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

While separating a three - compound mixture containing an acid( carboxylic acid - RCOOH), a base( amine - RNH₂) and a neutral organic compound(say benzene C6H6), the concept of different solubility of their protonated and non-protonated forms can be applied. The amine is first extracted by treating it with 5% HCl, and then the carboxylic acid is separated by treating it with a strong base like NaHCO₃. Finally the solvent is evaporated to get the neutral compound in solid form. The following steps are followed to separate these three compounds.

Step - 1:

The three - compound mixture is added in a non-polar solvent like dichloromethane as taken by the student. A non-polar solvent is chosen because the neutral compound(benzene here) will remain soluble in the solvent through out during the extraction of the acid and the base.

Step - 2:

Now the solution is treated with 5% HCl so that the amine is protonated and forms the aqueous layer.

R -NH₂ + HCl ----------------.> RNH₃⁺Cl^-  ( protonated base, forms aqueaus layer)

Now this aqueous layer is treated with NaOH to get the precipitation of R-NH₂, which is separated from the mixture.

RNH₃⁺Cl^- + NaOH ------------> R-NH₂ (ppt.) + Na+ + Cl- +H2O

Now the solution contains carboxylic acid and the neutral compound.

Step -3 :

In step 3, the solution is treated with a strong base like NaHCO₃ to protonate the carboxylic acid and forms the aqueous layer.

R-COOH +NaHCO₃   - --------------> R-COO^-Na⁺ (aqueous layer)+ CO2 + H2O

Now this aqueous layer is treated with HCl to get the precipitation of R-COOH, which is separated from the mixture.

Now both amine and carboxylic acid are separated leaving only the neutral cmpound in the solution.

Step-4:

In the final step the solvent(dichloromethane) is evaporated to get the solid neutral compound.