Download Alkenes from Alcohols

Alkenes from Alcohols
2-Methyl-1-butene and 2-Methyl-2-butene
INTRODUCTION
The dilute sulfuric acid catalyzed dehydration of 2-methyl-2-butanol (t-amyl alcohol)
proceeds readily to give a mixture of alkenes. The mechanism of this reaction involves the
intermediate formation of the relatively stable tertiary carbocation followed by the loss of a
proton either from a primary carbon atom to give 2-methyl-1-butene, or from a secondary
carbon to give 2-methyl-2-butene.
H2SO4
2-Methyl-2-butanol
--------->
<-------
2-Methyl-1-butene
+ 2-Methyl-2-butene
EXPERIMENTAL PROCEDURE
Pour 22.5 mL of 6 M H2SO4 into a 50-mL pear-shaped distillation flask. Place the flask
in an ice bath. Slowly add 15.0 mL of 2-methyl-2-butanol with swirling. Swirl the mixture
thoroughly.
Set up a fractional distillation apparatus, using a 20 x 150 mm test tube, clamped and
placed in an ice bath, to collect the distillate. The ice cooling of the distillate, as it is collected, is
necessary because the alkene product is volatile. Heat the flask slowly until the distillation of the
olefin is complete. Note: How will you know when the distillation is complete?
Transfer the distillate to a separatory funnel and shake with 5 mL of 10% sodium
hydroxide solution to remove any trace of sulfuric acid through a neutralization reaction. Draw
off the aqueous solution (the bottom layer) into a beaker labeled “acid waste” and save for later
neutralization and disposal. Add the pot residue from the distillation flask to the “acid waste”
beaker.
Dry the organic layer remaining in the separatory funnel adding a sufficient amount of
anhydrous sodium sulfate until the drying agent no longer clumps together. Filter with a cotton
plug through a stemless funnel.
Carefully decant the organic layer into a tared 18 x 150 mm test tube with stopper. Next, place a
small beaker on a balance and zero the balance. Place the test tube containing the product in the
beaker and record the mass to determine the yield of alkene product.
CALCULATIONS
Calculate the theoretical yield and then the percent yield of the alkene mixture.
UNSATURATION TESTS
Place four to five drops of 2-methyl-2-butanol in a small test tube. In another test tube
place four to five drops of your product. Do not confuse the test tubes. Take one tube and add
10 - 15 drops of a solution of bromine in methylene chloride, drop by drop. Repeat with the
second tube. Record your observations for each test tube. Analyze the meaning of your results
and comment on this in your summary.
IR Analysis of Product
Obtain an IR spectrum of your product and of your starting alcohol reagent. Analyze the various
peaks to determine the result(s) of your reaction.
CHEMICAL REACTIONS
Provide the reaction mechanisms for:
1. The formation of each alkene formed.
2. The bromine with each alkene formed.
CLEANING UP
The anhydrous sodium sulfate should be rinsed into the acid waste container. Adjust the
pH of the “acid waste” to between 6 and 10 using sodium carbonate and then flush down the
drain.
Wash the glassware from the red kit with soapy water, rinse with distilled water and then
with ethanol or acetone. Dry as thoroughly as possible and return to the red kit.
SUPPLEMENTAL QUESTIONS
1. Draw a mechanism for the dehydration of cis-4-methylcyclohexanol (chair conformer)
catalyzed by phosphoric acid.
2. Draw the Lewis structure of the major alkene product formed by the dehydration of the
following alcohols?
a. Cyclohexanol
b. 1-Methylcyclohexanol
c. 2-Methylcyclohexanol
d. 2,2-Dimethylcyclohexanol