Question

What is the purpose of shaking the ether layer with a saturated NaCl solution?

What is the purpose of adding CaCl2 to the ether layer?

In this experiment you separated a mixture of benzoic acid, 4-t-butylphenol, and 1,4-dimethoxybenzene. You were instructed to dissolve the mixture in tert-butyl ether and extract with sodium bicarbonate first. Why can’t you extract with sodium hydroxide first? Explain.

A mixture of lactic acid (pKa 3.86) and ascorbic acid (pKa 11.80) are dissolved in an organic solvent. Which acid can be extracted with NaHCO3? Explain.

Why is it necessary to remove the stopper from a separatory funnel when liquid is being drained from it through the stopcock?

Answer 1

NaCl absorbs water molecules. If there is some water in the mixture that is being separated (mixture is soluble in ether here), then by shaking the ether layer with saturated solution of NaCl, the water gets absorbed by the NaCl. This separates the organic layer and the aqueous layer.

To ensure that no water molecule is left in the organic layer, CaCl2 is added as a drying agent.

Sodium bicarbonate will react with benzoic acid and the corresponding product is soluble in aqueous solution. where as 4-t-butylphenol does not react with sodium bicarbonate. Thus benzoic acid can be separated from the mixture. If sodium hydroxide is used, it reacts with both benzoic acid as well as 4-t-butylphenol and the products that are formed are both soluble in aqueous layer. Thus the two cannot be separated from the mixture.

Lactic acid has smaller pKa value meaning it is strong acid compared to ascorbic acid (high pKa). Stronger acids can react with NaHCO3 and the product is soluble in aqueous layer. Thus lactic acid can be separated using NaHCO3.

If stopper is not removed then vaccum is created inside the funnel which creates upward pressure for the liquid which is difficult to drain out. Hence stopper should be removed in order to drain out the liquid.