Sublimation packings

We have seen that in a metal the atoms are close-packed, i.e. each metal atom is surrounded by a large number of similar atoms (often 12, or 8). The heat required to break up 1 mole of a metal into its constituent atoms is the heat of atomisation or heat of sublimation. Values of this enthalpy vary between about 80 and 800 kJ. for metals in their standard states these values indicate that the bonds between metal atoms can vary from weak to very strong. There is a rough proportionality between the m.p. of a metal and its heat of atomisation. so that the m.p. gives an approximate measure of bond strength. 

These model compounds can also be used in device fabrication, since thin films of appropriate thickness can be obtained by sublimation and subsequent deposition onto a substrate in vacuum. Electrical as well as optical properties of such devices have turned out to be strongly dependent on both the molecular packing within the crystallites and the polycrystalline morphology. Understanding and control of this aspect is one of the current scientific challenges. 

The packing energy of an organic crystal can be easily calculated by a lattice sum over pairwise interactions. The potential parameters for these calculations are summarized in Table 15. The packing energy is usually a quite accurate estimate of the crystal sublimation energy. 

Dichlorodibenzo- -dioxin. 2-Bromo-4-chlorophenol (31 grams, 0.15 mole) and solid potassium hydroxide (8.4 grams, 0.13 mole) were dissolved in methanol and evaporated to dryness under reduced pressure. The residue was mixed with 50 ml of bEEE, 0.5 ml of ethylene diacetate, and 200 mg of copper catalyst. The turbid mixture was stirred and heated at 200°C for 15 hours. Cooling produced a thick slurry which was transferred into the 500-ml reservoir of a liquid chromatographic column (1.5 X 25 cm) packed with acetate ion exchange resin (Bio-Rad, AG1-X2, 200-400 mesh). The product was eluted from the column with 3 liters of chloroform. After evaporation, the residue was heated at 170°C/2 mm for 14 hours in a 300-cc Nestor-Faust sublimer. The identity of the sublimed product (14 grams, 74% yield) was confirmed by mass spectrometry and x-ray diffraction. Product purity was estimated at 99- -% by GLC (electron capture detector). 

Wakayama, N., Inokuchi, H. (1967) Heats of sublimation of polycyclic aromatic hydrocarbons and their molecular packings. Bull. Chem. Soc. Japan 40, 2267-2271. 

We used all of the Cis hydrocarbons in Pedley and derived their enthalpies of sublimation by subtracting the recommended enthalpies of formation of the solid and the corresponding gaseous species. There was considerable variation in the sublimation enthalpies, as seemingly befits the diverse choice of compounds (and associated crystal packing) including such species as naphthacene, 6,6-diphenylfulvene, 3,4,5,6-tetramethylphenanthrene, [3.3]paracyclophane and n-octadecane. 

Heat transfer from the shelves to the sublimation front depends on the pressure and the distance between shelf and product (Fig. 1.58). Mass transfer (g/s) increases with the pressure, but also depends on the flow resistance of the already dry product and of the packing of the bones. If the maximum tolerable Tke is defined, the drying time depends only on the two processes mentioned above. It cannot be shortened under a given geometric situation and the chosen Tke. This method of Tkt control does not require thermocouples, and does not contaminate the product. 

The purification of ammonium chloride.—Crude sal ammoniac is usually contaminated with iron or tarry matters, and in consequence, the colour varies from yellow to red it can be purified by heating it in thin layers on an iron plate hot enough to drive off the water and free acid, and to carbonize most of the tarry products. The grey mass is then sublimed. The sublimation is conducted in cast-iron pots lined internally with firebricks, and covered with a lid made of slightly concave plates. The salt to be sublimed is well dried, and heated. The pots hold about half a ton, and the sublimation occupies about five days. The sublimate forms a solid fibrous crust about 4 inches thick. The crust is easily detached from the lid it is then broken up, separated from adhering dirt, and packed for the market in barrels or sacks. W. Hempel 9 proposed converting the crystalline salt into hard stone-like masses by press, between 50° and 100°. 

The greenish-black crystals of Bil3 are composed of a hexagonal close packed array of iodine atoms with bismuth atoms in the interstitial spaces, with six nearest neighbours at 3.07 A. It is prepared by heating the elements in stoichiometric quantities, and purified by subliming under reduced pressure. 

Analyses. Samples of reaction mixtures were frozen and sublimed into the vacuum line (10-6 mm Hg). The vapor was passed through a wide-bore U-trap at —45°C which collected acetic acid and volatile oxidation products and allowed any unreacted hexenes to pass. After further fractional condensation the hexene was transferred to a graduated tube to measure the volume and was finally analyzed by GLC using a 12 meter X 6 mm column packed with polypropylene glycol LB-550-X on Chromosorb W. The sublimation residue was analyzed by IR spectroscopy while the acetic acid condensate was subjected to a wet separation (5, 15) to recover the oxidation products for GLC analysis. 

Carbon dioxide is formed by the action of acids on carbonates and as a by-product of fermentation, the conversion of carbohydrates to ethanol by yeast. Because its triple point is at 5.1 atm, it cannot exist as a liquid at ordinary atmospheric pressure. Solid carbon dioxide, which is sold as dry ice, sublimes directly to the gas—a property that makes it convenient as a refrigerant and cold pack. 

A 37.5-g. (0.266-mole) sample of coumalic add (Note 1) is placed in a 30 x 10 cm. cylindrical flask attached horizontally to a 55 x 3 cm oven-heated Vycor tube (Note 2) loosely packed with 20 g. of fine copper turnings (Note 3). Following the Vycor tube successively are two ice-cooled 50-ml. receivers and a dry ice trap. The latter is connected to an efficient vacuum pump (Note 4). The system is evacuated, and the Vycor tube is heated to 650— 670° Then the flask containing the coumalic acid is heated with a nichrome wound heating jacket to 180°, and the temperature is allowed to rise slowly to 215°. During this time coumalic acid sublimes into the Vycor tube and a-pyrone distills into the ice-cooled receivers. The pressure is held below 5 mm (Note 5) The yield of pale yellow crude material is 18-19 3 g. (70-75%). 

When the water vapor is completely expelled, the end of the tube is fitted with a cork stopper bearing a straight calcium chloride tube loosely packed to prevent plugging. The nitrogen is now allowed to pass through bromine at the rate of approximately one bubble per second and then is directed into the reaction tube. As the reaction proceeds, a sublimate of zirconium bromide collects in the tube several centimeters beyond the furnace. 

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