Chapter XV

Aromatic Hydrocarbons

163. Preparation of Benzene from Benzoic Acid (Section 350). - In a 6-inch evaporating dish place 10 grams of sodium hydroxide and 25 cc. of water. Heat over a free flame and stir until the sodium hydroxide dissolves; then stir in gradually 12 grams of benzoic acid. Evaporate to dryness over a free flame, which is kept constantly in motion; this will take about 10 minutes. Grind the mixture of sodium benzoate and sodium hydroxide in a mortar, and transfer it to an 8-inch test-tube. Clamp the test-tube in an inclined position so that the mouth of the tube is slightly lower than the other end; this will prevent any water given off during the heating from running back into the tube and cracking it. Connect the tube with a condenser and receiver, and heat with a free flame kept constantly moving, until no more liquid distils over. Measure the volume of the benzene and calculate the number of grams and the percentage yield obtained. Separate the benzene from the water, dry it with calcium chloride, and distil, noting the temperature.

Benzene melts at 5.4°, boils at 80.4°, and has the specific gravity 0.8736 (20°/4°). The yield should be about 6 grams.

Note. - By converting the benzoic acid into sodium benzoate in the presence of an excess of sodium hydroxide, an intimate mixture of the two substances is obtained. In this condition the compounds enter into reaction more readily and at a lower temperature than does a mixture of the acid and soda-lime; such mixtures are commonly used in the preparation of hydrocarbons from acids.

The benzene prepared in this way contains a small amount of diphenyl, which may be isolated from the residue left after the distillation of the hydrocarbon. The residue on crystallization from alcohol yields crystals of diphenyl, which after two sublimations melt at 71°.

164. Properties of Benzene (Section 353). - (a) Test for thiophene in commercial benzene (Section 554). - To a few drops of sulphuric acid add a crystal of isatin, about 5 cc. of crude benzene, and shake.

(b) Test for carbon disulphide in benzene. - Add, to about 10 cc of crude benzene 2 drops of phenylhydrazine and set aside for some time. If carbon disulphide is present, a crystalline precipitate of the formula (C₆H₅NH.NH₂)₂.CS₂ will separate. If crude benzene is not available add 1 drop of carbon disulphide to 10 cc. of benzene, and test this mixture.

(c) Melting-point of benzene (Section 7). - Place about 50 cc. of benzene in a flask, insert a thermometer so that the bulb is covered, and place in a freezing mixture of ice and salt. When the benzene has become solid, remove the flask, and allow the mixture to melt partly. Note the temperature when about one-fourth of the benzene is liquid. Pour off the liquid, melt the benzene, and place the flask again in the freezing mixture, and let it stay until the benzene is solid. Remove the flask and determine the melting-point as before. Repeat until the melting-point remains constant. Pure benzene melts at 5.4°.

(d) Inflammability of benzene. - Burn a few drops of benzene in an evaporating dish. Is much soot formed?

(e) Benzene and sulphuric acid. - Shake 2 cc. of benzene with about 5 cc. of pure concentrated sulphuric acid. Is the hydrocarbon soluble? If the benzene is pure it will not markedly color the acid.

Shake together about 2 cc. of benzene and 5 cc. of fuming sulphuric acid (sp. gr. 1.89 at 20°) as long as heat is developed. Does the hydrocarbon dissolve? (Eq.) Pour the mixture slowly into a test-tube two-thirds full of ice. Does any insoluble substance separate?

In order to compare the behavior of aromatic hydrocarbons with that of the paraffins, repeat the above experiment using petroleum ether or gasoline in place of benzene.

(f) Benzene and nitric acid. - Caution. - As the reaction may become violent the mouth of the tube should be held away from the experimenter. Repeat experiment (e) above with both benzene and petroleum ether or gasoline, using fuming nitric acid (sp. gr. 1.48) in place of fuming sulphuric acid. When this test is applied to unknown substances, very small quantities should be used at first, since fuming nitric acid reacts with certain substances with explosive violence.

(g) Test for a double bond in benzene. - Apply the test with a solution of potassium permanganate. (See experiment 72d, page 50.)

(h) Benzene and bromine. - Add to about 10 cc. of benzene 1 cc. of a solution of bromine in carbon tetrachloride. Divide the solution into two parts; place one in direct sunlight and the other in the dark in your desk. Observe the two tubes after a few minutes. Compare the results with those obtained with gasoline (experiment 70c, page 47) and with amylene (experiment 74a, page 50).

