Reduced pressure sublimation

The common methods of purification, discussed below, comprise distillation (including fractional distillation, distillation under reduced pressure, sublimation and steam distillation), crystallisation, extraction, chromatographic and other methods. In some cases, volatile and other impurities can be removed simply by heating. Impurities can also sometimes be eliminated by tbe formation of derivatives from which the purified material is regenerated (see Chapter 2). 

Quinazoline [253-82-7] M 130.2, m 48.0-48.5 , b 120-121 /17-18mm, pK, -4.51 (aq H2SO4, anhydrous dication), pK 2.01 (anhydrous monocation), pK3 4.3 (equilibrium with 3,4-hydrated species), PK4 12.1 (hydrated anion). Purified by passage through an activated alumina column in benzene or pet ether (b 40-60°). Distd under reduced pressure, sublimed under vacuum and crystd from pet ether. [Armarego 7 Chem 1170 1961.]... 

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. 

Diphenyl carbonate (phenyl carbonate) [102-09-0] M 214.2, m 80 , 168 /15mnt, 306 /760inm. Purify it by distillation under reduced pressure, sublimation, or by gas chromatography with 20% Apiezon on Embacel, and... 

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. 

Most of the water is sublimated from the frozen mass by heating the product under reduced pressure. The operating conditions must be such that the product remains in a soHd state while sublimation is taking place. The completion of sublimation can be observed by an increase in product temperature. This increase occurs when the energy being introduced is no longer consumed by the latent heat of sublimation, but is absorbed by the product instead. 

The hexachloride, uranium hexachloride [13763-23-0], UCl, is best prepared by chlorination of UCl [10026-10-5] with SbCl. An alternative preparative approach is the disproportionation UCl [13470-21 -8] to UCl and UCl under reduced pressure. The obvious disadvantage of the second method is contamination by UCl, however, sublimation is a possible purification technique. Isostmctural with the hexafluoride, the hexachloride is monomeric with an octahedral arrangement of the chlorine atoms around the uranium center. 

In addition to distillation (73), BI can be purified by sublimation under reduced pressure (77). 

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. 

Further purification of azulene may be achieved by sublimation at reduced pressure, mp 99 C. The checkers found that mechanical losses, particularly as mentioned in Note 9, lead to reduction in yield with reduction in scale (0.1 mol, 39% yield 0.5 mol, 43% yield 0.8 mol, 79% yield). 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives (vide supra). 

Anthracene has also been crystd from EtOH, chromatographed through alumina in hot benzene (fume hood) and then vac sublimed in a pyrex tube that has been cleaned and baked at 100°. (For further details see Craig and Rajikan J Chem Soc, Faraday Trans 1 74 292 1978 and Williams and Zboinski J Chem Soc, Faraday Trans 1 74 611 1978.) It has been chromatographed on alumina, recrystd from n-hexane and sublimed under reduced pressure. [Saltiel J Am Chem Soc 108 2674 1986 Masnori et al. J Am Chem Soc 108 1126 1986.] Alternatively, it was recrystd from cyclohexane, chromatographed on alumina with n-hexane as eluent, and recrystd two more times [Saltiel et al. J Am Chem Soc 109 1209 1987]. 

Benzo[6]biphenylene [259-56-3] M 202.2, m dec >250. Purified by sublimation under reduced pressure. 

Dichloroanthracene [605-49-1] M 247.1, m 214-215°. Purified by crystn from MeOH or EtOH, followed by sublimation under reduced pressure. [Masnori and Kocbi J Am Chem Soc 107 7880 1985.]... 

Maleic anhydride [108-31-6] M 98.1, m 54 , b 94-96 /20mm, 199 /760mm. Crystd from benzene, CHCI3, CH2Cl2or CCI4. Sublimed under reduced pressure. [Skell et al. J Am Chem Soc 108 6300 7956.]... 

Nitronaphthalene [86-57-7] M 173.2, m 57.3-58.3 , b 30-40 /0.01mm. Fractionally distd under reduced pressure, then crystd from EtOH, aqueous EtOH or heptane. Chromatographed on alumina from benzene/pet ether. Sublimes in vacuo. 

Phthalic anhydride [85-44-9] M 148.1, m 132°, b 295°. Distd under reduced pressure. Purified from the acid by extracting with hot CHCI3, filtering and evaporating. The residue was crystd from CHCI3, CCI4 or benzene, or sublimed. Fractionally crystd from its melt. Dried under vacuum at 100°. [Saltiel 7 Am Chem Soc 108 2674 1986.]... 

Tetramethylammonium bromide [64-20-0] M 154.1, sublimes with dec >230°. Crystd from EtOH, EtOH/diethyl ether, MeOH/acetone, water or from acetone/MeOH (4 1) by adding an equal volume of acetone. It was dried at 110° under reduced pressure or at 140° for 24h. 

Ethylmercuric chloride [107-27-7] M 265.1, m 193-194°. Mercuric chloride can be removed by suspending ethylmercuric chloride in hot distilled water, filtering with suction in a sintered-glass crucible and drying. Then crystd from ethanol and sublimed under reduced pressure. It can also be crystd from water. 

B. Di-tert-butyl dicarbonate. A solution of 20.0 g. (0.076 mole) of di-i-butyl tricarbonate in 75 ml. of carbon tetrachloride is placed in a 600-ml. beaker fitted with a magnetic stirrer, and 0.10 g. (0.0009 mole) of freshly sublimed l,4-diazabicyclo[2.2.2]octane (DABCO) is added (Note 9). Rapid evolution of carbon dioxide begins at once. The reaction mixture is stirred at 25° for 45 minutes to complete the loss of carbon dioxide (Note 10), and then 35 ml. of water, containing sufficient citric acid to make the aqueous layer slightly acidic, is added. The layers are separated and the organic layer is dried over anhydrous magnesium sulfate and then concentrated at 25° with a rotary evaporator. The residual liquid is distilled under reduced pressure to separate 13.3-15.1 g. (80-91%) of di-butyl dicarbonate as a colorless liquid, b.p. 55-56° (0.15 mm.) or 62-65° (0.4 mm.) n T> 1.4071-1.4072 (Note 11). 

Prolonged heating under reduced pressure should be avoided, since 1,2,4-triazole tends to sublime. 

A solution of crude (53) in 1.5 liters of absolute ethanol is treated with 350 ml of 40% aqueous NaOH solution and refluxed for 7 hr. This solution is then cooled in ice, treated with 650 ml of 12 N HCl and allowed to stand at room temperature overnight. The mixture is stirred and refluxed for 1.5 hr, cooled and filtered. The precipitate is washed well with ethanol and discarded. The filtrate and washings are concentrated under reduced pressure, poured into water and filtered. The filtrate is made alkaline with NaOH solution saturated with K2CO3 and filtered. The precipitate is washed well with water, dried under reduced pressure and sublimed at 165° (0.1 mm) The sublimate is recrystallized from benzene-cyclohexane to yield 7.65g of (54) mp 193-195°. 

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