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Chapter XIX

Diazo Compounds

187. Preparation of Phenol from Aniline (Section 429). - Prepare some iodo-starch paper for use as an indicator for nitrous acid as follows: Grind about 1 gram of starch with a few cubic centimeters of water and pour the mixture into 200 cc. of boiling water. Cool the solution and dissolve in it a crystal of potassium iodide. Insert pieces of filter-paper into the solution, and hang them up to dry in a place free from acid vapors. The moist paper can be used in the following preparation.

In a 500 cc. beaker pour 25 cc. of concentrated sulphuric acid into 200 cc. of water, and add to the warm solution 20 grams of aniline slowly and with vigorous stirring. Cool, and add 100 grams of ice. Dissolve 16 grams of sodium nitrite in 100 cc. of water, and add the solution in small quantities, with stirring, to the solution of aniline sulphate, until the amine has been diazotized. This can be determined by testing the solution for nitrous acid, which will be present when the nitrite has been added in excess. The amount of nitrite which has been dissolved is a slight excess over that required for the reaction. When the larger portion of the nitrite has been added, stir vigorously, place a drop of the solution on a piece of iodo-starch paper. If free nitrous acid is present a blue spot will form. (Eq.) Continue the addition of nitrite until the solution, after thorough stirring and standing 15 to 20 seconds, produces an immediate blue color with the iodo-starch paper. Pour the solution of the diazonium salt into a flask, and heat it at 40°-50° on a water-bath for half an hour. Distil with steam (§28, page 18) and collect about 300 cc. of distillate. As long as any phenol distils over the distillate will give a color with a solution of ferric chloride. Saturate the solution of phenol with sodium chloride, and extract three times with ether, using 100 cc. each time. Let the ether solution stand over night with about one-fifth its volume of anhydrous sodium. Filter the ether, and distil it off from a flask on a water-bath (§34, page 22). Wash the drying agent with some of the recovered ether, and add this solution to the flask. Pour the residue into a small flask, and distil using an air-condenser. Collect the fraction boiling at 180°-183°. Calculate the percentage yield.

Phenol melts at 43°, boils at 183°; it is soluble in about 15 parts of water at 16°. The yield in this preparation should be about 15 grams.

Note. - In the preparation of diazonium salts it is advisable to keep the reaction-mixture cold during the addition of the nitrite solution, even in the case when the resulting salt is to be decomposed by water; in this way the formation of by-products is largely avoided.

A small amount of p-hydroxydiphenyl, C6H5.C6H4OH, is formed as the result of the condensation of the diazonium salt with some of the phenol formed in the reaction. The compound can be obtained by filtering the hot solution from which the phenol has been distilled. On cooling, p-hydroxydiphenyl separates out in plates which melt at 164°.

188. Preparation of Iodobenzene from Aniline (Sections 429, 405). - Mix together in a beaker 150 cc. of water, 12 cc. of concentrated sulphuric acid, and 10 grams of aniline. Cool the solution, and add 100 grams of ice. Diazotize the solution by adding a solution of 8 grams of sodium nitrite dissolved in 40 cc. of water in the way described in the above experiment (preparation of phenol). Pour the solution of the diazonium salt into a liter flask and add a solution of 22 grams of potassium iodide in 50 cc. of water. Place the flask in a water-bath heated to 70°-80° until the evolution of nitrogen ceases. Make the solution strongly alkaline by adding a solution of sodium hydroxide, and distil with steam as long as an oil comes over. It is necessary to have the end of the tube through which the steam enters the flask pass to the bottom of the latter, so that the steam passes through the heavy oil, and keeps it constantly in motion. Separate the iodobenzene, and dry it with calcium chloride. Filter it into a small flask and distil, using an air-condenser. Calculate the percentage yield.

Iodobenzene boils at 188° (corrected). The yield should be about 20 grams.

189. Preparation of p-Tolunitrile: Sandmeyer Reaction (Section 429). - Caution. - In this preparation cuprous cyanide is to be used. The compound is made from copper sulphate and potassium cyanide. As the reaction involves the liberation of cyanogen, a very poisonous gas, it should be carried out under the hood - with a good draught, and care should be taken not to breathe any of the gas liberated. Care should be exercised also in handling the potassium cyanide.

