussed. 

Selectivity Enhancement by Addition of Water-Miscible Organic Cosolvents... 

MeOOC COOMe Organic cosolvent HOOC COOMe... 

Organic cosolvent, v/v Relative rate of hydrolysis Enantiomeric excesses of (4)... 

Several other examples have been reported in the literature and most of them have been already revievred (see, for instance, [11]). Hovrever, it must be mentioned that, of course, the selectivity enhancement via addition of vater-misdble organic cosolvents may not be taken for granted, as sometimes this approach may be unsuccessfij] [11a]. 

Pal et al. explored an efficient Pd/C-catalyzed reaction of 2-iodophenols with terminal alkynes in water without the use of any organic cosolvents in the presence of PPh3, Cul, and prolinol to give the expected benzofuran products (Eqs. 4.42, 4.43).78 The hydroxyl group was well tolerated during reactions. When used as a base, prolinol afforded better yields of products than triethylamine, a common organic base, possibly due to its better miscibility with water. 

Arylsilanols, silanediols, and triols performed poorly under fluoride activation conditions, but instead required Ag20 (78X274 However, the cross-coupling of arylsilanediols and similar organosilicon reagents (formed in situ from the respective halosilanes) can be achieved under very mild conditions, using phosphine-free catalysts in water in the absence of any organic cosolvents.275... 

The organic compounds were dissolved or dispersed in an aqueous solution of the catalyst, with or without an organic cosolvent, and the net oxidations were carried out at applied potentials causing the oxidation of Ru11 to RuIV complex (0.6-0.8 V vs. SCE). It has been demonstrated that this electrocatalytic system is capable of providing a general and selective method for the oxidation of alcohols, aldehydes, cyclic ketones, and C—11 bonds adjacent to alkeneic or aromatic groups. 

Several microwave-assisted procedures have been described for soluble polymer-supported syntheses. Polyethylene glycol) (PEG)-supported aryl bromides have been shown to undergo rapid palladium(0)-catalyzed Suzuki couplings with aryl boronic acids in water (Scheme 12.16) [63], The reaction proceeded without organic cosolvent... 

Nzengung VA, Voudrias EA, Nkedi-Kizza P, Wampler JM, Weaver CE (1996) Organic cosolvent effects on sorption equilibrium of hydrophobic chemicals by organoclays. Environ Sci Technol 30 89-96... 

Nkedi-Kizza, P, Rao, P.S.C., Hornsby, A.G. (1985) Influence of organic cosolvent on sorption of hydrophobic organic chemicals by soils. Environ. Sci. Technol. 19, 975-979. 

A very large group of common acceptors are linear, branched or aromatic alcohols. The glycosides formed are widely applicable as, e.g., non-ionic detergents. The alcohol is mostly used as an organic cosolvent and the water... 

Influence of Organic Cosolvents. Rao et al. (49) have recently presented a solvophobic model for estimating the sorption of a hydrophobic solute from a mixed solvent. This model is based on the work of Yalkowsky et al. (27), who developed an empirical relationship between the solubility in a mixed solvent system, Sm, and that in pure water given by... 

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