Question

An overzealous student decided to conduct an extraction and recrystallisation experiment by removing all the dichloromethane at the very beginning and then separating the solid residue of benzoic acid and acetanilide by recrystallisation. Explain why this procedure would be ineffective.

Answer 1

Acetanilide is also solid compound but non-polar, soluble in organic solventsonly like dichloromethane.For separation of acetanilide and benzoic acid , the mixture is to be dissolved in dichloromethane and NaOH is added, which deprotonates benzoic acid forming sodium benzoate (soluble in water).Two layers are formed , organic layer with acetanilide and aqueous layer with sodium benzoate.-which can be separated .

Once the two layers are separated, water in the organic layer will be extracted using Na2SO4 and any left over acetanilide is extracted using dichloromethane, then solvent evaporated to give crude acetanilide.HCl is added to water layer to neutralize base and yield benzoic acid as crude solid. The crude solid is then purified by recrystallization.

So both organic solvent and aqueous solvent are relevant to carry out liquid -liquid base extraction of the components of the mixture.

If no organic solvent is there then during the neutralization of solid sodium benzoatewould yield benzoic acid , which will get mixed with acetanide.So no separation occurs.

For Recrystallization from solvent would not yield proper results as choice of suitable solvent for the purpose is diffecult , both are organic compounds with comparable solubility in organic solvents.

Recrystallization is effective in removing impurities from a compound when the solubility of the impurity in the solvent is high at all temperatures , while the crude compound dissolves only at elevated temperatures.On heating the compound dissolves, which crystallizes on cooling consequently,The impurity stay dissolved and gets separated on filtration.