Question

2. You have carried out a synthesis reaction and isolated the solid crude product. Scrutiny of its 1H NMR spectrum indicates the presence of an impurity, which you estimate to be approximately 6-8% of the total sample. You decide to purify it via recrystallization and know that you will first need to determine which solvent or solvent pair would be suitable. Describe what you would do and how you would evaluate your results. What properties of the solvent would you consider appropriate for the task? Briefly explain your rationale.

Answer 1

you have not mentioned exactly which compound. because recrystallization solvents will vary according to the polarity of compound. .

The smallest amount of hot solvent to fully dissolve the mixture, thus making a saturated solution. The solution is then allowed to cool. As the solution cools the solubility of compounds in solution drops. This results in the desired compound dropping (recrystallizing) from solution. The slower the rate of cooling, the bigger the crystals form.

For example if the compound is mid polar we can use minimum amount of ethanol and DCM mixture for recrystallization. If it is polar componund we can go for DMSO - Water mixture.

After selecting solvent pair we wil leave it to slow evaporation to form crystal.

As well if it is new compound then we have to try various solvent pairs.

The Purity of compound can be determined by melting point. If compound is ~99% pure then it will melt 1-2^oC range

The solid crystals are collected by filtration and the filtrate is discarded. If the solubility product of the impurity is exceeded, some of the impurity will co-precipitate. However, because of the relatively low concentration of the impurity, its concentration in the precipitated crystals will be less than its concentration in the original solid. Repeated recrystallization will result in an even purer crystalline precipitate. The purity is checked after each recrystallization by measuring the melting point, since impurities lower the melting point