Evaporation of the solvent

The theory of the action of drying agents has been considered in Section 1,20. We are now concerned with the practical methods for the removal of water from organic solids and liquids and from solutions of 

Small quantities of solids may be spread upon unglazed porcelain plates. The chief disadvantage of this method is the comparatively high cost of the porous plates, since they cannot be conveniently cleaned nor can the same area be used for different substances. However, a plate may be broken and used for small amounts of material. 

The best method for removing water (and also solvents of relatively low boiling point) adhering to solids is drying under reduced pressure. A vacuum desiccator is used for this purpose several forms are shown in Fig. II, 38, 1. These are fitted with the exception of ( ) either with 

When exhausting desiccators, a filter flask trap (see Fig. 77, 19, 2) should always be inserted between the desiccator and the pump. The vacuum should be applied gradually and should not exceed about 50 cm. of mercury for models (a), (6) and (d). These desiccators may withstand lower pressures, but it is generally considered unsafe to exhaust below this pressure unless the precaution be taken of surrounding the desiccator by a cage of fine-mesh steel wire collapse of the desiccator will then do no harm.J Models (c) and (e) may be exhausted to about 20 mm. of mercury a steel wire cage must be provided for this low pressure. 

II This dual tilling permits the absorption of both acid smd basic vapours which may be evolved. Thus an amine hydrochloride, which has been recrystallised from concentrated hydrochloric acid, may be readily dried in such a desiccator. If concentrated sulphuric acid alone were used, so much hydrogen chloride would be liberated that tlie pressure inside the desiccator would rise considerably, smd the rate of drying would be reduced. With sodium hydroxide present, however, the hydrogen chloride is removed, smd tho water is absorbed in the normal manner by the reagents but largely by the acid. 

It is frequently necessary to concentrate a filtrate in order to obtain a farther crop of crystals, or it may be necessary to concentrate a solution to a smaUer volume. If the solvent is water and the substance is not volatile in steam, simple evaporation on a large dish on a steam or water 

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Another method for the hydroxylation of the etliylenic linkage consists in treatment of the alkene with osmium tetroxide in an inert solvent (ether or dioxan) at room temperature for several days an osmic ester is formed which either precipitates from the reaction mixture or may be isolated by evaporation of the solvent. Hydrolysis of the osmic ester in a reducing medium (in the presence of alkaline formaldehyde or of aqueous-alcoholic sodium sulphite) gives the 1 2-glycol and osmium. The glycol has the cis structure it is probably derived from the cyclic osmic ester ... 

Endo-exo ratios of the micelle-catalysed reactions have been determined by adding 0.25 mmol of 5.1c and 0.5 mmol of 5.2 to a solution of 5 mmol of surfactant and 0.005 mmol of EDTA in 50 ml of water in carefully sealed 50 ml flasks. The solutions were stirred for 7 days at 26 C and subsequently freeze-dried. The SDS and CTAB containing reaction mixtures were stirred with 100 ml of ether. Filtration and evaporation of the ether afforded the crude product mixtures. Extraction of the Diels-Alder adducts from the freeze-dried reaction mixture containing C12E7 was performed by stirring with 50 ml of pentane. Cooling the solution to -18 C resulted in precipitation of the surfactant. Filtration and evaporation of the solvent afforded the adduct mixture. Endo-exo ratios... 

Note S. The glassware used for drying and evaporation of the solvent should be... 

To a mixture of 25 ml of water and 3 ml of 95% sulfuric acid were added 40 ml of DMSO. The mixture was cooled to 10°C and 0.20 mol of l-ethoxy-l,4-hexadiyne (see Chapter III, Exp. 51) was added with vigorous stirring in 15 min. During this addition, which was exothermic, the temperature of the mixture was kept between 20 and 25 0. After the addition stirring was continued for 30 min at 3S C, then 150 ml of water were added and six extractions with diethyl ether were carried out. The combined extracts were washed with water and dried over magnesium sulfate. Evaporation of the solvent in a water-pump vacuum, followed by distillation through a 25-cm... 

A mixture of 0.15 mol of 1-methylthio-1,2-pentadiene (see Chapter IV, Exp. 26), 100 ml of water, 40 ml of methanol (note 1) and 0.18 mol of NalO., was vigorously stirred. The temperature rose gradually, but was kept between 30 and 35°C by occasional cooling. After 1.5 h 500 ml of water were added to the white suspension and ten extractions with 30-ml portions of chloroform were carried out. The extracts were dried (without previous washing) over magnesium sulfate. Evaporation of the solvent in a water-pump vacuum (the last traces at 0.5-1 mmHg) gave the... 

Evaporation of the solvent gave the product as bright yellow crystals (27.6 g) in 74% yield. 

A solution of benzyl indole-5-carboxylate(1.0g, 3.98 mmol) and methyl 4-(bro-momethyl)-3-methoxybenzoate (2.06 g, 7.97 mmol) in dry DMF (10 ml) was heated at 80°C for 24 h. The reaction solution was cooled, poured into water (100 ml) and the product extracted with EtOAc (3 x 75 ml). The extract was washed with water and brine and dried over MgSO, . The product was obtained by evaporation of the solvent and purified by chromatography on silica gel using 1 4 EtOAc/hexane for elution. The yield was 1.11 g (32%) and some of the indole (30%) was recovered unreacted. 

Benzoylindoline (223 g, l.Omol) was dissolved in CH2CI2 (2.23 1) and Mn02 (261 g, 3.0 mol. Diamond"Shamrock grade M) was added. The mixture was heated at reflux and agitated for 18 h. The reaction mixture was filtered and the solid washed with hot CH2CI2 (200ml). Evaporation of the solvent left 7-benzoylindole. 

