Lab 3: Recrystallization Flashcards
What and how does recrystallization work?
Crude solid compounds, isolated from a chemical systhesis or from an extraction, are often purified by recrystallization, which is a method based on the differences in solubility of solids in a given solvent system. Since the solubility of a compound increases markedly with temperature, a saturated solution of a compound is made at an elevatedd temperature, and thus the same compound can be recovered, in solid form, by allowing the soltuion to cool.
Why does purification work?
The soluble impurities contaminating the main component are in much smaller concentration and hence will remain in soltuionn under both hot and cold conditions. The growing crystals of the chief component often reject them in crystallization process because the impurities do not meet the required geometry of the already forming crystal lattice.
How are insoluble impurities removed?
Insoluble impurities are usually removed while the solution is still at the elevated temperature by hot gravity filtration. the purified solid crystals are separated from the surrounding liquid (mother liquor) by suction or vacuum filtration.
What are the 4 basic requirements for a good recrystallization solvent?
1) Dissolve sparingly or not at all at room temperature, but readily at boiling point. And dissolve at a reasonable volume (no more than 10-20x eight of solid)
2) Cannot react chemically with the compound
3) Dissolve impurities exceedingly well or not at all
4) Idealy have a boiling point lower than the melting point of the solid to avoid oiling out
What is oiling out?
When a solvent has a boiling point higher than the melting point of the solid, it merely oils out (melts but does not dissolve).
How do you know when to take solution off the heat?
Until solid Just dissolves.
What is the difference between gravity filtration and hot gravity filtration?
Hot gravity filtration is very similar, but it requires the solution and all other glassware to be hot (usually preheated in lab).
Why are recrystallizations done in ground glass joint erlenmeyer flasks?
Because good crystal growth and expulsion of impurities requires space (depth) in the solution. Erlenmeyer is chosen so that it will contain as much solution as possible (half to two-thirds full) and even then to maximize the depth, you would turn the flask on a 45 degree angle while the crystals are forming. Ground glass joint flask can sustain a higher heat.
What are 4 advantages of using a flask? Can a beaker be used for recrystallization?
1) The narrow neck allows some refluxing of the solvent so that surface evaporation is minimized
2) They can be corked so that during crystallization the vapour and the liquid can equilibrate; the delicate choice of a definite amount of solvent will not be changed through evaporation.
3) They can be swirled to facilitate the dissolving of the solid
4) It is easy to remove crystals form flask.
A beaker is never used because it can’t be corked and can not obtain depth of solution.
When is a hot filtration used? What is used?
1) If there are insoluble impurities in the remaining solution (sand).
2) If there are coloured, soluble impurities which necessitates the use of Norit and Celite.
Hot gravity filtration requires additional solvent, glass funnel and erlenmeyer flask that is preheated.
Why do we need to add an additional amount of solvent when doing hot gravity filtration?
Excess solvent is used to prevent premature precipitation of the solid on the dunnel or in the receiver.
What is Norit used for? How does it work? What solvents does it work best in? What problem can occur?
Norit is used to remove coloured, soluble impurities. These impurities attach to the activated surfaces of the Norit by adsorption, and since Norit in not soluble in any solvent it and the absorbed impurites can be filtered off. The decolourizing action of Norit works best in aqueous and some alcohol solvents, but ineffective in hydrocarbon solvents. A problem can occur, such as the tiny Norit particles going through the filter paper.
What is Celite and what is it used for?
Celite is a white coarse powder made up of crushed diatomaceous earth. It is added to the solution at the same time Norit is added. The tiny particles of Norit absorb onto the Celite surfaces and are prevented from going through the filter paper.
What is the difference between crystallizing at room temperature and ice temperature? When is it necessary to use an ice bath?
At room temperature, better crystals form, but a lower yield. At ice temperature, there is smaller crystals, but a higher yield. An Ice bath is used when all other methods fail such as seeding and scratching.
What is the advantage of using a condenser during recrystallization?
Using a condenser is advantageous because it does not allows the reagents to escape the flask. Without it, too much vapour would escape the system when being heated. It also allows for the solution to be heated at a higher temperature.
