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Cyanogen and Related Compunds
119. Formation of Cyanogen (Section 188). - (a) From mercuric cyanide. - Caution. Cyanogen is a poisonous gas. Perform the experiment under a hood. Heat in a small dry test-tube over a free flame about 0.5 gram of mercuric cyanide. Ignite the gas generated. Note the unusual color of the different parts of the flame. (Eq.)
(b) From copper sulphate and potassium cyanide. - Caution. - Potassium cyanide is very poisonous. It should not be touched with the fingers; use a spatula or handle small pieces with pincers. Hood. - Add to 5 cc. of a cold saturated solution of copper sulphate a cold saturated solution of potassium cyanide until the precipitate first formed dissolves. (Eq.) Heat to boiling (Eq.) and ignite the gas evolved. Add a solution of potassium iodide to a solution of copper sulphate. (Eq.) Determine by an appropriate test if iodine is liberated.
120. Formation of Potassium Cyanide from Potassium Ferrocyanide (Section 193). - Heat to a high temperature in a porcelain crucible about 1 gram of potassium ferrocyanide. (Eq.) When cold add water, and heat to boiling. Pour off the solution and test for a cyanide according to the next experiment.
121. Test for a Soluble Cyanide (Section 194). - To 1 cc. of a dilute solution of potassium cyanide add 5 drops of a solution of sodium hydroxide and 5 drops of a solution of ferrous sulphate; heat to boiling, cool, and add dilute hydrochloric acid, drop by drop, until the solution just shows an acid reaction. If no color develops add 3 drops of a solution of ferric chloride. Potassium ferrocyanide is formed from the potassium cyanide and ferrous sulphate; the ferrocyanide and the ferric salt then form Prussian blue. (Eqs.)
Note. - It is usually not necessary to add ferric chloride, since the ferrous sulphate contains enough ferric salt to give the reaction. An excess of hydrochloric acid should be avoided since it interferes with the formation of the ferric ferrocyanide.
122. Action of Cyanides on the Soluble Salts of Heavy Metals (Section 193). - (a) Add a few drops of a solution of potassium cyanide to a solution of ferric chloride. (Eq.) Next add an excess of potassium cyanide (Eq.), acidify, and then add a solution of ferrous sulphate. (Eq.)
(b) Add a few drops of a solution of potassium cyanide to a solution of silver nitrate (Eq.); add an excess of the cyanide. (Eq.) Determine if silver chloride is soluble in potassium cyanide. Precipitate some silver cyanide and determine its solubility in nitric acid and in ammonia. How could you distinguish silver chloride from silver cyanide?
123. Properties of Potassium Ferrocyanide (Section 194). - (a) Conversion into hydroferrocyanic acid. - Add concentrated hydrochloric acid to about 10 cc. of a saturated solution of potassium ferrocyanide as long as a precipitate is formed. (Eq.) Pour off the liquid and place a part of the crystals on a porous plate. They turn blue due to oxidation.
(b) Decomposition with strong sulphuric acid. - In a test-tube moisten with 5 drops of water about 1 gram of powdered potassium ferrocyanide and add 5 cc. of concentrated sulphuric acid. Heat and apply a flame to the gas evolved. (Eq.)
(c) Ferrocyanides of heavy metals. - Add a solution of potassium ferrocyanide to solutions of salts of ferric iron, silver, copper, and mercury. (Eqs.)
124. Preparation of Potassium Ferricyanide from Potassium Ferrocyanide (Section 195). Dissolve 5 grams of potassium ferrocyanide in about 20 cc. of warm water, cool and add bromine in slight excess, drop by drop, with constant shaking. (Eq.) Evaporate to crystallization under the hood, and set aside the solution to crystallize.
125. Properties of Potassium Ferricyanide (Section 195). - (a) Ferricyanides of heavy metals - Add a solution of potassium ferricyanide to solutions of salts of ferric iron, ferrous iron, silver, copper, and mercury (Eq.)
(b) Potassium ferricyanide as an oxidizing agent. - In a small beaker dissolve 2 grams of lead nitrate in about 10 cc, of boiling water, and add a solution of sodium hydroxide until the precipitate first formed has dissolved. Next add a strong aqueous solution of 4 grams of potassium ferricyanide, and heat to boiling. (Eq.) Filter off the precipitate, test the filtrate for a ferrocyanide and determine if the precipitate is lead dioxide, PbO2. Add hydrochloric acid to a little of the substance and heat. (Eq.)
Note. - (b) An alkaline solution of potassium ferricyanide is a valuable mild oxidizing agent, which is frequently used in the oxidation of organic compounds. The oxidizing power is equivalent to that represented in the following equation:
2K3Fe(CN)6 + 2KOH = 2K4Fe(CN)6 + H2O + O
126. Preparation of Methyl Cyanide (Section 197). - Weigh directly into a dry 100 cc. distilling flask 20 grams of phosphorus pentoxide and add 20 grams of powdered acetamide; stir the mixture and shake it. Close the flask with a cork bearing a thermometer, connect with a condenser, and use a 25 cc. distilling flask as a receiver. Heat cautiously with a small flame, kept constantly in motion, as long as any liquid distils. Add to the distillate about 2 grams of phosphorus pentoxide, and redistil, collecting the portion which boils at 80°-82°. Weigh the distillate and calculate the percentage yield obtained.
Methyl cyanide (acetonitrile) boils at 81.6°. The yield obtained in the above experiment should be 50-60 per cent of the theoretical.
127. Hydrolysis of Methyl Cyanide (Section 197). - (a) Heat about 1 cc. of methyl cyanide with a dilute solution of sodium hydroxide, and note the odor of the gas evolved. (Eq.)
(b) In a small round-bottomed flask mix 3 cc. of water and 6 cc. of concentrated sulphuric acid; cool the mixture and add 5 grams of methyl cyanide. Connect the flask with a return condenser and boil gently for 15 minutes. (Eq.) Dilute with about 5 cc. of water and distil off about 5 cc. Test the distillate for acetic acid. (See experiment 91, page 64.) Make a part of the contents of the flask strongly alkaline with sodium hydroxide and note the odor.
128. Formation of an Isocyanide (Section 198). - Repeat the carbylamine test for primary amines (experiment 113c, page 87), in which an isocyanide is formed, or apply the test to aniline, C6H5NH2, which is a primary amine containing the phenyl radical, C6H5; phenyl isocyanide, C6H5NC, is formed. Proceed as follows: Hood. - Warm together 2 drops of aniline, 3 drops of chloroform, and 2 cc. of an alcoholic solution of potassium hydroxide. (Eq.) Caution. - As phenyl isocyanide has a very disagreeable odor and is poisonous, the tube containing it should be washed thoroughly under the hood.