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Aromatic Aldehydes, Ketones, and Quinones
208. Properties of Benzaldehyde (Sections 479-482). - (a) Oxidation of benzaldehyde. - Leave a few drops of benzaldehyde on a watch-glass exposed to the air for a few days. (Eq.)
Grind a pinch of starch with a few cubic centimeters of cold water and pour the mixture into about 50 cc. of boiling water. Cool, and dissolve in the solution a crystal of potassium iodide. Divide the solution into two portions. Shake one of these with 2 drops of benzaldehyde and expose both to sunlight for some time. Explain. (See Section 480.)
(b) Test for the aldehyde group in benzaldehyde. - Shake up a few drops of benzaldehyde with about 15 cc. of water, and test parts of the solution with Fehling's solution, Tollen's reagent, and Schiff's reagent. Compare the results with those obtained with acetaldehyde. For the test with phenylhydrazine see experiment 192d, page 158. If this experiment has not been performed, do it now.
(c) Benzaldehyde and sodium hydrogen sulphite. - Shake about 1 cc. of benzaldehyde with 5 cc. of a saturated solution of sodium hydrogen sulphite. Filter off the crystals by suction - and wash them with a little alcohol until odorless. Dissolve the crystals in warm water; to one-half of the solution add a solution of sodium carbonate, and to the other dilute sulphuric acid. Explain the cause of the appearance and odor produced.
(d) Benzaldehyde and sodium hydroxide. - Shake up a few drops of benzaldehyde with a solution of sodium hydroxide, and then warm the mixture. Is a resin formed? Compare the results with those obtained in the case of acetaldehyde (experiment 108d, page 83). For the behavior of benzaldehyde with alkalies see experiment 193, page 160.
(e) Benzaldehyde and ammonia: hydrobenzamide. - Mix together 3 cc. of benzaldehyde, 20 cc. of alcohol, and 10 cc. of a concentrated solution of ammonia. Set aside the mixture until the product which separates becomes solid. Heat on the steambath, and add alcohol until the solid dissolves. Set aside to crystallize. Determine the melting-point of the crystals. Compare the behavior of formaldehyde, acetaldehyde, and benzaldehyde with ammonia.
Hydrobenzamide, (C6H5CH)3N2, crystallizes from alcohol in octahedra which melt at 110°.
209. Preparation of Benzophenone (Sections 487 and 348). - In a dry 500 cc. flask put 25 grams of benzene, 25 grams of benzoyl chloride, 50 cc. of carbon bisuiphide, and 30 grams of anhydrous aluminium chloride. The aluminium chloride should not be exposed to the air any longer than necessary, as it rapidly absorbs moisture. Connect the flask with a reflux condenser and heat it on a water-bath. The reaction is complete when a few drops of the solution in the flask, when treated with a little water, give oil which does not have the disagreeable odor of benzoyl chloride. The time required is from 3 to 4 hours. Cool the flask in water, and pour the contents into about 300 cc. of water to which has been added 20 cc. of concentrated hydrochloric acid. Separate the layer of carbon bisulphide in which the benzophenone is dissolved, and evaporate off the solvent on the steam-bath in the hood, away from any flames. If no steam-bath is available, place the solution in a flask and distil off the solvent on a water-bath through a long condenser. Add 50 cc. of alcohol and 2 grams of bone-black, boil for about 5 minutes, and filter hot. Wash the bone-black with a few cubic centimeters of hot alcohol. When the filtrate is cold, add cold water until a slight permanent cloud is produced, and set aside to crystallize. If an oil separates, scratch the inside of the beaker just below the surface of the solution with a glass rod. At the end of some hours or at the next exercise, filter off the crystals by suction and wash them twice (§12, page 7) with a mixture, of 2 volumes of alcohol and 1 of water. A further amount of benzophenone can be obtained by adding to the filtrate from the crystals cold water until the solution clouds, and proceeding as above.
Weigh the product and calculate the percentage yield from the benzoyl chloride used. Write equations for all the reactions involved, including those into which the aluminium chloride enters.
Benzophenone melts at 48°, and boils at 306.1° (corr.). The yield in the preparation is about 20 grams.
210. Preparation of Benzophenoneoxime (Section 488). - In a flask provided with a reflux condenser dissolve 5 grams of benzophenone in 50 cc. of alcohol, and add a solution of 5 grams of hydroxylamine hydrochloride in 15 cc. of water; to this add a solution of 10 grams of sodium hydroxide in 15 cc. of water, and heat the mixture on a water-bath for an hour. Pour the product into about 200 cc. of water, filter if necessary, and add to the filtrate dilute sulphuric acid until the solution is just acidic to litmus paper. After an hour filter off the benzophenoneoxime.
Weigh the product when dry and calculate the percentage yield, which should be nearly theoretical. Recrystallize the oxime from about 25 cc. of hot alcohol.
Benzophenoneoxime melts at 141°.
