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BACKGROUND
Principle:
In the first step catalytic reduction of nitrobenzene is occurring forming N-phenylhydroxylamine in presence of zinc and ammonium chloride with sufficient water. In the second step rearrangement is occurring which involves the conversion of an N- phenylhydroxylamine into the corresponding 4-aminophenol through treatment with an aqueous mineral acid such as sulfuric acid (Bamberger rearrangement).1
Aim:
To prepare p-aminophenol from nitrobenzene.
Reaction:
Step 1:
Step 2:
Mechanism:
Step 1:
Step 2:
Use:
It is used in the industrial synthesis of the common NSAID, paracetamol.
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REQUIREMENTS
Chemicals: Ammonium chloride,
Nitrobenzene
Zinc powder
NaCl
Ether
Benzene
Conc. sulphuric acid
Sodium bicarbonate
Magnesium sulphate
Apparatus: Conical flask
Beaker
Thermometer
Pipette
Glass rod
Buchner funnel
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PROCEDURE
Step 1: Preparation of n-Phenyl hydroxyl amine from Nitrobenzene
In a 2 litre beaker, equipped with a thermometer and mechanical stirrer, place 25 g of ammonium chloride, 800 ml of water and 50 g (41.6 ml, 0.41 mol) of redistilled nitrobenzene. Stir the mixture vigorously, and add 59 g (0.83 mol) of zinc powder of 90 percent purity during about 15 min; the rate of addition should be such that the temperature rapidly rises to 60-65 °C and remains in this range until all the zinc has been added. Continue the stirring for a further 15 min, by which time the reduction is complete as is shown by the fact that the temperature commences to fall. Filter the warm reaction mixture at the pump to remove the zinc oxide, and wash it with 100 ml of hot water. Place the filtrate in a conical flask, saturate it with common salt (about 300 g) and cool in an ice bath for at least 1 hour to ensure maximum crystallisation of the desired product. Filter the pale yellow crystals of phenylhydroxylamine with suction and drain well. The yield of crude, dry product is about 38 g; this contains a little salt and corresponds to about 29 g (66%) of pure phenylhydroxylamine as determined by its separation from inorganic materials by dissolution in ether. The substance deteriorates upon storage and is therefore used immediately for a secondary preparation. If required perfectly pure, it may be recrystallised from benzene-light petroleum (b.p. 40-60 °C) or from benzene alone; the resulting pure compound is somewhat more stable and has a melting point of 81 °C.
Step 2: Preparation of p-Aminophenol from n-Phenyl hydroxyl amine
Add 4.4 g of recrystallised phenylhydroxylamine to a mixture of 20 ml of concentrated sulphuric acid and 60 g of ice contained in a 1-litre beaker cooled in a freezing mixture. Dilute the solution with 400 ml of water, and boil until a sample, tested with dichromate solution, gives the smell of quinone and not of nitrosobenzene or nitrobenzene (10-15 min). Neutralise the cold reaction mixture with sodium hydrogen carbonate, saturate with salt, extract twice with ether and dry the ethereal extract with magnesium sulphate or anhydrous sodium sulphate. Distil off the ether; p-aminophenol, m.p. 186 °C, remains. The yield is 4.3 g (98%).
Calculation:
Here limiting reagent is nitrobenzene; hence yield should be calculated from its amount.
Molecular formula of nitrobenzene = C6H5NO2
Molecular formula of p-aminophenol = C6H7NO
Molecular weight of glycine = 123 g/mole
Molecular weight of p-aminophenol = 109 g/mole
Theoretical yield:
123 g nitrobenzene forms 109 g p-aminophenol
Therefore, 50 g nitrobenzene will form …….? (X) g p-aminophenol
X =(109 ×50)/123 = 44.3 g
Theoretical yield = 44.3 g
Practical yield = ————- g
% Yield = (Practical Yield)/(Theoretical Yield) × 100
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CONCLUSION
p-aminophenol was synthesized and the percentage yield was found to be………..%
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REFERENCES
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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. 955.
