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Chem 333 Lab Lecture

Chem 333 Lab Lecture. Exercise F.1 Recrystallization of an Impure Solid Technique C Melting Point. Purpose. Learn how to recrystallize an organic solid. Learn how to measure melting point of organic solids. Recrystallization. Method for purifying solid compounds

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Chem 333 Lab Lecture

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  1. Chem 333 Lab Lecture Exercise F.1 Recrystallization of an Impure Solid Technique C Melting Point

  2. Purpose • Learn how to recrystallize an organic solid. • Learn how to measure melting point of organic solids.

  3. Recrystallization • Method for purifying solid compounds • Used throughout CHEM 334/338 to purify reaction products.

  4. Steps in Recrystallization • Dissolve crude solid in minimum amount of hotsolvent (saturated solution) • Let solution cool to room temperature, so that crystal lattice reforms slowly. • Cool solution in an ice bath to minimize solubility (maximize recovery). • Suction filter the pure solid, impurities stay in solution. • Let solid air dry to remove traces of solvent. • Weigh pure crystals • Determine melting point of pure crystals

  5. Solubility Curves Reference: www.fofweb.com/Subscription/Science/Physical/P0812.jpg

  6. Solvent Characteristics • Dissolve all of the compound when the solvent is hot • Dissolve very little or none of the compound when the solvent is at room temperature • Have different solubilities for the compound and the impurities • Have a lower boiling point than the melting point of the compound • Have a relatively low boiling point • Be inexpensive, non-toxic, non-reactive, and odorless

  7. Dissolve Impure Solid Add hotsolvent until solid dissolves: (use minimum amount of hot solvent)

  8. Pour a small amount of the hot solvent into the flask containing the solid.

  9. Swirl the flask to dissolve the solid.

  10. Place the flask on the hot plate to keep the solution hot.

  11. If the solid is still not dissolved, add a small amount more hot solvent and swirl again. Keep solution hot.

  12. Recrystallize Solid Let solvent cool slowly to room temperature, so crystal lattice can reform. Cool in ice bath to make sure that the maximum amount of solid has crystallized. (filtrate gets cold!)

  13. When all of the solid has dissolved, let the solution cool. Do not disturb it.

  14. As the solution cools crystals will form in the flask.

  15. When the solution has cooled to room temperature, put it in an ice bath to finish the crystallization process.

  16. Isolate Pure Crystals Suction filter pure crystals to separate from solvent and impurities.

  17. Suction Filtration Apparatus.

  18. Melting Point • Physical property • Identify an unknown • Characterize reaction product • Estimate purity • Melting point range • temperature at which 1st drop of liquid appears to temperature at which all solid has been converted to liquid. • 1 - 2°C for pure compounds • ALWAYS report as a range

  19. Theory of Melting Point • Effect of impurities • Less energy needed to disrupt crystal lattice when impurities are present. • Generally results in a lower melting point with a broader range. Pure solid: Tight crystal lattice Impurities disrupt crystal lattice

  20. Melting Point Measurement • Load sample into a capillary tube • 2-3mm of material in tube is sufficient. • Too much material will broaden range & may increase melting point. • Put capillary tube into Mel-Temp • Control heating rate • Heat rapidly until 10-15°C from expected m.p. • Reduce rate to 1-2°C/minute • Refer to heating rate curves in lab

  21. Melting Point Measurement • Data Collection • Record identity of compound being tested • Record melting point range • temperature when 1st drop of liquid appears and when all solid is converted to liquid • Put used capillary in broken glass box • Never remelt a sample

  22. Prelab Preparation • Read Technique F • Recrystallization & Filtration • Sublimation section not required • Read Technique C • Melting Point

  23. Prelab Preparation • Prepare Notebook • Title, procedural reference, procedural changes • Table of reagents • Structure, relevant physical property, solubility in H2O, hazards of acetanilide & benzoic acid • Prelab quiz

  24. In Lab • Measure melting point of at least 1 of the known solid samples provided. • Record • Name of compound(s) used • True melting point • Observed melting point • Observations • Color changes • Gas evolution • Sublimation

  25. In Lab • Recrystallize unknown solid • Record • Unknown number • Mass of impure unknown • Approximate volume of H2O • Observations • Anything that may affect recovery or purity • Air dry recrystallized unknown

  26. After Lab • Weigh air dried recrytallized solid. • Determine melting point of recrystallized solid. • Identify unknown solid based upon melting point.

  27. After Lab • Calculate % error of melting points • A single temperature literature value represents the upper temperature of the experimental melting point range. • Example • Literature value: 122.4ºC • Experimental data: 119 - 121º

  28. After Lab • Calculate Percent Recovery

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