CH 228
What Did You Do?
What Did You Observe?
What Does It Mean?
Acid-Catalyzed Dehydration of cis/trans-2- and 4-Methylcyclohexanol
Question of the Day: Which alkenes form from the dehydration of commercial mixtures of
cis- and trans-2-methylcyclohexanol and cis- and trans -4-methylcyclohexanol?
Figure 1. Structures of cis- and trans-2- and 4-Methylcyclohexanol
You learn in lecture that compounds with leaving groups sometimes proceed to an alkene by an E1
mechanism and sometimes by an E2 mechanism. In this reaction you may observe different products.
In this experiment half of the class will perform an acid-catalyzed dehydration reaction of cis- and
trans-2-methylcyclohexanol and the other half will do the same reaction with cis- and trans-4methylcyclohexanol.
Key Techniques:
 Distillation
 GC analysis
 IR analysis
Key Concepts:
 Stability of carbocations
 Distillation of immiscible liquids
 Hydride shifts
 Zaitsev (Saytzeff) rule
 Percent yield of each individual product
Objectives:
 Propose a mechanism for the dehydration consistent with the GC data
 Recognize the differences in the IR spectra of products and starting material
1
Apparatus:
 Distillation set-up with 50 mL round-bottom flask as reaction vessel and sample vial as
receiver (Figure 2)
 ½" Magnetic stir bar
 Heating source (sand bath on hot plate/stirrer)
 Magnetic stir bar retriever
 Disposable pipets
 Three sample vials with sealable top (on side shelf)
 Gas Chromatograph
 FTIR
Distillation Procedure:
Obtain your equipment from the stockroom. Add 25 mL of 2- or 4-methylcyclohexanol (whichever
your TA has assigned for you), a ½-inch magnetic stir bar, 5 mL 85% phosphoric acid and 5 drops
of concentrated sulfuric acid to a 50 mL round-bottom flask. Turn on the stirrer motor such that the
two liquid layers mix. The solution may darken and warm up.
Label the three sample receiving vials as “Fraction 1 – methylcyclohexene,” “Fraction 2 –
methylcyclohexene,” and “Fraction 3 - methylcyclohexene”. Assemble a distillation apparatus (Figure
2). Wrap the heated portion of your distillation apparatus with aluminum foil from side shelf (3-inch
wide strip NOT pictured below) above the liquid level in the flask. Heat the reaction flask in a sand
bath on low heat (less than 50%, not literally “low” setting on your hot plate) and distill the volatile
product into a sample vial. In order to obtain good (high) yield of product, it is important not to cause
decomposition of the starting material and/or product by using high heat. This low heating also helps
prevent distillation of starting material before it has reacted.
Figure 2. Distillation Apparatus
Collect the first 5 mL of distillate in the vial labeled “Fraction 1” and replace with the vial labeled
“Fraction 2.” Collect the second 5 mL of distillate in the vial labeled “Fraction 2”. Collect the third
5 mL of distillate in the vial labeled “Fraction 3”. Note the boiling point range of each fraction as it
is collected write this in your notebook. The total volume of a vial is 20 mL, so 5 mL in a vial is
2
about ¼ full. If you want to be more accurate, measure 5 mL of water with your 10 mL graduated
cylinder, pour the water into a vial, and draw a line on the side of the vial with the sharpie in your
desk drawer. Use this vial to estimate the “5 mL mark” with your sample vials.
If there are two layers in the distillate, remove the top layer (organic product) with a disposable
pipet and place it into a new vial. Label that vial with the fraction number. Do not try to get all the
liquid, you will only need a few mL to perform your analyses.
Add anhydrous sodium sulfate (enough to cover the bottom of the vial containing your product, onegrain thick) in order to remove any residual water. Cap the vials and save them. We will analyze them
by GC and IR. If there is enough time, do the analyses this week. If not, place the vials in a beaker to
keep them from tipping over and do the analyses next period.
Clean Up
Use the ground-glass stopper in the blue bin to stopper the distillation round-bottomed flask (the
one which may have black residue in it) after it has cooled and you have dissembled the apparatus.
Take the stoppered flask and rinse with water several times. Put each rinse in the “Acid Waste”
bottle in the chemical waste hood. Next, rinse the flask with acetone and put that into the large
“Organic Waste” bottle. Finally, rinse the flask with dichloromethane (DCM) to remove the last of
the black crud and put that into the “Halogenated Organic Waste” bottle. It may take a couple of
DCM rinses. Finally, wash the flask with the pink soap in the spray bottle with a little water and
use your test tube brush. Rinse all your ground-glass equipment with acetone followed by water
before returning it to the stockroom. The product of this reaction possesses an obnoxious odor.
Please minimize exposure of the liquid product to the laboratory environment (and your fellow
students). Keep it in the hood at all times.
Analysis:
IR analysis: Run an IR spectrum on your product (only analyze one of your products, infrared
cannot distinguish between the fractions): Place several drops of the liquid product on a salt plate,
and place the other salt plate on top. Place the two salt plates together into the salt plate holder and
obtain your spectrum. (See CH 228 web site:
http://chemistry.clemson.edu/organic/Labs/InstrumentDocuments.html “FTIR.”
Compare your spectrum with a provided spectrum of the starting material. (On wall in lab) Are
any peaks missing which were originally in the starting material? Have any new peaks appeared?
What does this imply? (Answer in your notebook.)
GC analysis: You will inject a small amount (1.0 μL) of each of your samples into the preprogrammed GC. How many products were formed in the reaction? What are the relative
percentages of each product? How much starting material is still present? (Answer in your
notebook and in the post-lab.) Your TA will help you operate this instrument. You can also see
http://chemistry.clemson.edu/organic/Labs/InstrumentDocuments.html “Gas Chromatograph”
Compare the chromatograms from the GC analysis of the dehydration of your isomer of
methylcyclohexanol with chromatograms of other groups with the other isomer. We have authentic
commercial samples of 1- and 4-methylcyclohexene to compare with your product.
3