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Chapter XVI

Nitro Compounds and Sulphonic Acids

170. Preparation of Nitrobenzene (Sections 383, 385). - To 80 cc. of concentrated sulphuric acid, contained in a one-half liter flask, add 70 cc. of concentrated nitric acid cautiously with shaking. Keep the mixture cool by placing the flask in cold water. To the cold mixture of acids add slowly and with vigorous shaking 50 grams of benzene from a small flask in portions of about 2 cc. The addition should take about one-half hour. From time to time test the temperature of the mixture which should be kept warm (40°-50°). If the temperature rises above 50°, place the flask in cold water. When all the benzene has been added, connect the flask with a reflux air-condenser and place it in a water-bath the temperature of which is about 60°. Shake the flask vigorously about every 5 minutes. At the end of an hour cool the flask in running water, pour the contents into a separatory funnel, and separate the upper layer of nitrobenzene from the acids. Wash with about 100 cc. of water. As nitrobenzene is heavier than water the lower layer is separated this time and shaken vigorously with a dilute solution of sodium hydroxide until the aqueous layer shows an alkaline reaction. The nitrobenzene is finally washed again with water, tested for free acid, which should not be present, separated, and warmed on the steam-bath with about 10 grams of anhydrous calcium chloride until the turbid liquid has become clear and no drops of water are visible. Decant or filter the liquid into a distilling flask, and distil using an air condenser. Reject the first part of the distillate which contains benzene and is apt to be cloudy due to the presence of a trace of water, and collect what distils at 204°-208°. Calculate the percentage yield obtained from the benzene.

Nitrobenzene boils at 207° (uncorrected), melts at 3°, and has the specific gravity 1.204 at 20°/4°. The yield should be about 60-70 grams.

Note. - A low yield of nitrobenzene may be the result of one of the following causes: Either the nitration was effected at too low a temperature and a quantity of benzene is recovered in the final distillation, or the temperature of the mixture reaches too high a point and a large amount of dinitrobenzene is formed. In the latter case there will be an appreciable residue in the flask when the temperature reaches 207° in the final distillation. This result is apt to occur if the benzene is added too rapidly at first, and the mixture of acid kept at too low a temperature; little nitration occurs, and when the mixture is heated in the water-bath, the reaction takes place rapidly, and the heat generated causes the temperature to rise above 60°, with the result that dinitrobenzene is formed. The effect on the result of the temperature at which nitration is carried out is marked, as can be seen by comparing with the above preparation that of dinitrobenzene described below.

It is necessary to wash the nitrobenzene free from nitric acid. If this is not done, the acid which remains dissolved in the nitrobenzene will react further with it when the product is distilled. Brown vapors of oxides of nitrogen will be given off, dinitrobenzene will be formed, and an explosion may occur. The residue of dinitrobenzene should not be distilled; it is apt to decompose violently when the flask is nearly empty. It may be dissolved out of the flask and crystallized from alcohol.

171. Properties of Nitrobenzene (Sections 384, 391) - (a) Solubility of nitrobenzene. - Describe the odor of nitrobenzene. Determine whether nitrobenzene is soluble in alcohol, ether, benzene, dilute hydrochloric acid, and a solution of sodium hydroxide. Mix about 2 cc. of nitrobenzene with 5 cc. of concentrated sulphuric acid and pour the mixture into water. How could you most readily separate into its constituents a mixture of benzene and nitrobenzene?

(b) Reduction of nitrobenzene to aniline. - Place 2 cc. of nitrobenzene and about 3 grams of granulated tin in an 8-inch test-tube, and add with constant shaking, in portions of 1 cc., 5 cc. of concentrated hydrochloric acid. The acid should be added at such a rate that the tube becomes hot, but care should be taken to avoid a violent reaction. Do not add the next portion of acid until the solution begins to cool. Finally, boil the solution for about 3 minutes, shaking the tube constantly. Cool under running water and add a strong solution of sodium hydroxide (1:2) until the precipitate first formed has largely dissolved (about 20 cc.). Remove with a pipette a few of the oily drops which separate. Put 1 drop of the oil on a watch-glass and place near it a drop of concentrated hydrochloric acid; bring the drops together by touching them with a glass rod. (Eq.) Aniline hydrochloride, C5H5NH2.HCl, is formed as a crystalline solid. Shake up a few drops of the aniline with 2 cc. of water and add a few drops of bromine water. Tribromaniline, Br3C6H2NH2, is precipitated.

