Objective: To prepare p-bromo acetanilide from acetanilide

Apparatus/Glassware Required: Conical flasks, beaker, volumetric flask, measuring cylinder, suction pump and Buchner funnel

Chemicals Required: Acetanilide, bromine, sodium metabisulphite and glacial acetic acid

Principle: The principle involved in the synthesis of p-bromo acetanilide is nuclear bromination. Bromination of acetanilide occurs at the para position due to the amine substituent. This substituent provides resonance stabilization to the carbocations created by ortho and para addition. Since the amine provides steric hindrance at the ortho position, bromination of acetanilide occurs at the para position. The function of the catalyst is to increase the electrophilic activity of the halogen.

For chemical reaction (Click download  tab)

Procedure:

1. Dissolve 6.75 g of finely powdered acetanilide in 22.5 mL of glacial acetic acid in a 500 mL conical flask.
2. In another small flask dissolve 2.65 mL (8.5 g) of bromine in 12.5 mL of glacial acetic acid, and transfer the solution to a burette or separating funnel.
3. Add the bromine solution slowly and with constant shaking to ensure thorough mixing; stand the flask in cold water.
4. When all the bromine has been added, the solution will have an orange colour due to the slight excess of bromine; a part of the reaction product may crystallize out.
5. Allow the final reaction mixture to stand at room temperature for 30 minutes with occasional shaking.
6. Pour the reaction product into 200 mL of water; rinse the flask with about 50 ml of water.
7. Stir the mixture well and if it is appreciably coloured, add just sufficient sodium metabisulphite solution to remove the orange colour.
8. Filter the crystalline precipitate with suction on a Buchner funnel, wash thoroughly with cold water and press as dry as possible with a halide glass stopper.
9. It is recrystalline from dilute methanol.

Uses: Analgesic, Antipyretic.

Note: Colourless crystals; Practically insoluble in cold water; Sparingly soluble in hot water; Soluble in benzene, chloroform, ethyl acetate; Moderately soluble in alcohol; Melting Point 166-168 °C.

Precautions:

1. The concentrated acids are corrosive. Gloves should be worn when transferring these reagents.
2. When working with bromine keep the safety rules with special care. The valve of the dropping funnel should be bromine-tight (for greasing a grinded glass valve, apply sulfuric or phosphoric acid). Notice the color change on dropping the bromine.
3. During the bromine addition period keep the temperature at 25-35°C by proper cooling. Lower temperature results in decreased reaction rate and conversion, higher temperature results in higher amounts of undesired by-products.