Sublimation cold fingers

Sublimation. This is a most useful process for small-scale work as the losses are comparatively small. This can be performed (a) In a long narrow tube sealed at one end. The material is shaken to the closed end of the tube, which is then inserted horizontally in a metal-heating block (Fig. 50) (b) In the cold-finger device (Fig. 35, p. 62)-... 

Sublimation under reduced pressure. The so-called vacuum sublimation may be carried out in the apparatus of Fig. II, 45, 4. The cold finger is fitted into the larger tube by means of a rubber stopper... 

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. 

Azelaic acid [123-99-9] M 188.2, m 105-106". Crystd from H2O (charcoal) or thiophene-free benzene. The material cryst from H2O was dried by azeotropic distn in toluene, the residual toluene soln was cooled and filtered, the ppte being dried in a vacuum oven. Also purified by zone refining or by sublimation onto a cold finger at 10" torr. 

Sodium bis(trimethylsilyl)amide (hexamethyl disilazane sodium salt) [1070-89-9] M 183.4, m 165-167 (sintering at 140 ). It can be sublimed at 170 /2 Torr (bath temp 220-250°) onto a cold finger, and can be recrystd from CgHg (sol lOg in lOOmL at 60°). It is slightly soluble in Et20 and is decomposed by H2O. [Chem Ber 94 1540 1961.]... 

The submitters reported a melting point of 114-116°. The checkers obtained analytically pure material with a recovery of 80% after decolorization with activated carbon and recrystallization from 2-3 ml. of hexane at 0°. The product was also purified with comparable efficiency by sublimation at 85-90° (10 mm.). A small amount of a yellow, volatile impurity was removed from the cold finger before the product began to sublime. The melting point of the product after purification by the checkers was 110-112°. The reported melting point is 114-116°. 

Fig. 10. Apparatus for the sublimation of decaborane. (A) cold finger (B) ground-glass joint for connection to high-vacuum system (C) high-vacuum stopcock ...

Figure 88 shows two forms of sublimation apparatus. Note all the similarities. Cold water goes in and down into a cold finger upon which the vapors... 

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. 

When everything has settled down, slowly begin to heat the bottom of the sublimator, if necessary. You might see vapors coming off the solid. Eventually, you ll see crystals of purified solid form on the cold finger. Since you ll work with different substances, different methods of heating will have to be used. Ask your instructor. 

The checkers noted the presence of an oil which also distilled onto the cold-finger during the sublimation. The easiest way to remove this oil is to press the sublimed cyclopropenone between two sheets of filter paper. The melting point is recorded after having removed the oil in this manner. However, this oil poses no hindrance in the next step. Alternatively, the submitters report that pure cyclopropenone can be obtained more rapidly but in slightly reduced yield by recrystallization of the crude product from pentane at low temperature (ca. —70°). 

Dimethyldibenzotellurophene. Powder tellurium (0.94 g 7.4 mmol) is thoroughly mixed with 2.61 g (6.1 mmol) of pure 4,4-dimethyl-2,2 -biphenyldiyl mercury, the mixture is placed into a sublimation apparatus equipped with a cold finger, and slowly heated under vacuum in a metal bath to 260-270°C. Colourless needles begin to sublime at 228°C. The temperature is held at 260°C for 12 h. The sublimate is recrystallized from a mixture of 300 mL of methanol and 10 mL of carbon tetrachloride. Yield 1.38 g (79%) m.p. 158°C. Dibenzotellurophene was similarly obtained in 82% yield. 

TelluranthreneP Finely powdered tellurium (2.3 g, 18 mmol) and 2.5 g (9.0 mmol) of o-phenylenel mercury are intimately mixed by grinding in a mortar, the mixture is placed in the well of a sublimation apparatus, which is then evacuated to <1 torr. The well of the sublimation apparatus is heated at 250°C in a bath of Wood s metal for 10 h, and the cold finger is cooled with dry ice. The sublimed solids are dissolved in chloroform, the solution is decanted through a filter, the solvent is evaporated, and the residue is recrystaUized from carbon tetrachloride or ethanol/benzene. Yield 1.05 g (57%). m.p. 179°C. 

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). 

GLC analysis shows that the product Is contaminated by small amounts of diol. If desired, purer material could be obtained by sublimation at 35-40 C/0.01 mm onto a cold finger kept at 0°C. ... 

Tris(ethylene)platinum(0) is stable for several hours at 20° under 1 atm of ethylene and keeps for many weeks at -20°.6 In the absence of ethylene, decomposition to metallic platinum occurs in minutes at room temperature. The complex is quite volatile, with a vile smell, and sublimes slowly at 20° in an atmosphere of ethylene onto a cold finger at 0°. 

LiCl-free BuNTe(n-N Bu)2TeN Bu is obtained via vacuum sublimation. In a typical experiment a sublimation apparatus is charged with 1.770 g of the crude product. Sublimation is conducted at 90-95°C/10 mbar with the cold finger at 18°C. Over 24 h pure BuNTe(p-N Bu)2TeN Bu sublimes onto the cold finger as a yellow-orange solid (1.239 g). 

Difluoromcthylcnecydopropane (20, R = F 2.30 g, 25.6 mmol) was sealed under vacuum in a thick-walled 165-mL glass tube. The tube was heated to 312 C for 3.5 h, then cooled to liquid N2 temperature when opened and the contents (1.75 g, 76%) were transferred to a cold finger under vacuum. A quantity of polymeric material remained in the tube. The material that collected on the cold finger was thawed and trap-to-trap distilled to yield 0.38 g of a ca. 70 30 mixture of 20 (R = F) and 2,2-difiuoromcthylenecyclo-propane (by I9F NMR) and a white solid residue. The solid residue was sublimed under reduced pressure to give crystalline 21 (R = F) yield 0.68 g (30%) mp 64-65 C. 

Trimethylamine oxide (Aldrich) must be freshly sublimed for the procedure that follows. Typically, 2 g of trimethylamine oxide is sublimed in a cold finger apparatus (60 °C, 0.05 torr, 16h). Dry trimethylamine oxide is very hygroscopic. It is important to handle it in a dry atmosphere using suitably dried solvents. 

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. 

Dimethoxyazoxybenzene [1562-94-3] M 258.3, m 165. Crystd from hot 95% EtOH, dried, then sublimed in a vacuum onto a cold finger. 

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