2. In preparation for an important synthesis you are attempting to recrystallize

Question

2. In preparation for an important synthesis you are attempting to recrystallize a starting material from benzene. You begin by dissolving 4g of material in 100 mL of boiling benzene. After filtering off some insoluble impurities, you allow the solution to cool to room temperature. You then cover the flask, pop it in the freezer, and head out to lunch. When you return, expecting to find beautiful needle-like crystals, you instead discover a clear solution and no precipitate. What went wrong? How might you remedy the situation?

Explanation / Answer

Ans: There are many conditions or precautions we should consider before chosing a solvent for recrystallization/

a. No solubility or very less solubility of solute at normal/room temperature, but solute should be prefectly soluble at boiling point of solvent.

b. impurities should be insoluble in the solvent, irrespective of temperature scale of solvent.

c. Boiling point should not be that much high. Hence removal of solvent from the crystal should be done.

d. Boiling point of solvent should be lower than the melting point of solute

e. solvent should not interact with the solute chemically.

Possible reasons for for not forming the crystal might be.

Given= a clear solution is present

Note: Ignoring the formation of an oil etc, and ignoring the chemical reactions. The below thing can be happened with the recrystallization procedure.

1. In some case a supersaturated solution of solute-solvent is formed or too much solvent is still present to oppose the crystallization.

Sol- a seed crystal can be added or some of the solvent can be evaporated to get the crystals.

Related Questions

Navigate

• Browse (All)
• Browse 2
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.