Solvents, boiling points removal

Diacetone alcohol has been recently claimed as a solvent for polyimide preparation. According to the patent,107 ODA was dissolved in the solvent and then PMDA added to the solution. The patent claims good storage stability and easy removal of the solvent (boiling point 166°C). 

The maximum feasible operating temperature must be a sensible margin below the solvent boiling point, to enable insoluble contaminants to be removed from a pre-dissolved solution of the Crude API, see section 1.1.4. 

Reactions above room temperature usually require modifications to the standard equipment set-up. In some instances the reaction can be performed in a sealed tube, usually made of thick-walled glass. The reaction mixture is placed in the tube which is then sealed, placed in an oven and heated to the appropriate temperature (Fig. 9.22). After the reaction is complete, the tube is cooled, opened and the contents removed. Such a technique is employed when temperatures in excess of the solvent boiling point are required, or for reactions involving extremely volatile compounds. 3. R.E. Rondeau, J. Chem. Eng. Data, 1966, 11, 124. 

As an additional precaution for the very delicate CB determinations a nitrogen gas flow of 10 mL/min through the condenser should be maintained during extraction. The flask is then cooled to approximately 0 °C, preferably in a freezer, before the solvent is removed by rotary evaporation at 0.1 hPa. TTie boiling chips stay in the flask, because removal is a potential source of contamination. At the low temperature of the solvent they will not induce boiling. A layer of ice growing on the outside of the flask retards the evaporation of the solvent (boiling point 76 °C at atmospheric pressure). It may be removed by immersion in a water-bath at 30 °C after 15 min. It is important to allow for this delay as aqueous acetonitrile tends to bump or boil explosively. 

Solvents with boiling points below 90-95°. A steam bath or water bath should be employed. Alternatively, the apparatus of Fig. 77,13, 3, but with a Alter flask as receiver, may be used the end of the rubber tubing attached to the tubulure is either placed in the sink or allowed to hang over the bench. If a distillation is ultimately to be conducted in the flask from which the solvent is removed, the apparatus depicted in Fig. 77,13, 4 is recommended the distilling flask may be replaced by a Claisen flask or a Claisen flask with fractionating side arm, particularly if the subsequent distillation is to be conducted under diminished pressure. 

It must be borne in mind that in spite of the fact that the solvents have normal boiling points below 90-95°, they cannot always be completely removed by heating on a steam or water bath when they form part of mixtures with less-volatile liquids. Simple distillation may lead to mixtures with higher boiling points than the individual solvents, so that separation of the latter may not be quite complete. In such cases the distillation should be completed with the aid of an air bath (Fig. 77,5,3) or an oil bath the Are hazard is considerably reduced since most of the solvent will have been removed. 

The most desirable characteristics of a solvent for recrystalhsation are (a) a high solvent power for the substance to be purified at elevated temperatures and a comparatively low solvent power at the laboratory temperature or below (6) it should dissolve the impurities readily or to only a very small extent (c) it should yield well-formed crystals of the purified compound and (d) it must be capable of easy removal from the crystals of the purified compound, i.e., possess a relatively low boiling point. It is assumed, of course, that the solvent does not react chemically with the substance to be purified. If two or more solvents appear to be equally suitable for the recrystallisation, the final selection will depend upon such factors as ease of manipulation, inflammability and cost. 

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... 

Place a mixture of 25 5 g. of n-valerio acid (Sections 111,83 and 111,84), 30 g. (37 -5 ml.) of dry n-propyl alcohol, 50 ml. of sodium-dried benzene and 10 g. (5-5 ml.) of concentrated sulphuric acid in a 250 ml. round-bottomed flask equipped with a vertical condenser, and reflux for 36 hours. Pour into 250 ml. of water and separate the upper layer. Extract the aqueous layer with ether, and add the extract to the benzene solution. Wash the combined extracts with saturated sodium bicarbonate solution until effervescence ceases, then with water, and dry with anhydrous magnesium sulphate. Remove the low boiling point solvents by distillation (use the apparatus of Fig. II, 13,4 but with a Claisen flask replacing the distilling flask) the temperature will rise abruptly and the fi-propyl n-valerate will pass over at 163-164°. The yield is 28 g. 

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... 

Butyl stearate [123-95-5] M 340.6, m 26.3 , d 0.861. Acidic impurities removed by shaking with 0.05M NaOH or a 2% NaHC03 soln, followed by several water washes, then purified by fractional freezing of the melt and fractional crystn from solvents with boiling points below 100°. 

Toluene is commonly used. It can be dried by molecular sieves or direct distillation from calcium hydride into the reaction flask. Solvent stored over calcium hydride for several days is usually sufficiently dry to decant directly into the reaction flask, but distillation gives more consistent results. Any solvent with a boiling point sufficiently high to melt sodium is satisfactory. The submitters have also used methyl-cyclohexane and xylene in acyloin condensations. After the sodium is dispersed, the high-boiling solvent can be removed and replaced with anhydrous ether (as noted by the submitters) or can be retained and used in combination with ether (checkers). 

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