(c) Reduction of nitrobenzene to phenylhydroxylamine (Section 424). - Dissolve 3 drops of nitrobenzene in 2 cc. of water and 2 cc. of alcohol, and add about 6 drops of a 10 per cent solution of calcium chloride and a pinch of zinc dust. Heat to boiling for one-half minute and filter. Add to the filtrate a strongly ammoniacal solution of silver nitrate. The phenylhydroxylamine, C6H5NHOH formed as the result of the reaction reduces the silver salt to metallic silver.

Notes. - (a) Many nitro compounds are insoluble in sodium hydroxide but impart a marked color to the solution of the alkali. Concentrated sulphuric acid is a valuable reagent to separate many nitro compounds from substances which are insoluble in the acid. The change in appearance of the nitrobenzene after solution and precipitation is due to the fact that when a solution of an oil insoluble in water is poured into water, the oil separates in minute globules which cause a milky appearance.

(b) The reduction of nitro compounds to amines, which are soluble in dilute acids, is a reaction of importance in their identification. It should be noticed, however, that substances other than nitro compounds can be reduced to amines. Among these are hydroxylamines, and azo, hydrazo and azoxy compounds (Sections 425, 426). The test is, nevertheless, of value in the identification of nitro compounds.

(c) Nitro compounds are reduced in neutral solution to hydroxylamine derivatives; the reaction takes place more readily in the presence of a little calcium chloride. The hydroxylamines reduce an ammoniacal solution of silver nitrate. Nitroso, azo, and azoxy compounds behave in a similar manner.

172. Preparation of m-Dinitrobenzene (Section 386). - To 5 grams of benzene contained in a 100 cc. flask, fitted with a cork and a piece of glass tubing about 2 feet long to serve as a reflux condenser, add slowly in small portions, shaking the flask constantly, a cooled mixture of 18 cc. of concentrated nitric acid and 36 cc. of concentrated sulphuric acid. The acid should be added at first in portions of about 1 cc.; about 10 minutes are required for the addition of the mixture. After the main reaction has ceased, remove the condenser and boil the mixture gently over a free flame for 5 minutes. Cool the contents of the flask to about 80° and pour the product slowly with stirring into about 200 cc. of water. Filter by suction, and wash twice with water. Dissolve the dinitrobenzene in 40 cc. of hot alcohol and set aside to crystallize. Filter off the crystals by suction and wash with 10 cc. of cold alcohol. Dry the crystals on a porous plate, and determine their melting-point. By adding 10 cc. of water to the filtrate a small additional amount of dinitrobenzene can be obtained. Calculate the percentage yield.

m-Dinitrobenzene crystallizes in colorless, odorless needles, which melt at 90°, and boil at 297°. It is readily soluble in hot alcohol; it dissolves in about 28 parts of alcohol at 20°. The yield in the preparation should be about 9 grams.

Note. - Small quantities of benzene and other hydrocarbons can be readily identified by converting them into solid nitro derivatives the melting-points of which can be determined. As little as 2 or 3 drops of benzene is sufficient for the identification of the hydrocarbon in this way. In working with such a small quantity proceed as follows: Mix 3 drops of benzene and 1 cc. each of concentrated sulphuric acid and concentrated nitric acid. Boil the mixture for one-half minute. Cool, and pour slowly into 10 cc. of water. Shake, filter by suction (§42, page 28), and wash with water. Dissolve in a boiling mixture of 4 cc. of water and 4 cc. of alcohol. Set aside to crystallize, filter, wash with 5 cc. of cold 50 per cent alcohol, and dry on a porous plate. The compound prepared in this way melts at 89°-89.5°.

