Drying, of crystals

Drying of crystals. Whenever possible crystals should be dried in a smdl vacuum desiccator containing a suitable desiccant. The latter depends upon the nature of the solvent used c/. p. 19). However for most purposes anhydrous calcium chloride is satisfactory. If a hydrocarbon has been used in the recrystallisation, a few thin fresh shavings of paraffin wax are efficacious. 

Filtration and drying of crystals. The crystals are usually separated from the mother liquor by filtration with suction in a Buchner fuimel. A filter paper is fitted into the flat part of the funnel and moistened with some of the solvent, and suction is applied before the mixture of crystals and solution is added. When most of the liquid has drained from the funnel, the suction is discontinued and a little of the filtrate is used to rinse the adhering crystals from the beaker in which crystallization takes place. Suc-... 

Drying of Crystals.—When crystals have been freed from the mother-liquor they must be dried. This may be effected (1) at the ordinary temperature by the gradual evaporation of the solvent in the air, and (2) at higher temperatures by heating on a water-bath or in an air-bath, In the first case the crystals are spread... 

Crystallisation by Evaporation. — If a compound is so easily soluble in all solvents that it will only crystallise out on partial evaporation, then, in order to get good crystals, a solution, not too dilute, is made, by the aid of heat if necessary, and filtered from the impurities remaining undissolved. In this case, as a crystallisation vessel, one of the various forms of shallow dishes— the so-called crystallising dishes — is used, in which the solution is allowed partially to evaporate. In order to protect the vessel from dust, it is covered with a funnel or watch-glass, in the manner indicated under Drying of Crystals. fn crystallising... 

The easy dewatering and drying of crystals requires that the crystals be of a uniform and sufficiently large size. This is, therefore, the major theme of this chapter. 

The aim of the earlier discussion was to explain how ays-tals of a desired size could be produced. Furthermore, the CSD should be as narrow as possible for easy drying. In principle, the drying of crystals can be carried out in the same way as that of any particulate material. However, there are some cases when the crystalline structure itself poses problems in drying. We will briefly discuss these cases. 

Drying Removal of a volatile solvent from a solid in the presence of a non-condensable gas, normally air Wide range from drying of crystals to drying of biscuits... 

For the filtration of very small quantities of crystals, the simple apparatus shown in Fig. 46 is often used. It consists of a fine glass rod (sometimes termed a filtration nail ) which is flattened at one end, the flattened surface being preferably roughened. It fits as shown into a small funnel which replaces F (Fig. 45). A circular piece of filter-paper is cut e-g.y with a clean sharp cork-borer) so as to fit completely and snugly over the flat end. After draining, the nail is raised and the filter-paper and crystals are removed with forceps and dried. 

The tribromobenzene obtained in this way should be entirely free from unchanged tribromoaniline. To test its purity, dissolve a small quantity in hot dry benzene and pass in hydrogen chloride gas from a Kipp s apparatus no trace of crystals of tribromoaniline hydrochloride should appear. Note also that although the m.p.s of the two compounds are almost identical, that of the recrystallised product from the above preparation is considerably depressed by admixture with tribromoaniline. 

A) Extract the mixture with about 40 ml. of chloroform, in which the free base is very soluble. Run off the lower chloroform layer, dry it with potassium carbonate as in (a), and then add carbon tetrachloride slowly with stirring to the filtered chloroform solution until the base starts to crystallise out. Allow to stand for a short time (t.e., until the deposition of crystals ceases) and then filter at the pump as the crystals lose the last trace of solvent, they tend as before to break up into a fine powder, the deep green colour becoming paler in consequence. 

In order to dry the crystals, the Buchner funnel is inverted over two or three thicknesses of drying paper (i.e., coarse-grained, smooth surfaced Alter paper) resting upon a pad of newspaper, and the crystalline cake is removed with the aid of a clean spatula several sheets of drying paper are placed on top and the crystals are pressed flrmly. If the sheets become too soiled by the mother liquor absorbed, the crystals should be transferred to fresh paper. The disadvantage of this method of rapid drying is that the recrystallised product is liable to become contaminated with the Alter paper flbre. 

