Sublimation flasks

Tetrachlorodibenzo- -dioxm- C, U.L. 2,7-Dichlorodibenzo-p-dioxin- C, U.L. (0.500 gram) was stirred with 10 ml of chloroform containing trace amounts of FeCls and L and heated to reflux temperature while chlorine gas was passed into the mixture for 18 hours. After cooling, the white insoluble product was collected by filtration and triturated with 15 ml of boiling chloroform. The insoluble portion was transferred to a sublimation flask where it was vacuum-sublimed at 140 °C. The sublimate was recrystallized from 2.5 ml of anisole and washed with chloroform. The product weighed 0.229 gram and contained 2.9 /xCi of radioactivity per mg. 

Vacuum sublimation is a convenient purification technique, frequently used in synthetic organometallic chemistry. There are many types of sublimation flasks, varying chiefly in degree of sophistication of design. The main practical problem with vacuum sublimation is the inefficient transfer of heat from heat source to the sample being sublimed. Virtually all set ups use an oil- or metal bath and rely on conduction as the main mechanism of heat transfer. Under the vacuum conditions of the usual sublimation (10 - 10 mm Hg) both... 

We use a sublimation set-up as sketched below (Figure 1). A Philips IR lamp (375 W/ 220 V) connected to a variable resistor is sufficient for most sublimations. The lamp is protected from the sublimation flask by a metal gauze screen. The lamp is placed in a metal cylinder with reflecting walls and the sublimation flask is inserted through a conical reflector which can be adjusted to fit the flask. Dimensions are not critical and there is ample opportunity for improvisation e.g. use of aluminum foil as a disposable reflector. 

Controlling the temperature within a narrow range is rather difficult. Trial and error will establish the temperature achieved for a certain type of sublimation flask at a specified applied voltage. We have determined the temperature in the center of a sublima-... 

It is observed that the temperature achieved strongly depends on both the diameter of the sublimation flask, and on the fit of the flask in the conical reflector top entrance. 

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 method described here is cheaper and safer than the conventional methods (silicon oil or Woods metal bath). In addition it is much cleaner since no oil or metal stick to the walls of the sublimation flask after it is removed from the sublimer. The main advantages are the high sublimation rates and higher temperatures that can be reached relative to the conventional techniques. 

Our first method began by subliming 14C-radiolabeled dioxin onto the cold finger of a small sublimation flask. The sublimate was... 

The high surface to volume ratio of dioxin film afforded by the sublimation flask allows equilibrium to be reached in a short time, and allows easy, complete separation of the solid dioxin from the equilibrated solution. The values obtained were consistent with those obtained with radiolabeled dioxin. 

The oxime is freely soluble in water and in most organic liquids. Recrystallise the crude dry product from a minimum of 60-80 petrol or (less suitably) cyclohexane for this purpose first determine approximately, by means of a small-scale test-tube experiment, the minimum proportion of the hot solvent required to dissolve the oxime from about 0-5 g. of the crude material. Then place the bulk of the crude product in a small (100 ml.) round-bottomed or conical flask fitted with a reflux water-condenser, add the required amount of the solvent and boil the mixture on a water-bath. Then turn out the gas, and quickly filter the hot mixture through a fluted filter-paper into a conical flask the sodium chloride remains on the filter, whilst the filtrate on cooling in ice-water deposits the acetoxime as colourless crystals. These, when filtered anddried (either by pressing between drying-paper or by placing in an atmospheric desiccator) have m.p. 60 . Acetoxime sublimes rather readily when exposed to the air, and rapidly when warmed or when placed in a vacuum. Hence the necessity for an atmospheric desiccator for drying purposes. 

Dissolve 1 g. of anthracene in 10 ml. of glacial acetic acid and place in 50 ml. bolt head flask fitted with a reflux water-condenser. Dissolve 2 g. of chromium trioxide in 2 ml. of water and add 5 ml. of glacial acetic acid. Pour this solution down the condenser, shake the contents of the flask and boil gently for 10 minutes. Cool and pour the contents of the flask into about 20 ml. of cold water. Filter off the crude anthraquinone at the pump, wash with water, drain well and dry. Yield, 1 g. Purify by re crystallisation from glacial acetic acid or by sublimation using the semi-micro sublimation apparatus (Fig. 35, p. 62, or Fig. 50, p. 70). 

A simple apparatus for sublimation in a stream of air or inert gas is shown in Fig. II, 45, 2.. 4 is a wide-necked conical flask provided with... 

A simple apparatus for sublimation in a stream of air or of inert gas is shown in Fig. II, 45, 3.. d is a two-necked flask equipped with a narrow inlet tube B with stopcock and a wide tube C 12-15 inm. in diameter. The latter is fitted to a sintered glass crucible and the usual adapter and suction flask E. A well-fitting filter paper is placed on the sintered glass filter plate to collect any sublimate carried by the gas stream. 

