Organic Chem Lab 3
Seeding
Adding a small crystal from the pure compound
Time of refluxing
Begins when the reaction starts to boil
Recrystallization Procedure
Begins with dissolving the solid in an appropriate hot recrystallization solvent. Often a decolorizing agent (activated carbon, NORIT) is used at this point to remove soluble colored impurities. Filtered The solution is then allowed to cool slowly first to room temperature and then put on ice to induce more crystals to form. Collection of Crystals
How can crystallization be then induced?
By lightly scratching the tip of a glass stirring rod on the side of the receiving flask for a few minutes or by "seeding"
Norit
Decolorizing carbon or activated carbon which removes colored impurities. Norit is added to the hot solution and stirred for a couple of minutes. Hot gravity filtration is conducted to removed the norit
What happens if norit is added to a solution near the boiling point
It may blow up violently
Ideal Recrystallization Solvent
One that dissolves a large amount of the substance to be recrystallized in hot solvent and only a small amount in cold solvent
Usually a simple solubility test is conducted in
a test tube at different temperatures whereby a smaller amount of the recrystallized substance
Why is recrystallization successful?
because most substances are more soluble in hot solvents as opposed to cold solvents and impurities usually have different solubility compared to other compounds
Slowly cooling the solution results in
crystals being large compared to rapid cooling which results in smaller crystals
Another characteristic for selecting a recrystallization is
the volatility of the solvent. A solvent with a low boiling point can be removed easily through evaporation. A solvent with an extremely high boiling is difficult to remove and often requires heating under a vacuum
The addition of too much recrystallization solvent can result in
A reduction in the yield and can prevent recrystallization from occurring at all
It is filtered using
hot gravity filtration in order to remove impurities
Technique that uses heat while preventing the loss of solvent
reflux
Recrystallization
the most widely used method in organic chemistry to purify a substance. As the name suggests it uses a substance that has already crystallized but then dissolves it in a hot solvent and crystallizes it again to remove the impurities. The principle works because most substances are more soluble in hotter solvents than in cold ones. Also, impurities have different solubility than the parent compound
What is your dry mass after distillation?
1,4-dimethoxybenzene with crystals in RBF
Care must be taken to only use a small amount because large amounts can result in
Loss of product
Recrystallization solvent for 1,4-dimenthoxybenzene
Methanol
The melting point of the recrystallized substance should
NOT be significantly lower than the boiling point of the recrystallization solvent. The reason for this being that the substance being recrystallized will melt as opposed to dissolve when the recrystallization solvent is being boiled. As a result, the solid will form a liquid that is no longer soluble in the solvent. This is often referred to as oiling out and as a result the liquid will not recrystallize
How are the impurities removed?
Often at the boiling point of the solvent impurities are insoluble at this temperature and are removed via hot gravity filtration
Why is recrystallization effective at removing impurities?
The recrystallization solvent is heated until the boiling point of the solvent
What would the reason be if no crystals form?
The solution is supersaturated
Purpose of Recrystallization
To remove impurities from a crystalline substance
Recrystallization solvent for Benzoic Acid
Water
Heating under reflux involves
heating the liquid at its boiling point and then using a condenser that has cold water rushing along side of its wall to cause the solvent vapors to condense and flow back into the reaction vessel
Rapid cooling causes
impurities to be trapped in the crystals. The crystals are formed and then collected through vacuum filtration and washed with ice-cold solvent.
Heating is generally done to
increase the rate of most organic reactions
