Dehydration of 2-methylcyclohexanolIntroduction:
Alkenes are found as starting materials in the industrial production of a variety of important products
including plastics, fuels and cleaning agents. While most of the commercially important alkenes are
made from petroleum, they can also be synthesized in ways that are a bit more conducive to an
educational lab setting. (Commercial ethylene is made by steam cracking of aliphatic hydrocarbons
which requires temperatures on the order of 800 °C.) Alcohols can be used as starting materials in the
preparation of a variety of different classes of compounds including alkenes. Alkenes can be formed
under fairly mild conditions by heating an alcohol in the presence of an acid (typically concentrated
sulfuric or phosphoric acid) to yield the alkene and water, thus this is referred to as a dehydration
reaction (See Figure 1).
Figure 1: Dehydration of 2-propanol to form propene and water
The mechanism for this reaction is fairly straightforward. A hydroxide ion is not a very good leaving
group, so we have to do something to make it a better leaving group. By protonating the hydroxyl group
(recall that the reaction is performed with sulfuric or phosphoric acid) we create a much better leaving
group, i.e. water, and the reaction proceeds via an E1 mechanism as shown below in Figure 2.
Figure 2: Mechanism for dehydration
One complication can arise if the alcohol that you are using can give different alkene products
depending upon which carbon is deprotonated in the final step. For instance, dehydration of 2-butanol
will yield the more substituted alkenes (i.e. B and C) and the less substituted alkene A. If either the
more substituted or the less substituted alkenes predominate, then we would say that this reaction
exhibits regioselectivity. This example also highlights an additional complication that when possible, you
will observe cis/trans isomers as well. Again, if either the cis or trans product predominates, we would
say that there is stereoselectivity.
Figure 3: Dehydration of 2-butanol
Description of the Experiment:
You will perform a dehydration experiment using 2-methylcyclohexanol as your starting material and
85% phosphoric acid as your acid catalyst. You will isolate the alkene product(s) of this reaction and use
gas chromatography (GC) to determine which alkene is the major product of the reaction.
85% phosphoric acid
5% sodium bicarbonate
1. Create a table of important physical properties of reagents and possible products in your lab
2. Write out the reaction that you are performing and the potential product(s) that you expect to
3. Prepare a flow chart of the steps that you will take in today’s lab.
Aluminum Heat Transfer Block
Hickmann Still with Side-Arm
– 2-methylcyclohexanol is a flammable liquid.
– 85% phosphoric acid is a corrosive material. Appropriate PPE (Personal Protective Equipment)
must be worn at all times.
– Aqueous solutions can be poured down the drain with plenty of water.
– Organic liquids should be disposed of in the non-halogenated waste container.
– Solids should be disposed of in the solids waste container.
Set Up the Reaction
Place an aluminum heat transfer block on your stirring hot plate. Add 1 mL of 2-methylcyclohexanol to a
3 mL reaction vial equipped with a magnetic stir bar. Cool the solution in an ice bath and then add 0.3
mL of 85% phosphoric acid. Place the reaction vial in the appropriately sized hole in the aluminum plate
and clamp the reaction vial to the ring stand. Equip the reaction vial with a Hickmann still w/side arm
attachment, a Claisen adapter (another clamp should be placed here to keep your apparatus from falling
over!), thermometer adapter, thermometer and condenser (the thermometer will go in the side of the
Claisen adapter that allows the thermometer to extend into the Hickmann still, the condenser can go in
the other side). Maintain a slow trickle of water thru the condenser and begin heating the reaction.
Develop a gentle distillation of material into the Hickman still, from which you should remove materials
frequently (as it fills up), combining the distilled materials into a 15 mL centrifuge tube. Keep an eye on
the temperature of the distillation and make a note of the temperature range over which you collect
materials. Once the temperature drops (after it reaches approximately 100 °C) you should immediately
stop heating the reaction, allow the apparatus to cool to room temperature (let it cool while you go on
to work up the reaction, etc.), rinse the residual material into the non-halogenated carboy with WATER,
wash your glassware and return all check-out items to the stockroom.
Wash the organic layer with 2 x 1 mL of 5% aqueous sodium bicarbonate solution, being sure to retain
the organic layer! Dry the organics over anhydrous magnesium sulfate. Isolate the organics from the
drying agent by using a disposable pipette and a filter tip pipette, filtering into a pre-weighed 3-mL
Obtain a weight of your product mixture so that you can determine a percent yield (this is a yield of the
alkene mixture). Determine the product(s) formed in this reaction by the use of GC. Since all of the
products are alkenes with identical molecular weights and similar structures, we can assume that their
thermal conductivities are not appreciably different. Thus, the areas measured for each compound can
be assumed to be proportional to the amount of the total isolated product for each compound. Using
this data, you can calculate an approximate yield of each component.
Be sure to include all relevant data including:
– Crude weight
– Crude percent yield
– GC Data (copy of chromatogram)
– Final weight and percent yield of each product
– Discuss which product was the major product. Write a mechanism for the formation of this
product. Why did it predominate?
Results: I made a template of the results/mechanisms and posted on D2L. Copy that into your results section. You need to do all
mechanism for both alcohols. I also posted an example of a correct GC for 2-methylcyclohexanol and example calculations you need
to include from the GC chromatograph you printed. Show calculations for %mass and amount of moles for each peak (major and
minor product peaks).
Discussion: State purpose and introduce the concept of E1. Explain conditions which SN1 and E1 compete and explain how we can
isolate one over the other. Be sure to talk about carbocation formation and solvents. BE DETAILED- make sure to include reaction
Conclusion: Restate purpose and concept(s), restate %mass of products and conclude which is major (again), and end with a closing
Have a great weekend!