Chemistry 241
Exp 7. Cyclohexene from Cyclohexenol
To read more about green chemistry, try:
http://www.rsc.org/Publishing/Journals/gc/index.asp
Dehydration reactions:
http://dl.clackamas.cc.or.us/ch106-03/intramol.htm
Separatory funnel use:
http://orgchem.colorado.edu/hndbksupport/ext/extprocedure.html
Drying agents:
http://orgchem.colorado.edu/hndbksupport/drying/drying.html
http://www.brynmawr.edu/Acads/Chem/mnerzsto/drying_agents_1999.htm
This synthesis reaction is the reverse of the hydration addition reactions that we
are studying now in chapter 4. As a matter of fact, every step in this reaction is
potentially reversible. We will use Le Chatelier’s principle and remove the cyclohexene
as it is formed to drive this reaction towards products.
This reaction is green in two respects. First, it uses phosphoric acid instead of
sulfuric acid, which is much stronger, more dangerous to handle, and more likely to
damage your starting material. Secondly, this reaction can be carried out without solvent,
as the reactants are miscible in each other. This greatly reduces waste.
Mechanism for the dehydration of cyclohexanol
H
O
OH
H
H
OH2
H
H2O
:
Safety precautions: Although green chemistry reduces safety precautions, it is
difficult to eliminate them all together. Cyclohexene is easily flammable. Phosphoric
acid is not strong like sulfuric acid, but skin contact should be avoided. Any spills should
be cleaned immediately.
This is our second synthesis, the procedure for synthesis prelabs will be followed
again. You need a balanced reaction showing stereochemistry and a table of reactants,
like the one below. Don’t forget to fill it out in your PreLab using the amounts in the
instructions.
Table of Reagents and Products
name of
b.p. m.w.
mL density grams moles ratio of moles
compound
used (g/mL) used
used
theory used
cyclohexanol
100.2
0.948
1
1
H3PO4
(85% w/w)
cyclohexene
-
98.0
-
82.1
0.811
-
Limiting reagent _________________________________
Figure 1. Fractional distillation apparatus, column packed with copper Only the clamp
holding the round bottom flask is tight. The remaining clamps are loose (support only).
The (U-shaped) Clausen Adapter will be replaced with a 3-way connector today.
Reaction Instructions:
1. Prepare a hot water bath in a 250 mL beaker on a hot plate/stirrer.
2. Place 0.075 moles cyclohexanol, 1.75 mL of 85% H3PO4, and a small magnetic
stirrer in a 25 mL round bottom flask.
3. Set up the flask with a fractionating column, distillation head, thermometer,
condenser, vacuum adaptor, and receiving flask. Connect a calcium chloride
drying tube via rubber tubing to the vacuum adaptor. Only the bottom clamp
should be tight. The rest should be loose, but supportive.
4. Heat at a gentle reflux for about five minutes, then heat the flask more vigorously
to distill the mixture into the collection flask. Keep going until only 1 mL is left
in the distillation flask, then remove the heat source. Record the boiling point.
The water bath should get hot enough to distill your material. If not, switch to a
sand bath.
Isolation:
5. Place the distillate into a separatory funnel and wash with 5 mL distilled water.
Carefully separate the layers and place the organic layer in a clean and dry
Erlenmeyer flask.
6. Remove any visible water droplets before adding the sodium sulfate drying agent.
Add a little of the drying agent, let stand for five minutes, swirling occasionally.
A saturated drying agent will clump together. If the drying agent is in one big
lump and no agent is loose, the sodium sulfate is saturated and more needs to be
added and a further five minutes must be waited. When all water has been
eliminated, the liquid will be clear and some sodium sulfate will not be clumpy.
Characterization.
7. Determine the mass of your product and its refractive index and IR spectra.
8. Determine theoretical and percent yield.
Post lab analysis:
Analyze purity and yield of your product. In other words, what was your percent
yield? Do you think that is good or bad? If your percent yield is poor, what do you think
happened that reduced your yield? What do you think you could do next time to improve
your yield? Did the color, refractive index, and bp of your product match the literature
bp, color and refractive index? Was it even close? Is the IR give the expected peaks and
no extra signals? Mark and identify key peaks on the IR.
Post lab questions:
1. Complete the following reactions. Draw the most stable alkene formed without
hydride or methyl shifts. You may use information on stability of alkenes found
in chapter four.
OH
H+
heat
a.
OH
H+
heat
b.
OH
H+
heat
c.
2. Draw mechanisms for the reactions in 1a and 1c, above, using H3O+ as your acid.