Question

The organic layer can also be extracted with NaOH (aq) instead of using potassium carbonate. However, sometimes, initially two layers form in the separatory funnel when the base is first added to the organic layer. But during extraction, the solution has been reported to become warm. When the separatory funnel is put back into the ring stand to let separation occur, no separation is observed not even after standing for 5 minutes. What happened? Explain using a chemical reaction.

Answer 1

Sodium hydroxide (NaOH) is a strong Base compared with potassium carbonate (K2CO3).  

C6H5NH2 + CH3I + NaH C6H5NHCH3 + C6H5N(CH3)₂ + C6H5NH2

In the above reaction the N-alkylation of aniline in presence of sodium hydride base. In this reaction 3 different products found in the reaction mixture. One is starting material not completely reacted and will be remained some quantity. And dialkylated product and mono alkylated product will be formed in their respective ratios.

To isolate only dialkylated product we dilute with water and extract with suitable solvent to get complete isolation of the dialkylated product.

To isolate mono alkylated product we dilute with saturated K2CO3 solution and extract with suitable solvent to get complete isolation of mono alkylated product.

To isolate starting aniline we dilute with 1M NaOH solution and extract with suitable solvents to recover the unreacted starting material completely (the same process will be repeated to get good yields).

Here in this case there is no clear separation will be observed due to the Solubility of the substance in both the layers and trying to dissolve the compound, so that the competition between the layers results no clear separation of the layers.

These three processes has to be done sequentially to get clear and complete results.

While using NaOH aqueous solution in extractions there may be possibility of formation of any other side products, if there is presence of Base sensitive groups present.

Whenever there is presence of Chiral centres, we always avoid the extraction with aqueous NaOH solution. The chirality will be disturbed.

Aqueous NaOH solutions are highly basic and polar, if we use these solutions in extractions organic substances almost soluble in the solution, both organic and aqueous layers trying dissolve the same compound, so that no clear separation of layers will be found. NaOH trying to react with organic substances at room temperature (strong base) which are base sensitive, meanwhile heat will be generated.

So that heat generation taking place due to reaction of Strong base(NaOH) with some sensitive organic groups.

The tendency to dissolve the compounds present in both layers causes no clear separation between the layers. NaOH causes dissolution inorganic substances in organic layer, causes no clear separation.