Desiccators, calcium chloride

The chloroform solution Is washed with water (2 x 250 ml) and stirred with glacial acetic acid (250 ml) for 1 hr (Note 6). The solution that results Is washed with water (2 x 250 mL), and the chloroform phase is treated with 1.5 L of hexane and kept at 2 C for 48 hr. The material which precipitates Is collected and dried at 0.1 ram In a desiccator (calcium chloride) for 5 hr to give p-methoxyphenyllead triacetate (20-22 g, 35-40%) as pale yellow crystals, mp 138-139°C (Note 7). The product may be kept for at least 3 weeks If stored at 2°C In a sealed container. 

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. 

Both forms sublime very readily, even at room temperature a small sample on exposure to the air will completely volatilise in a short time, particularly on a warm day or if the sample is exposed to a gentle current of air. Hence the above method for rapid drying. A sample confined in an atmospheric desiccator over calcium chloride rapidly disappears as the vapour is adsorbed by the calcium chloride. A sample of the hexahydrate similarly confined over sodium hydroxide undergoes steady dehydration with initial liquefaction, for the m.p. of the hydrated-anhydrous mixture is below room temperature as the dehydration proceeds to completion, complete resolidification occurs. 

Aluminium isopropoxide can be obtained as a fine powder from technical sources. When the bottle has once been opened however, the stopper should be firmly replaced and covered with wax more conveniently, the stoppered bottle can be kept in an atmospheric desiccator over calcium chloride or sodium hydroxide, preferably in the dark. 

Assemble in a fume-cupboard the apparatus shown in Fig. 67(A). Place 15 g. of 3,5-dinitrobenzoic acid and 17 g. of phosphorus pentachloride in the flask C, and heat the mixture in an oil-bath for hours. Then reverse the condenser as shown in Fig. 67(B), but replace the calcium chloride tube by a tube leading to a water-pump, the neck of the reaction-flask C being closed with a rubber stopper. Now distil off the phosphorus oxychloride under reduced pressure by heating the flask C in an oil-bath initially at 25-30, increasing this temperature ultimately to 110°. Then cool the flask, when the crude 3,5-dinitro-benzoyl chloride will solidify to a brown crystalline mass. Yield, 16 g., i.e,y almost theoretical. Recrystallise from caibon tetrachloride. The chloride is obtained as colourless crystals, m.p. 66-68°, Yield, 13 g Further recrystallisation of small quantities can be performed using petrol (b.p. 40-60°). The chloride is stable almost indefinitely if kept in a calcium chloride desiccator. 

The beat method of drying, if time permits, is to place the crystals in a desiccator containing an appropriate substance (usually anhydrous calcium chloride, silica gel, or concentrated sulphuric acid) to absorb the solvent. More eflicient and more rapid drying is obtained with the aid of a vacuum desiccator (see Section 11,38 and Fig. 77, 38, 1). 

Anhydrous magnesium sulphate. This is an excellent, neutral desiccating agent and is inexpensive. It is rapid in its action, chemically inert and fairly efficient, and can be employed for most compounds including those (esters, aldehydes, ketones, nitriles, amides, etc.) to which calcium chloride is not applicable. 

Commercial diethyl carbonate may be purified by the following process. Wash 100 ml. of diethyl carbonate successively with 20 ml. of 10 per cent, sodium carbonate solution, 20 ml. of saturated calcium chloride solution, and 25 ml. of water. Allow to stand for one hour over anhydrous calcium chloride with occasional shaking, filter into a dry fiask containing 5 g. of the same desiccant, and allow to stand for a further hour. Distil and collect the fraction boiling at 125-126°. Diethyl carbonate combines with anhydrous calcium chloride slowly and prolonged contact should therefore be avoided. Anhydrous calcium sulphate may also be used. 

An alternative procedure is to leave the syrupy residue in a vacuum desiccator over anhydrous calcium chloride and siUca gel, and to Alter ofl the successive Crops of crystals as they separate. These are washed with light petroleum, b.p. 40-60°, spread on a porous tile and recrystallised. 

