Chapter XVII

Halogen Derivatives of Aromatic Hydrocarbons

178. Preparation of Brombenzene (Section 403). - To 50 grams of benzene in a 200 cc. flask add 35 cc. (105 grams) of bromine. Fit the flask with a cork stopper through which passes a glass tube, bent twice at right angles. The longer arm of the tube which passes through the cork should be about 2 feet long. Place below the end of the shorter arm a flask containing 100 cc. of water to absorb the hydrobromic acid formed in the reaction; the end of the tube should be just above the surface of the water. Put the flask containing the benzene and bromine in a pail or large beaker containing ice-water, and add to the flask two clean iron tacks or nails. Allow the reaction to proceed until the next laboratory exercise. Wash the product with water, and with a little sodium hydroxide if necessary, until any excess of bromine has been removed. Place the liquid in a separatory funnel, separate it, and dry it with anhydrous calcium chloride. Fractionate three times, collecting the following fractions: up to 120°, 120°-150°, 150°-160°, 160°-200°. The fraction boiling at 150°-160° is practically pure brombenzene. Calculate the molecular proportions in which the reacting substances were used in the preparation and the percentage yield obtained. Determine the specific gravity of the brombenzene (§56, page 37).

The residue boiling above 200° contains p-dibrombenzene. It may be poured while still hot onto a watch-glass, and crystallized from alcohol. (See directions under the preparation of dibrombenzene.)

Put the hydrobromic acid collected as a by-product in the reaction into the bottle provided for hydrobromic acid residues. This preparation yields a large amount of hydrobromic acid, as one-half of the bromine used is converted into the acid. On distillation the aqueous solution yields an acid than can be used conveniently for the preparation of alkyl halides.

Brombenzene boils at 156°, and has the specific gravity 1.491 at 20°. The yield in this experiment should be from 60 to 65 grams.

Note. - In order to obtain a good yield of brombenzene, it is advisable to keep the mixture of benzene and bromine cold during the reaction. The product obtained consists of a mixture of benzene, brombenzene, and dibrombenzene. The effect of the temperature on the reaction is clearly seen by comparing the method used in this preparation with that employed to prepare dibrombenzene. See the experiment below.

179. Preparation of p-Dibrombenzene (Section 404). - Hood. - To 5 grams of benzene in a small round-bottomed flask add 10 cc. of bromine. Close the flask with a cork through which passes a piece of glass tubing about 3 feet long to serve as a reflux condenser. Place the flask in the hood, drop into it a tack or piece of clean iron filing, and allow the reaction to proceed spontaneously to completion; this usually takes about 30 minutes. Then heat the flask cautiously over a free flame for 2 minutes to expel most of the excess of bromine. Add about 50 cc. of water and heat to boiling, shaking vigorously. Cool until the dibrombenzene solidifies, and decant off the liquid. Repeat the washing with hot water twice, adding to the liquid the last time about 50 cc. of a solution of sodium hydroxide. Cool and wash the crystals once more with cold water, decant off the water, add 75 cc. of alcohol and a little bone-black, and heat to boiling. Filter the hot solution, and add 20 cc. of water. Set aside to, crystallize. Separate the crystals and determine their melting-point. Calculate the percentage yield obtained.

p-Dibrombenzene crystallizes in colorless leaflets, which melt at 89° and boil at 219°. The yield should be about 13 to 14 grams.

180. Properties of Aromatic Halogen Compounds (Section 401). - (a) Solubility of halogen compounds. - Determine whether brombenzene is soluble in the following: Water, alcohol, ether, concentrated sulphuric acid, dilute hydrochloric acid, and a solution of sodium hydroxide.

(b) Inflammability of halogen compounds. - Wrap a strip of copper gauze about 1 cm. wide around a piece of wire to serve as handle, so that a roll about as thick as a lead pencil is formed. Pour a few drops of the liquid or place a few crystals on the gauze and place it for an instant in a flamee Remove the gauze and note whether soot is formed by holding the burning substance in front of a piece of white paper. Determine whether the following substances burn, and whether soot is produced: Benzene, brombenzene, dibrombenzene, benzyl chloride, ethyl bromide, chloroform, carbon tetrachloride. What conclusions can be drawn from the experiment? (See the note to experiment 164d, page 129.)