Dissolve 50 grams of copper sulphate in 200 cc of hot water, place the solution in a 2 liter flask on a water-bath heated to boiling under the hood, and add in small portions a solution of 55 grams of potassium cyanide dissolved in 100 cc. of water. Leave the solution heating on the water-bath, and prepare the diazo compound as follows: Add 20 grams of p-toluidine to a mixture of 45 cc. of concentrated hydrochloric acid and 100 cc. of water, and stir. Place the vessel containing the solution in cold water, add 100 grams of ice, and diazotize with a solution of 16 grams of sodium nitrite in 60 cc. of water, taking precaution to avoid an excess of the nitrite solution. (See experiment 187 above.) Hood. - Pour the diazo solution into the hot solution of cuprous cyanide; add about 10 cc. at a time and shake vigorously. Avoid running the solution down the side of the flask; it should fall directly into the solution of the cyanide. This precaution is taken to avoid as much as possible the formation of cresol, CH3.C6H4.OH, as the result of the interaction of the diazo compound with water. The addition of the diazo solution should take about 15 minutes. Leave the flask on the waterbath for about 15 minutes more, and when the evolution of nitrogen ceases, distil with steam under the hood. If the tolunitrile solidifies in the condenser, turn off the water in the latter for a short time; the steam will warm the water in the condenser, and the nitrile will melt. Distil as long as an insoluble substance comes over in appreciable amounts. If only a small amount of cresol has been formed the nitrile will solidify in the receiver. Separate the crystals and let them dry spontaneously on a porous plate. As tolunitrile melts at 38°, the crystals should not be put in a warm place. Weigh the crystals, and calculate the percentage yield. Save the preparation for a future experiment.

If much cresol has been formed during the reaction the product will not solidify. In this case, extract with a small amount of ether, shake the ethereal solution with an equal volume of a solution of sodium hydroxide, and dry over calcium chloride. Pour off the solution, distil off the ether, and distil, using an air-condenser.

p-Tolunitrile melts at 38° and boils at 218°. The yield should be about 13 grams.

190. Preparation of Diazoaminobenzene (Section 430). - Dissolve 10 grams of aniline in a mixture of 100 cc. of water and 20 cc. of concentrated hydrochloric acid. Add 100 grams of ice. When the solution is cold add a solution of 8 grams of sodium nitrite in 40 cc. of water. Dissolve 10 grams of aniline in a mixture of 7 cc. of concentrated hydrochloric acid and 50 cc. of water, and add 50 grams of ice. Pour the solutions of aniline hydrochloride slowly with stirring, into the solution of the diazo compound. (Eq.) Dissolve 25 grams of hydrated sodium acetate in 50 cc. of water, and add the solution to the mixture. At the end of half an hour, add about 300 cc. of water, stir, and filter by suction; wash the precipitate with cold water. Press the diazoaminobenzene on a porous plate, and let it stand until dry; do not heat it. Weigh the product and calculate the percentage yield.

Crystallize a little of the compound from petroleum ether as follows: Digest for 2 or 3 minutes 2 grams of the diazoaminobenzene with 100 cc. of petroleum ether, filter hot, evaporate to about 50 cc., and set aside the solution to crystallize.

Diazoaminobenzene crystallizes in yellow plates, which melt at 98°; it explodes when heated above its melting-point. It is insoluble in water, and is soluble in benzene, ether, and alcohol. The yield obtained in the above preparation is about 15 grams.

Note. - The sodium acetate is added to interact with the excess of hydrochloric acid present:


The salt is frequently added to solutions containing strong acids when it is desired to bring about a reaction which does not take place in the presence of such acids. (See diazoamino compounds, Section 430.)

191. Preparation of Aminoazobenzene (Section 431). - Mix in a small beaker 10 grams of diazoaminobenzene, 20 grams of aniline, and the aniline hydrochloride prepared as follows: Dissolve 5 cc. of aniline in 10 cc. of concentrated hydrochloric acid, cool in running water, filter by suction, press on a porous plate.

Allow the salt to dry for about 5 minutes. Heat for 1 hour on a water-bath the mixture so that the temperature of the latter is between 50°- and 60°; stir frequently. Dissolve the product in 50 cc. of glacial acetic acid, and pour the solution slowly into 400 cc. of water, stirring vigorously. After about 5 minutes filter by suction, wash with water and set aside to dry.

Convert a part of the aminoazobenzene into the chloride as follows: Dissolve 2 grams of the compound in 10 cc. of alcohol and pour the solution into a boiling mixture of 20 cc. of concentrated hydrochloric acid and 200 cc. of water. Boil for 10 minutes and set aside to crystallize. Filter and wash the crystals with water containing some hydrochloric acid.

