1. Organic Chemistry Lab II, Spring 2009
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