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Chapter XVIII

Aromatic Amines

182. Preparation of Aniline (Section 415). - Provide a 1 liter flask with a cork and tubes arranged for distillation with steam (§28, page 18); the tubes will be needed later in the preparation and should be ready for use. Place in the flask 25 grams of nitrobenzene and 50 grams of granulated tin, and connect it with a reflux air-condenser. Add to the mixture 125 cc. of concentrated hydrochloric acid in the following way: Add about 10 cc. of the acid and shake vigorously. If reaction does not take place in a minute or two, warm gently until there is an evolution of hydrogen and the reaction produces sufficient heat to keep the flask warm. When the reaction subsides, add another portion of 10 cc. of acid, and shake. Continue in this way until about one-half the acid has been added. If the reaction becomes too vigorous and there is chance of vapor escaping from the flask, place the latter in cold water. Care must be taken not to keep the flask so cold that the reaction does not proceed rapidly.. The last half of the acid may be added in portions of about 20 cc. When all the acid has been added, heat the flask on a boiling water-bath, shaking occasionally, until no drops of nitrobenzene can be seen floating on the surface of the liquid. The odor of nitrobenzene should also disappear. From 1/2 to 1 hour is required to complete the reaction in this way[1]. Add to the hot solution 50 cc. of water, place the flask in cold water, and add a solution of 90 grams of sodium hydroxide in 150 cc. of water until most of the precipitate first formed has dissolved and the solution shows a strong alkaline reaction. The mixture must not be cooled too strongly; it is necessary only to keep it from boiling in order to prevent loss of aniline. Distil with steam. The aniline passes over at first as a colorless oil. As the compound is somewhat soluble in water (about 1 part in 30), the distillation should be continued after the drops cease to form; change the receiver at this point and collect about 100 cc. of the solution. Combine the two distillates and saturate the solution with sodium chloride, using 20 grams of salt for each 100 cc. of solution. Extract twice with ether using about 200 cc. each time (§31, page 20). Separate the ether, place it in a bottle, and add 3 or 4 sticks of potassium hydroxide. Stopper the bottle and let the solution stand over night. Filter the solution into a dry flask. The water extracted from the ethereal solution forms with the potassium hydroxide a saturated solution of the alkali; care should be taken not to allow this to get into the flask. Distil off the ether on a water-bath (see §34, page 22). Transfer the residue to a small dry flask, wash out the first flask with a few cubic centimeters of the dry ether which has just been distilled off, and add the solution to the aniline. Distil over a free flame using an air-condenser, heating cautiously until the small amount of ether has come over, and collect the fraction boiling at 180°-184°. Calculate the percentage yield obtained.

Aniline boils at 182° (183.7° corrected), and has the specific gravity 1.024 at 16°. The yield should be from 85 to 90 per cent.

183. Properties of Aniline (Sections 411, 415). - (a) Solubility of aniline. - Determine whether aniline is soluble in the following: water, dilute sodium hydroxide, dilute hydrochloric acid, concentrated sulphuric acid, alcohol, and benzene.

(b) Salts of aniline. - Add 2 cc. of aniline to 5 cc. of water and add 1 cc. of concentrated sulphuric acid. Heat to boiling and set the solution aside to crystallize. (Eq.)

Heat together in a small beaker about 5 cc. of aniline, 5 cc. of water, and 10 cc. of concentrated hydrochloric acid. When the solution is cold, filter off the crystals by suction; do not wash them with water in which they are very soluble. Dry the crystals in a beaker on the steam-bath. The salt will be needed for the following experiments.

(c) Double salts containing aniline - Make a saturated solution of about 1 gram of aniline hydrochloride in a little water. To one-half of the solution add a few drops of a solution of platinum chloride, and to the other a few drops of saturated solution of stannous chloride, and cool. (Eqs.)

(d) Base-forming property of aniline. - Make a saturated aqueous solution of aniline by shaking about 1 cc. of the amine with 15 cc. of water. Test the clear solution with litmus paper, and add some of it to a solution of aluminium sulphate or chloride. Compare the results with those obtained with methylamine (experiment 113b, page 86). Explain.

(e) Hydrolysis of salts of aniline. - Dissolve a little aniline hydrochloride in water and test the solution with litmus paper. Is the result in accord with those obtained in (d) above? Explain.

(f) Reaction of salts of aniline with bases. - Add to a concentrated aqueous solution of aniline hydrochloride a solution of sodium hydroxide. (Eq.)

(g) Aniline and acetyl chloride. - Add acetyl chloride, drop by drop, to 1 cc. of aniline as long as an evident reaction takes place. Add about 15 cc. of water and heat to boiling, filter hot, and cool in running water. (Eq.) Filter off the precipitate by suction, wash twice with cold water, and dry the crystals on a porous plate for half an hour. Determine the melting-point of the crystals. Acetanilide melts at 112° Amines are frequently identified by converting them into acetyl derivatives.

