1. What could you do to lower the errors for recrystallization? 2. What could yo
ID: 845672 • Letter: 1
Question
1. What could you do to lower the errors for recrystallization?
2. What could you do to lower the errors for column chromatography?
3. Which technique, recrystalization or column chromatograohy, gave you the purest compound as determined by your melting points? Explain your answer.
4. Which technique, recrystalization or column chromatograohy, gave you the highest yield based on your percent yield calculations? Explain your answer.
5. What are potential errors for each technique that can lead to a loss of yield or poor purification? which of these error did you encounter?
a) Recrystallization b) Column Chromatography
Explanation / Answer
1)recrystallization. Some salts are actually less soluble in hot water! Ideally, your cmpd must have good solubility when soln hot and very little when cold. You must expect some loss that is how you get your cmpd pure. And it is not unusual to do repeated recrystallizations to get your cmpd analytically pure (a la Madame Curie)
(2) You went and used too much solvent, so half your product is still in the mother liquor! So dissolve in minimum amount of hot solvent filter hot and cool slowly for nice xtls. Can add a second solvent (form two layers) to complete pptn, for example, add hexane a nonpolar solvent to a solution of the cmpd in polar CH2Cl2 (this was our method of choice for our particular cmpds). Can vac mother liquor down and place in fridge to get a second crop of xtls.
But don't forget you don't want your impurities to come out as well so your impurities must not be sparingly soluble in your recrystallization solvent.
(3) You haven't allowed for complete rectstallization. Put your mother liquor back in the fridge overnight and see if you get a second batch (you often do (can seed with an xtl of first batch.
(4) You could even have a a supersaturated soln (no product) which is where scratching the sides of your recrystallization flask with a glass rod is used (it works). Can also seed as well.
Recrystallization is a skill that comes with practice. What many don't realize is that to be a skilled chemist you have to have excellent lab technique besides A grades on your academic courses.
2)
Column chromatography in chemistry is a method used to purify individual chemical compounds from mixtures of compounds. It is often used for preparative applications on scales from micrograms up to kilograms. The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process. The latter prevents cross-contamination and stationary phase degradation due to recycling.
The classical preparative chromatography column, is a glass tube with a diameter from 5 mm to 50 mm and a height of 5 cm to 1 m with a tap and some kind of a filter (a glass frit or glass wool plug
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