Pure Solid Compounds: Molecules held together in rigid formations by intermolecular forces.

Transcription

1 Recrystallization: Purification of Solid Compounds Pure Solid Compounds: Molecules held together in rigid formations by intermolecular forces. Types of Intermolecular Forces? 1. Van der Waal s forces London or dispersion forces Dipole-dipole forces 2. Hydrogen bonds Impure Compounds? Impurities buried inside the crystalline lattice The Process: to use a hot solvent to surround the molecules in the crystalline lattice, allowing the heat to loosen and break apart the intermolecular forces, until the lattice is no longer intact. Once the lattice is gone, the impurities caught inside the lattice can dissolve into the solvent, or float off into the solvent. Then the lattice is reformed, slowly so as to prevent trapping the impurities back into the crystal again. 1

2 Factors to Consider: 1. The Solvent Cannot react with the solid that is being recrystallized Should not dissolve the solid when cold but should dissolve the solid when hot Should either dissolve the impurities when cold or hot (a.k.a. soluble impurities) or should not dissolve them when cold OR hot (a.k.a. insoluble impurities). Should be relatively volatile. The boiling point of the solvent should be lower than the melting point of the solid (to avoid oiling out ). The perfect amount of solvent forms the ideal saturated solution. Use too little and you have a supersaturated solution and run the risk of crystals forming too soon, before you are ready for them. Use too much solvent and you no longer have a saturated solution. There is no such thing as a perfect solvent. All solvents will dissolve a solid compound to some slight extent when cold. The more solvent you use, the more the compound will stay permanently dissolved in the solvent, leading to lower amount recovered from the solvent. Thus: the goal is to use the barest minimal amount of solvent. H N CH 3 O Example: Acetanilide Solubility: 1 gram per 20 ml of hot water and 0.11 grams per 20 ml of cold water So to dissolve 2 grams of acetanilide, how much hot water do you need? 1 gram into 20 ml so 2 grams into 40 ml of hot water If you use 40 ml of hot water and you COOL it so it is now COLD water, how much of the acetanilide will stay dissolved? 0.11 grams remain in 20 ml so 0.22 grams remain in 40 ml of cold water Can you ever get back ALL of the 2 grams of acetanilide from the water? 0.22 grams will stay dissolved leaving you with only 1.78 grams MAXIMUM to be recovered. Use as little solvent as possible. You want a saturated solution (as close to supersaturated as you can get!) in order to get the maximum amount of crystals to form. Make sure the solvent is HOT when you want to dissolve something. 2

3 Make sure the solvent is COLD when you DON T want the crystals to dissolve! 2. The Hot Plate This may sound stupid but it DOES get hot. The glassware on it also gets very hot. Electrical cords should not be near the hotplate when it is hot, or they will melt. Please note that the hotplates will have two controls on them, one for heat and one for a magnetic stirrer. Turning on the magnetic stirrer will not make it hot. 3. Boiling Stones The glass container typically used for a recrystallization process is the Erlenmeyer flask. The inside of the Erlenmeyer flask is a smooth surface which will not allow vapor bubbles to form, causing the solution to begin to superheat. At some point, a massive vaporization will occur all at once. Your solution will explode out of the top of your Erlenmeyer (a.k.a. bumping ). This is generally not considered to be a good thing. The purpose of a boiling stone is to provide a rough surface on which the liquid s vapor bubbles can form. This promotes a smooth and even boiling process. Always add boiling stones to a cool solution. What would happen if you added it to one you had already heated to HOT? The solution would suddenly be able to form vapor bubbles and would do so, frothing out the top of your flask always be sure the solution/flask is not HOT when adding boiling stones. How many boiling stones do you think you need to do this correctly? Only 2 or 3 are necessary to get the job done You do not need to add 25 extra is not better Contrary to popular belief, extra boiling stones do not make something boil faster. Heat does that. Always use anytime you are heating liquids, unless told otherwise. 4. Activated Charcoal (a.k.a. carbon, decolorizing charcoal, decolorizing carbon, powdered graphite, activated charcoal/carbon) Used to remove colored impurities from a solid compound during a recrystallization process Colored impurities are typically molecules with large Pi electron systems. These Pi electrons can adhere to the large surface area of the activated charcoal. Once the impurities are clinging to the charcoal, you can then filter away both the charcoal and the impurities. 3

4 Do not add activated charcoal to a hot solution the carbon particles will act as thousands of tiny boiling stones and cause an eruption to occur out of your flask! Warning: Your own molecule may (does!) have Pi electrons and therefore may also wish to adhere to the carbon. The more you use, the lower your yield tends to be. This is unavoidable. 5. Hot Gravity Filtration This type of filtration is used primarily to remove insoluble impurities (those little black floatie things that never dissolve). We use fluted filter paper for this process. Fluted filter paper is preferred over the standard cone because it provides a larger surface area for the solvent to travel through quicker. Why is quicker important? To remove INSOLUBLE impurities, you must heat to dissolve the desired compound AND keep it hot and dissolved as you filter out the insoluble impurities. In order to keep it hot dissolved, the solvent must stay HOT. If the solvent starts to cool, what will happen..? Your crystals will begin to crystallize out in the funnel instead of staying dissolved in the solution this will dramatically LOWER your results and cause a disaster in your filter paper! For this same reason, a short stem funnel is always used instead of a long stem funnel. The longer the stem, the longer the solution must travel through it and the more likely it will be to cool down and crystallize in the stem. WORK FAST and keep everything HOT!! Caution: Beware of solvent-eating filter papers!! Dry filter paper will change your concentration as the filter paper will absorb water from your solution, resulting in a more concentrated solution. What was once happily saturated, becomes supersaturated suddenly, and crystals start to form in the filter paper 6. Vacuum Filtrations Used to rapidly remove liquids (the solvent) from solids (the new crystals). Used to remove SOLUBLE impurities (those that dissolved in the hot or cold solvent at the start of the process). The Büchner funnel has a flat bottom with a lot of surface area exposed for drainage of the solvent. The use of a vacuum pulls the solvent through the holes and away from the solid crystals quickly. Very often, the crystals are left in the Büchner funnel with the vacuum running, long after the solvent is gone, in what we call the process of air drying. 4

5 Be sure to select the correct size filter paper for your Büchner funnel (should easily fit inside the funnel and still cover all the holes). If you wish, you may seat the filter paper, just be sure to use whatever solvent you are going to be filtering, so no other form of contamination occurs. 5