Question

(3) A student accidentally uses toluene to triturate their crude product instead of methanol. Moments after heating, their crude product dissolves completely.

1. Explain why the result was undesirable, drawing comparisons between recrystallization and trituration.
2. Propose a solution to recover the student’s product involving a technique utilized in this lab.
3. Explain the difficulty using the technique proposed in (b) with the solvent that the student has dissolved their product in.

d. Propose a solution to the problem above.

Answer 1

(3) a. Lets see what is it means by tituration and crystallization.

Tituration: It is one of the purification methods of crude chemical compounds containing soluble liquids. In this process the solvent is chosen where the desired compound is soluble and the impurity is insoluble or vice versa. Therefore one of them are soluble and another remains insoluble. On filtration or centrifugation both will separated be from the mixture and we will get the purecompound.

recrystallization: This is also a purification techniques where the impure compound is purified by dissolving them into a solvent. Here either desired compound or impuriy forms crystals (generally desired compound forms crystel) and other remains insoluble in the solvent. In this way the impurities separates out.

The student supposed to dissolve the crude product in methanol where one of the impurities or pure product should be insoluble and gives pure product after separation. Now when the student dissolved the crude product in the toluene by heating, it was completely dissolved. It means both desired product and impurities were soluble and hence on recrystallization both will be back again and the product will remain impure. This is why the result was undesirable. The tituration technique is better technique than recrystallization to purify the compound.

b. The solution is simple. The students just need to evaporate the toluene either by heating or evaporation at room temperature (depending on the stability of the compound at high temperature) to dryness to recover the sample.

c. As already mentioned in toluene both the desired product and impurities are soluble, it is not possible to precipitate any of them in toluene which is performed using the precipitation technique above (b) where one of the the pure product or impurity was insoluble.

d. Now the student has the same crude product in hand which he/she had before dissolving the crude product in toluene. The student can purify it by tituration (which he/she was supposed to do before) by dissolving in methanol to get pure product. In order to get higher purity the student can repeat the tituration techniques 2-3 more times. This will solve the problem.