Organic 211L Recrystallization. Additional Handout. Recrystallize. Why? To separate a compound from impurities.

Transcription

1 Organic 211L Recrystallization Additional Handout Recrystallize. Why? To separate a compound from impurities. How to recrystallize (generally): Dissolve your solute in the least amount of solvent necessary. Remove impurities if they are insoluble by a hot filtration. Or if you have soluble impurities (or after removing the insoluble impurities) chill the solution to recrystallize the solid. After a solid has formed, vacuum filter the solid from the liquid using a Buchner funnel. a) Find a good recrystallization solvent. A good recrystallization solvent is one that does not dissolve the solute when cold (around room temperature or colder) but does dissolve the solute when warm (preferably near boiling). For solids, the solubility of solutes increases as the temperature increases. A good recrystallization solvent should not be toxic or react with your solute. A way to test for a good recrystallization solvent would be to put a small amount of solute in a test tube and then add a set amount of solvent at room temperature. The test tube would then be put in a beaker of boiling (or warm) water. Be careful to not be above the boiling point of the solvent. For example, methanol boils at 64.7 o C and putting a test tube of methanol in boiling water would boil off the methanol. Remember: Like dissolves like. One solvent may not give the properties you need. Two (or more) solvents may be required. You could use methanol to get your sample to dissolve and then add water (second solvent must be miscible with the first one) to decrease the solubility of your solute. b) Find the least amount of solvent necessary. There are two ways to find the least amount necessary. i) If you know the identity of the sample you are recrystallizing, you may look it up in books or the Internet. For example, benzoic acid on Wikipedia says (hot water); 0.34 g/100 ml (25 o C), THF 3.37 M, ethanol 2.58 M, methanol 2.91 M} ii) Determine it experimentally by adding the same amount of solute to many test tubes. You would then add different amounts of solvent to each tube. For example, you could add 0.1 grams of solute to a test tube. You would then add 5 mls, 10 mls, 15 mls, etc. of solvent. Heating the tubes up and then chilling them back down would return you solid. You would see which tube gave you the highest percent recovery and use that amount of solvent. c) Recrytallize your solvent depending upon whether your solute has soluble or insoluble impurities.

2 Soluble impurities This means the impurities dissolve in the solute and do not reform a solid upon cooling. Insoluble impurities This means the impurities never dissolve in the solute even with heating and must be removed by hot filtration. C Insoluble impurities 1) Fold a fluted filter paper by folding a filter paper in half. Now, fold it in half again. Now, fold it in half again. One more time. Now open the filter paper to a half moon and fold each visible fold in half by going back and forward. 2) ALWAYS RECRYSTALLIZE IN AN ERLENMEYER. Why? Because the solvent leaving the flask hits the sides of the flask and returns back into the flask. Will you lose some solvent? Sure, but not near as much as if you recrystallized in a beaker. Add the solute into an Erlenmeyer. 3) Add the least amount of solvent into the Erlenmeyer at room temperature and add a stir bar. 4) At the same time as step # 2, add an Erlenmeyer of some solvent onto the hotplate as well. 5) Turn heat up on hotplate. Temperatures on hot plate do not correspond to your solution. A higher number means a warmer plate. If you turn the number up too high, the porcelain top WILL crack and you will be charged for a stirring hot plate ($800). 6) Heat your sample until all of your solute dissolves. THIS IS VERY TOUGH TO CALL. When you do you know when your sample has dissolved and all that is left are impurities? Good question. 7) Add your fluted filter paper to a powder funnel (this is a funnel with a wide neck so it won t plug.) Put the powder funnel and filter paper in a beaker. Pour your hot solution carefully into the filter paper and the hot solution will go into the beaker. Your solution will cool down going into the beaker so some solid will form in the beaker. 8) Some solid will form in the fluted filter paper on the powder funnel. You take the extra hot solvent from step # 4 and pour some of it (as little as possible) on the fluted filter paper while over the beaker to wash it into the beaker. This beaker will not contain more than the least amount of solvent possible. 9) Concentrate your sample. How? Assume you put 10 mls in step # 3. Also, assume you added 5 mls in step # 8. You would put your beaker on a hot plate and boil off the solvent (IN A HOOD) back down to 10 mls. 10) Chill your beaker down to 5 o C or so. The colder the better.

3 11) Vacuum filter your solid through a Buchner funnel (shown below). The vacuum will be furnished by attaching a hose to the filter flask and attaching it to a water aspirator. Turn the water on full to get the full pressure. WATCH FOR LEAKS OF WATER ON THE FLOOR. The filter paper must fit EXACTLY in the Buchner funnel and cannot go around the sides (or your product will escape to the filtrate.) The liquid that comes through is called the filtrate or MOTHER LIQUOR. We will not use standard taper joints as shown in the drawing below but will use a FILTER ADAPTER. Filtrate or Mother Liquor 12) Let your solid dry for a week or more (depending upon solvent.) 13) DISCARD THE MOTHER LIQUOR IN THE APPROPRIATE PLACE.

4 C) Soluble Impurities 1) ALWAYS RECRYSTALLIZE IN AN ERLENMEYER. Why? Because the solvent leaving the flask hits the sides of the flask and returns back into the flask. Will you lose some solvent? Sure, but not near as much as if you recrystallized in a beaker. Add the solute into an Erlenmeyer. 2) Add the least amount of solvent into the Erlenmeyer at room temperature and add a stir bar. 3) Turn heat up on hotplate. Temperatures on hot plate do not correspond to your solution. A higher number means a warmer plate. If you turn the number up too high, the porcelain top WILL crack and you will be charged for a stirring hot plate ($800). 4) Heat until all the solute dissolves. This is much easier to determine than with insoluble impurities. 5) Chill your beaker down to 5 o C or so. The colder the better. 6) Vacuum filter your solid through a Buchner funnel (shown above). The vacuum will be furnished by attaching a hose to the filter flask and attaching it to a water aspirator. Turn the water on full to get the full pressure. WATCH FOR LEAKS OF WATER ON THE FLOOR. The filter paper must fit EXACTLY in the Buchner funnel and cannot go around the sides (or your product will escape to the filtrate.) The liquid that comes through is called the filtrate or MOTHER LIQUOR. It will contain your impurities. 7) Let your solid dry for a week or more (depending upon solvent.) CALCULATIONS: Percent recovery To calculate percent recovery, you use the following equation. Percent Recovery Amount Recovered Amount Started With You should know two solubilities. The solubility of the solute when in cold solvent and the solubility of the solute when in hot solvent. For example: Assume benzoic acid dissolves 1 g/ 5 mls of solvent when the solvent is at 98 o C. Assume benzoic acid dissolves 1 g/ 45 mls of solvent when it is at 4 o C. Question # 1 Assuming you use the solubility numbers above and that you have 5 grams of material to recrystallize. What is the least amount of solvent necessary?

5 We use the hot solubility numbers. 1g/ 5 mls. We multiply by 5 (since we have 5 grams) and 25 mls would be the least amount of solvent necessary. Question # 2 What is the maximum percent recovery? Now, we use the cold solubility numbers. 1 g/ 45 mls If we used 25 mls in question 1 as our least amount necessary, we would set up the equation below and solve for X. 1 g X 45 mls 25 mls X = 0.55 grams. This is the amount of material still dissolved in our solution. So remembering the percent recovery equation, 0.55 grams still dissolved, and our starting amount of 5 g, we solve for percent recovery. Percent Recovery 5 g g 5 g Percent Recovery = 89 %