Potassium hydride (1 eq.) was washed with hexanes and suspended in anhydrous ether at 0°C. 7-Bromoindole was added as a solution in ether. After 15 min, the solution was cooled to — 78°C and t-butyllithium (2 eq.) which had been precooled to — 78°C was added by cannula. A white precipitate formed. After 10 min DMF (2 eq.) was added as a solution in ether. The reaction mixture was allowed to warm slowly to room temperature and when reaction was complete (TLC) the suspension was poured into cold 1 M H3PO4. The product was extracted with EtOAc and the extract washed with sat. NaHCOj and dried (MgS04). The product was obtained by evaporation of the solvent and purified by chromatography on silica gel (61% yield). 

The addition of (BuO)3B and PhsP to an aqueous solution of 2-aminothiazole mixed to 15% impure substances, followed by evaporation of the solvent and sublimation of the residue, provides 97.8% pure 2-aminothiazole (1573). 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

Although simple solutions can be examined by these electrospray techniques, often for a single substance dissolved in a solvent, straightforward evaporation of the solvent outside the mass spectrometer with separate insertion of the sample is sufficient. This situation is not true for all substances. Peptides, proteins, nucleotides, sugars, carbohydrates, mass organometallics, and many... 

More recent versions of this type of probe include some refinements, such as the provision of a wick to aid evaporation of the solvent and matrix from the probe tip (Figure 13.5). Such improvements have allowed greater flow rates to be used, and rates of 1 to 10 ml/min are possible. For these sorts of low flow rates, minibore LC columns must be employed. 

In some cases, it may be convenient to dissolve a solid and present it for analysis as a solution that can be nebulized and sprayed as an aerosol (mixed droplets and vapor) into the plasma flame. This aspect of analysis is partly covered in Part B (Chapter 16), which describes the introduction of solutions. There are vaporization techniques for solutions of solids other than nebulization, but since these require prior evaporation of the solvent, they are covered here. There are also many solid samples that need to be analyzed directly, and this chapter describes commonly used methods to do so. 

In production, anhydrous formaldehyde is continuously fed to a reactor containing well-agitated inert solvent, especially a hydrocarbon, in which monomer is sparingly soluble. Initiator, especially amine, and chain-transfer agent are also fed to the reactor (5,16,17). The reaction is quite exothermic and polymerisation temperature is maintained below 75°C (typically near 40°C) by evaporation of the solvent. Polymer is not soluble in the solvent and precipitates early in the reaction. 

Extrusion Processes. Polymer solutions are converted into fibers by extmsion. The dry-extmsion process, also called dry spinning, is primarily used for acetate and triacetate. In this operation, a solution of polymer in a volatile solvent is forced through a number of parallel orifices (spinneret) into a cabinet of warm air the fibers are formed by evaporation of the solvent. In wet extmsion, a polymer solution is forced through a spinneret into a Hquid that coagulates the filaments and removes the solvent. In melt extmsion, molten polymer is forced through a multihole die (pack) into air, which cools the strands into filaments. 

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

Adhesives. Contact adhesives are blends of mbber, phenoHc resin, and additives suppHed in solvent or aqueous dispersion form they are typically appHed to both surfaces to be joined (80). Evaporation of the solvent leaves an adhesive film that forms a strong, peel-resistant bond. Contact adhesives are used widely in the furniture and constmction industries and also in the automotive and footwear industries. The phenoHc resins promote adhesion and act as tackifiers, usually at a concentration of 20—40%. In solvent-based contact adhesives, neoprene is preferred, whereas nitrile is used in specialty appHcations. The type and grade of phenoHc resin selected control tack time, bond strength, and durabiHty. 

Using hexamethylphosphoramide as the solvent, only the second reaction occurs. Disilane also reacts with potassium in 1,2-dimethoxyethane to form KS1H3, although S1H4 and nonvolatile polysHanes are also produced (28,31). Pure crystalline KSiH prepared from SiH and potassium in 1,2-dimethoxyethane has been obtained by slow evaporation of the solvent. WhenHquid ammonia is used as the solvent, only a small fraction of SiH is converted into metal salt most of the SiH undergoes ammonolysis (32). 

Because the evaporation of the solvent is an endothermic process, heat must be suppHed to the system, either through conduction, convection, radiation, or a combination of these methods. The total energy flux into a unit area of coating, is the sum of the fluxes resulting from conduction, convection, and radiation (see Heat exchange technology, HEAT thansfer). 

Film thickness is an important factor iu solvent loss and film formation. In the first stage of solvent evaporation, the rate of solvent loss depends on the first power of film thickness. However, iu the second stage when the solvent loss is diffusion rate controlled, it depends on the square of the film thickness. Although thin films lose solvent more rapidly than thick films, if the T of the dryiug film iucreases to ambient temperature duriug the evaporation of the solvent, then, even iu thin films, solvent loss is extremely slow. Models have been developed that predict the rate of solvent loss from films as functions of the evaporation rate, thickness, temperature, and concentration of solvent iu the film (9). 

Methyl 2-hydroxy-2-carbomathoxy-4-haptenoate (3), A solution of dimethyl mesoxalate 2 (1.46 g, 10 mmol] and 1-pentene 2 (0 70 g, 10 mmol) in CH2CI2 was heated at 140 C lor 16 h. The solvent was reirroved and the residue distilled under reduced pressure. The fraction collected between 90 and 105°C (0 5 torr) was diluted with Et20 (20 mL), washed with water and dned The residue after evaporation of the solvent, gave on dislillation 1 55 g of 3 (62%), bp 89-90 C (0 2 torr). 

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