Synthesis of Alkenes 1: Miniscale Synthesis of
Alkenes Via the Acid-catalyzed Dehydration of
3,3-dimethyl-2-butanol
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Organic Chemistry Lab II, Spring 2009
Dr. Milkevitch
March 1 & 3, 2010
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 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
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
Step 1: Protonation of hydroxyl group
Step 2: Ionization to carbocation
Step 3: Deprotonation to give the alkene
Mechanism for This Reaction
Step 1: Protonation of the alcohol
Step 2: Formation of the carbocation
Rearrangement
Hydrogens
available for
elimination
Products
Products: Don’t Forget
Formed from secondary carbocation
Which Product Prevails?
•More substituted alkene predominates
•Called Zaitsev’s Rule
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!!
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
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-2butanol)
Accomplish a IR spectrum
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?
Miniscale Apparatus for Distillation