Dissolve 2 5 g. of hydroxylamine hydrochloride and 4 g. of crystallised sodium acetate in 10 ml. of water in a small flask or in a test-tube. Warm the solution to about 40° and add 2 5 g. of cyclohexanone. Stopper the vessel securely with a cork and shake vigorously for a few minutes the oxime soon separates as a crystalline solid. Cool in ice, filter the crystals at the pump, and wash with a little cold water. RecrystaUise from light petroleum, b.p. 60-80°, and dry the crystals upon filter paper in the air. The yield of pure cycZohexanone oxime, m.p. 90°, is 2 -5 g. 

Method B. In a 500 ml. round-bottomed flask, provided with a reflux condenser protected by a cotton wool (or calcium chloride) drying tube, place 59 g. of succinic acid and 102 g. (94-5 ml.) of redistilled acetic anhydride. Reflux the mixture gently on a water bath with occasional shaking until a clear solution is obtamed ca. 1 hour), and then for a further hour to ensure the completeness of the reaction. Remove the complete assembly from the water bath, allow it to cool (observe the formation of crystals), and finally cool in ice. Collect the succinic anhydride as in Method A. The yield is 45 g., m.p. 119-120°. 

C. Fumaric acid from furfural. Place in a 1-litre three-necked flask, fitted with a reflux condenser, a mechanical stirrer and a thermometer, 112 5 g. of sodium chlorate, 250 ml. of water and 0 -5 g. of vanadium pentoxide catalyst (1), Set the stirrer in motion, heat the flask on an asbestos-centred wire gauze to 70-75°, and add 4 ml. of 50 g. (43 ml.) of technical furfural. As soon as the vigorous reaction commences (2) bvi not before, add the remainder of the furfural through a dropping funnel, inserted into the top of the condenser by means of a grooved cork, at such a rate that the vigorous reaction is maintained (25-30 minutes). Then heat the reaction mixture at 70-75° for 5-6 hours (3) and allow to stand overnight at the laboratory temperature. Filter the crystalline fumaric acid with suction, and wash it with a little cold water (4). Recrystallise the crude fumaric acid from about 300 ml. of iif-hydrochloric acid, and dry the crystals (26 g.) at 100°. The m.p. in a sealed capillary tube is 282-284°. A further recrystaUisation raises the m.p. to 286-287°. 

Dissolve 57 g. of dry malonic acid in 92 5 ml. of dry P3rridine contained in a 500 ml. round-bottomed flask, cool the solution in ice, and add 57 g. (70 ml.) of freshly distilled n-heptaldehyde (oenanthol) with stirring or vigorous shaking. After a part of the aldehyde has been added, the mixture rapidly seta to a mass of crystals. Insert a cotton wool (or calcium chloride) tube into the mouth of the flask and allow the mixture to stand at room temperature for 60 hours with frequent shaking. Finally, warm the mixture on a water bath until the evolution of carbon dioxide ceases (about 8 hours) and then pour into an equal volume of water. Separate the oily layer and shake it with 150 ml. of 25 per cent hydrochloric acid to remove pyridine. Dissolve the product in benzene, wash with water, dry with anhydrous magnesium sulphate, and distil under reduced pressure. Collect the ap-nonenoic acid at 130-13272 mm. The yield is 62 g. 

Into a 1-litre beaker, provided with a mechanical stirrer, place 36 - 8 g. (36 ml.) of aniline, 50 g. of sodium bicarbonate and 350 ml. of water cool to 12-15° by the addition of a little crushed ice. Stir the mixture, and introduce 85 g. of powdered, resublimed iodine in portions of 5-6 g, at intervals of 2-3 minutes so that all the iodine is added during 30 minutes. Continue stirring for 20-30 minutes, by which time the colour of the free iodine in the solution has practically disappeared and the reaction is complete. Filter the crude p-iodoaniline with suction on a Buchner funnel, drain as completely as possible, and dry it in the air. Save the filtrate for the recovery of the iodine (1). Place the crude product in a 750 ml. round-bottomed flask fitted with a reflux double surface condenser add 325 ml. of light petroleum, b.p. 60-80°, and heat in a water bath maintained at 75-80°. Shake the flask frequently and after about 15 minutes, slowly decant the clear hot solution into a beaker set in a freezing mixture of ice and salt, and stir constantly. The p-iodoaniline crystallises almost immediately in almost colourless needles filter and dry the crystals in the air. Return the filtrate to the flask for use in a second extraction as before (2). The yield of p-iodoaniline, m.p. 62-63°, is 60 g. 

