Separation techniques soluble organic compounds

WATER-AIR EQUILIBRATION. McAuliffe (6) introduced a multiple phase equilibrium procedure for the qualitative separation of hydrocarbons from water-soluble organic compounds. For n-alkanes, more than 99% were found to partition in the gas phase after two equilibrations with equal volumes of gas and aqueous solution. Cycloalkanes require three equilibrations to be essentially completely removed, and oxygen-containing organic compounds (e.g., alcohols, aldehydes, ketones, and acids) remain in the aqueous layer. Thus, after equilibration with equal volumes of gas, an immediate clue is given regarding the identification of the compound. More details of this technique can be found in Chapter 7. 

Extraction is a useful technique for separating compounds having different solubility properties. Carboxylic acids can be separated from other organic compounds by extraction, because aqueous base converts a carboxylic acid into a water-soluble carboxylate anion (19.12). 

Coagulation - This is a process for separation of PTFE (polytetrafiuoroethylene) solids from its dispersion. The emulsion or dispersion containing this polymer (dispersion polymerization) has to be broken (destabilized) in order to cause precipitation of PTFE particles. Dilution to reduce solids concentration below 20%, addition of water-soluble organic compounds, and addition of soluble inorganic salts are the common techniques used to break PTFE emulsions. 

There are two main aims in applications of PTLC in organic geochemistry (1) assessment of the bulk group composition of soluble organic matter by its fractionation and (2) separation of a particular selected group of compounds with geochemical meaning. The important factor in technique selection should be the repeatability of... 

Distillation is a suitable technique for the isolation of volatile organic compounds from liquid samples or the soluble portion of solid samples [24,27-30]. The physical basis of separation depends on the distribution of constituents between the liquid mixture and the vapor in equilibrium with that mixture. The more volatile constituents are concentrated in the vapor phase, which is collected after condensation. The effectiveness of the separation is dependent on the physical properties of the... 

Ion-pairing techniques are also used to separate weak acids and bases but additionally they find application in the separation of other ionic compounds. The methods originated in the field of solvent extraction. An ionised compound (A+q) that is water soluble can be extracted into an organic solvent by using a suitable counter ion (B q) to form an ion-pair, according to the equation ... 

Reducing the number of fluorines on the fluorous tag also provides a general solution to the reaction solvent problem as the number of fluorines is reduced, the solubility in organic solvents tends to go up. Of course, the solubility in fluorous solvents tends to go down at the same time, and the residual tag must strike a balance between too many fluorines (low solubility in organic solvents) and too few fluorines (cannot easily be separated from organic compounds). However, thanks to the technique of fluorous solid-liquid extraction, there is surprisingly broad latitude here. 

The technique of solvent extraction has long been used in organic chemistry for concentrating and purifying some substances. In the case of organic compounds, the separation process is simple, in many cases being based only on differences in the solubility of the compounds in different solvents. 

Isolation of the feeding factor for M. sexta was a far more difficult task. Whereas 2-tridecanone is a simple, stable molecule soluble in organic solvents, the feeding factor is water soluble, occurs at trace levels in plant tissue, and is easily hydrolyzed under mild alkaline or acidic conditions with subsequent loss of biological activity. The isolation of such a compound was a formidable obstacle requiring a departure from the more classical approach of hydrolysis or chemical derivatization followed by isolation of the lipophilic product. The necessity that pure substance be isolated with retention of biological activity required some basic research in modem separation techniques to develop a suitably mild isolation strategy. 

When the separation procedures described in detail above are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Section 2.26) or a mixture of non-volatile solids may frequently be separated by making use of the differences in solubilities in inert solvents. The progress of such separations may be monitored by application of the various chromatographic techniques detailed in Section 2.31, or indeed these techniques may be employed on the preparative scale for effecting the separation itself (e.g. flash chromatography, p. 217). The techniques of counter current distribution, fractional crystallisation or fractional sublimation (Section 2.21) may also be employed where appropriate. 

Calystegines, in common with other classes of polyhydroxy alkaloids, are highly water-soluble and cannot be isolated by conventional alkaloid separation techniques which employ add-base pardoning methods with non-hydroxylic organic solvents [20], Ion-exchange chromatography is therefore extensively employed for separation of the alkaloid fraction from neutral and addic compounds present in the extract [21,22]. 

Various physical and chemical properties useful to understand the solubility of RTlLs have been smdied, among which dielectric properties are crucially important. However, there are, at least, two problems in the study of dielectric properties. One problem concerns the experimental techniques and the other, the scientific aspects. Furthermore, there arises a basic question about how the permittivity derives, assunting that ILs are homogeneous. This is related to the interconnection polar to non-polar domains as predicted by computer simulation and evidenced by experiments. In addition, anomalous phase separation behaviour has been reported for binary systems of RTILs with some organic compounds. 

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