Dissolve 5 g. of hydroxylamine hydrochloride in 10 ml. of water in a small conical flask and add a solution of 3 g. of sodium hydroxide in 10 ml. of water. Cool the solution in cold or ice water, and add 6 g. (7-6 ml.) of acetone slowly. Cool the flask, shake well, and leave overnight, during which time the oxime may crystallise out. If no crystals appear, cork the flask and shake vigorously when the acetoxime usually separates as colourless crystals. Filter the crystals at the pump, dry rapidly between filter paper (yield 2- 6 g.) and determine the m.p. (59°). Extract the filtrate with two 20 ml. portions of ether, and remove the solvent a further 0 - 5 g. of acetoxime (m.p. 60°) is obtained. Recrystallise from light petroleum, b.p. 40-60° CAUTION inflammable) to obtain the pure acetoxime, m.p. 60°. Acetoxime sublimes when left exposed to the air. 

Mix 100 g. of ammonium chloride and 266 g. of paraformaldehyde in a 1-litre rovmd-bottomed flask fitted with a long reflux condenser containing a wide inner tube (ca. 2 cm. diameter) the last-named is to avoid clogging the condenser by paraformaldehyde which may sublime. Immerse the flask in an oil bath and gradually raise the temperature. The mixture at the bottom of the flask liquefies between 85° and 105° and a vigorous evolution of carbon dioxide commences at once remove the burner beneath the oil bath and if the reaction becomes too violent remove... 

Impure quinone may be purified by placing it in a distilling flask attached to a condenser and passing a rapid current of steam into the flask the quinone Sublimes and collects in the receiver. It is separated from the water by filtration and dried the m.p. is 116°. The vapour has a penetrating odour and attacks the eyes. 

In a 250 ml. conical flask, fitted with an air condenser of wide bore, place 50 g. (51 -5 ml.) of acetonylacetone (see Section V,9, Note 2) and 100 g. of ammonium carbonate (lump form). Heat the mixture in an oil bath at 100° until effervescence stops (60-90 minutes) some ammonium carbonate (or carbamate) sublimes into the condenser and this must be pushed back into the reaction mixture by means of a stout glass rod. Replace the air condenser by a Liebig s condenser with wide bore inner tube and reflux the mixture gently (bath temperature, 115°) for a further 30 minutes dissolve the solid which has sublimed into the condenser in about 5 ml. of hot water and return the solution to the reaction mixture. 

For further purification the material may be recrystallized from hot water, or dissolved in alkali and reprecipitated, or distilled under reduced pressure or sublimed. Each of these operations involves considerable loss of product, either through solubility or through decomposition by heat. The best-appearing product is obtained by distillation under reduced pressure. The crude acid is distilled from a Claisen flask with a delivery tube set low in order that the acid need not be heated much above the boiling j)oint. The product boiling at i4i-i44°/20 mm. is ])ure wliitc and melts at 125-132° (Note 8). The yield of distilled acid is about 75-85 per cent of the weight of the crude acid. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

The cooled contents of the 2S0-ml. flask containing ferrous chloride (Note 6) are added to the cold sodium cyclopentadienide solution while passing a stream of nitrogen through both flasks. The combined mixture is stirred for 1.25 hours at a temperature just below reflux. Solvent is removed by distillation, and the ferrocene is extracted from the residue with several portions of refluxing petroleum ether (b.p. 40-60°). The product is obtained by evaporation of the petroleum ether solution. Ferrocene may be purified by recrystallization from pentane or cyclohexane (hexane, benzene, and methanol have also been used) or by sublimation. The 3ueld is 31-34 g. (67-73%) (Note 7), m.p. 173-174°. 

The organic layer is separated, evaporated on a steam bath, and the dark semicrystalline residue is distilled with steam to remove biphenyl. The contents of the steam-distillation flask are then extracted with ether (Note 3), and the ethereal layer is separated, dried over magnesium sulfate, and percolated through a short column of chromatographic alumina (Notes 4 and 5). Evaporation of the ethereal solution gives crude triphenylene which is sublimed at 175-180° and 0.1 mm. pressure. After rejection of an initial sublimate of impure biphenyl, the sublimed material forms nearly colorless crystals, m.p. 186-194° (Note 6). Yield 8-9 g. (53-59%). It may be further purified by recrystallization from a mixture of methylene chloride and pentane yielding colorless crystals, m.p. 199° (Note 7). 

Commercial selenium dioxide gives more consistent results when freshly sublimed material is used. Place the oxide (50 g) in a 7-cm porcelain crucible upon which is set a 250-ml filter flask cooled by running through it a stream of water. The crucible is heated with a low flame until sublimation is complete (20-30 minutes). After cooling, the sublimed selenium dioxide is scraped from the flask and is stored in a stoppered bottle. 

In a 100-ml flask is placed a mixture of 19.5 g (0.18 mole) of freshly sublimed, pulverized selenium dioxide, 15 g (0.10 mole) of df/-camphor and 15 ml of acetic anhydride. The flask is fitted with a magnetic stirrer and a condenser, and the mixture is heated to 135° on an oil bath with stirring for 16 hours. After cooling, the mixture is diluted with ether to precipitate selenium, which is then filtered off, and the volatile materials are removed under reduced pressure. The residue is dissolved in ether (200 ml), washed four times with 50-ml portions of water and then washed several times with saturated sodium bicarbonate solution (until the washes are basic). The ether solution is finally washed several times with water, then dried, and the ether is evaporated. The residue may be purified by sublimation at reduced pressure or recrystallized from aqueous ethanol (with clarification by Norit, if necessary). The product is yellow, mp 197-199°. 

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