Crystallize the rest of the aminoazobenzene as follows: Dissolve the substance in alcohol in the proportion of 1 gram of aminoazobenzene to 10 cc. of alcohol. Heat to boiling and add water until the solution begins to cloud; set aside to crystallize. Filter off the crystals and wash them with a mixture of equal volumes of alcohol and water.

Aminoazobenzene crystallizes in yellow prisms, melts at 127°, and distils undecomposed above 360°. It crystallizes well from hot water in which it is difficultly soluble.

192. Preparation and Properties of Phenylhydrazine (Section 433). - (a) In a beaker pour slowly, with stirring, 20 grams of aniline into 170 cc. of concentrated hydrochloric acid. Cool the beaker to 0° in a freezing mixture of ice and salt, and add, drop by drop, from a separatory funnel, a mixture of 20 grams of sodium nitrite dissolved in 100 cc. of water, until a drop of the solution when diluted with a few drops of water produces a blue color with iodo-starch paper. (See experiment 187, page 153.) The mixture should be stirred during the addition of the sodium nitrite. Dissolve 120 grams of hydrated stannous chloride in 100 cc. of water, cool the solution with ice-water, and pour it slowly into the solution of the diazo compound. After an hour filter off by suction the precipitate of phenylhydrazine hydrochloride, using a porcelain funnel or a filter-plate and hardened filter-paper. Remove as much of the mother-liquor as possible by pressing the crystals, and then transfer them to a flask and add an excess of a solution of sodium hydroxide. Extract with ether, dry over anhydrous potassium carbonate, and distil off the ether. The resulting oil can be used for the following experiments. If it is desired to keep the phenylhydrazine, it should be distilled; as it decomposes slightly on distillation it is better to distil it under diminished pressure (§24, page 14). The compound can also be purified by freezing it and pouring off the liquid which does not solidify.

Phenylhydrazine is nearly colorless when pure; it melts at 17.5°, boils at 242°, and has the specific gravity 1.097 at 23°. The yield in this preparation should be about 10 grams.

(b) Conversion of phenyihydrazine into benzene. - Dissolve 35 grams of copper sulphate in 100 cc. of water in a 500 cc. flask connected with a reflux condenser. Heat the solution to boiling, remove the flame, and drop into the solution from a separatory funnel suspended in the condenser 10 grams of phenylhydrazine dissolved in 10 cc. of acetic acid and 50 cc. of water. As soon as the phenylhydrazine has been added, arrange the condenser for distillation, and distil as long as an oil comes over. Separate the oil and shake it with anhydrous calcium chloride until no drop of water can be seen, and the benzene is clear. Distil the oil and note its boiling-point.

Benzene, the product of the reaction, boils at 80.4°. The yield should be about 6 grams.

Add 2 drops of phehylhydrazine to Fehling's solution.

(c) Formation of a urea derivative from phenylhydrazine: phenyl-semicarbazide. - Dissolve 1 cc. of phenylhydrazine in 1 cc. of glacial acetic acid and 5 cc. of water, and add a solution of 0.5 gram of potassium cyanate dissolved in 2 cc. of water. Heat to boiling, cool, and shake. Filter off the precipitate, wash with cold water, and crystallize it from boiling water. Phenylsemicarbazide melts at 172°.

(d) Formation of a phenylhydrazone. - Dissolve 2 drops of phenylhydrazine in 12 cc. of alcohol, and add 12 cc. of water. Add to the solution 2 drops of benzaldehyde and boil for half a minute. Cool, shake thoroughly, and filter by suction (§43, page 29). Wash with 10 cc. of 50 per cent alcohol, and recrystallize the precipitate from 25 cc. of boiling 50 per cent alcohol. Cool, filter off the crystals, and wash them with 10 cc. of 50 percent alcohol. Dry the crystals, and determine their melting-point.

Benzalphenhydrazone is a white crystalline compound which melts at 156°.

Notes. - (b) The reaction between phenylhydrazine and copper sulphate takes place according to the following equation:

C6H5NH.NH2 + CuSO4 = C6H6 + N2 + Cu + H2SO4

(c) Phenylhydrazine is a derivative of ammonia, and its salts, therefore, react in many cases like ammonium salts and the salts of amines. The preparation of phenylsemicarbazide from phenylhydrazine salts is analagous to the preparation of urea from ammonium salts; the equations for the two reactions are similar. (See Section 180.)





(d) Phenylhydrazine is a valuable reagent for identifying aldehydes. The method of preparing the phenyihydrazones is illustrated by this experiment.

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