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Stereoselective Reduction of Ketones with Sodium Borohydride: Making a Diol Microscale Reduction of Benzil

Stereoselective Reduction of Ketones with Sodium Borohydride: Making a Diol Microscale Reduction of Benzil. Organic Chemistry Lab II, Spring 2010 Dr. Milkevitch March 22 & 24, 2010. Today’s Experiment. Take a look at some chemistry of the C=O group Reduce the C=0 group to an alcohol

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Stereoselective Reduction of Ketones with Sodium Borohydride: Making a Diol Microscale Reduction of Benzil

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  1. Stereoselective Reduction of Ketones with Sodium Borohydride: Making a DiolMicroscale Reduction of Benzil Organic Chemistry Lab II, Spring 2010 Dr. Milkevitch March 22 & 24, 2010

  2. Today’s Experiment • Take a look at some chemistry of the C=O group • Reduce the C=0 group to an alcohol • Using the common reagent: • Sodium borohydride • Purpose: • To illustrate the production of an alcohol by the reduction of a ketone • To use TLC and IR spectroscopy to characterize your results

  3. Background • Alcohols : • Useful class of organic compounds • Readily available • Cheap, relatively non-hazardous • Can be converted to other compounds • Reducing agents: • agents that add H: to compounds • Reduce carbonyl group to an alkoxide • Subsequent protonation yields an alcohol _

  4. Two Useful Hydride Agents • Sodium Borohydride: NaBH4 • Lithium Aluminum Hydride: LiAlH4 • Called complex hydrides • Don’t have simple structure like NaH or LiH • Today: Use NaBH4 • Safe to use • Highly selective reducing agent

  5. Uses of Sodium Borohydride • Reduces aldehydes to primary alcohols, ketones to secondary alcohols • Very selective: • Only reactive towards aldehydes/ketones • Will not reduce carboxylic acids or esters

  6. Mechanism: Hydride Reduction of Carbonyl Group

  7. Mechanism: Reduction of Benzil Step 1:

  8. Mechanism: Reduction of Benzil Step 2: Benzoin • Step 3: Repeat of Steps 1, 2 • Reaction at the other C=O

  9. Mechanism: Reduction of Benzil Final Product: hydrobenzoin But there is more than one product!!!!!

  10. Multiple Products Possible • Remember, C=O is planar • Hydride can attack carbon of C=O from above or below the plane • Therefore, you can have different stereoisomers

  11. What are the Multiple Products? • Hydrobenzoin has 2 chiral centers! • Have a meso compound • Pair of enantiomers Meso Pair of enantiomers

  12. Procedure • Weigh out 200 mg of benzil into a 25 ml erlenmeyer flask • Add 2 ml of 95% ethanol, swirl to mix • Add 75 mg of sodium borohydride • Let stand 10-15 min with occasional swirling • Yellow color should disappear as benzil is reduced • Add 2 ml of ddH2O slowly • Heat to boiling, solution should be clear and colorless • Filter off any insoluble impurities if necessary • Use a pasteur filter pipet • See me or the lab TA to make one • Transfer the hot, clear solution to a clean 25 ml erlenmeyer flask, using a filter pipet • Add an additional 2 ml of hot ddH2O • Allow solution to crystallize undisturbed • Cool in ice bath, vacuum filter • Wash with minimal cold ddH2O • Weigh your product

  13. Characterization • Do a melting point determination • TLC: dissolve 1-3 mg of your product in ethyl acetate • Dissolve 1-3 mg of (+/-)benzoin in ethyl acetate also • Acquire a TLC plate (in oven) • Spot the TLC plate with small amounts of your product and (+/-)benzoin • Run TLC plate in 9:1 methylene chloride/ethyl acetate • This mobile phase is located in the hood • Stain with iodine (see Dr. M) • Acquire an IR spectrum • Interpret your results

  14. Your Report • Formal Report not required • Complete experiment worksheet

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