Hood. - Mix 5 cc. of benzene with 2 cc. of bromine. Add a small tack or clean piece of iron filings to the mixture. From time to time shake the mixture and breathe across the mouth of the tube. (Eq.) Let the mixture stand under the hood until the next exercise and then pour it into water. Is the liquid heavier than water? Explain.

Notes. - (d) When aromatic compounds burn they produce a large amount of soot. Soot is also formed when unsaturated compounds and paraffin derivatives which contain alkyl radicals with four or more carbon atoms are burned. The simpler paraffin derivatives do not produce soot on burning. The behavior of an unknown substance on ignition is frequently determined as a preliminary test in its identification. The test is best made by putting some of the liquid or solid on a small roll of copper gauze, to which a piece of wire is attached to serve as a handle.

(e and f) The determination of the behavior of substances with sulphuric acid and with nitric acid is a valuable aid in their identification. Aromatic compounds yield, in general, soluble suiphonic acids with fuming sulphuric acid, and insoluble or difficultly soluble nitro-compounds with fuming nitric acid.

(g) The unsaturation of benzene and other aromatic hydrocarbons is of quite a different kind from that of ethylene. Aromatic hydrocarbons do not readily react with potassium permanganate in the cold.

165. Preparation of Ethylbenzene: Fittig Synthesis (Section 347). - (a) In a round-bottomed one-half liter flask, which is placed in a vessel containing cold water, put 200 cc. of ether dried over sodium (see experiment 95b, page 70) and 27 grams of sodium in the form of a wire or thin shavings. Connect the flask by means of a tightly fitting stopper with a reflux condenser through which water is passing. In order to guard against any water entering the flask, wrap the stopper and the neck of the flask with a towel. Caution. - Read §41, page 27, carefully. When hydrogen is no longer evolved, add through the condenser a mixture of 60 grams of brombenzene and 60 grams of ethyl bromide. At the end of about an hour turn off the water from the condenser, and let the reaction proceed for at least over night. Connect the flask with a condenser and receiver, and distil off the ether on a water-bath. Distil the residue directly with a large smoky flame, which is kept constantly in motion, as long as any liquid passes over. Fractionate (see §23, page 12) the product twice, and keep the fraction boiling at 133°-137°. Calculate the percentage yield obtained from the brombenzene used. Add about 50 cc. of alcohol to the flask containing the excess of sodium, and let the mixture stand at least one-half hour.

Ethylbenzene boils at 135°, and has the specific gravity 0.883 at 0°. The yield in the preparation should be 25 to 28 grams.

(b) Ethylbenzene may be considered as a substitution-product of ethane and of benzene. Determine whether the hydrocarbon shows the characteristic properties of a paraffin or an aromatic hydrocarbon. State in your notes what tests were applied and the results in each case.

Note. - (a) In the preparation of hydrocarbons by the Fittig synthesis the halides used are often diluted with ether in order to moderate the reaction; the volume of ether used ordinarily is twice that of the halogen compounds. Benzene and petroleum ether are also used as diluents, especially in the ease of very active substances, when it is desired to have the reaction take place very slowly. When reaction takes place sluggishly, the mixture without diluents can be heated on a water-bath or in an oil-bath. The reaction between halides and sodium is catalyzed by a few drops of ethyl acetate or methyl cyanide.

The rates at which the halides react with sodium are different, and, consequently, an excess of the more reactive halide is used when a hydrocarbon containing two radicals is prepared. In the preparation of ethylbenzene the quantity of ethyl bromide required theoretically for 60 grams of brombenzene is 41 grams; it has been found that a better yield is obtained if 60 grams of ethyl bromide are used. An excess of sodium is also used, as the metal becomes coated with sodium bromide, and thus is prevented from further action. The amount of sodium equivalent to 60 grams of brombenzene and 60 grams of ethyl bromide is 21.5 grams; 27 grams of the metal are used.

If all the substances used have not been carefully dried, the hydrogen formed as the result of the reaction between sodium and water reduces a part of the halides to hydrocarbons.

In the preparation of hydrocarbons containing two different radicals, the product obtained is usually a mixture; in addition to ethylbenzene, some diphenyl and butane are formed in the preparation described above.

