George Mason University Organic Chemistry Worksheet

Dehydration of 2-Methylcyclohexanol: Gas ChromatographicAnalysis of the Resulting Product Mixture
Objective:
To carry out the acid-catalyzed dehydration of 2-methylcyclohexanol to produce a
mixture of products and to analyze this mixture using gas chromatography in order to
determine the %composition of the products.
Discussion: First give a description of the reaction in general and give a reaction
mechanism (you’ll need to do this by hand – good practice for the exam). The
mechanism is given in the powerpoint. Make sure you draw arrows showing electron
flow.
Then write something like this: There are three products possible from the acid catalyzed E1
dehydration of 2-methylcyclohexanol . 1- methylcyclohexene, 3-methylcyclohexene, and
methylenecyclohexane. Note that the last product, methylenecyclohexane is produced via a
rearrangement of the initially formed carbocation and is produced in such a low yield that it
will be ignored, The major product is predicted to be the most substituted alkene, 1methylcyclohexene according to Saytseff’s (or Zaitzev’s) rule.
Give a Rxn equation showing the two products.
The products will be separated from the reaction mixture by simple distillation.
Since the distillate will be a mixture of isomeric products, GC will be used to analyze the
product mixture to determine which product is the major product.
All of the product in this experiment will not be collected so a % yield will not be calculated.
Relevant Physical Constants: Fill in the following table. The bp’s of the products and
starting material (from the search engine for industry – The Chemical Book) are provided.
Name
2-methylcyclohexanol
1-methylcyclohexene
3-methylcyclohexene
85% Phosphoric acid
Sodium sulfate,
Anhydrous
Formula
MW
BP
C
171-173
110
104
Procedure:
Describe the procedure followed. Especially describe the use of the
Hickman still. Make sure that you indicate where the distillate collects in the setup and how
you collect it for drying. Provide a figure of the apparatus used. Chemix does not have a
Hickman still head. So, either draw the apparatus by hand or use a screen shot and crop it to
show just the apparatus. (At the end of this document there is a screenshot of one. You can
“cut and paste” it.) Make sure you include analysis by GC. Explain what is meant by the
retention times and how you estimate the areas under the peak.
Data: Complete the table below.
Compound
Retention time (min.)
Area under peak
% of compound in
sample
1-methylcyclohexene
3-methylcyclohexene
methylenecyclohexane
Air
Show all calculations:
Include the chromatogram and label the identities of all the peaks on the chromatogram.
Results/Conclusions:
Explain how you identified the peaks in the gas chromatogram. What is the major
product of the reaction studied in this lab? What is the minor product? What are their relative
amounts? How do you calculate the relative amounts. Draw the structures of the products.
Which of the products would you have predicted to be the major product? Explain your
reasoning. Did the GC results support your prediction?
In your discussion, make sure you do the following. Identify the major and minor
product signals in the chromatogram and their bps. Since the products have a similar
polarity, the difference in their elution from the column should depend on their bps. Predict
the order of elution based on bp. Comment on whether the actual GC results support this
prediction. Determine the areas under the signals and calculate the relative amounts of each
component in the reaction mixture.
Structures and equations you may find useful.
H3PO4
-H2O
OH2
OR
OH2
Rxn Equation
H3PO4
major product
1-methylcyclohexene
minor product
3-methylcyclohexene
Dehydration of 2-Methylcyclohexanol: Gas
Chromatographic Analysis of the Resulting
Product Mixture
Alkenes can be produced by the acid-catalyzed dehydration of an
alcohol. Sulfuric or phosphoric acid is commonly used to catalyze the
dehydration.
The E1 pathway is summarized below and will be the major pathway that we
will consider.
H
C H3
CH3
CH3
OH
H3PO4
O
H
-H2O
C H3
H
OH2
H
C H3
H
OR
H
C H3
C H3
OH2
H
H
A third product, methylenecyclohexene, can be formed. It proceeds via
rearrangement of the carbocation that is initially formed followed by
elimination. The compound is formed in very low yield and is not
considered in this lab.
H3PO4
-H2O
major product
1-methylcyclohexene
minor product
3-methylcyclohexene
very minor product
The products will be separated from the reaction mixture by
distillation. Since the distillate will be a mixture of isomeric
products, gas chromatography (GC) will be used to analyze the
product mixture to determine which product is the major product.
H3PO4
All of the product in this experiment will not be
collected so a % yield will not be calculated.
H2O
Hickman Still Head –
used in this lab
because a
microdistillation is
required.
Short pathlength for condensation
Distillate collects here
Rxn vessel
Heat source
The next step before analysis by GC is drying of the sample. Drying
means “getting rid of water”. This is done by adding a small amount of
a drying agent – an anhydrous solid that will absorb water from the
mixture. The drying agent in this procedure was sodium sulfate, NaSO4.
Gas Chromatographic (gas-liquid chromatographic) Analysis
Major product
Minor product
A Gas Chromatograph
What happens in the column?
Look at your data: We need to know 1) the retention times of the
two components and 2) the areas under each signal, which
corresponds to the amount of the component present
Major product
Minor product
Air
What is this peak?
On your chromatogram look at the two peaks.
1) The retention time can be read directly from the chromatogram. It
corresponds to the time the component spent on the column. Read
it from the abscissa of your chromatogram from time = 0 to the
peaks of the signal.
Retention time
2) You need to approximate the area under the two signals. For each signal, measure the height of the
signal from the baseline to the peak. Then measure the
width at ½ the height of the signal. Use a ruler! Blow up the figure if necessary to see it.
Approximate the area of a signal as the height of the signal x the width of the signal at ½ its height.
baseline
To approximate the area under a
chromatogram.
To calculate the percent composition of a component
% of component B =
area B________
X 100
area A + area B + area C
What will determine the order of elution of the two products
from the chromatography column?
When predicting retention order in gas chromatography, the
overriding factor is a comparison of the boiling points. The compound
with the lowest boiling point elutes first.
When two compounds have close boiling points (within 5oC or less) it
becomes important to consider the polarity of the compounds and
the polarity of the stationary phase. For this reaction, the polarity of
the products are similar, but their bps differ.
bp = 110℃
bp = 104℃
So the one that is more volatile (has the lower bp) elutes first.