Preparation of Dry Ice

Preparation of Dry Ice. Put a bag made from a dense fabric onto the valve of a cylinder with liquid carbon dioxide and discharge a strong stream of the gas. What is observed Measure the temperature of the solid carbon dioxide— dry ice. Put a small piece of the ice into a beaker and carefully add 3-5 ml of ethyl ether or ethanol extinguish all burners ). Again measure the temperature. Write down the result. 

Cholestenone. Place a mixture of 1 0 g. of purified cholesterol and 0-2 g. of cupric oxide in a test-tube clamped securely at the top, add a fragment of Dry Ice in order to displace the air by carbon dioxide, and insert a plug of cotton wool in the mouth of the tube. Heat in a metal bath at 300-315° for 15 minutes and allow to cool rotate the test-tube occasionally in order to spread the melt on the sides. Warm with a few ml. of benzene and pour the black suspension directly into the top of a previously prepared chromatographic column (1) rinse the test-tube with a little more benzene and pour the rinsings into the column. With the aid of shght suction (> 3-4 cm. of mercury), draw the solution into the alumina column stir the top 0 -5 cm. or so with a stout copper wire to... 

To a solution of ethylnagnesium bromide in 350 ml of THF, prepared from 0.5 mol of ethyl bromide (see Chapter 11, Exp. 6) was added in 10 min at 10°C 0.47 mol of 1-hexyne (Exp. 62) and at 0°C 0.47 mol of trimethylsilylacetylene (Exp. 31) or a solution of 0.60 mol of propyne in 70 ml of THF (cooled below -20°C). With trimethyl si lylacetylene an exothermic reaction started almost immediately, so that efficient cooling in a bath of dry-ice and acetone was necessary in order to keep the temperature between 10 and 15°C. When the exothermic reaction had subsided, the mixture was warmed to 20°C and was kept at that temperature for 1 h. With 1-hexyne the cooling bath was removed directly after the addition and the temperature was allowed to rise to 40-45°C and was maintained at that level for 1 h. 

A solution of methylmagnesium bromide in 150 ml of diethyl ether, prepared from 0.5 mol of methyl bromide (see Chapter II, Exp. 5) was subsequently added in 20 min with cooling at about 20°C. After the addition the mixture was warmed for 2 h under reflux (the thermometer and gas outlet were replaced with a reflux condenser), a black slurry being formed on the bottom of the flask. The mixture was cooled in a bath of dry-ice and acetone and a solution of 30 g of ammonium chlori.de in 200 ml of water was added with vigorous stirring. The organic layer and four ethereal extracts were combined, dried over potassium carbonate and subsequently concentrated in a water-pump vacuum. Careful distillation of the residue through a 40-cm... 

SFE of fruits and vegetables and meat products has been reported, but the sample preparation techniques necessary to obtain reproducible results are extremely time consuming. Solid absorbents such as Hydromatrix, Extrelut " anhydrous magnesium sulfate or absorbent polymers are required to control the level of water in the sample for the extraction of the nonpolar pesticides. Without the addition of Hydromatrix, nonpolar pesticides cannot penetrate the water barrier between the sample particles and the supercritical CO2. The sample is normally frozen and the addition of dry-ice may be required to reduce losses due to degradation and/or evaporation. Thorough reviews of the advantages and limitations of SFE in pesticide residues... 

Although the reaction of alkali acetylides with oxirane proceeds slowly in liquid ammonia 15], it is an excellent method for preparing "homo-propargylic alcohols in quantities of 1 mol or more [2). Since oxirane is very volatile, considerable losses could occur if it is allowed to be swept along with the escaping ammonia vapour. One solution is to carry out the reaction under reflux, using a special condenser filled with dry ice and acetone. This would require regular addition of dry ice over a period of at least 12 h. It is much simpler to mix the acetylide... 

In a well-ventilated hood, to a rapidly stirred mixture of 50.2 gm (0.546 mole) of dinitrogen tetroxide and 123 gm (1.5 mole) of anhydrous sodium acetate in 150 ml of carbon tetrachloride prepared at Dry Ice temperature is added, over a 1 hr period, a solution of 46.5 gm (0.546 mole) of 2-pyrrolidone in 50 ml of... 