166. Preparation of Diphenylmethane (Section 358). - As a large amount of hydrochloric acid is formed in the preparation the apparatus should be set up under a hood. In a 500 cc. flask provided with a reflux condenser, place 120 grams of benzene and an aluminium-mercury couple which is prepared as follows: Cut up 2 grams of aluminium foil into strips about 1 inch by 0.5 inch and allow them to stay in a solution of mercuric chloride, made by dissolving 1 gram of the salt in 200 cc. of water, for 8 to 10 minutes; a film of mercury is deposited on the aluminium. Wash the couple thoroughly with water, then with alcohol, ether, and finally with benzene. Into the upper end of the condenser place a separatory funnel containing 60 grams of benzyl chloride; allow the chloride to drop very slowly into the flask. At the end of an hour heat the flask on a water-bath for about 15 minutes. Pour the contents of the flask into an equal volume of water, which contains a little sodium hydroxide, shake, separate the benzene solution, and extract the aqueous layer once with a little benzene. Combine the benzene solutions, and dry them with calcium chloride. Pour off the solution into a distilling flask, and distil, using a condenser, until the temperature of the vapor reaches 150°. Remove the condenser, attach a short air condenser, distil, and collect the fraction which boils at 250°-300°. Redistil and collect the portion boiling at 255°-265°. Record the weight obtained, and calculate the percentage yield from the benzyl chloride. The yield should be about 35 grams.

Diphenylmethane melts at 26° and boils at 262°.

Note. - In the preparation of certain compounds by condensation as the result of the elimination of chlorine and hydrogen, an aluminium-mercury couple gives better results than aluminium chloride. It is probable that little aluminium chloride is first formed by the action of the metal on the organic halogen compound; the presence of mercury makes the aluminium more active.

167. Oxidation of Diphenylmethane to Benzophenone (Sections 358, 487). - In the identification of organic substances they are frequently oxidized and the products isolated. The following procedure is an example of one commonly used. Dissolve 5 grams of diphenylmethane in 10 cc. of glacial acetic acid and add a solution of crystalline chromic anhydride prepared by dissolving 4.5 grams of the anhydride in 5 cc. of water, and adding 30 cc. of glacial acetic acid. Let the mixture stand for half an hour, and then warm for an hour on the steam-bath. Pour the product into 100 cc. of water, filter off the oil through a moist filter paper, dissolve it in 20 cc. of hot alcohol, and add cold water until the solution clouds. Set aside to crystallize.

If an oil separates, rub it against the side of the beaker with a glass rod. A form of benzophenone which crystallizes with difficulty, is produced as the result of the oxidation of diphenylmethane. If a sample of benzophenone is available a trace can be used to seed the oil if it does not crystallize when rubbed sharply against the beaker. Determine the melting-point of the crystals. Benzophenone melts at 48°.

Note. - In oxidizing compounds with chromic anhydride, a slight excess is used over that required for the oxidation. Two molecules of the anhydride furnish three atoms of oxygen. If the compound which is insoluble in water is to be oxidized, acetic acid is often used as a solvent. As chromic anhydride is difficultly soluble in glacial acetic acid, it is first dissolved in a little water and acetic acid added to the solution. Oxidation takes place more readily in the presence of a small amount of sulphuric acid, which is often used as a catalytic agent.

168. Formation of Hexaphenylethane (Section 362). - In a test-tube put about 20 cc. of ethyl acetate or ether, 1 gram of triphenylchlormethane and 5 grams of finely granulated zinc. Close the tube with a tightly fitting cork, and shake frequently during 10 minutes. Dissolve a little iodine in ethyl acetate. Filter off the solution from the zinc rapidly through a fluted filter-paper into two test-tubes; to one add the iodine solution drop by drop. (Eq.) Shake the other solution in contact with air, (Eq.) set the tube aside, and examine it in a few minutes.

169. Properties of Naphthalene (Sections 369-371). - (a) Solubility of napthalene. - Test the solubility of naphthalene in hot and in old water, alcohol, and ether.

(b) Naphthalene and bromine. - Add bromine, drop by drop, to a few crystals of naphthalene. Compare the ease of substitution in the case of this hydrocarbon with that of benzene.

(c) Compound of naphthalene and picric acid. - Caution. - Picric acid stains the hands; it should be handled carefully. Dissolve 0.1 gram of the hydrocarbon and 0.2 gram of picric acid in 5 cc. of boiling alcohol. Allow the solution to cool spontaneously, and collect the yellow needles on a filter by suction (see §42, page 28) and wash them three times with cold alcohol, using 1 cc. of alcohol each time. Dry the crystals on a porous tile for half an hour in the air, and then determine their melting-point. The compound of naphthalene and picric acid has the formula C₁₀H₈.C₆H₂(NO₂)₃OH; it melts at 150.5°.

Note. - (b) Naphthalene and bromine react readily without the presence of a halogen carrier. The reaction can be used to prepare anhydrous hydrogen bromide.

(c) Picric acid, which is trinitrophenol, forms crystalline addition-products with a number of organic compounds; these are frequently made as an aid in the identification of certain hydrocarbons. (Sec anthracene, Section 372.)