CHEM 321: AN E1 REACTION: CYCLOHEXENE FROM CYCLOHEXANOL
(A PARTNERS EXPERIMENT)
Elimination always competes with substitution. This is because any nucleophile is also a base on account of its
(relatively) rich electron supply. Since a base is present, abstraction of a hydrogen ion is a possibility instead of
attack at the electron-deficient carbon atom:
Nuc
H-Nuc
Nuc
H
elimination
C
substitution
C
C
C
C
C
Substitution is the favored process at low temperature, but elimination predominates if the reaction temperature is
high. Just as with substitution, there are two elimination pathways: E1 and E2. In the unimolecular pathway,
substitution and elimination share the same slow step: a carbocation intermediate must form. This means that the
rate at which substrate is consumed is fixed regardless of what product (elimination or substitution) forms.
However, the predominant product of a particular unimolecular reaction depends on what happens after the slow
step. In this experiment, the temperature is high, and elimination predominates. Strong acid is added in order to
change the alcohol's poor leaving group (―OH) into a good leaving group (water) so as to favor a carbocation
intermediate. Phosphoric acid is used instead of other common strong acids for two major reasons:
(a) the anion of phosphoric acid (H2PO4‾ ) is a poor nucleophile, so the tendency toward substitution is
minimized, favoring elimination; and
(b) sulfuric acid, normally a logical choice as a strong acid, favors polymerization and degradation of the
alcohol.
The elimination reaction is shown below.
H
O H
H
O H
H+
H
H
H 2O
H
H
H
H
H
H
base
H
H
base = neutral cyclohexanol
Carbocations are extremely reactive, so other products should be expected. The cation might be attacked by the
oxygen of an unprotonated cyclohexanol, forming dicyclohexyl ether. Or, the π-cloud of cyclohexene might attack
the cation, leading to a dicyclohexyl cation which could form a larger (polymeric) alkene upon loss of a proton.
Normally, one should consider the possibility of rearrangement. Unsubstituted cyclohexyl cations don't rearrange
since no cation of greater stability can form.
EXPERIMENTAL SECTION
 Partners!
CAUTION: Phosphoric acid is a strong acid that can cause serious skin burns and damage
clothing. Immediately wipe up any spills and wash skin or clothing with lots of water
followed by a little sodium bicarbonate solution, then a final rinse with more water.
CAUTION: Cyclohexene is a skin irritant. Avoid skin contact. It is also volatile and very flammable.
Don't breathe its vapors. Watch out for ignition sources in its vicinity.
[In the protocol that follows, the reacting mixture is heated not only (1) to favor elimination
but also (2) to drive off the more volatile alkene and water that results from the elimination as
it forms, an application of Le Châtelier’s principle, and (3) to minimize side reactions. Why
does the alkene have a significantly lower boiling point than the parent alcohol?]
Weigh cyclohexanol (20 g, may need to be melted) into a tared 100 mL boiling flask and add a boiling chip. Add about 6
mL of 85% phosphoric acid. Assemble apparatus for simple distillation with a 50 mL receiving flask. After you are sure
the thermometer is properly positioned, insulate the upper part of the boiling flask and distillation adapter. Heat the mixture
at a gentle boil, making sure the vapor temperature does not exceed 100° C. If the vapor temperature rises too high,
unreacted cyclohexanol will distill over. You must be rather patient. The reaction is fairly slow, and you just have to wait
while the alkene forms. The distillate (the liquid that collects in the receiving flask) will have a cloudy layer on top and a
clear layer below. The reaction is over when the liquid remaining in the boiling flask becomes cloudy, viscous, and slightly
yellow.
Pour the distillate from the receiving flask into a 125 mL separatory funnel and discard the lower layer. Wash the organic
layer with 10-15 mL of 5% sodium bicarbonate solution to neutralize any acid that may have been entrained in the vapor
stream during distillation. Discard the new lower layer. Decant the product (organic layer) carefully into a dry 50 mL
Erlenmeyer flask and rid it of visible water. Dry it with a little anhydrous CaCl2 by swirling the stoppered flask slowly for
about 10 minutes. Use a Pasteur pipette to transfer the liquid (leaving solids behind) into a dry 50 mL boiling flask (if the
organic layer got cloudy from the CaCl2 dust particles, remove those by gravity filtration), add a boiling chip, and distill
through dry glassware into a dry pre-weighed 25 mL receiving flask. Record the temperature when 1) the first drop drips
into the receiving flask, 2) there is a constant drip rate into the receiving flask, and 3) the drip rate slows down. What is the
accepted boiling range of the desired product? (You should have this info in Table 1 of your pre-lab. Keep this range in
mind as you distill over the product.) If you distill too much beyond the boiling range of the desired product, it will be
contaminated with the side product. If the distillate appears cloudy, then dry it over about 10-15 pellets of anhydrous
CaCl2, swirl it until clear, and decant the liquid into a preweighed, dry, glassware. Record the mass of product.