Sulphonic Acids

173. Preparation of Sodium Benzenesulphonate (Section 392). - In a 250 cc. flask place 50 grams of fuming sulphuric acid which contains 8 to 10 per cent of sulphur trioxide (sp. gr. 1.90), connect the flask with a reflux condenser, and add, drop by drop, from a separatory funnel suspended in the condenser, 20 grams of benzene. During the addition of the benzene, which should take from 10 to 15 minutes, the flask should be shaken vigorously every minute or two. If all the benzene does not dissolve after continuous shaking for a few minutes, heat the mixture on a water-bath until solution is complete.

The sulphonic acid formed can be separated in the form of a salt by either of the methods described below. In the first method (a), which in the case of benzenesulphonic acid is simpler, advantage is taken of the fact that sodium benzenesulphonate is insoluble in a solution of sodium chloride. The second method (b) is the one commonly used to isolate sulphonic acids. It is based on the fact that the calcium salts of sulphonic acids are soluble in water, whereas calcium sulphate is very difficultly soluble. In certain cases it is advisable to prepare the barium salts.

(a) Dissolve 65 grams of sodium chloride in 250 cc. of water, and filter the solution if necessary. Measure off 200 cc. of the solution, and reserve the rest for washing the crystals of sodium benzenesulphonate. Pour into 200 cc. of the salt solution, slowly and with stirring, the mixture of benzenesulphonic acid and sulphuric acid. Filter off from the warm solution the diphenylsulphone, (C6H5)2SO2, which separates, and cool the filtrate in cold water, using ice if necessary. If crystals do not separate, scratch the side of the vessel containing the solution with a glass rod. Let the crystals stand with the mother-liquor for about 15 minutes to insure the complete separation of the salt. Filter by suction, and drain off as much of the liquid as possible by pressing the solid down firmly with a spatula. Disconnect the filter-flask from the pump, and cover the salt with a part of the sodium chloride solution which was reserved for this purpose. When the liquid has penetrated into the solid, remove it by the aid of the pump; repeat the washing with more of the salt solution. Draw off as much of the liquid as possible, and transfer the sodium benzenesulphonate to a porous plate to dry. The compound prepared in this way contains about 0.5 per cent of sodium chloride. Weigh the product and calculate the yield. About 30 grams of sodium benzenesulphonate should be obtained.

(b) Pour the mixture of sulphuric acid and benzenesulphonic acid into 500 cc. of water in a large evaporating dish, and heat it to boiling. Add precipitated calcium carbonate, which has been rubbed to a thick paste with water, until the solution no longer shows an acid reaction. Filter through a cotton cloth filter to separate most of the calcium sulphate, and wash the precipitate with hot water. If a small amount of solid passes through the filter the solution need not be refiltered, as the precipitate will be removed in a later filtration. Evaporate the solution to about one-half its volume, and add just enough of a solution of potassium carbonate to precipitate the calcium and convert the salt into potassium benzenesulphonate. If it is difficult to determine when the calcium has just been precipitated, filter off a few cubic centimeters of the solution from time to time, and add to the filtrate a drop of the solution of potassium carbonate. An excess of potassium carbonate should be avoided, for a sample of pure potassium benzenesulphonate is desired. Filter off the calcium carbonate, wash it with a little hot water, evaporate the solution to crystallization, and let it cool. Filter off the crystals and dry them on a porous plate. The filtrate on evaporation will yield a further quantity of the salt. Calculate the percentage yield obtained.

Potassium benzenesulphonate crystallizes in lustrous plates, which effloresce in the air. The salt is very soluble in water, and melts above 300° with decomposition.

Note. - (b) The free sulphonic acid can be obtained from the calcium salt by adding to its solution just enough sulphuric acid to precipitate the calcium as sulphate, evaporating the solution to a small volume, and placing it in a desiccator to evaporate over sulphuric acid. In the preparation of free sulphonic acids in this way it is better to prepare the barium salt, as barium sulphate is less soluble in water than calcium sulphate, and the free acid is not contaminated with a small amount of sulphate.

