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Synthesis of Alkenes 1: Miniscale Synthesis of Alkenes Via the Acid-catalyzed Dehydration of 3,3-dimethyl-2-butanol PowerPoint Presentation
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Synthesis of Alkenes 1: Miniscale Synthesis of Alkenes Via the Acid-catalyzed Dehydration of 3,3-dimethyl-2-butanol. Organic Chemistry Lab II, Spring 2009 Dr. Milkevitch March 1 & 3, 2010. Elimination Reactions. Today : Conduct an elimination reaction

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Synthesis of Alkenes 1: Miniscale Synthesis of Alkenes Via the Acid-catalyzed Dehydration of 3,3-dimethyl-2-butanol

Organic Chemistry Lab II, Spring 2009

Dr. Milkevitch

March 1 & 3, 2010

elimination reactions
Elimination Reactions
  • Today: Conduct an elimination reaction
    • One of the fundamental reactions of organic compounds
  • What you will learn/observe:
      • Review elimination reactions
      • Learn how to synthesize an alkene
        • Acid catalyzed dehydration of an alcohol
      • Look at the products formed
      • See which product will predominate
elimination reactions1
Elimination Reactions
  • Let’s Review:
    • Elimination reaction: a fundamental organic reaction
      • Two species are eliminated from a substrate
      • Elimination mean’s they are gone, gone, gone
        • NOT a substitution
  • Basic mechanism:
another kind of elimination reaction
Another Kind of Elimination Reaction
  • A dehydration reaction
    • Multiple step reaction
    • Loss of water, forming a carbocation
    • Alkene formation results
  • Mechanism: E1 mechanism
    • E1 elimination of a protonated alcohol
  • Reaction of alcohols
    • Acid catalyzed
    • Forms an alkene and water
general mechanism acid catalyzed dehydration of an alcohol
General Mechanism: Acid-Catalyzed Dehydration of an Alcohol

Step 1: Protonation of hydroxyl group

Step 2: Ionization to carbocation

mechanism for this reaction
Mechanism for This Reaction

Step 1: Protonation of the alcohol

Step 2: Formation of the carbocation

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Rearrangement

Hydrogens

available for

elimination

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Products: Don’t Forget

Formed from secondary carbocation

which product prevails
Which Product Prevails?
  • More substituted alkene predominates
  • Called Zaitsev’s Rule
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Procedure

  • Construct your simple distillation apparatus
  • Use a 25 ml RB flask
  • Place 6 ml of 3,3-dimethyl-2-butanol into this flask
    • Include a magnetic stir bar
  • Add 6 ml of 85% phosphoric acid to this flask
  • Place on apparatus
  • Start stirring and heating
  • Collect distillate in a graduated cylinder
    • In an ice bath
  • Distillate should be cloudy
  • Don’t let the temperature rise above 75 deg C!!
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Procedure II

  • Complete distillation
  • Turn off heat, let apparatus cool
  • Disassemble distillation apparatus
    • Clean with acetone
    • Set aside to dry
  • Transfer distillate to a small separatory funnel
    • Drain lower aqueous layer into a beaker
  • Wash organic layer with 20 ml of saturated salt soln
  • Drain off lower aqueous layer
  • Pour off organic layer into a 25 ml erlenmeyer flask
  • Dry with anhydrous magnesium sulfate
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Procedure III

  • Gravity filter into a clean, dry 10 ml RB flask
  • Reassemble distillation apparatus
  • Distill again
    • Cool receiving flask in an ice bath
  • Weigh a small vial, record weight of this flask
  • When distillation is done, transfer distillate to this vial
  • Reweigh the distillate in the vial
  • Do a GC analysis
    • Determine the % of your alkenes
    • Determine the % of your starting material (3,3-dimethyl-2-butanol)
  • Accomplish a IR spectrum
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Your Report

  • Formal Report Required
  • Make sure to include the mechanism of the reaction in your report (separate section)
  • Results Section:
    • Make sure to state all results
      • Prove that you made the alkenes
    • Include GC chromatogram (annotated)
    • Determine % of your alkenes
    • IR spectrum, annotated (if completed)
  • Conclusions section:
    • Was the experiment successful?
      • How much of the alkenes are produced?
      • Any leftover 3,3-dimethyl-2-butanol?
    • If unsuccessful, any ideas why?