Sublimation water aspirator

If you re going to pull a vacuum in the sublimator, do it now. If the vacuum source is a water aspirator, put a water trap between the aspirator and the sublimator. Otherwise you may get depressed if, during a sudden pressure drop, water backs up and fills your sublimator. Also, start the vacuum slowly. If not, air, entrained in your solid, comes rushing out and blows the crude product all over the sublimator, like popcorn. 

Better yields are obtained at low pressures (preferably below 5 mm.) because of more efficient sublimation of coumalic acid. The submitters report that a water aspirator could be used with crude (unrecrystallized) coumalic acid to avoid damage to the vacuum pump by untrapped corrosive vapors, and that yields of a-pyrone averaged 45% in this modification. The checkers used a mechanical pump with an efficient sodium hydroxide trap in all runs. 

To a solution of cycloheptatriene (15.0 mL, 12.0 g, 130 mmol) and freshly distilled diglyme (60 mL) was added Cr(CO)g (11.1 g, 50.4 mmol), and the reaction mixture was heated at reflux for 4 h. The Cr(CO)g, that sublimed into the condenser, was periodically scraped back into the reaction flask. The reaction mixture was allowed to cool to room temperature and most of the diglyme was removed by distillation at reduced pressure (water-aspirator). Purification of the resultant viscous material via flash-chromatography (silica gel hexane/ether 10 1) afforded 7.48 g (65%) of 5.5a as a red solid, mp 129-130 °C. 

Two common types of sublimation apparatus are shown in Figure 2.57. In each case there is a chamber that may be evacuated using a water aspirator, house vacuum, or vacuum pump and a cold-finger in the center of the vacuum chamber to provide a surface upon which the sublimed crystals may form. The cold-finger is cooled by water (Fig. 2.57a) or another cooling medium such as ice/water or dry ice/acetone (Fig. 2.57b). A simple sublimation apparatus may be assembled... 

The 15-mL centrifuge tube and adapter are placed in the flask so that the tip of the centrifuge tube is about 3 to 8 mm above the bottom of the flask (Fig. 13). The flask is clamped with a three-prong clamp. Many substances sublime at atmospheric pressure (see following experiment). For vacuum sublimation, the sidearm of the flask leads to an aspirator or vacuum pump for reduced pressure sublimation. The vacuum source is turned on and the centrifuge tube filled with ice and water. The ice is not added before applying the vacuum so atmospheric moisture will not condense on the tube. 

Why apply the vacuum to the sublimation system before you turn on the cooling water to the water condenser 5-50. Why place a water trap in the vacuum line when using an aspirator to obtain the vacuum ... 

When no more caffeine will sublime onto the cold finger, remove the heat, shut off the aspirator and the cooling water to the cold finger, and allow the apparatus to cool to room temperature under reduced pressure. Once cooled, carefully vent the vacuum and return the system to atmospheric pressure. Carefully remove the cold finger from the apparatus. 

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