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Senin, 02 Agustus 2010

PHENYLHYDRAZINE


Place 130 ml. of concentrated hydrochloric acid in a 1 -5 litre round-bottomed flask, equipped with a mechanical stirrer and immersed in a freezing mixture of ice and salt. Start the stirrer and, when the temperature has fallen to about 0°, add 60 g. of finely-crushed ice (1), run in 47-5 g. (46-5 ml.) of pure aniline during about 5 minutes, and then add another 60 g. of crushed ice.
Dissolve 35 g. of sodium nitrite in 75 ml. of water, cool to 0-3°, and run in the cold solution from a separatory funnel, the stem of which reaches nearly to the bottom of the flask. During the addition of the nitrite solution (ca. 20 minutes), stir vigorously and keep the temperature as near 0° as possible by the frequent addition of crushed ice. There should be a slight excess of nitrous acid (potassium iodide- starch paper test) at the end of 10 minutes after the last portion of nitrite is added.

In the meantime, prepare a sodium sulphite solution as follows. In a 2-litre beaker or bolt-head flask place 50 g. of sodium hydroxide (2) and add 500 ml. of water. When the sodium hydroxide has dissolved, add 112-5 g. of recrystallised sodium bisulphite (3), and stir mechanically until the solid has dissolved. Cool the resulting solution to about 25° and add a few drops of phenolphthalein indicator solution. Introduce small quantities of sodium bisulphite until the pink colour of the solution just disappears, then stir in a further 12 g. of sodium bisulphite (the total weight required should not exceed 135-140 g.). Cool this solution, with stirring, to about 5° by immersion in an ice bath, then add about 60 g. of crushed ice. Run in the ice-cold diazonium solution as rapidly as possible, while stirring vigorously.

The reaction mixture immediately acquires a bright orange-red colour. Slowly heat the solution to 60-70° on a water bath and maintain this temperature for 30-60 minutes, i.e., until the colour becomes quite dark. Acidify the solution to litmus with con- centrated hydrochloric acid (40-50 ml. are required) ; continue the heating on a boiling water bath until the colour becomes much lighter and in any case for 4-6 hours. If any solid is present, filter the solution. To the hot, clear solution add, with stirring, 500 ml. of concentrated hydrochloric acid ; cool, first in running water, and then in a freezing mixture to 0°. The phenylhydrazine hydrochloride separates as yellowish or pinkish crystals. Collect them on a Buchner funnel, drain, wash with 25 ml. of dilute hydrochloric acid (1:3) , and press well with a large glass stopper (4). Liberate the free base by adding to the phenylhydrazine hydrochloride 125 ml. of 25 per cent, sodium hydroxide solution. Extract the phenyl- hydrazine with two 40 ml. portions of benzene, dry the extracts with 25 g. of sodium hydroxide pellets or with anhydrous potassium carbonate : thorough drying is essential if foaming in the subsequent distillation is to be avoided. Most of the benzene may now be distilled under atmo- spheric pressure, and the residual phenylhydrazine under reduced pressure. For this purpose, fit a small dropping funnel to the main neck of a 100 ml. Claisen flask (which contains a few fragments of porous porcelain) and assemble the rest of the apparatus but do not connect the " Perkin triangle " to the pump. Run in about 40 ml. of the benzene, solution into the flask, heat the latter in an air bath so that the benzene distils over steadily. Allow the remainder of the benzene solution to run in from the dropping funnel as fast as the benzene itself distils over. When all the benzene solution has been introduced into the flask, close the stopcock on the funnel, and continue the heating until the temperature on the thermometer reads about 90°. Allow to cool. Replace the dropping funnel by a rubber stopper carrying a capillary tube reaching to the bottom of the flask, and distil under diminished pressure.
Collect the phenylhydrazine at 137-138°/18 mm. (or at 119-120°/12 mm.). The yield of almost colourless liquid is 70 g. ; it crystallises on cooling in ice and then melts at 23°. Phenylhydrazine slowly darkens on exposure to light.

CAUTION. Phenylhydrazine is highly poisonous and produces unpleasant burns in contact with the skin. Wash off immediately any liquid which has come into contact with the skin first with 2 per cent, acetic acid, then with soap and water.

Notes.
(1) External cooling may be dispensed with if more ice is added.
(2) This weight assumes 100 per cent, purity of the sodium hydroxide. If the commercial solid is used, its purity should be determined and a corresponding adjustment made in the weight.
(3) The sodium sulphite solution may also be prepared by dissolving 100 g. of pure (or a corresponding quantity of commercial) sodium hydroxide in about 125 ml. of water, and then diluting to 750 ml. The flask is cooled in running water, a few drops of phenolphthalein indicator are added, and sulphur dioxide passed in until the pink colour just disappears (it is advisable to add a further 1-2 drops of the indicator at this point) and then for 2-3 minutes longer. It is best to remove a sample for test from time to time, dilute with 3-4 volumes of water, and test with 1 drop of phenolphthalein.
(4) If desired, the phenylhydrazine hydrochloride may be purified by recrystallisation. The crude hydrochloride is boiled with 6 times its weight of water and a few grams of decolourising carbon. After filtering, a volume of concentrated hydrochloric acid equal in volume to one-third of the solution is added, and the mixture cooled to 0°. Pure white crystals are obtained in 85-90 per cent, yield.

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