Sydney Schramm
A report showcasing analytical skills:
Alkenes from Alcohols: Analysis of a Mixture by Gas Chromatography
Introduction & Theoretical Background
Sulfuric acid
O
O
S
O
O
H
H
Molar Mass: 98.08 g/mol
Molecular Formula: H2SO4
Melting Point: 10ºC
2-methyl-2-butanol
H3C
H3C
CH3
OH
Molar Mass: 88.15 g/mol
Molecular Formula: C5H12O
Melting Point: -12ºC
2-methyl-1-butene
CH3
H3C
CH2
Molar Mass: 70.13 g/mol
Molecular Formula: C5H10
Melting Point: -137ºC
2-methyl-2-butene
CH3
H3C
CH3
Molar Mass: 70.13 g/mol
Molecular Formula: C5H10
Melting Point: -134ºC
This experiment used sulfuric acid as a catalyst and 2-methyl-2-butanol as a substrate in a
dehydration reaction to give a mixture of alkenes that can be analyzed using gas
chromatography. This reaction is an elimination reaction, more specifically, an E1 reaction (2).
Each step of the reaction can be reversed, and different isomers can be produced by the alkene.
In the reaction, involving 2-methyl-2-butanol, a stable tertiary carbocation is produced (see
figure below) (1). Depending on the loss of a proton from a primary carbon or a secondary
carbon, either 2-methyl-2-butene or 2-methyl-1-butene will be produced, as shown below (2).
Loss of a proton from a primary carbon yields 2-methyl-1-butene and loss of a proton from a
secondary carbon yields 2-methyl-2-butene (2).
H3C
OH
H+
H2O
+
H3C
fast
CH3
H3C
H2O
+
CH3
H3C
HSO4-
+
H3C
slow
H3C
CH3
+
H3C
+
C
H3C
H
H
O
CH3
H3C
CH3
slow
+
C
H3C
H3C
CH2
H
H
H
2-methyl-1-butene
O
CH3
H3C
+
C
H3C
CH3
slow
H3C
CH3
H
H
H
2-methyl-2-butene
O
E1 vs. E2 Reactions
E1
E2

Rate depends upon concentration of
alkyl halide (1)

Rate depends upon concentration of
alkyl halide and base (1)

Prefers high temperatures (1)

Prefers high temperatures (1)

Two step unimolecular process with
one transition and two intermediate
states (1)

One step bimolecular process with one
transition state (1)

Products can be determined by
Zaitsev’s Rule. (1)

Hydride and methyl rearrangements are
impossible (1)

Prefers strong base (1)

Must occur with antiperiplanar
stereochemistry (1)

Products can be determined by
Zaitsev’s Rule. (1)

Hydride and methyl rearrangements are
possible (1)

Strong base not required (1)

The reaction examined by this experiment follows Zaitsev’s rule which indicates that in
an elimination reaction with a possibility of having more than one alkene product, the product
with greater stability will be the major product (1). Alkenes with greater substitution are usually
more stable than alkenes that are lesser substituted. Thus, as observed by this experiment, 2methyl-2-butene, which is more substituted than 2-methyl-1-butene (see structures above), will
be the major product (2). The more the alkyl groups attached to the sp2 carbons, the more
substituted are the alkenes (1). Conditions that favor the observance of Zaitsev’s rule is the
presence of a small base; a large base will likely follow and produce the lesser substituted
“Hoffman Product.” Side products may have included H2SO4, H3O+, and H2O (2) also a
competing substitution reaction may have produced the following.
Substitution
H
O
CH3
H3C
H
CH3
H3C
+
C
OH
CH3
CH3
2-methyl-2-butanol (Start)
To analyze the concentrations of the components of the solution produced by the reaction
of 2-methyl-2-butanol, gas chromatography is used. It can help determine the relative amounts
of different materials in a mixture or the impurities in a mixture (2). On the copy, retention time
(minutes) and integral (percent area) are indicated. Higher peaks indicate a lesser volatile
component, and a lower peak indicates a more volatile component (2). There is a gaseous
mobile phase of helium or nitrogen; the stationary phase involves a non-volatile high boiling
liquid (2). As the crude sample is injected into the apparatus, its components are separated based
on temperature, polarity, and rate of gas flow (2).
Experimental
Detector Temperature= 25 °C
Column Temperature= 44 °C
Injection Temperature= 41 °C
Pre-weigh Vial (in Calcium Chloride)= 23.714
Vial + Collected Material= 24.393
Theoretical yield: (1.60g 2-methyl-2-butanol) / (88.15g/mol) = .018 mol
0.018 mol x (70.13g/mol) = 1.26 g mixture
Actual yield: (.679g mixture) / (88.15 g/mol) = 0.0077 mol
0.0077 mol x (70.13g/mol) = 0.5401 g mixture
Percent yield = actual/theoretical x 100% = 42.87%
Minor product = 9.49791% composition
Major product = 90.50211% composition
% yield for minor product= (.0949)( 0.0077 mol) = 0.00073 mol
Mass = 0.00073 mol (70.13g/mol) = 0.051g
% yield for major product is = (.9050211)( 0.0077 mol) = 0.00696 mol
Mass = 0.00696 mol (70.13g/mol) = 0.488g
Discussion
This experiment was successful in verifying Zaitsev’s rule; 2-methyl-2-butene was
produced as the major product from 2-methyl-2-butanol, as it is more substituted than the
product 2-methyl-1-butene. The gas chromatography verified this. Most substituted alkenes
have a transition state with a lower energy. The more substituted the product, the more stable it is
and the least volatile. The lesser substituted product will be less stable and will be more volatile.
The larger peak confirmed that the mixture was 90.50211% 2-methyl-2-butene, which is the
more stable and most substituted alkene. The mixture also included 9.49791% 2-methyl-1butene.
Possible errors include over distillation with the range of 30 to 45 degrees Celsius and not
capping the product vial quickly enough; products may have evaporated leading to an incorrect
yield. Another issue could have occurred when preparing the solution. If the proper
measurements were not used, the process would not have been as accurate as it should have
been. Also, the vial may not have been cooled in ice when mixing reagents of sulfuric acid and
alcohol and after distillation; the highly volatile products may have been lost. The theoretical
percentage for the collected mixture would have been 90% 2-methyl-2-butene and 10% 2methyl-1-butene.
References
1. Wade, G; L. Organic Chemistry. 6th edition: Prentice Hall: New Jersey, 2006.- Organic Book
2. Weldegirma, Solomon. Experimental Organic Chemistry Laboratory Manual. Boston:
Houghton Mifflin Company, 2008.