Halogenoalkanes
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I need some to help me in the preparation of 2-chloro-2-methylpropane
1) Why does adding anhydrous calcium chloride to reaction mixture increase the yield?
2) What causes the pressure build up in the separating funnel when the reaction mixture is being shaken?
3) What is the purpose of sodium hydrogencarbonate?
4) What was the purpose of washing the organic layer with water after washing with sodium hydrogencarbonate?
5) Why should the reagents used for this washing procedure should be cold?
6) Anhydrous sodium sulphate plays no part in the reaction. Why then it is used?
Corrie writes ...
1. Adding the calcium chloride to the reaction mixture, forming a fairly concentrated solution in the aqueous layer, improves the separation of the two layers by (a) decreasing the solubility of the halogenoalkane product in the aqueous layer, and (b) increasing the density of the aqueous layer relative to the organic layer.
2 & 3. Sodium hydrogencarbonate is added to the crude product to remove any remaining hydrochloric acid by reacting with it. I'll leave you to write an equation. Carbon dioxide is one of the products of this reaction, so a gas pressure builds up in the separating funnel. Can you see why you would not use NaOH solution for this neutralisation?
4. The organic layer is washed with water to remove any remaining sodium hydrogencarbonate and the water soluble products from its reaction with acid present, as above.
5. One reason will be to do with the solubility of the product in the water layer and the other will be to do with any possible reaction between the product and water.
6. Finally, after washing, the product will contain traces of water. Anyhydrous (i.e. without water) sodium sulphate is a drying agent and will remove the remaining water from the product, forming hydrated sodium sulphate, which can be filtered off.
Risk assessment
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updated: 25 June 2007