174. Identification of a Sulphonic Acid by Conversion into a Phenol (Section 399). - Melt about 1 gram of sodium hydroxide in a small iron or porcelain crucible, and add to the fused mass about 0.5 gram of sodium benzenesulphonate. Keep the mixture just above its melting-point, and do not let it char. Stir occasionally during 5 minutes. Cool, dissolve in water, acidify with dilute hydrochloric acid, and note the odor produced. Filter and add bromine water, drop by drop. Write equations for all the reactions involved in the test.

175. Preparation of Benzenesulphonyl Chloride (Section 395). - Convert all of the salt of benzenesulphonic acid obtained in experiment 173 above into benzenesulphonyl chloride as follows: Hood. Place the dry salt in a flask and add phosphorus pentachloride, which should be weighed in the hood, in the proportion of 3 parts by weight of the salt to 4 parts by weight of the chloride. Heat the mixture on the steam-bath, with occasional shaking, for half an hour. Cool, and add to the liquid about ten times its volume of ice-water. Shake about every 10 minutes in order to facilitate the reaction of the phosphorus oxychloride present with water. At the end of an hour pour off the water and wash the oil twice with water by decantation. Separate the benzenesulphonyl chloride and calculate the percentage yield.

The chloride can be used without further purification for the experiments given below. If it is desired to preserve the chloride for future use (see experiment 186, page 152), it should be taken up in ether, dried over calcium chloride and distilled under reduced pressure (§24, page 14) after the ether has been removed.

Benzenesulphonyl chloride is a colorless liquid which distils at 246°-247° with decomposition, and undecomposed at 120° under a pressure of 10 mm. It melts at 14.5°.

176. Preparation of Benzenesulphonamide (Section 396). - Put into a test-tube about 1 cc. of benzenesulphonyl chloride and add about 5 cc. of strong ammonia. Shake until a solid is formed and the odor of the chloride has disappeared. Pour off the liquid and wash twice with water by decantation. Add about 20 cc. of water; heat to boiling until the substance has dissolved. Filter, if necessary, and set the solution aside to crystallize. When cold, filter by suction, wash with cold water, and dry the amide for half an hour on a porous plate. Determine the melting-point of the amide. If the crystals do not melt sharply, recrystallize them from hot water.

Benzenesulphonamide crystallizes from hot water in needles, which melt at 156°.

Note. - Sulphonamides are frequently made in the identification of sulphonic acids or their salts. The preparation can be carried out with small quantities in a few minutes. Proceed as follows: Warm together in a testtube on a steam-bath about 0.5 gram of the salt with an equal volume of phosphorus pentachloride, until the mixture liquefies. Cool, pour 5 cc. of water into the tube, warm gently, and shake for about 1 minute. Pour off the liquid and wash twice by decantation with .5 cc. of cold water. Add 5 cc. of ammonia and proceed as described in the experiment above.

177. Preparation of p-Toluenesulphonic Acid (Section 397). - To 25 grams of toluene in a 200 cc. flask add 25 cc. of pure concentrated sulphuric acid, and place on the steam-bath. Allow the mixture to stand until the toluene has dissolved. This will require about 2 hours if the flask is shaken once in a while. Let the solution cool; before it solidifies pour it, with stirring, into 100 cc. of pure concentrated hydrochloric acid. Heat the mixture on the steam-bath until the solid dissolves. Set aside to crystallize. Filter by suction, and wash the crystals with concentrated hydrochloric acid. Press the crystals to remove as much of the mother-liquor as possible and dry them on a porous plate. Weigh the toluenesulphonic acid and calculate the percentage yield from the toluene used.

p-Toluenesulphonic acid crystallizes from water, in which it is very soluble, in long colorless needles. It is less soluble in concentrated hydrochloric acid. It melts at 104°-105°. The yield should be about 25 grams. The amide can be prepared by the method described in experiment 176; it melts at 136°-137°.

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