Question

during work up of the oxidation product mixture, washes of the toluene solution was saturated aqueous NaCl solution we're done to remove as much of the Starks catalyst as possible. why was possible contamination of the crude product with the tetralkylammonium chloride salt a concern during column chromatography

5. During work-up of the oxidation product mixture, washes of the toluene solution with sat. aq. NaCl solution were done to remove as much of the Stark's Catalyst as possible. Why was possible contamination of the crude product with the tetralkylammonium chloride salt a concern during column chromatography?

Answer 1

A Stark's catalyst is a quaternary ammonium salt used as a phase transfer catalyst for the oxidation reaction like catalytic oxidation of cyclohexene to 1,6-hexanedioic acid. It is an example of green chemistry,

Stark's catalyst is an ionic liquid having formula as below,

R₃-N⁺CH^₃Cl^- where R is mixture of C₈ (octyl) and C₁₀ (decyl) linear chains, of which C₈ chain is predominant.

As these tetralkylammonium salt has good solubility in both the solution phases (organic layer and aqueous layer). As it is used as catalyst therefore after the reaction is completed the catalyst need to be recycled or removed from the product formed. Here in the given case the product is soluble in toluene (organic layer) which is also contaminated with catalyst tetralkyl ammonium salt.

The removal of tetralkylammonium salt from the crude reaction mixture is possible with the washing of the toluene layer with NaCl solution. As the concept of the ion pair phenomenon, in which the tetralkylammonium cation shall form the ion pair with its anion (Cl^-) from the NaCl solution. Thus the Stark's catalyst can be removed from the crude reaction mixture by repeat washing of the toluene layer by NaCl solution.

If not removed completely during work up of the reaction, due to the good solubilty of tetralkylammonium salt in organic solvents. The column chromatography purification of the product from the oxidation reaction will be difficult separate from the the tetralkylammonium salt. We need to develop other methods for the purification of the product.

The product obtained from the oxidation reaction is dicarboxyalic acid which will also form dia tetralkylammonium salts. Therefore removal of dia tetralkylammonium salts from the product by the salt exchange method using ion pair formation (washing of toluene with saturated aqueous solution of NaCl) is very necessary before column chromatography purification.