Sublimation cold-finger condenser

Sublimation.—This is a valuable means for the purification of many organic substances and is admirably adapted for small scale work since losses are generally small. Sublimation may be carried out either at atmospheric pressure or under reduced pressure. The apparatus shown in Fig. XII, 2, 20 is simple and effective the sublimate is collected on the cold finger condenser. 

The crude product is purified by decolorization with charcoal and recrystallization from 100-150 ml. of diisopropyl ether to give about 19-21 g. (27-30%) of colorless needles, m.p. 70-72°. This material is sufficiently pure for most purposes. Further purification may be achieved by further recrystaUization from diisopropyl ether and/or sublimation at 60° (0.1 mm.) onto a cold-finger condenser to give material melting at 73-74° (Note 9). 

Thz larger sublimator (Fig. 11.15a) consists of a tube with a side arm which fitted with a cold-finger condenser, and it is used as follows ... 

Put some vacuum grease on the joint of the cold-finger condenser and fit it into the sublimator (there should be a gap of approximately 1cm between the solid and the condenser). 

Figure 115 shows three forms of sublimation apparatus. Note all the similarities. Cold water goes in and down into a cold finger on which the vapors from the crystals condense. The differences are that one is larger and has a ground glass joint. The sidearm test tube with cold-finger condenser is much smaller. To use them,... 

For simplicity, we have chosen long Schlenk tubes (Figure 3), without the conventional cold finger condenser. Deposition of crystals takes place on the cold walls of the sublimation flask 1-2 cm above the conical reflector. 

The checker finds that upon distillation the product crystallizes directly on the cold finger condenser (sublimation apparatus) and can be removed by scraping under anhydrous conditions. 

Sublimations can be carried out under pressure or vacuum with equal ease. The technique is useful only if impurities associated with the component being sublimed have a substantially different vapor pressure at the sublimation temperature. It is often used as a final purification step in the preparation of an analytical sample. A simple but effective apparatus is shown in Fig. 2-6 the sample is placed in the bottom of the outside tube, it is heated, and crystals of sublimate collect on the large cold-finger condenser. A simple tube such as that illustrated in Fig. 2-7 can be used for... 

Put the crude solid into the bottom of the sublimator. How much crude solid This is rather tricky. You certainly don t want to start with so much that it touches the cold finger. And since as the purified solid condenses on the cold finger it begins to grow down to touch the crude solid, there has to be really quite a bit of room. I suggest that you see your instructor, who may want only a small amount purified. 

Sublimation differs from ordinary distillation because the vapour condenses to a solid instead of a liquid. Usually, the pressure in the heated system is diminished by pumping, and the vapour is condensed (after travelling a relatively short distance) on to a cold finger or some other cooled surface. This technique, which is applicable to many organic solids, can also be used with inorganic solids such as aluminium chloride, ammonium chloride, arsenious oxide and iodine. In some cases, passage of a stream of inert gas over the heated substance secures adequate vaporisation. 

Methoxycarbonylamino Tellurium Pentafluoride1 0.11 g (3.4 mmol) of methanol and 1.08g (4.1 mmol) of isocyanato tellurium pentafluoride are condensed into a 100 ml stainless steel cylinder. The mixture is stirred overnight at 20°, the cylinder is evacuated to constant weight, and the solid residue is sublimed at 30°/0.075 torr onto a cold finger at —30° yield 0.24 g (25%) m.p. 77". 

Mercury Bis[pentafluorotelluro trifluoromethylamide]3 14.2 g (44.6 mmol) of dichloromethyleneamino tellurium pentafluoride are condensed into a 100 ml stainless steel autoclave containing 48.0 g (201 mmol) of mercury (II) fluoride. The mixture is stirred and heated at 60° for4days. The autoclave is chilled to — 25° and any volatile materials removed at this temperature under vacuum. The remaining solid is sublimed at 60" onto a cold-finger at — 25° yield 17.8 g (98%) m.p. 67-68°. 

In addition to showing that solvent-free melt and sublimation crystallization conditions offer an attractive route to new polymorphs, a CSD survey of these methods of crystallization and the frequency of Z was performed (Table 3-5). There is a dramatic increase in the occurrence of Z > 3 crystal structures when melt or sublimation crystallization conditions are used [20]. The occurrence of high Z in melt crystallization and sublimation methods is ascribed to the rapid cooling of the hot liquid or vapor (100-300° C) in the open flask or on the cold finger (kinetic phase), conditions under which hydrogen-bonded clusters are likely to condense in a pseudo-symmetric crystalline arrangement. On the other hand, the slower nucleation process of solution crystallization gives the frequent situation of Z < 1 (88% hits). 

Various types of apparatus for microsublimation have been described in the literature for instance, a simple arrangement of suction tube and cold finger illustrated in Figure 25 may be conveniently used here, too, the distance between the evaporator and the condenser can be regulated. When sublimation is complete, the apparatus is carefully taken apart and the sublimate is removed from the surface of the condenser by means of a spatula or by use of a solvent. 

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