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Lecture 14

Lecture 14. IR/NMR LG Problems Ch 8: Column Chromatography Lecture Problem 5 Due Next Lecture This week in lab: Ch 7: TLC, PreLab Due Quiz 4 Spec Unknown Progress Check Next week in lab: Ch 8 PreLab Due Quiz 5 (last quiz given in lab). Lab Guide Problem 11.53(f).

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Lecture 14

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  1. Lecture 14 • IR/NMR LG Problems • Ch 8: Column Chromatography • Lecture Problem 5 Due Next Lecture This week in lab: • Ch 7: TLC, PreLab Due • Quiz 4 • Spec Unknown Progress Check Next week in lab: • Ch 8 PreLab Due • Quiz 5 (last quiz given in lab)

  2. Lab Guide Problem 11.53(f) Propose a structure that is consistent with the following data: C3H6O2 (HDI = 1) 1H NMR: 1.27 ppm (t, 3H) 2.66 ppm (q, 2H) 10.95 ppm (s, 1H) IR: 1715 cm-1 3500-3000 cm-1 Possible compound(s):    Solution: Double check structure. Make sure splitting patterns and ppm values make sense as well as integration values. Also, make sure the numbers and types of atoms match the MF!

  3. NMR & IR Problem Propose a structure that is consistent with the following data: C4H7ClO2 1H NMR: 1.5 ppm (t, 3H) 4.0 ppm (s, 2H) 4.3 ppm (q, 2H) IR: 1740 cm-1 1200 cm-1 Possible compound(s): Solution: Double check structure. Make sure splitting patterns and ppm values make sense as well as integration values. Also, make sure the numbers and types of atoms match the MF!

  4. Ch 8: Column Chromatography Experiment 2 Lab Periods: • 1st You will perform your first synthetic experiment - the oxidation of fluorene to fluorenone - and will monitor the reaction by TLC. • 2nd  You will separate the fluorene - fluorenone mixture by column chromatography monitored by TLC.

  5. Ch 8: Column Chromatography Experiment PreLab: Be sure to include the reaction mechanism with conditions, limiting reagent identified, theoretical yield of fluorenone, in the Diagrams of Special Apparatus/Reactions section.

  6. Oxidation of Fluorene to Fluorenone • Fluorene can be oxidized by oxygen in the air. • Use a phase-transfer catalyst, Stark’s catalyst. • Allow the reaction to go half-way to completion - monitor by TLC; could take 5 to 30 minutes. • Work-up: liquid/liquid extraction • Purification: column chromatography

  7. Phase Transfer Catalysis - How does this work? Oxidation mechanism given in class.

  8. Hints on 1st Synthetic Expmt: FluoreneFluorenone • Be very careful with the 10 M NaOH!  It will burn you badly if it touches your skin. • Reaction times will vary - Stirring the reaction VIGOROUSLY speeds up the reaction and shortens time. • You are monitoring the reaction by TLC until it is approximately half-way complete.  You will judge this based upon UV intensity of the S. M. and Prod. spots on the developed TLC plate.  • Run TLC 5 minutes after reaction starts. • As with the TLC experiment, check your TLC plate under the UV lamp for spot intensity BEFORE developing, BUT be sure toluene is evaporated! • Be sure to label anything you leave in the hood to evaporate or dry.

  9. Column Chromatography Pack column with alumina (dry pack or slurry method). Alumina will be combined with hexanes. Allow alumina to settle - avoid air bubbles. Add the sample - by pipette. Sample will be dissolved in a minimal amount of solvent. Sample should be a small narrow band on the alumina. Elute your sample. Collect fractions in small Erlenmeyers. Test each fraction by TLC. Combine “like” fractions. As the mobile phase drains, replenish with fresh mobile phase. Never let the column go dry! Always have mobile phase above the top of the column!

  10. Hints on Column Chromatography of Fluorene/Fluorenone • Flash chromatography (use of nitrogen gas on top of the column) can be used to help push the mobile phase through the column more quickly. Do flash the whole time. • Once the fluorene has come off the column, switch to a more polar mobile phase such as 50% dichloromethane in hexanes to elute the fluorenone more quickly.

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