Solvents searching

In the past, the search for suitable two-phase solvent systems entirely relied on a laborious and time-consuming trial-and-error method that has often discouraged the users of HSCCC, while the method for systematic solvent search has not been reported. In this article, we introduce a method for the systematic selection of suitable two-phase solvent systems for HSCCC and its application to the separation of antibiotics and dyes. 

If the above solvent search reaches the solvent system of n-hexane/methanol/water (2 1 1), which is suitable for the most hydrophobic compounds, and a more hydro-phobic solvent system is required, one may reduce the amount of water from the above solvent system and/or replace methanol with ethanol. Some useful solvent systems for the extremely hydrophobic compounds are M-hexane/ethanol/water (6 5 2) and w-hexane/methanol (2 1). On the other hand, if the solvent search reaches the solvent systems of w-butanol/water or ferf-butyl methyl ether/butanol/acetonitorile/water (2 2 1 5), which are suitable for the most hydrophilic compounds, and a still more hydrophilic solvent system is required, the above solvent system may be modified by the addition of acid or salt trifluoroacetic acid (TFA) or ammonium acetate has been successfully used. 

One reaction is found after performing this search it is shown in Figure 5-29. Analysis of the reaction conditions by retrieving the catalyst/solvent and conditions 011117, reading the given literature for more information, solves the problem... 

The input to a minimisation program consists of a set of initial coordinates for the system. The initial coordinates may come from a variety of sources. They may be obtained from an experimental technique, such as X-ray crystallography or NMR. In other cases a theoretical method is employed, such as a conformational search algorithm. A combination of experimenfal and theoretical approaches may also be used. For example, to study the behaviour of a protein in water one may take an X-ray structure of the protein and immerse it in a solvent bath, where the coordinates of the solvent molecules have been obtained from a Monte Carlo or molecular dynamics simulation. 

It is quite common to do the conformation search with a very fast method and to then optimize a collection of the lowest-energy conformers with a more accurate method. In some cases, single geometry calculations with more accurate methods are also performed. Solvent effects may also be important as discussed in Chapter 24. 

The molecular mechanics force fields available include MM+, OPLS, BIO+, and AMBER. Parameters missing from the force field will be automatically estimated. The user has some control over cutoff distances for various terms in the energy expression. Solvent molecules can be included along with periodic boundary conditions. The molecular mechanics calculations tested ran without difficulties. Biomolecule computational abilities are aided by functions for superimposing molecules, conformation searching, and QSAR descriptor calculation. 

The Courtaulds Tencel Process. The increasing costs of reducing the environmental impact of the viscose process coupled with the increasing likelihood that the newer cellulose solvents would be capable of yielding a commercially viable fiber process led Courtaulds Research to embark on a systematic search for a new fiber process in the late 1970s. 

With its flexible and logical search language, REACCS can retrieve molecular stmctures, the atoms and bonds that are transformed ia a reaction, relative and absolute stereochemistry, the role (reactant, product, solvent, or catalyst) of a molecule ia a reaction, reaction data (eg, temperature and yield), hterature references, and keyword descriptions of reaction types. 

Many antioxidants ia these classes are volatile to some extent at elevated temperatures and almost all antioxidants are readily extracted from their vulcanizates by the proper solvent. These disadvantages have become more pronounced as performance requirements for mbber products have been iacreased. Higher operating temperatures and the need for improved oxidation resistance under conditions of repeated extraction have accelerated the search for new techniques for polymer stabilization. Carpet backiag, seals, gaskets, and hose are some examples where high temperatures and/or solvent extraction can combine to deplete a mbber product of its antioxidant and thus lead to its oxidative deterioration faster (38,40). 

Sulfolane was first described ia the chemical Hterature ia 1916. It has been noted for its exceptional chemical and thermal stabiUty and unusual solvent properties. The search for a commercial process to produce sulfolane began about 1940. Market development quantities became available ia 1959. Siace then, both the use of and the appHcations for sulfolane have iacreased dramatically. 

It is possible to go beyond the SASA/PB approximation and develop better approximations to current implicit solvent representations with sophisticated statistical mechanical models based on distribution functions or integral equations (see Section V.A). An alternative intermediate approach consists in including a small number of explicit solvent molecules near the solute while the influence of the remain bulk solvent molecules is taken into account implicitly (see Section V.B). On the other hand, in some cases it is necessary to use a treatment that is markedly simpler than SASA/PB to carry out extensive conformational searches. In such situations, it possible to use empirical models that describe the entire solvation free energy on the basis of the SASA (see Section V.C). An even simpler class of approximations consists in using infonnation-based potentials constructed to mimic and reproduce the statistical trends observed in macromolecular structures (see Section V.D). Although the microscopic basis of these approximations is not yet formally linked to a statistical mechanical formulation of implicit solvent, full SASA models and empirical information-based potentials may be very effective for particular problems. 

The importance of the solvent, in many cases an excess of the quatemizing reagent, in the formation of heterocyclic salts was recognized early. The function of dielectric constants and other more detailed influences on quatemization are dealt with in Section VI, but a consideration of the subject from a preparative standpoint is presented here. Methanol and ethanol are used frequently as solvents, and acetone,chloroform, acetonitrile, nitrobenzene, and dimethyl-formamide have been used successfully. The last two solvents were among those considered by Coleman and Fuoss in their search for a suitable solvent for kinetic experiments both solvents gave rise to side reactions when used for the reaction of pyridine with i-butyl bromide. Their observation with nitrobenzene is unexpected, and no other workers have reported difficulties. However, tetramethylene sulfone, 2,4-dimethylsulfolane, ethylene and propylene carbonates, and salicylaldehyde were satisfactory, giving relatively rapid reactions and clean products. Ethylene dichloride, used quite frequently for Friedel-Crafts reactions, would be expected to be a useful solvent but has only recently been used for quatemization reactions. ... 

Figure 8.12(b) shows that if dichloromethane (5j = 3.1, Sy = 1.61) or diethyl ether (5-p = 2.8, Sy = 4.08) are used, then the change in solvent strength is not very large. If, however, selectivity values are considered (Figure 8.12(c)) it is apparent that the selectivity value decreases when dichloromethane is used and increases when diethyl ether is used. It can be concluded that selectivity values must also be considered in the search for suitable solvents for "D. 

Successful recrystallization of an impure solid is usually a function of solvent selection. The ideal solvent, of course, dissolves a large amount of the compound at the boiling point but very little at a lower temperature. Such a solvent or solvent mixture must exist (one feels) for the compound at hand, but its identification may necessitate a laborious trial and error search. Solvent polarity and boiling point are probably the most important factors in selection. Benzhydrol, for example, is only slightly soluble in 30-60 petroleum ether at the boiling point but readily dissolves in 60-90° petroleum ether at the boiling point. 

Until one develops a feel for recrystallization, the best procedure for known compounds is to duplicate a selection in the literature. For new compounds, a literature citation of a solvent for an analogous structure is often a good beginning point. To assist in the search, Table A3.4 lists several of the common recrystallizing solvents with useful data. The dielectric constant can be taken to be a rough measure of solvent polarity. 

By clicking the appropriate buttons on the form, the user can combine molecular structure queries of sample, CSP and solvent, using operators AND, OR, NOT with data queries in one search. A query for the search of chiral separations of alpha-aromatic acids on any polysaccharide phases coated on silica gel providing an alpha value superior to 1.2 is shown in Eig. 4-4. 

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