Question

A student is working on the separation of a three-compound mixture, consisting of a carboxylic acid, amine, and neutral compound, and starts by extracting the dichloromethane solution of the mixture with 10% NaHCO3 rather than 5% HCl. Assuming the student recognizes the error before proceeding further, would the student still be able to separate and identify the components of the mixture? Describe what changes would have to be made to the remainder of the procedure in order to do so.

Answer 1

A. The three-component mixture is first dissolved in ether. Water containing HCl is added and the mixture is shaken. This allows the R-NH₂ to become converted to R-NH₃⁺ and dissolve in the water.

However as you have already added NaHCO3 , I suggest neutralize it with excess of HCl

B. The mixture is carefully shaken to allow completion of the reaction.

C. The water containing the R-NH₃⁺ is transferred to a 2^nd container and purified to remove any non-amine compounds.

D. The amine is isolated. By adding aqueous NaOH the HCl is neutralized and the conjugate acid is converted back to the amine. The resulting cloudiness is due to the re-formation of the amine which is not very soluble in water. The crude amine is isolated by filtration.

E. The ether layer contains the carboxylic acid and the neutral compounds. These fractions are separated into its components by adding 1 mL of 3 N aqueous NaOH.

F. The bottom layer of water containing the R-COO₂^- ions is transferred to another container. These negatively charged ions are balanced with Na⁺ ions left from the reaction of NaOH.

G. This aqueous layer is purified by removing any non-acids from it.

H. The bottom layer of water containing the R-COO₂^- ions is transferred to a 2^nd container. These negatively charged ions are balanced with Na⁺ ions left from the reaction of NaOH. This aqueous layer is purified by removing any non-acids from it.The crude acid is isolated by filtration. Thus the solid carboxylic acid is isolated

I The non-acid compound remaining in the ether layer is then purified and isolated. It is first purified by addition of pure water, then the ether layer is separated from the water and dried and then the ether is removed by evaporation