Evaporation of solvent

during preparative organic work, substances have very often to be isolated from dilute solution, this operation is one of the commonest. Ether is distilled from the steam or water bath through 

In order to avoid loss and conflagration due to the volatility of the ether, a filter flask, connected to the condenser with a cork, is used as receiver, and a length of rubber tubing hanging down below the bench is attached to the side tube of the receiver for safety. 

Naked flames must never be allowed on the bench when work with ether or any inflammable solvent is in progress. 

Small amounts of easily evaporated liquids can be removed directly from a test tube or a small flask heated on the water bath. On each occasion the test tube is filled to a depth of 2-3 cm. only, and is replenished from time to time during the boiling on the water bath it must be shaken continuously or stirred with a thin glass rod. All preliminary tests with solutions are carried out in this simple way previous to an examination of the residues. For the latter purpose solutions of substances liable to decompose are left exposed to the air in watch-glasses or small crystallising basins so that the solvent may evaporate. 

When it is necessary to remove completely such solvents as alcohol or benzene it is not sufficient to use the steam or water bath alone, because the boiling point rises higher and higher as the concentration of the solution increases even ether causes difficulties. In this case, when the solvent ceases to distil, an oil bath or, more frequently, a vacuum is used. It is sufficient to mount a capillary tube, place the flask in a deep porcelain basin or in an enamelled basin maintained at a moderate temperature, and connect directly 

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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. 

The particle-beam interface (LINC) works by separating unwanted solvent molecules from wanted solute molecules in a liquid stream that has been broken down into droplets. Differential evaporation of solvent leaves a beam of solute molecules that is directed into an ion source. 

Having removed the larger droplets, it may remain only to encourage natural evaporation of solvent from the remaining small droplets by use of a desolvation chamber. In this chamber, the droplets are heated to temperatures up to about 150 C, often through use of infrared heaters. The extra heat causes rapid desolvation of the droplets, which frequently dry out completely to leave the analyte as small particles that are swept by the argon flow into the flame. 

The rate of evaporation of solvent from droplets can be increased by blowing a drying gas across the stream. Nitrogen is frequently used as the drying gas. 

Evaporation of solvent from a spray of electrically charged droplets at atmospheric pressure eventually yields ions that can collide with neutral solvent molecules. The assemblage of ions formed by evaporation and collision is injected into the mass spectrometer for mass analysis. 

To assist evaporation of solvent, the argon stream carrying the aerosol can be passed through a heated tube called a desolvation chamber, operated at temperatures up to about 150°C. 

Precipita.tlon. An ink may also be caused to dry by precipitation of its binder rather than by evaporation of solvent. This can be accompHshed by a dding a diluent, such as water in the form of steam or humidity, to a hygroscopic solvent ink system, which causes the solubiHty of the resin in the ink film to decrease sharply and causes it to precipitate when its tolerance for the diluent is reached. Eurther drying is accompHshed by absorption of the solvents into the stock and then by evaporation. Another form of precipitation setting is the quick-set mechanism. This utilizes resins held in solution in a relatively poor solvent, by means of a small amount of an exceUent solvent (called a sweetener) blended with it. When the ink film is printed on the paper, an amount of the solvents is absorbed reducing the content of the sweetener solvent to a point which causes the resins to precipitate and the ink to set. 

Infrared and Microwave Inks. These ate inks which have been formulated to absorb these radiant energies. The energy causes the inks to heat and dry through the partial evaporation of solvent. Absorption of the ink into a porous substrate can also be part of the overall drying mechanism with these inks. They have not found wide commercial success due to the variabiHty of the it absorption with ink color and the energy inefficiency of microwave systems in drying nonwater-based inks. 

The heHcoidal stmcture of such Hquid crystals can be carried to the soHd state by cross-linking (119,120) or by careful evaporation of solvent (121,122). Underivatized ceUulose can also form ordered mesophases (123,124), and gel films precipitated from lithium chloride—dimethyl acetamide retain some mesophase stmcture (122). 

