Download Gas Chromatography: Analyzing Alkene Isomers David L. Flanigan

Gas Chromatography: Analyzing Alkene Isomers
David L. Flanigan
CHM2211L
February 20, 2004
Introduction:
In this experiment a secondary alcohol (2-methyl-2-butanol) was dehydrated
using a dilute solution of sulfuric acid. The reaction resulted in a mixture of
alkenes that was purified by distillation and analyzed using gas chromatography.
Theoretical Background:
Alcohols can be dehydrated using catalytic acid conditions to give alkenes.
H+
OH
R
H 2O
R
Alkenes
Alcohols
Water
Ease of alcohol dehydration follows the general trend: 3° > 2° > 1°. This trend is
based on the fact that most acid catalyzed alcohol dehydrations proceed through
the E1 mechanism. In order for the elimination to occur a good leaving group and
a base sufficiently strong enough to remove a β-proton must be present. The
hydroxyl group is not a good leaving group. To enhance its leaving potential it
must be transformed into a good leaving group. This is accomplished by using
strong mineral acids. The strong acid donates a proton (Brønstead-Lowry acid) to
the oxygen of the hydroxyl group which is the proton acceptor (Brønstead-Lowry
base).
R
O
H OH2
H
R
poor LG
H
O
H
H2O
good LG
Good leaving groups must be weak bases after leaving and water is a good
example. In the presence of heat and an ionizing solvent the leaving group
departs and a carbocation is formed. This is the aspect of the dehydration that
allows 3° alcohols to dehydrate easier than 1° based on ease of carbocation
formation.
H
O
R
-H2O
H
R
carbocation
When the carbocation is formed the β-proton is now activated toward removal
resulting in elimination
H
R
H2O
R
alkene
The β-proton is removed by water forming the alkene and regenerating the acid
catalyst.
Some alcohols can form mixtures of alkenes upon dehydration. For example,
when 2-methyl-2-butanol is exposed to catalytic acid dehydration conditions two
products are formed. They are 2-methyl-2-butene and 2-methyl-1-butene.
H3O+
HO
2-methyl-2-butanol
2-methyl-1-butene
2-methyl-2-butene
This is caused by two different types of β-proton that are susceptible to removal
upon carbocation formation.
H 2O
Ha
Hb
Upon removal of proton Ha the disubstituted alkene is formed. When proton Hb is
removed the trisubstituted alkene is formed.
H 2O
Ha
Hb
The ratio of products formed is based on the stability of alkenes formed. Since
the more highly substituted alkene is more stable it is formed as the major
product as stated by Saytzeff’s Rule.
The product mixture can then be analyzed using gas chromatography to confirm
the product distribution.
Data and Results:
Contents of reaction flask:
Product distilled at 28-40°C
Theoretical Yield Calculation:
1.0 mL water
0.5 mL conc. H2SO4
1.1 mL of 2-methyl-2-butanol
(den. = 0.805, MW = 88.15)
1.1 mL alcohol X
0.805 g alcohol
1 mL alcohol
X
1 mol alcohol
88.15 g alcohol
Theoretical Yield:
Actual Yield :
X
1 mol alkene
1 mol alcohol
X
70.1 g alkene
1 mol alkene
= 0.70 g alkene
0.70 g alkene mixture
0.52 g alkene mixture
Percent Yield Calculation:
% Yield =
0.52 g alkene mix
0.70 g alkene mix
% Yield:
Boiling Point:
GC Data:
2-methyl-1-butene
2-methyl-2-butene
X 100 = 74%
74%
2-methyl-1-butene
2-methyl-2-butene
RT
3.489
4.176
31 °C
39 °C
Area
Area %
25961682
16.375
132578322 83.625
Discussion/Conclusion:
When 2-methyl-2-butanol is subject to acid catalyzed dehydration conditions two
alkene products are formed. Based on the gas chromatography data the ratio of
2-methyl-2-butene to 2-methyl-1-butene is approximately 84:16.
It was predicted that two different alkene products would form upon dehydration
of an alcohol and that the more substituted alkene would be the major product
formed. This data also confirms that Saytzeff’s Rule holds true.