Question

A shelf containing several chemicals, Salicylic acid, 4-(4-Hydroxyphenyl)butan-2-one (a.k.a. Raspberry Ketone) and 4-methylaniline, fell and the vials shattered. The white powders are hopelessly mixed in a dustpan. You are promised a raise if you can devise a procedure that a less qualified chemist will follow to separate the three compounds. You must describe what extraction steps are to be taken at each point of the separation and how to isolate and purify each compound again as a solid. To ensure that you get the raise, you will also need to suggest how to determine the purity of each recovered sample.

Answer 1

The three organic compounds to be separated are Salicylic acid, 4-(4-Hydroxyphenyl)butan-2-one (a.k.a. Raspberry Ketone) and 4-methylaniline which are acidic, neutral and basic compounds respectively. Salicylic acid will form salt with a base (NaOH) and 4-methylaniline with an acid (HCl). The water soluble ionic salts are insoluble in less polar organic solvents (dichloromethane, DCM). 4-(4-Hydroxyphenyl)butan-2-one would not react with either acid or base. Hence, physical separation can be achieved by liquid/liquid extraction strategy using water and dichloromethane. The mixture of chemicals is transferred from one solvent into another in a separating funnel.

Procedure:

Dissolve the mixture of white powders from dustpan in DCM. Transfer it into a separating funnel. Extract the mixture with aqueous HCl. Only 4-methylaniline will react to form a salt and dissolve in the aqueous layer. This layer should be removed into a beaker. The remaining DCM layer now contains Salicylic acid and 4-(4-Hydroxyphenyl)butan-2-one. Then the DCM layer is basified with NaOH solution resulting in the formation of the ionic compound of Salicylic acid. It will dissolve in the aqueous layer and removed into another seperate beaker. The remaining DCM layer now contains only neutral compound 4-(4-Hydroxyphenyl)butan-2-one.

(1) The beaker containing an aqueous layer containing the ionic salt of 4-methylaniline is treated with NaOH pellets to regenerate 4-methylaniline. (litmus test may confirm the basicity of the solution). The basic solution is then transferred into a separating funnel and extracted with DCM. The top aqueous layer is discarded and the DCM layer dried with anhydrous Na₂SO₄. The DCM is removed on a rotatory evaporator and pure 4-methylaniline obtained.

(2) The Salicylic acid is regenerated from the beaker containing an aqueous layer of it upon treatment with concentrated HCl. And then following the step as in (1) gives pure Salicylic acid.

(3) The 4-(4-Hydroxyphenyl)butan-2-one is obtained by drying the final DCM layer on rotatory evaporator.

The purity of each recovered sample can be determined by TLC in comparison with standard pure samples. Further, if there is an access to NMR, the purity may be confirmed.