Recrystallization Spring Fall 09

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Recrystallization Spring Fall 09

  1. 1. Purifying Acetanilide by Recrystallization
  2. 2. Definition <ul><li>Recrystallization: purification process used to remove impurities from organic compounds which are solid at room temperature. </li></ul><ul><li>Principle: </li></ul><ul><ul><li>Solubility of a compound in a solvent increases with temperature. </li></ul></ul><ul><ul><li>As the solution cools, crystals form and grow. </li></ul></ul><ul><ul><li>Molecules in a crystal have a greater affinity for molecules of the same kind than for impurities. </li></ul></ul>
  3. 3. <ul><li>Criteria for selecting recrystallizing solvent </li></ul><ul><li>The compound being purified must be insoluble in solvent at room temp but soluble in boiling solvent . </li></ul><ul><li>Solvent’s boiling point must be lower than the compound’s melting point . (Why?) </li></ul><ul><li>An abundant quantity of crystals must be recoverable from the cool solvent. </li></ul>
  4. 4. Choosing a Recrystallization Solvent <ul><li>Options: </li></ul><ul><ul><li>Impurities should be soluble in the chosen solvent at all temperatures so that they stay in solution. </li></ul></ul><ul><ul><li>OR </li></ul></ul><ul><ul><li>Impurities should be insoluble at all temperatures so they can be filtered from the hot solution. </li></ul></ul><ul><li>Example: A compound X has the following characteristics in solvents A, B, and C. </li></ul>Note: 1- Be careful that your compound does not melt before the solvent boils. 2- Make sure that you record your observations in a chart like the one shown here. Soluble Soluble Soluble Solvent’s Boiling Point Insoluble Insoluble Soluble Room Temperature C B A
  5. 5. Step 1 – Choosing the Right Solvent Petroleum Ether Water Ethanol Acetone Dissolves at Room Temperature? YES NO STOP Bad Solvent Dissolves? YES NO STOP Bad Solvent CRYSTALS? YES Good Solvent NO Bad Solvent Heat Ice-water bath Sample added to solvent
  6. 6. Step 2 – Dissolving the Compound <ul><li>Dissolving the Compound </li></ul><ul><ul><li>Dissolve the compound in a MINIMUM volume of boiling solvent. </li></ul></ul><ul><ul><li>After all of the compound dissolves, add 5% excess to prevent premature crystallization. (For example: if 10 mL of boiling solvent is required to just dissolve a compound, 0.5 mL will be added) </li></ul></ul>1 - Heat some solvent to boiling. Place the solid to be recrystallized in an Erlenmeyer flask. 2 - Add small amount (1 mL) of the hot solvent into the flask. (Use a Pasteur pipet) 3 - Swirl the flask 4 - Place the flask on the hot plate to keep the solution warm 5 - Repeat steps 2 to 4 until you have dissolved the compound, and then add the 5% excess.
  7. 7. Step 3 - Decolorizing <ul><li>Decolorizing the Solution </li></ul><ul><ul><li>Add a spatula tip of activated carbon (or decolorizing carbon) after the solution is removed from the heat and has stopped boiling !!! Stir in the carbon and then allow it to sit briefly. </li></ul></ul><ul><ul><li>Activated carbon has a surface area that adsorbs dissolved organic substances: avoid adding too much because carbon can also adsorb your compound!! </li></ul></ul><ul><li>Hot Filtration – Right After Decolorizing </li></ul><ul><ul><li>The hot solution is filtered (in this step) by gravity filtration through a funnel (stemless) containing a fluted filter paper. Keep the paper and funnel warm to avoid crystals forming during filtration. </li></ul></ul>
  8. 8. Decolorizing the Solution <ul><li>Assemble the gravity filtration apparatus using a beaker. </li></ul><ul><li>Preheat: Boil 20 mL of solvent (boiling chip) and pour the boiling solvent over filter paper. Leave the apparatus on hot plate. </li></ul><ul><li>Remove the solvent used to preheat the apparatus from the beaker fast and then filter the solution containing the carbon. </li></ul><ul><li>Let the filtrate cool at room temperature, and then in an ice-water bath. </li></ul>
