Answer to Question #141651 in Physical Chemistry for Hadden

Question

Ethanal and Carbon disulphide are miscible liquid which forms a two-component and two-phase system.
Although the mixture cannot be separated into the two components by fractional distillation, chemical methods can be used. (i). Suggest a reason why the mixture cannot be separated by fractional distillation. (ii). Briefly describe chemical reactions for obtaining one of the two components from the mixture.

Answer 1

i) These two substances have near boiling points (ethanal 20 ^oC and carbon disulphide 46 ^oC) and form azeotrope.

ii) Mixture can be treated with sodium bisulfite water solution. Product of the reaction between ethanal and sodium bisulfite will be extracted to water phase. Then ethanal can be regenerated:

This reaction will be the best. Also, reactions between ethanal and nitrogen nucleophiles (for example hydroxylamine) can be used. And then carbon disulphide can be distilled.