ANALYSIS
Think of two chemical tests to characterize the structure of the product and perform them along with positive and
negative controls (name the chemicals used!). Each test should require only 1-2 drops of product.
Run an IR spectrum of your product using the ATR accessory of the IR instrument. Work rapidly because the product
evaporates fast.
CLEAN UP
The first boiling flask residue should be washed down the drain with plenty of water. The second boiling flask residue goes
in the “non-halogenated mixed organics” bottle. Pour your product into the "student prep cyclohexene" bottle.
MAJOR LABORATORY REPORT (no longer than 5 pages, 1.5-2.0 spacing, including figures and tables)
Please pay attention to the general outline, required formatting, and sample for the major lab report in your lab syllabus.
You will have more data to discuss in your report than is provided in the sample. Guidelines for this report:
 Title
 Source Reference
 Introduction containing:
o the experiment objective(s) (what was the purpose of performing the experiment?);
o the context (the experiment that was performed as a means through which the purpose could be met – pertinent
concepts and techniques included);
o the rationale (why is this objective worthwhile?); and
o Figure 1 should include the experimental E1 reaction (add in mechanism arrows) AND potential side reactions
(mechanisms with arrows). Use ChemDraw.
 Experimental methods (refer to previous experiment handouts for examples of writing a concise experimental protocol)
 Results presenting actual data/observations that support whether the goal was met or not (e.g., mp/bp range, IR spectra,
chemical tests, appearance, percent yield, etc.)
 Discussion containing:
o a reiteration of the objective,
o comments on how these data compare to published values and what conclusions can be drawn,
o plausible reasons for percent yield deviations from 100 percent,
o chemistry concepts supporting results/interpretations (including figures when appropriate, such as reactions for
chemical tests), and
o a summary sentence indicating whether or not objectives were met
 References (and appropriate citations throughout the report)
 Appendix containing:
o calculations with work, protocols for chemical tests, additional supporting figures (ie chem test reactions)
For experiments 1-6, objectives were stated for you. In this lab report AND in each subsequent lab report, it is up to you to
use your scientific knowledge to generate the objectives. Here are some concepts to consider (weave into report):
1. What reaction mechanism did we wish to favor in this experiment? Is there any mechanism that is in competition
with the desired reaction mechanism? If so, what is it, and how did the experimental design mostly avoid this
competing reaction?
2. What chemical principle was exploited to “push” or “pull” the desired reaction towards the products? Why was it
important to maintain the vapor temperature near 100 °C during the reaction? What was the purpose for using a
dilute NaHCO3 solution to wash the crude product?
3. What methods were employed to characterize the product?
REMEMBER THESE GUIDELINES FOR ALL IR SPECTRA:
Label and attach the IR spectra to your lab report, or insert them into the report document. Your RESULTS section
should direct the reader to IR spectra (labeled as FIGURES):
How do I label an IR spectrum?
 With an appropriate figure caption (see example) and figure number under the figure
 The correct name of the chemical, spelled out, is in the caption
 The molecular structure of the chemical (so the reader may see what functional groups are present – include
on experimental product as well, with the structure of the expected product)
 Circle individual peaks you choose to discuss in your report, and tell what vibration is the likely cause of
each peak. You need to assign enough significant peaks to allow the reader to see that the product is what it
is claimed to be and is pure (or contaminated) as claimed in the report. Label each chosen peak explicitly
with the name of the responsible molecular vibration. Examples: O-H stretch, sp2 C-H stretch, C=O stretch.
These assignments should be chosen to confirm that the molecule is of the class you claim it to be. You
should not use the sp3 C-H stretch unless it is a main significant signature peak.
 Figure caption includes a citation to a bibliographic citation in your REFERENCES section
What are the IR spectra that I should include in my report?
 IR spectrum of experimental product
 Published IR spectrum of expected major product, citation included
 Published IR spectra of any potential contaminants or side products that would show characteristic IR peaks,
citation(s) included
 To decide whether contamination from specific compounds is significant, evaluate the relative intensities of
pertinent peaks in the product spectrum along with intensities of those peaks in the spectra of the starting
materials and/or plausible side products.
Where can I find published IR spectra? (These resources are on the course webpage under “Useful Links” as well.)
 Spectral Database for Organic Compounds, SDBS. URL: http://sdbs.db.aist.go.jp
 National Institute of Standards and Technology, NIST Chemistry WebBook. URL: http://webbook.nist.gov./
PRE-LAB QUESTIONS
1. Explain Le Chatelier’s principle in your own words, briefly. How will it be used in this experiment?
2. During the liquid-liquid extraction, following the protocol, you will wash the organic layer with 5% sodium
bicarbonate, then discard the lower layer that forms. Considering this, what is the solvent used to prepare 5%
sodium bicarbonate solution?
3. The functional group of interest in the product does not show a very strong presence in the IR spectrum. Why
would it still be useful to collect its IR spectrum? Remember, characterization tests allow you determine the
presence or absence of different functional groups.