Question

Recrystallization/Melting Point

2-ethoxybenzamide is the unknown.

1. Adding enough solvent to achieve a “saturated” solution is necessary for recrystallization.

a. What is the problem with adding too much solvent?

b. What is the problem with adding too little solvent?

2. Why is fluted filter paper preferred over standard conical filter paper for the filtration of the recrystallized solid?

3. What is the purpose of adding boiling stones when heating solutions to a boil?

4. Name 2 requirements that must be considered when choosing a recrystallization solvent.

5. What effect would impurities have on the melting point of your unknown solid (2 effects)?

Answer 1

1) Recrystallization is the rechnitec used for purification and separation of solid organic compound. The choice and quantity of solvent is crucial components of this technique.

Choose the solvent in such a way that solid compound partially soluble at room temperature and completely soluble at hot condition or at boiling temperature of the solvent.

Addition of enough or required amount of solvent is very important because it makes the saturated solution.

A) If too much of solvent is added for recrystallization then most of organic compound gets dissolved and chances of evaporation of compound with solvent is increases.

B) If too little solvent is added during the recrystallization it creates even more worse situation than previous one. Firstly solid compound do not dissolves completely and after cooling this solution crystals starts appearing very rapidly. Random size of crystals are obtained. Impure components get not separated properly it comes with crystals. Even complete recrystallization is not occurred. Yield of the compound is very less. Finally melting point is not obtained exact or correct and difficult to identify the unknown compound.

Overall in both cases yield of the product affected largely means getting less yield and in second cases impurity of compounds remains partially with crystals of compound.

Therefore use of enough solvent to make the solution saturated for proper recrystalization is required.