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Principle: Aniline undergoes nucleophilic substitution with bromine, even in cold. The bromine atoms enter at the two ortho positions and the para position with the formation of 2,4,6-tribromoaniline. Here presence of bromine atoms in tribroboaniline, reduces the basic properties of the amino group, and salts even with strong acids are almost completely hydrolysed in presence of water.
Aim: To prepare 2, 4, 6-tribromoaniline from aniline.
Use: It is used in organic synthesis of pharmaceuticals, agrochemicals and fire-extingushing agents.
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Chemicals: Aniline – 4 ml.,
Bromine – 6.4 ml.,
Dilute hydrochloric acid,
Apparatus: Buchner flask set as shown in the figure,
Filtration set etc.
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The apparatus is assembled as shown in fig. A 500 ml. Buchner flask A is fitted with a cork through which passes a glass delivery tube B reaching nearly to the bottom of A. B is then connected by a short piece of rubber tubing to the side-arm of a 150 ml. distilling-flask C, care being taken to ensure that the two glass tubes touch one another inside the rubber connection. A narrow glass tube D is then fitted as shown so that it reaches within 2-3 cm. of the bottom of C.
4 ml. aniline is dissolved in 10 ml. of dilute hydrochloric acid in A, and diluted with 200 ml. of water. 6.4 ml. of bromine is placed in C carefully, and it is covered with about 40 ml. of cold water. The position of the tube D is adjusted until it touches the surface of the bromine layer. A is connected to a suction water-pump, so that a steady stream of bromine vapour carries from C into A, where the greyish-white tribromoaniline soon begins to separate. The contents of A is shaken occasionally for even distribution of tribromoaniline. When the water in C becomes almost colourless (about 40 minutes), due to evaporation of bromine, the current of air is stopped, and tribromoaniline is filtered at pump, washed well with water and drained. The product is recrystallised from rectified spirit, using animal charcoal.
|Calculation of Yield:
A g.mol of Aniline with 3 g.mol of Bromine yields a g.mol of Tribromoaniline
i.e. 93 g of Aniline with 3 ×159 g of Bromine yields Tribromoaniline = 329.8 g
4.1 g of Aniline shall yield =
(329.8 / 93) × 4.1 = 14.54 g Tribromoaniline
Hence, theoretical yield =14.54 g.
If reported Practical yield is 8.5 g, then, % yield
= × 100
= (8. 5 / 14.54) × 100 = 58.46 %
A solution is prepared by adding 8.4 ml of bromine in 20 ml glacial acetic acid. Another second solution is prepared adding 5 ml of aniline in 20 ml of glacial acetic acid in a conical flask. The first solution is added drop wise to the second solution from a dropping funnel. During addition, the flask is shaken time to time and cooled in ice. A yellow coloured mass is obtained. It is poured into excess of water. The separated product is filtered, washed with water and recrystallised from rectified spirit. The yield is about 10 g (55.6%).
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The synthesized 2,4,6-tribromoaniline was obtained as colourless crystals, % yield: 58.46 and of m.p. 120°.
[ps2id id=’references’ target=”/][ps2id id=’1′ target=”/]
- Mann F. G., Saunders B. C., Practical Organic Chemistry, 4th Edition, Dorling Kindersly India Pvt. Ltd., New Delhi, Page No. – 165.
- Vogel’s Textbook of Practical Organic Chemistry by Brian S. Furniss, Antony J. Hannaford, Peter W. G. Smith & Austin R. Tatchell; Fifth Edition; Page No.- 909.
- Comprehensive Practical Organic Chemistry: Preparation and Quantitative Analysis By V. K. Ahluwalia, R. Aggarwal; Page No. 108.