Question

In the williamson ether synthesis of n-butyl methyl ether from 1-bromobutane, sodium, and ethanol, how could the purity as well as the percent yield of the product be improved?

Answer 1

I think, you meant to us Methanol insted of ethanol in synthesis of n-butyl methyl ether from 1-bromobutane, and sodium.

In the Williamson ether synthesis first step is to create sodium methoxide from sodium and methanol, which is crucial step as the reactant as well as product of this step are reactive in nature and absorbs moisture.

Addition of sodium metal to methanol produces Sodium methoxide. Sodium metal is oxidized to sodium cations and the hydrogen atoms of the -OH groups are reduced to hydrogen gas. Methanol is used in excess amount as it is used as solvent as well. This reaction is exothermic in nature, hence reflux condenser is used. Also to avoid spillover, sodium metal can be added in small quantity (small pieces). After, complete sodium is reacted excess methanol is distilled out under nitrogen atmosphere to avoid moisture contamination. This removal of excess methanol helps in increasing further step rate of reaction with 1-bromobutane.

1-bromobutane is added slowly to the above sodium methoxide solution under nitrogen. Reaction proceeds with sodium bromide precipitation. After completion of 1-bromobutane addition, reaction is refluxed to complete the reaction and hence to improve the yield.

Work-up of this reaction is also very crucial. First water is added to dissolve sodium bromide, resulting in two phases, organic phase contains n-butyl methyl ether and aqua phase contains mixture of methanol and n-butyl methyl ether and sodium bromide. These two phases can be separated in a separatory funnel, although the separation would be poor. Further distillation of water phase is needed, as the product n-butyl methyl ether and methanol form an azeotrope which boils at 56-58°C, and hence can be separated easily. The distillate contains n-butyl methyl ether and methanol, while the residue contains water, sodium bromide, and the bulk of the methanol. Further, an aqueous extraction of the methanol can be done as most of the solvent methanol was left behind in the distillation step. To the aqueous solution of calcium chloride is used instead of only water. CaCl2 salts out the ether. After extraction, the ether is dried with NaCl or CaCl2. These salts form solid molecular complexes with both methanol and water; therefore, any residual methanol and water are removed from the product.

Finally, the extracted ether is redistilled to produce pure n-butyl methyl ether (boil point 65-68 °C). If the distillate is cloudy or not clear, means there is still water in product. Hence, it has to be dried and redistilled again till you get clear liquid. If it boils at 56°C, means it contains ether-methanol azeotrope. In such situation, resultant should be rewashed with aqueous calcium chloride, re-dried, and redistilled.