Hydrolysis of benzyl cyanide to phenylacetamide. In a 1500 ml. three-necked flask, provided with a thermometer, reflux condenser and efficient mechanical stirrer, place 100 g. (98 ml.) of benzyl]cyanide and 400 ml. of concentrated hydrochloric acid. Immerse the flask in a water bath at 40°. and stir the mixture vigorously the benzyl cyanide passes into solution within 20-40 minutes and the temperature of the reaction mixture rises to about 50°, Continue the stirring for an additional 20-30 minutes after the mixture is homogeneous. Replace the warm water in the bath by tap water at 15°, replace the thermometer by a dropping funnel charged with 400 ml. of cold distilled water, and add the latter with stirring crystals commence to separate after about 50-75 ml. have been introduced. When all the water has been run in, cool the mixture externally with ice water for 30 minutes (1), and collect the crude phenylacetamide by filtration at the pump. Remove traces of phenylacetic acid by stirring the wet sohd for about 30 minutes with two 50 ml. portions of cold water dry the crystals at 50-80°. The yield of phenylacetamide, m.p. 154-155°, is 95 g. RecrystaUisation from benzene or rectified spirit raises the m.p. to 156°. 

Method 2. Place a mixture of 126-5 g. of benzyl chloride, 76 g. of thiourea and loO ml. of rectified spirit in a 500 ml. round-bottomed flask fitted with a reflux condenser. Warm on a water bath. A sudden exothermic reaction soon occurs and aU the thiourea passes into solution. Reflux the resulting yellow solution for 30 minutes and then cool in ice. Filter off the white crystals and dry in the air upon filter paper. Concentrate the filtrate to half its original volume and thus obtain a further small crop of crystals. The yield of crude hydrochloric acid as in Method 1 the m.p. is raised to 150°, although on some occasions the form, m.p. 175°, separates. 

Sodium anthraquinone-p-sulphonate ( silver salt ). Place 60 g. of fuming sulphuric acid (40-50 per cent. SO3) in a 250 or 500 ml. round-bottomed flask and add 50 g. of dry, finely-powdered anthra-quinone (Section IV,145). Fit an air condenser to the flask and heat the mixture slowly in an oil bath, with occasional shaking, so that at the end of 1 hour the temperature has reached 160°. Allow to cool and pour the warm mixture carefully into a 2 litre beaker containing 500 g. of crushed ice. Boil for about 15 minutes and filter off the unchanged anthraquinone at the pump. Neutralise the hot filtrate with sodium hydroxide and allow to cool, when the greater part of the sodium anthra-quinone-p-sulphonate separates as silvery glistening plates ( silver salt ). Filter these with suction and dry upon filter paper or upon a porous plate. A second crop of crystals may be isolated by concentration of the trate to half the original volume. The yield is 40-45 g. 

There are some alternatives to this HCI generator type of crystallization. There are, of course, canisters of HCI gas that can be purchased. Also, one can crystallize with very concentrated (fuming) HCI by pouring the stuff directly into the ether/freebase [26]. Regular 35% HCI can do this too, but the water content may dissolve the MDA.HCl or make the crystals sticky which means that the chemist will have to dry the solution by removing the water. 

The first stage crystals are rich in pentaerythritol linear formal and may be treated (60,61) to convert this species to pentaerythritol and formaldehyde, which can then be recovered. The concentrated Hquors obtained after redissolving are then recrystallized and filtered prior to drying of the final product. 

Manufacture. Phosphoms pentachloride is manufactured by either batch or continuous processing. In the former, the phosphoms trichloride usually dissolves in carbon tetrachloride before being treated with chlorine. A mixture of ca one part of phosphoms trichloride to one part of carbon tetrachloride is introduced to a water-jacketed vessel that contains an efficient stirrer and a tight cover with a redux condenser. The chlorine is passed into the vessel below the Hquid level, and crystals of phosphoms pentachloride form in the Hquid. When the reaction is completed, the suspension of crystals of phosphoms pentachloride in the carbon tetrachloride is drawn out of the vessel and the crystals are filtered and then dried by circulating hot water through the jacket of the filter. The clarified carbon tetrachloride is returned to the reaction vessel. 

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