Chapter 15, Problem 2Q

Interpretation Introduction

Interpretation:

The better solvent among water and methanol for recrystallization of the compound should be identified.

Concept introduction:

Recrystallization is a process in which an impure crystalline solid dissolved in hot solvent precipitates back when the solvent is cooled and that leads to the formation of pure solid crystals. For successful recrystallization, maximum solubility of solute in the hot solvent is essential and solubility in the cold solvent should be minimum for the maximum yield of crystals.

If solute will have good solubility in cold solvent, there will be loss of compound on recrystallization.To minimize this loss, the choice of the recrystallization solvent is important.

Characteristics of good recrystallization solvent are as follows:

• Solvents boiling point must be smaller than compounds melting point.
• At room temperature, the compound taken for purification must should be insoluble in the solvent.
• It must have low solubility at lower temperatures and high solubility at a higher temperature.
• Recovery of the crystal should be maximum from the cool solvent.
• For effective recrystallization,the solubility of organic solids in the solvent should not be too large because otherwise, it will be difficult to recover the compound.
• The solubility of organic solid should not be too small, else a huge volume of solvent will be needed to dissolve the compound.