Question

I did an experiment where it was recrystallizing fluorene contamiinated with fluorenone. I need to discuss these questions about what typically happens:

1. Discuss how the following factors affect the yield and purity: i.) quantity of solvent, rate of cooling, absence of cooling- and explain why the yield or the purity improves or worsens.

2. Explain why it is unable to seperate fluorenone from fluorene by extraction

Answer 1

In recrystallization factors affecting on yield and purity:
1) Solvent: Appropriate solvent selection is one of the main factors which affects the yield and eventually purity of the recrystallization process. Even after taking appropriate solvent the quantity of that matters a lot. We know as the volume of the solvent increases the amount of substance soluble also increases. This means we take an excess of solvent then the amount of desired substance soluble in the solvent will also increase. In this case, there is no problem with purity, but it results in poor yield. And if we take less solvent than needed, there is always a good chance to coprecipitate the impurity along with the desired compound, hence we loose on purity. But yield may be comparable.
2) The rate of cooling: Recrystallization is a thermodynamic process, where we need to let system attain an equilibrium. If we do not then the purity of recrystallized product be hampered. If we cool the hot solution fast by maybe putting it in ice cold water, then impurity as well gets trapped into our desired crystals. Hence, it is advised to let the solution cool at room temperature. This will improve the purity.
3) The absence of cooling: As discussed above, if we do not let cool the solution at room temperature than the crystallization will be fast, in fact, it will no longer be a crystallization, instead it will be simple precipitation. This will definitely let impurities as well get precipitated along with the desired product.