15/05/2008
Chemistry 231 – Experiment 11
Cyclohexene from Cyclohexanol
Lee
1
Larry Lee
Partner: Ichiro Suzuki
Objective:
The purpose of this experiment is to isolate Cyclohexene from
Cyclohexanol by sulphuric acid catalyzed dehydration reaction.
Procedure:
Please refer to Chemistry 230/231 lab manual, Camosun College, revised
July 2005, pages 43-451 {note: use of superscript to give source of information]
Data:
Chemical reaction:
OH
H2SO4 heat
+
H 2O
Table 1: List of reagents and amounts used for the preparation of Cyclohexene {note:
descriptive title}
Reagent
Cyclohexanol
Dilute sulphuric
acid
~50:50 water
Molar mass
(g/mol)
100.2 g/mol
Suggested mass
or volume
10.4 mL
Mass or
volume used
11 mL
Mol used
Notes/ potential hazards2
0.106 mol
98 g/mol
4 mL
4 mL
-
Density = 0.962 g/mL
Colourless viscous liquid
(bp 160.8 oC),
Flammable, skin irritant
18 M conc sulphuric
Add water to acid to
dilute. Very good
dehydrating agent. Clear
colourless and odourless
liquid. Very corrosive.
{Note: It is not necessary to include reagents used as washes}
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Experiment 11 Cyclohexene from Cyclohexanol
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Table 2: Yield and physical data for Cyclohexene isolated. {note: descriptive title}
Product
Cyclohexene
Table 3.
Compound
Cyclohexanol
Molar mass
(g/mol)
Grams
obtained
82.1 g/mol
% Yield
4.15 grams
47.7%
Bp oC
Expt.
80-84
Notes2
Lit.2
82.8
Clear colourless and
odourless liquid. Some
oily droplets was
observed in the final
isolation.
Infrared spectral analysis of Cyclohexanol and Cyclohexene
Absorption
frequency (cm-1)
3400 – 3600 cm-1
Functional
group
O-H
Strong, v .broad
2929 cm-1
C-H alkane
Strong, broad
3023 cm-1
=C-H
Medium, sharp
2933 cm-1
C-H alkane
Medium
Absent
C=C
Very weak
Cyclohexene
Intensity
Literature
reference3
3400-3650 cm-1
Strong, broad
2850 – 2960 cm-1
Medium
3020-3100 cm-1
Medium
2850 – 2960 cm-1
Medium
1640-1680 cm-1
Medium
Calculations:
% Yield calculation:
Since mole of Cyclohexanol : mole of Cyclohexene is 1:1, then 0.106 mol of
Cyclohexanol theoretically gives 0.106 mol of Cyclohexene.
Theoretical mass of Cyclohexene = (0.106 mol Cyclohexene)(82.1g/mol Cyclohexene) = 8.70 g
% Yield = mass experimental
mass theoretical
x 100%
=
4.15 grams x 100 = 47.7 %
8.70 grams
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Experiment 11 Cyclohexene from Cyclohexanol
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Discussion: The sulphuric acid catalyzed dehydration of cyclohexanol to cyclohexene proceeds
by an E1 mechanism. After protonation of the alcohol, formation of a carbocation occurs. This is
the slow step or rate determining step of the reaction. The formation of the alkene product occurs
by elimination of a hydrogen on a β-carbon to the carbocation. The mechanism is shown below3:
O
O S
OH
H
O
H O S
O
H
O H
O
O H
O
H
H
+
H 2O
+
O
O
O S
O
O H
H O S
O H
O
The reaction can also proceed in the reverse direction because the carbocation can undergo an
SN1 reaction with water.
As a result of this, the reaction is in equilibrium. To force the reaction
to proceed towards the E1 product, water and the alkene are collected by distillation. {discuss the
reaction and observations}.
In this experiment, the initial distillation of the cyclohexene
produced a cloudy and acidic product.
After neutralization and extraction of the aqueous
solution, the crude cyclohexene was redistilled to give a clear colourless liquid, containing a few
immiscible droplets which can be attributed to water. {discuss appearance of the product}
Table 1 summarizes the reagents and amounts of reagents used in the preparation of
cyclohexene from cyclohexanol, while Table 2 gives the yield and physical data for cyclohexene.
The yield of cyclohexene was low because the reaction is in equilibrium between the SN1 and E1
products {discuss yield, side reactions, potential errors that are embedded in the experiment, not
personal errors}.
On the final distillation of cyclohexene, the product was collected at a boiling
range of 80-84oC which is very close to the boiling point of pure cyclohexene at 82.8oC2 {discuss
any physical properties}.
Both cyclohexanol and cyclohexene were characterized by IR
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Experiment 11 Cyclohexene from Cyclohexanol
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spectroscopy. {discuss product characterization by instrumental methods} Figure 1 and 2 show
the infrared spectrum of cyclohexanol and cyclohexene, respectively, while Table 3 is a summary
of the characteristic absorption frequencies of relevant functional groups.
Of note, the C=C
absorption, characteristic of alkenes at about 1620 cm-1, was absent. The reason for this can be
attributed to the symmetrical nature of the cyclohexene, and there is no significant dipole change
on IR excitation of the molecule.
Evidence to support the isolation of cyclohexene was provided
by the absence of an alcohol absorption at 3400-3650 cm-1 and the presence of =C-H absorption
characteristic of alkenes at 3023 cm-1.
Conclusion:
A yield of 4.15 grams and 47.7% of cyclohexene with a boiling range of 80-84oC
was isolated from sulphuric acid catalyzed dehydration of cyclohexanol.
{note: Avoid the use of slang, avoid personalizing the report with “I”, “we”, “us”, “our”. Use
third person passive past tense.}
References
1.
R.Raap, N.Khalifa. Organic Chemistry Experiments for Chemistry 230 and 231.
Camosun College, Department of Chemistry and Geoscience, Victoria, BC, Revised
July 2005. pages 43-45.
2.
Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/. Accessed May 15,
2008.
3.
McMurry, J. Organic Chemistry, 7th ed.: Brooks Cole, 2008.
{Include IR spectrum – Labelled figures. Do not include the printed table}
{Citation is standard ACS (American Chemical
Society)http://www.lib.uwaterloo.ca/discipline/chem/acs_ref.html#websites }
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Experiment 11 Cyclohexene from Cyclohexanol
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