  9. 9. Folding a Fluted Filter Paper 1- Fold the paper in half. Crease sharply. 2- Fold in half again. 3- Fold each quarter in half again, with all creases facing up. 4- Fold each eighth in half again but this time make the folds in the opposite direction to the one you have been doing.
  10. 10. Step 4 – Forming Crystals <ul><li>Recrystallizing Pure Compound </li></ul><ul><ul><li>The solution is now allowed to slowly cool at room temperature. (If crystal formation occurs too rapidly, impurities may become trapped in the crystals). </li></ul></ul><ul><ul><li>Then the solution is further cooled in an ice-water bath to maximize crystal formation. </li></ul></ul><ul><ul><li>TIP: If crystals do not form, induce crystallization yourself: </li></ul></ul><ul><ul><li>- scratch the inside of the flask with a glass stirring rod </li></ul></ul><ul><ul><li>- or add a seed crystal of pure compound </li></ul></ul>
  11. 11. <ul><li>Collecting, Washing, and Drying the Crystals </li></ul><ul><ul><li>Vacuum filtration is used to separate the crystals from the mother liquor. Pre-weigh your filter paper and record mass. </li></ul></ul><ul><ul><li>Clamp the filter flask to a stand. A Büchner funnel is fitted to the flask with a rubber adaptor. Place filter paper in the funnel, and finally connect the flask by tubing to a vacuum source. </li></ul></ul>Step 5 – Isolating Crystals
  12. 12. Isolating Crystals Disconnect the vacuum. Carefully remove the filter paper and solid from the funnel. Set the filter cake on a watch glass to dry. Wet filter paper with solvent. Turn on vacuum. The solution containing the crystals is poured onto the filter. After the mother liquor has been pulled through the filter, wash crystals with small portions of ice cold solvent .
  13. 13. Isolating Crystals <ul><li>Sandwich your crystals between two filter papers. Let your crystals dry for 1 week in your lab drawer. </li></ul><ul><li>Next Week </li></ul><ul><li>Measure and record the mass and melting point. </li></ul><ul><li>Submit your crystals to the TA in a labeled vial. </li></ul><ul><li>Note: Label your vial with your name, TA’s last name, mass of compound, and MP </li></ul>
  14. 14. Step 6 – Recovery Analysis (Next Week) <ul><li>Percent Recovery is calculated by dividing the mass of the recrystallized compound by the mass of the crude compound before recrystallization. </li></ul><ul><li>mass of recrystallized compound, g </li></ul><ul><li>% Recovery = ————————————————— x 100 </li></ul><ul><li> mass of crude compound, g </li></ul><ul><li>Assessing Purity: </li></ul><ul><ul><li>Observe color </li></ul></ul><ul><ul><li>Measure melting point </li></ul></ul>
  15. 15. Procedure Cautions <ul><li>Wear your goggles at all time. </li></ul><ul><li>Chemicals: </li></ul><ul><ul><li>Acetanilide is toxic and irritating. Don’t touch it with your bare hands use appropriate tools to handle it. Avoid inhaling dust. </li></ul></ul><ul><ul><li>All of the organic solvents are flammable, keep them away from heat sources. </li></ul></ul><ul><li>Be careful to avoid burns from the hot solvents, hot glassware, or hot plates! </li></ul><ul><li>Good to know if you want to have a good grade: </li></ul><ul><ul><li>Moving a flask from the hot plate to a cold bench may cause breaking. Place your flask on a paper towel instead of directly on the bench. </li></ul></ul><ul><ul><li>Adding activated carbon to a boiling solution, may cause boiling over. Let it cool a little bit before adding the carbon. </li></ul></ul><ul><ul><li>Be quick at the hot filtration step to avoid premature crystallization in the filter. </li></ul></ul><ul><ul><li>You will be graded on your percent recovery and purity. </li></ul></ul><ul><li>Have Fun! </li></ul>

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