Table of Contents
Why does scratching the side of a flask induce crystallization?
Scratching causes cracks and crevices on the surface of the flask (though microscopically). These will act as favorable sites for nucleation, which leads to the formation of crystals.
What can go wrong in a recrystallization?
For that reason, the following problems commonly occur: if too much solvent is added in the recrystallization, a poor or no yield of crystals will result. If the solid is dissolved below the boiling point of the solution, too much solvent will be needed, resulting in a poor yield.
What will happen if recrystallization is allowed to proceed too quickly?
The solubility of the compound in the recrystallization solvent is important. If the cooling process is allowed to proceed slowly, nearly pure crystals of the compound will form. If the solution is cooled too quickly, the impurities will precipitate out of the solution along with the desired product.
What is usually the most challenging part of recrystallization?
solution. Typical problems: Adding too much solvent so that the product does not crystallize later. Filtering the hot solution too slowly so that the solvent cools and the solid starts crystallizing in the funnel and/or on the sides of glassware.
What does scratching a flask with a glass rod do to promote recrystallization?
You can often aid crystallization in stubborn cases by scratching the inside of the flask with a glass rod at the air-solvent junction. This scratching flecks off small shards of glass that form a surface upon which the solid can crystallize.
What was the purpose of scratching the surface of the test tube with a glass rod once the toluene cyclohexane solution has cooled down?
If crystals do not form spontaneously, chill the tube in an ice- water bath. If this fails, scratch the inside of the test tube with a glass rod to induce crystal growth. To reiterate, the solvent should dissolve the solute (solid compound) when hot and give a high recovery of good crystals when cold.
Why does oiling out occur in recrystallization?
Oiling out mostly occurs when the integration of solute molecules into the crystal lattice is kinetically hindered, delayed or the system experiences very high supersaturation.
How does recrystallization affect melting point?
Impurities found in the solid will tend to change the melting point by causing it to “spread” out into larger range. Observing a small quantity of the solid as it is heated carries out the measurement of melting point….
| alcohol | formula | Solubility (g/100 ml H2O) |
|---|---|---|
| n-pentanol | CH3CH2CH2CH2CH2CH2CH2OH | 0.1 |
How does scratching the walls of the flask promote crystal formation?
One theory is that scratching initiates crystallization by providing energy from the high-frequency vibrations. Another theory is that tiny fragments of glass are dislodged during scratching that provide nucleation sites for crystal formation. Seed crystals create a nucleation site where crystals can begin growth.
Why is it important to cool slowly during recrystallization?
It is important to slowly cool the flask first to room temperature and then in ice-water. A rushed crystal formation will trap impurities within the crystal lattice. Furthermore, the resulting crystals will be smaller.
What is the purpose of recrystallization?
Recrystallization is the most important method of purifying nonvolatile organic solids. Recrystallization involves dissolving the material to be purified (the solute) in an appropriate hot solvent. As the solvent cools, the solution becomes saturated with the solute and the solute crystallizes out (reforms a solid).
How is a solution formed in recrystallization?
In recrystallization, a solution is created by dissolving a solute in a solvent at or near its boiling point. At this high temperature, the solute has a greatly increased solubility in the solvent, so a much smaller quantity of hot solvent is needed than when the solvent is at room temperature.
What are the common problems in recrystallization?
For that reason, the following problems commonly occur: if too much solvent is added in the recrystallization, a poor or no yield of crystals will result. If the solid is dissolved below the boiling point of the solution, too much solvent will be needed, resulting in a poor yield.
Why does recrystallization not work at a cooler temperature?
At the cooler temperature, the solution is saturated at a much lower concentration of solute. The solute that can no longer be held in solution forms purified crystals of solute, which can later be collected. Recrystallization works only when the proper solvent is used.
Why is the product recovery less than 100\% in recrystallization?
Note that in any recrystallization some of the desired product is sacrificed and the recovery will be less than 100\%. This is because even at the lower temperatures the desired compound has some finite solubility in the recrystallization solvent and is thus lost when solvent and soluble impurities are removed.