(c) Comparison of the behavior of compounds containing the halogen joined directly to the benzene ring with those containing the halogen in a side-chain. - Read carefully Section 401, 402. Compounds of the two classes behave differently with an alcoholic solution of potassium hydroxide. In order that conclusive results may be drawn from the test, it is necessary to have a solution of the alkali which is free from halogen. As all samples of commercial potassium hydroxide contain potassium chloride, prepare a solution by dissolving about 1 gram of the hydroxide in 20 cc. of absolute Alcohol; filter from the undissolved carbonate and chloride and use the clear filtrate for the tests[1]. Test a part of the solution so prepared for a halide as follows: Dilute with water, acidify with nitric acid, and add silver nitrate. Add 3 drops of benzyl chloride to 5 cc. of the alcoholic potassium hydroxide solution, and boil gently for 2 minutes. Dilute with water, acidify with pure dilute nitric acid, and add a few drops of a solution of silver nitrate. Repeat the experiment, using brombenzene. In order not to confuse with silver bromide the cloudiness produced when the brombenzene is precipitated from the alcoholic solution by the addition of water, add 3 drops of brombenzene to 5 cc. of alcohol and dilute with the amount of water used in the test. Compare the appearance of the solution with the one which was heated. If the unchanged precipitated organic halogen compound interferes with the observation of the silver halide, it can be removed by shaking the solution with ether, and adding the silver nitrate to the clear aqueous solution.

Note. - (c) In applying this test it is necessary to determine first whether the compound contains any free halogen acid, which has been produced as the result of decomposition on standing. Shake a little of the substance with water, and test the aqueous solution with silver nitrate. If a precipitate is formed, the halide must be washed with water until the latter gives no test for halides.

181. Preparation and Properties of Triphenyichlormethane (Section 409). - (a) Place in a 500 cc. flask 25 grams of carbon tetrachloride, 50 grams of benzene, 100 cc. of carbon disulphide, and 30 grams of anhydrous aluminium chloride. It is important to avoid exposing the aluminium chloride to the air any longer than necessary. Attach to the flask by means of a stopper a small drying tube containing anhydrous calcium chloride, and leave the flask in the hood for at least 2 days. At the end of this time the mixture is poured onto 200 grams of ice, and stirred occasionally until the dark-colored addition-product of aluminium chloride and triphenylchlormethane has decomposed, and the latter salt has dissolved in the carbon disulphide. Separate the carbon disulphide, and filter it through a paper over which carbon disulphide has been poured. Add to the filtrate anhydrous calcium chloride, and shake occasionally until the solution is clear. Filter the solution through a dry filter-paper into a distilling flask. Attach the flask to a water-jacketed condenser, and distil off the solvent on a water-bath. Pour the residue into a beaker, and when it is cool, add 20 cc. of petroleum ether. Allow the mixture to stand for 15 minutes. Filter by suction, and wash the triphenylchlormethane with 20 cc. of petroleum ether. Let the crystals stand in the air for a few minutes until dry. Weigh the product and calculate the percentage yield from the carbon tetrachloride used.

Triphenyichlormethane melts at 109°-111°. It can be crystallized from boiling ligroin, in which it is readily soluble hot and difficultly soluble cold. Since triphenylchlormethane is decomposed when heated with water, anhydrous solvents should be used. The compound can be obtained in a very pure condition by recrystallizing it from boiling acetyl chloride. The yield in this preparation is about 85 per cent.

Note. - If the dark red oil which separates at first in the reaction is decomposed at once by water, benzophenone chloride, (C₆H₅),CCl₂, is obtained. This red oil, which is a molecular compound of benzophenone chloride and aluminium chloride, reacts on standing with more benzene and is converted into a substance of the composition (C₆H₅)₃CCl.AlCl₃, which is a crystalline solid. Water decomposes this addition-product into its constituents.

(b) Reaction with water. - Powder about 0.1 gram of triphenylchlormethane and shake it in a test-tube with water. Let the mixture stand a few minutes and then test the aqueous solution for hydrochloric acid with litmus paper and with silver nitrate.

(c) Reaction with sulphuric acid. - In a small beaker treat about 2 grams of triphenylchlormethane with 5 cc. of concentrated sulphuric acid, and shake until solution is complete. Is hydrogen chloride given off? Compare the results with those obtained with ethyl bromide (experiment 132a, page 98). Pour the solution into 50 cc. of water, filter off by suction the precipitate of triphenylcarbinol, wash with water, and dry it. Add a small amount of the carbinol to a little concentrated sulphuric acid on a watch-glass. Grind thoroughly some of the carbinol in a mortar with a little concentrated hydrochloric acid for about 1 minute. Remove the product with a spatula to a porous plate and let it dry. Add the compound to a little concentrated sulphuric acid. Write equations for all reactions which take place.

(d) Reaction with silver nitrate. - Dissolve a bit of triphenylchlormethane in a little acetone and add an alcoholic solution of silver nitrate. (Eq.)

(e) Reaction with metals. - Repeat experiment 168, page 133.


[1] A solution which can also be used for the test may be prepared by dissolving about 0.5 gram of clean sodium, free from any crust, in 20 cc. of alcohol and adding a few drops of water.