Preparation of Sodium Sulphide. Assemble an apparatus as shown in Fig. 67. Put 0.2 g of finely triturated highly pure sulphur into test tube 1 with a ground-glass joint, and lower the tube into a beaker with a mixture of dry ice and acetone. Connect a source of dry ammonia (see Fig. 62a) to cock... 

Allylic chlorides Actually the reaction of HOG with highly substituted alkenes is a convenient route to allylic chlorides if CH2G2 is used as the organic cosolvent. The reagent is prepared by addition of dry ice to calcium hypochlorite (70%) in water. The reaction of 1-methylcyclohexene is typical (equation I). Chlorohydrins arc the main products only in the case of 1-alkenes and 1,2-disubstituted alkenes. 

In the pail are placed 1000 g. (544 cc.) of concentrated sulfuric acid and 300 cc. of glacial acetic acid (Note 2), and this mixture is chilled by Dry Ice until the temperature is o° to — 50. Next, 500 g. (585 cc.) of />-cymene (Note 3) is added from the dropping funnel with vigorous stirring, any rise of temperature being prevented by the addition of Dry Ice (Note 4). Concurrently the nitrating mixture is prepared from 369 g. (262 cc.) of nitric acid (sp. gr. 1.42) and 1000 g. (544 cc.) of concentrated sulfuric acid, and cooled to 0-5° by the direct addition of small pieces of Dry Ice (Note 4). 

Ammonium carbamate is prepared from dry ice and liquid ammonia [14]. These conditions are very similar to the conditions under which we have observed the formation of amine salts. To some readers, ammonium carbamate may seem to be an exotic compound. In fact, it is manufactured industrially on a multiton scale, because on heating (usually at 100-185°C) ammonium carbamate is converted to urea and water [14-16]. Urea is important for both the agricultural and the plastics industries. The ammonium carbamate is not always isolated during urea preparation. Instead, the reactions are carried out under conditions where the carbamate is just an intermediate. Ammonium carbamate is only moderately stable and it gradually loses ammonia in air. Although the data are sparse, the rate of decomposition of carbamates in solution seems to decrease as the volatility of the parent amine decreases [17]. Free carbamic acids in solution do not decompose spontaneously to free amine and C02. Instead, the acid ionizes by reaction with water the proton is transferred from the hydronium ion to the amine and then decomposition occurs [17]. Acids catalyze the decomposition. 

A more common method for preparing a low temperature bath is to mix an organic substance with either dry ice or liquid nitrogen. Dry ice (C02, —78 °C) can be added in small lumps to the solvent until a slight excess of dry ice remains. Alternately, liquid nitrogen (N2, —196 °C) can be poured into the solvent until a slush is formed that consists of the solid-liquid mixture at its melting point. 

These quantities are convenient for hydrogenation in a Parr low-pressure apparatus if a larger apparatus is available, it is unnecessary to divide the material. The palladium-on-carbon catalyst is prepared by Hartung s method,3 using sufficient Norit to give a 5% catalyst. A few chips of Dry Ice should be placed in the bottle before the catalyst is added to provide an inert atmosphere. The checkers used a catalyst obtained from Baker and Company, Inc., 113 Astor Street, Newark, New Jersey. 

High-Boiling Residue Prepare a cooling coil from copper tubing [about 6.1 m x about 6 mm (od)] to fit into a suitable vacuum-jacketed flask. Immerse the cooling coil in a mixture of dry ice and acetone in a vacuum-jacketed flask, and connect one end of the tubing to a sample cylinder (see Sampling... 

Preparation of Sodium Sulphide. Assemble an apparatus as shown in Fig. 67. Put 0.2 g of finely triturated highly pure sulphur into test tube 1 with a ground-glass joint, and lower the tube into a beaker with a mixture of dry ice and acetone. Connect a source of dry ammonia (see Fig. 62a) to cock 3 of the apparatus, open cock 4, and fill the apparatus with ammonia. Put 2 g of metallic sodium thoroughly purified of oxide films and washed with benzene and diethyl ether onto filter 2. Pass ammonia through the apparatus until test tube 1 is filled with liquid ammonia up to one-third of its volume, after which disconnect the apparatus for preparing ammonia. 

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