Heat a suspension of 22 g. of the diacetate in a mixture of 120 ml. of concentrated hydrochloric acid, 190 ml. of water and 35 ml. of alcohol under reflux for 45 minutes. Cool the mixture to 0°, filter the solid with suction, and wash with water. Purify the crude aldehyde by rapid steam distillation (Fig. II, 41, 3) collect about 1500 ml. of distillate during 15 minutes, cool, filter, and dry in a vacuum desiccator over calcium chloride. The yield of pure o-nitrobenzaldehyde, m.p. 44—45°, is 10 g. The crude solid may also be purified after drying either by distillation under reduced pressure (the distillate of rather wide b.p., e.g., 120-144°/3-6 mm., is quite pure) or by dissolution in toluene (2-2-5 ml. per gram) and precipitation with light petroleum, b.p. 40°-60° (7 ml. per ml. of solution). 

To prepare crystalline monoperphthalic acid, place the thoroughly dry ethereal solution (4) in a distilling flask equipped with a capillary tube connected with a calcium chloride or cotton wool drying tube, and attach the flask to a water pump. Evaporate the ether without the application of heat (ice will form on the flask) to a thin syrup (about 150 ml.). Transfer the syrup to an evaporating dish, rinse the flask with a little anhydrous ether, and add the rinsings to the syrup. Evaporate the remainder of the ether in a vacuum desiccator over concentrated sulphuric acid about 30 g. of monoperphthalic acid, m.p. 110° (decomp.), is obtained. 

Prepare acetophenonephenylhydrazone by warming a mixture of 20 g. of acetophenone (Section IV, 136) and 18 g. of phenylhydrazine on a water bath for 1 hour. Dissolve the hot mixture in 40 ml. of rectihed spirit, and shake or stir to induce crystallisation. Cool the mixture in ice, filter and wash with 12 ml. of rectified spirit. Dry in a vacuum desiccator over anhydrous calcium chloride for at least half an hour. The yield of phenylhydrazone, m.p. 105-106 , is 28 g. 

Place an intimate mixture of 125 g. of powdered, anhydrous zinc chloride and 26-5 g. of acetophenonephenylhydrazone in a tall 500 ml. beaker in an oil bath at 170°. Stir the mixture vigorously by hand. After 3-4 minutes the mass becomes hquid and evolution of white fumes commences. Remove the beaker from the bath and stir the mixture for 5 minutes. Then stir in 100 g. of clean, white sand in order to prevent solidification to a hard mass. Digest the mixture for 12-16 hours on a water bath with 400 ml. of water and 12 ml. of concentrated hydrochloric acid in order to dissolve the zinc chloride. Filter off the sand and the crude 2-phenylindole, and boil the solids with 300 ml. of rectified spirit. Treat the hot mixture with a little decolourising carbon and filter through a pre-heated Buchner funnel wash the residue with 40 ml. of hot rectified spirit. Cool the combined filtrates to room temperature, filter off the 2-phenylindole and wash it three times with 10 ml. portions of cold alcohol. Dry in a vacuum desiccator over anhydrous calcium chloride. The yield of pure 2-phenylindole, m.p. 188-189°, is 16 g. 

Pure commercial ethyl acetate is allowed to stand for 2 days over anhydrous calcium chloride, the desiccant removed by filtration, and the ester is then finally dried over anliydrous calcium sulphate for several hours. 

Dry A.R. hydrated cadmium chloride to constant weight at 110° grind finely, dry again for 2-3 hours at 110° and then place in a screw-capped bottle and keep in a desiccator over calcium chloride. 

After drying or decomposing a sample, it should be cooled to room temperature in a desiccator to avoid the readsorption of moisture. A desiccator (Figure 2.9) is a closed container that isolates the sample from the atmosphere. A drying agent, called a desiccant, is placed in the bottom of the container. Typical desiccants include calcium chloride and silica gel. A perforated plate sits above the desiccant, providing a shelf for storing samples. Some desiccators are equipped with stopcocks that allow them to be evacuated. 

By-product water formed in the methanation reactions is condensed by either refrigeration or compression and cooling. The remaining product gas, principally methane, is compressed to desired pipeline pressures of 3.4—6.9 MPa (500—1000 psi). Einal traces of water are absorbed on siHca gel or molecular sieves, or removed by a drying agent such as sulfuric acid, H2SO4. Other desiccants maybe used, such as activated alumina, diethylene glycol, or concentrated solutions of calcium chloride (see Desiccants). 

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