211. Beckmann's Rearrangement of an Oxime (Section 488). - In a small flask dissolve 2 grams of benzophenoneoxime in 30 cc. of ether dried over sodium, and add slowly in small portions 3 grams of finely divided phosphorus pentachloride. At first it may be necessary to cool the solution; toward the end of the reaction the mixture should be heated on the steam-bath until the phosphorus pentachloride has dissolved. When this has taken place, pour the ethereal solution with vigorous stirring into 400 cc. of cold water. Stir until a solid has formed, and then filter it off by suction and wash with cold water. Crystallize the benzanilide from the smallest amount of boiling alcohol, and determine the melting-point of the dried crystals. Write equations for the reactions involved in the rearrangement.
Benzanilide melts at 160°.
212. Preparation of Quinone (Section 492). - Mix in a large beaker 250 cc. of water, 50 cc. of pure concentrated sulphuric acid, and 50 grams of sodium bichromate or the equivalent weight of potassium bichromate. When the salt has dissolved, set the beaker in cold water and cool the solution to 20°. Mix in a beaker 150 cc. of water and 10 grams of aniline; pour into the mixture, with stirring, 10 cc. of pure concentrated sulphuric acid. Cool the solution to 20° and add it, stirring vigorously, in four portions to the solution of sodium bichromate. The beaker in which the reaction takes place should be surrounded by cold water and the temperature should be kept between 35° and 40°. About 6 to 8 minutes are required for the addition of the solution of aniline sulphate. When this has been accomplished, remove the beaker from the water and let it stand for about 15 minutes. Transfer the product to a large flask, and cool it in running water to about 15°. Add to the flask 300 cc. of ether and shake. Let the flask stand for a few minutes. In order to facilitate the separation of the ether into a distinct layer, add to it 25 cc. of acetone, and give to the flask a gentle rotary motion. In a few minutes decant off carefully as much of the ether as possible into a separatory funnel; if some of the aqueous layer flows into the funnel it can be separated. Draw off the ethereal solution, place it in a flask, and distil the, ether off from a water-bath through a long water-jacketed condenser (see §34, page 22). Extract the solution of quinone once more with this recovered ether, and evaporate it off as before. Crystallize the residue of quinone from the smallest possible amount of boiling ligroin, or distil it with steam in the ordinary way except that no water is put into the flask containing the quinone. Dry the crystals of quinone in a desiccator; the compound is volatile at room temperature, and if left exposed to the air for a number of hours an appreciable quantity is lost.
Quinone melts at 116°. The yield in this preparation is 5 to 6 grams.
213. Properties of Quinone (Section 492). - (a) Reduction of quinone. - Shake up a crystal of quinone with 10 cc. of water. Note the odor and color of the solution; add to one-half of it a solution of sodium hydrogen sulphite. (Eq.) To the other half add a dilute solution of potassium iodide and a few drops of dilute hydrochloric acid. (Eq.)
(b) Unsaturation of quinone. - Dissolve a few crystals of quinone in 5 cc. of carbon tetrachloride and add, drop by drop, a solution of bromine in carbon tetrachloride. (Eq.)
(c) Formation of quinhydrone. - Make a saturated solution of quinone in water and add to the solution, drop by drop, a strong aqueous solution of hydroquinone. Examine the tube after about 1 minute. If crystals have not formed, repeat, using a more concentrated solution of hydroquinone. (Eq.) Add a solution of ferric chloride to a strong solution of hydroquinone. (Eq.) Add an ammoniacal solution of silver nitrate to an aqueous solution of hydroquinone.
214. Preparation of Anthraquinone (Section 494). - In a 250 cc. round-bottomed flask provided with a reflux condenser place 5 grams of anthracene and 50 cc. of glacial acetic acid. Heat the liquid to boiling over a wire gauze and add, drop by drop, from a separatory funnel placed in the upper end of the condenser, a solution made by dissolving 10 grams of chromic acid anhydride in 10 cc. of water and then adding 25 cc. of glacial acetic acid. Boil the solution for 10 minutes after the oxidizing agent has been added, and then allow the mixture to cool spontaneously to the temperature of the room. Finally cool under running water. Filter by suction, and wash the crystals twice with a mixture of 2 volumes of water and 1 volume of glacial acetic acid, using 20 cc. of the mixture each time. Finally wash twice with water and allow the crystals to dry in the air.
Sublime a small amount of the dried anthraquinone (§35, page 23), and crystallize a little from boiling alcohol.
Anthraquinone sublimes at about 250° in yellow needles which melt at 273°. The yield in this preparation is 5 grams.
Boil together for about half a minute a trace of powdered anthraquinone with a little zinc dust and 10 cc. of a solution of sodium hydroxide. Filter the solution and shake it with air. Anthraquinone is partly reduced by zinc and sodium hydroxide to a red salt of the composition:
This salt is oxidized by air to anthraquinone. The reaction is a valuable one in the identification of this substance.
Compare the properties of anthraqiiinone with those of benzoquinone. Does the former oxidize an acidified solution of' potassium iodide? Is it reduced by sulphurous acid?
Note. - After oxidation by chromic acid in acetic acid solution, the product is usually precipitated by pouring the solution into water. In the case of anthraquinone it is convenient to allow the mixture to cool as the compound crystallizes well from glacial acetic acid.