Question

Let's assume you have a mixture of equal parts benzoic acid, sand and toluene that are intimately mixed together (the crude material is a mixture of solid and liquid). Propose a procedure to separate these three components using any of the chemicals commonly available (you've used all of the "common" chemicals in the lab by now) in the organic chemistry teaching laboratories and observing the typical limitations with the instruments and techniques (such as not being able to distill or rotovap liquids with boiling points above 100°C).

My answer to this question is as follows:

1. For this mixture, we can work off a few key points.
   1. Toluene is insoluble in water, so when mixed, two distinct layers will form
   2. Benzoic acid can be dissolved in NaOH to form benzoate salt which is soluble
   3. Benzoic acid is soluble in water when heated and will crystallize when cooled (5)

First thing we can do is to mix the mixture with NaOH in a separating flask. The benzoic acid will dissolve in the NaOH and will be separated from the toluene. Because sand is heavy, it will be found in the bottom layer with the sodium benzoate. Separate the two layers. At this point, what is left in the separating flask should be isolated toluene. Proceeding to the bottom layer, neutralising it with diluted HCl will convert sodium benzoate back into benzoic acid and producing water and NaCl. Heating the solution will dissolve the benzoic acid because benzoic acid is soluble in water when heated. Once dissolved, carefully filter the solution, which will leave the sand isolated in filter paper, and proceed to put the filtered solution in an ice bath to crystallize. Once crystallized, mix with water to fully dissolve all the NaCl left and vacuum filter, leaving us with isolated benzoic acid.

We can assess the purity of the benzoic acid and toluene by performing various tests. We can perform melting point determination for benzoic acid and compare experimental values with credible literature values, the closer it is, the purer. We can also perform a TLC plate for the toluene and observe the spots. Seeing more than one spot will indicate contaminants. Finally, performing IR spectra for both compounds can reveal the functional groups found in each isolated compound. Seeing functional groups on IR graph that matches each compound indicates purity.

I lost two marks for this question and the comments are:

-1 for not washing organic layer with NaCl solution
-1 for not using drying agent to remove residual water from organic layer

I was wondering what the comments meant? Is the top organic layer not already isolated?

Answer 1

Yes it is isolated but we wash with NaCl to remove ionic impurities from the organic layer which can lead to impure product.

Though we have extracted, a little amount of water can remain in the organic layer which can be removed by drying agent such as Na2SO4 or MgSO4. So washing with NaCl(brine) and drying with drying agents are necessary.