(h) Aniline and bromine. - Add bromine-water, drop by drop, to an aqueous solution of aniline until the solution becomes yellow. Tribromaniline, which is precipitated, crystallizes from alcohol and melts at 116°.

(i) Aniline and nitrous acid. - Dissolve about 1 cc. of aniline in 2 cc. of concentrated hydrochloric acid and 5 cc. of water. Cool the solution in running water. Add a cold solution of about 0.5 gram of sodium nitrite in 5 cc. of water. Shake, and heat gently. Observe that a gas is given off, and note the odor of the solution. (Eq.)

(j) Aniline and ferric chloride - Add to an aqueous solution of aniline a few drops of a dilute solution of ferric chloride.

(k) Aniline and hypochlorites. - Add to a dilute aqueous solution of aniline a solution of bleaching powder or of sodium hypocblorite.

(l) Aniline and potassium bichromate. - Mix on a watch-glass a drop of aniline with 3 drops of concentrated sulphuric acid, and add 2 drops of a solution of potassium bichromate. Note the color, and heat on the steam-bath for a few minutes.

(m) Aniline and methyl iodide - Mix together 3 cc. of aniline and 2 cc. of methyl iodide, and let the mixture stand in a stoppered tube until the next exercise. (Eq.) Remove the crystals, dry them on a porous plate, and then treat them with a strong solution of sodium hydroxide. (Eq.) Note the odor. Is it like that of aniline?

Notes. - (a) The solubility of many amines in dilute hydrochloric acid serves to distinguish them from other nitrogen compounds. Amines which contain a number of strongly negative groups are insoluble in the acid.

(g) Acetyl chloride is a useful reagent to distinguish primary and secondary amines from tertiary amines. Alcohols and phenols also react with acetyl chloride.

(h) Many amines give a precipitate of an insoluble substitution-product when treated with bromine water; certain phenols give a similar reaction.

184. Properties of Methylaniline (Sections 412, 420). - (a) Methylaniline and acetyl chloride. - Add acetyl chloride, drop by drop, to about 1 cc. of methylaniline, add water, and heat to boiling. Cool. (Eq.) The acetyl derivative formed melts at 102°.

(b) Methylaniline and nitrous acid. - Repeat experiment 183i, page 150, using methylaniline in place of aniline. Is an oil formed? Compare the results in the two experiments. (Eq.)

185. Properties of Dimethylaniline (Sections 413, 422). - (a) Dimethylaniline and acetyl chloride. - Repeat experiment 184a, page 151, and compare the results obtained when primary, secondary, and tertiary amines are treated with acetyl chloride.

(b) Dimethylaniline and nitrous acid: nitroso-dimethylaniline. - Dissolve 5 grams of dimethylaniline in a mixture of 10 cc. of concentrated hydrochloric acid and 40 cc. of water. Cool the mixture in ice-water, and add, slowly with stirring, a solution of 3 grams of sodium nitrite in 20 cc. of water. At the end of 15 minutes filter off the salt by suction, wash with dilute hydrochloric acid, and crystallize a little of it from warm (about 60°) dilute hydrochloric acid. Shake up the rest with an excess of a solution of sodium hydroxide. Dissolve a little of the precipitate in ether, filter the solution, and let it evaporate. Boil some of the free amine with sodium hydroxide, note the odor of the gas given off, and test it with moist litmus paper. Write equations for all the reactions.

186. Use of Benzenesulphonyl Chloride in Distinguishing the Three Types of Amines (Section 395). - Shake together for 2 or 3 minutes 0.5 cc. of aniline, 50 cc. of a strong aqueous solution of sodium hydroxide (1 to 4) and 2 cc. of benzenesulphonyl chloride. Warm gently until the odor of the chloride disappears. Filter and add hydrochloric acid to the filtrate. (Eqs.)

Repeat the experiment using methylaniline and then dimethylaniline. Test the solubility in dilute hydrochloric acid of the substances separated in these two cases by filtration. Write equations for all reactions, and state clearly how the reagent serves to distinguish from one another the three classes of amines.

Note. - A few primary amines yield products with benzenesulphonyl chloride which do not dissolve in a dilute solution of sodium hydroxide. This is due to the fact that a difficultly soluble sodium salt is formed, which is partially hydrolyzed. Such compounds are converted into sodium salts by strong solutions of the base.

[1] If the preparation can not be completed in one exercise it is well to stop at this point; if the alkali to liberate the aniline is added, and the mixture allowed to cool, it is necessary to heat the flask to boiling before the. contents are distilled with steam. The distillation requires about an hour.

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