re crystallization

RecrystallizationPrepared by:

N. K. Nakhal .

Al-Azhar UniversityFaculty of pharmacy

Recrystallization is a process of

purifying compounds, by

separate molecules of impurities

from molecules of a substance.

Recrystallization

Definition :

Principle of recrystallization

The “trick" of the technique is to use

a solvent in which the solid is

sparingly soluble at low tempe. and

quite soluble at higher temperature

(at the boiling point of the solvent).

Types of recrystallization

There are different recrystallization techniques that can be used :

* Multi-solvent recrystallization.

* Single-solvent recrystallization.

Single-solvent recrystallization.

Single solvent recrystallization is the most basic and commonly used for recrystallization method. An ideal solvent does NOT dissolve the solid at room temperature BUT dissolves the solid well in hot solvent.

Multi - solvent recrystallization.

When one ideal solvent couldn’t be found for recrystallization its possible to use mix. of 2 solvent.Solvent 1 the sample should be soluble at room temp.

Solvent 2 1 the sample should be insoluble at Bp.The 2 solvent should be miscible at all ratio

Single-solvent recrystallizatio

n.

Teqnique of recrystallization

The steps in the recrystallization of a compound are:

1. Choosing

solvent.2. Dissolving of the sample .

3. Decolarization and Hot

filtration.4. Cooling / Crystallization.

5. Collecting / Washing / Drying.

A recrystallization solvent should, ideally, have

the following properties:

• Dissolve at hot, not dissolve at cold.

• Dissolve impurites at room temp. or not dissolve

at hot.

• Has a relatively low boiling point .

• Does not react with the compound being

purified.

1. Choosing a solvent

1. Choosing a solvent

• Choose a solvent

Insoluble at room temperature

Soluble at boiling point

Common Slovents

2. Dissolving sample

Add hot solvent until the sample dissolves.

This makes impurities that were locked inside the impure crystals available for removal.

MINIMUM

Too much solvent and your crystals don’t come back.

One key word in recrystallization is

• Add decolorizing charcoal if colored impurities are present.

• Filter the resulting solution while it is still hot.

3. Decolarization and Hot

filtration.

• Cool the solution and obtain crystals of your desired compound.

Soluble impurities will remain in solution.

4. Cooling / Crystallization

PATIENCE!!!

The other key word in recrystallization is:

• Perform suction filtration to isolate the compound.

• Drying the crystals you’ve isolated.

5. Collecting /Washing / Drying

• Wash with cold solvent.

Lab - Hints

• Use suitable amount of solvent.

• Impurities should be either more soluble or less soluble than the compound being recrystallized.

• Erlenmeyer flasks are preferred over beakers for recrystallization...

• You must be judicious in your use of decolorizing carbon

Lab - Hints

• Be sure the oven temp. is below the MP of the compound.

• Do not rapidly cool the hot solution ???

• what should you do if the dissolved compound fails to crystallize from the solution on cooling ???

Lab - Hints

Why you should not cool the hot solution

rapidly

???

Lab - Hints

If you cool the solution too quickly.

Lab - Hints

But if you let a crystallization proceed slowly.

Why you do recrystallization

3.When you synthesize a new compound or perform a separation, it never goes 100% and cleanly so you need to get it away from other chemicals.

1.Chemicals come from the manufacturer impure.2.Chemicals may decompose over time or oxidize and form other compounds.

THANK YOU

Any question??