Removal of Solvents

Removal of water from gases may be by physical or chemical means, and is commonly by adsorption on to a drying agent in a low-temperature trap. The effectiveness of drying agents depends on the vapour pressure of the hydrated compound - the lower the vapour pressure the less the remaining moisture in the gas. 

For further useful information on mineral adsorbents and drying agents, go to the SigmaAldrich website, under technical library (Aldrich) for technical bulletin AL-143. 

P2O5 BaO Mg(C104)2, CaO, MgO, KOH (fused), cone H2SO4, CaS04, AI2O3 KOH (pellets), 

Ba(C104)2, ZnCl2, ZnBr2 CaCl2 (technical) CUSO4 Na2S04, K2CO3. 

Drying agents that combine irreversibly with water include the alkali metals, the metal hydrides (discussed in Chapter 2), and calcium carbide. 

Traces of water can be removed from solvents such as benzene, 1,2-dimethoxyethane, ethyl ether, CH2CI2, pentane, 

Where substances are sufficiently stable, removal of solvent from recrystallised materials presents no problems. The crystals, after filtering at the pump (and perhaps air-drying by suction), are heated in an oven above the boiling point of the solvent (but below this melting point of the crystals), followed by cooling in a desiccator. Where this treatment is inadvisable, it is still often possible to heat to a lower temperature under reduced pressure, for example in an Abderhalden pistol. This device consists of a small chamber which is heated externally by the vapour of a boiling solvent. Inside this chamber, which can be evacuated by a water pump or some other vacuum pump, is 

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Alkvl Azides from Alkyl Bromides and Sodium Azide General procedure for the synthesis of alkyl azides. In a typical experiment, benzyl bromide (360 mg, 2.1 mmol) in petroleum ether (3 mL) and sodium azide (180 mg, 2.76 mmol) in water (3 mL) are admixed in a round-bottomed flask. To this stirred solution, pillared clay (100 mg) is added and the reaction mixture is refluxed with constant stirring at 90-100 C until all the starting material is consumed, as obsen/ed by thin layer chromatographv using pure hexane as solvent. The reaction is quenched with water and the product extracted into ether. The ether extracts are washed with water and the organic layer dried over sodium sulfate. The removal of solvent under reduced pressure affords the pure alkyl azides as confirmed by the spectral analysis. ... 

For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

The wastewater produced in this process consists mostly of water used in cleanup and propellant conveyance and sorting operations. Techniques such as the use of activated carbon and biological treatment are being investigated for the removal of solvents and dissolved organic compounds (143). 

Methods for isolation of the product polycarbonate remain trade secrets. Feasible methods for polymer isolation include antisolvent precipitation, removal of solvent in boiling water, spray drying, and melt devolatization using a wiped film evaporator. Regardless of the technique, the polymer must be isolated dry, to avoid hydrolysis, and essentially be devoid of methylene chloride. Most polycarbonate is extmded, at which point stabiUzers and colors may be added, and sold as pellets. 

Concentration. The concentration of fmit juice requites removal of solvent (water) from the natural juice. This is commonly done by evaporation, but the derived juices may lose flavor components or undergo thermal degradation during evaporation. In freeze concentration, solvent is crystallized (frozen) in a relatively pure form to leave behind a solution with a solute concentration higher than the original mixture. Significant advantages in product taste have been observed in the appHcation of this process to concentration of certain fmit juices. 

The final purification steps are responsible for the removal of the last traces of impurities. The volume reduction in the earlier stages of the separation train are necessarv to ensure that these high-resolution operations are not overloaded. Generally, chromatograjmy is used in these final stages. Electrophoresis can also be used, but since it is rarely found in process-scale operations, it is not addressed here. The final product preparation may require removal of solvent and drying, or lyophihzation, of the product. 

Inadequate removal of solvent leading to unwanted reaction in downstream equipment or in subsequent steps. 

In a 500-ml. round-bottomed flask fitted with a reflux condenser are placed 16.2 g. (0.08 mole) of dry a-naphthylthiourea (Note 1) and 180 ml. of redistilled chlorobenzene. The flask is heated at the reflux temperature by means of an electric heating mantle. Evolution of ammonia begins almost at once, and all of the solid dissolves after 30-45 minutes. The solution is maintained at reflux for 8 hours (Note 2) and then evaporated on a steam bath at water-pump pressure to remove all of the chlorobenzene. The residue crystallizes on cooling and is extracted with four 30-ml. portions of boUing hexane (Note 3). Removal of solvent from the combined hexane extracts affords pale yellow crystals of naphthyl isothiocyanate, m.p. 58-59°. The yield is 12.7-13.0 g. (86-88%). Recrystallization from hexane (9 ml. of hexane for 1 g. of solute) gives colorless needles, melting point unchanged (Note 4). 

Cortisone. 17a,21-Cyclopentylidenedioxypregn-4-ene-3,ll,20-trione(0.25g) in methanol (5 ml) is refluxed with a few drops of 1 JV hydrochloric acid for 15 min. Partial removal of solvent and dilution with water yields cortisone 0.2 g, 97% mp 217-220°. 

After removal of solvent by distillation under reduced pressure to 55°C/20 mm, the residue is 23.0 grams crude base (95% theory) as a pale yellow liquid. A sample of the crude base distills with some decomposition at 105° to 112°C/0.8 mm. 

The eluate was titrated to pH 7 with 0572N NaOH (63 ml). Removal of solvent left a gum, which was boiled with methanol (400 ml) for 20 minutes. The solid insoluble inorganic phosphate was filtered off and washed with methanol (200 ml). The slightly cloudy filtrate was filtered again, and evaporated to dryness in vacuo. The residual gum dissolved readily in water (40 ml) and on addition of acetone (600 ml) to the solution a mixture of sodium salts of hydrocortisone 21 -phosphate separated as a white solid. This was collected after 2 days, washed with acetone and dried at 100°C/0.1 mm/2 hrto constant weight. Yield4.45g. 

Solid PtH2X2(PEt3)2 (X = Cl, Br) is isolated by removal of solvent at -20°C the order of stability is Cl > Br > I and solutions decompose at room temperature, eliminating H2. Monohydrides PtHX3(PR3)2 are less stable [178],... 

Removal of solvent from the extracts leaves a residue that is purified by dry-column chromatography.2 The residue is dissolved in 40 ml. of acetone in a 300-ml., round-bottomed flask, 30 g. of silica gel (Note 8) is added, and the acetone is removed with a rotary evaporator. The resulting solid mixture is placed on top of 360 g. of dry silica gel (Note 8) packed in flexible nylon tubing (Note 9), and the column is developed with 420 ml. of 10 1 (vjv) benzene-acetone. Approximately 150 ml. of solvent drips from the bottom of the column toward the end of development, and this eluent is collected in 25-ml. fractions and checked for product by thin layer chromatography (Note 10). The column itself is then cut into 2-cm. sections, the silica gel in each section is eluted with three 25-ml. portions of ethyl acetate, and the eluent from each section is analyzed by thin-layer chromatography (Note 10). Combination of all the product-containing fractions yields 1.2-1.5g. (40-47%) of the benzylated compound as an oil, n 1.6083 (Notes 11 and 12). 

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