The reactions of oxiranes with thiocyanate ion or with thiourea are usually done in homogeneous solution in water, alcohols or alcohol-acetic acid. The use of silica gel as a support for potassium thiocyanate in toluene solvent is advantageous for the simple work-up (filtration and evaporation of solvent) (80JOC4254). A crown ether has been used to catalyze reactions of potassium thiocyanate. 

A slight variation of the above method accounts for increases in the solvent content of the gas stream between the inlet and the outlet of the tower and assumes that the evaporation of solvent tends to cool the hquid. This procedure offsets a part of the temperature rise that... 

The mixture is distilled until most of the ether has been removed and then refluxed for 8 hr. Ethyl acetate is added to decompose the excess reagent and concentrated aqueous sodium sulfate is then added until the precipitate begins to adhere to the sides of the flask. Finally ca. 100 g of solid sodium sulfate is added, the salts are removed by filtration and washed well with dioxane. Evaporation of the solvent gives a solid residue which is dissolved in 60 ml of chloroform and shaken with 3.5 g of manganese dioxide for 16 hr. Subsequently another 3.5 g of manganese dioxide is added and shaking continued for a further 16 hr. The solid is removed by filtration and washed well with hot chloroform. Evaporation of solvent and crystallization of the residue from acetone-hexane affords 0.51 g (72%) of 17a-hydroxy-17jff-ethylandrost-4-en-3-one mp 145-148°. 

The use of HMDS (ca. 1.5 mmol) and saccharin (0.01 mmol) per mmol of substrate in refluxing dichloromethane or chloroform has been recommended (5) for easy silylation of carboxylic acids, including azetidin-2-one-4-carboxyIic acids. Clear solutions result, i.e., no ammonium salts are present at completion of the reaction, and consequently the silyl esters can be obtained by direct distillation, or merely by evaporation of solvent. 

The evaporation of solvents under reduced pressure should be performed carefully with vigorous stirring at room temperature. An oil pump protected with a dry ice trap and equipped with a manometer is used. Initially the pressure should be adjusted to prevent excessive foaming it is reduced progressively to approximately 5 mm. 

A mixture of fluorene (1.5 g, 9 mmol), alumina-supported copper(II) bromide (30 g), and carbon tetrachloride (80 ml) was placed in a 200 ml round-bottomed flask and stirred with a Teflon-coated magnetic stirring bar at 80°C for 5 h. The product mixture was filtered, and the spent reagent was washed with carbon tetrachloride (30 ml). Evaporation of solvent from the combined filtrate under reduced pressure yielded 2.84 g (97 %) of 2.7-dibromofluorene as a pale yellow solid having iH NMR and IR spectra identical with those of an authentic sample, mp 157-159°C (lit. mp 162-163°C (ref. 17)). The purity was > 96 % (GC). 

The terminology of L-B films originates from the names of two scientists who invented the technique of film preparation, which transfers the monolayer or multilayers from the water-air interface onto a solid substrate. The key of the L-B technique is to use the amphiphih molecule insoluble in water, with one end hydrophilic and the other hydrophobic. When a drop of a dilute solution containing the amphiphilic molecules is spread on the water-air interface, the hydrophilic end of the amphiphile is preferentially immersed in the water and the hydrophobic end remains in the air. After the evaporation of solvent, the solution leaves a monolayer of amphiphilic molecules in the form of two-dimensional gas due to relatively large spacing between the molecules (see Fig. 15 (a)). At this stage, a barrier moves and compresses the molecules on the water-air interface, and as a result the intermolecular distance decreases and the surface pressure increases. As the compression from the barrier proceeds, two successive phase transitions of the monolayer can be observed. First a transition from the gas" to the liquid state. 

To the general public, the term plastic has become s Tionymous with polymer. More precisely, a plastic is the type of pol Tner that hardens on cooling or on evaporation of solvent, allowing it to be molded or extruded into specific shapes or spread into thin films. 

FIGURE 11.20 Densitogram of alkaloid fraction from Fumaria officinalis herb extract chromatographed in system silica/PrOH -i- AcOH -i- CHjClj (4 1 5) (a) fraction introduced with applicator with evaporation of solvent (b) fraction introduced from the edge of the layer with the eluent distributor. 

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