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

1. 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

2. 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

3. 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:

4. 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

5. General Mechanism: Acid-Catalyzed Dehydration of an Alcohol Step 1: Protonation of hydroxyl group Step 2: Ionization to carbocation

6. Step 3: Deprotonation to give the alkene

7. Mechanism for This Reaction Step 1: Protonation of the alcohol Step 2: Formation of the carbocation

8. Rearrangement Hydrogens available for elimination

9. Products

10. Products: Don’t Forget Formed from secondary carbocation

11. Which Product Prevails? • More substituted alkene predominates • Called Zaitsev’s Rule

12. 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!!

13. 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

14. 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

15. 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?

16. Miniscale Apparatus for Distillation