Synthesis of Cyclohexene The Dehydration of Cyclohexanol .
The general approach towards carrying out an organic reaction:
(1) Write out the balanced reaction, using structural formulas.
(2) Construct a table of relevant information for reactants and products – e.g., MPs, BPs, MWs,
densities, hazardous properties.
(3) Calculate the correct molar ratios of reactants. Convert moles to grams and milliliters.
(4) Mix correct amounts of reactants, solvents, catalysts in correct order to give specific
concentrations. Possibly heat or cool or irradiate with UV light, allow to react for necessary
amount of time, possibly follow reaction progress using chromatography (e.g., TLC, GC) or
spectroscopy (e.g., IR, NMR).
(5) After reaction is complete, the reaction mixture is usually a complex mixture of desired
product(s), byproducts, unreacted starting materials, solvents, and catalyst. Product may be light,
heat, or air (2 ) sensitive.
(6) Separation and purification steps (so-called reaction work-up). Some combination of
extractions, distillations, recrystallizations, chromatography, etc are used for the work-up.
(7) Identify product(s) using spectroscopy (IR, NMR, etc), chromatography (GC, TLC, etc),
physical properties (MP, BP, etc), and occasionally chemical tests.
Cyclohexene. In the presence of a strong acid, an alcohol can be dehydrated to form an alkene. The acid
used in this experiment is 85% phosphoric acid and the alcohol is
cyclohexanol. The phosphoric acid is a catalyst and as such increases the rate of reaction
but does not affect the overall stoichiometry. It can be seen from the balanced reaction
that 1 mole of alcohol produces 1 mole of alkene. The theoretical yield of alkene in moles
is therefore equal to the number of moles of alcohol used.
85% H 3O 4
1 The first step in the mechanism is protonation of the alcohol group by the acid (slightly exothermic).
The second step is the loss of water to form the carbocation (highly endothermic). The final step is
removal of a beta hydrogen by base (water) to form the alkene (exothermic). The overall reaction is
General Approach. The reaction is carried out in a fractional distillation apparatus. As the alcohol and
acid are heated, alkene and water are produced and co-distill into a collection vial. As in any
distillation, unless precautions are taken, some of the product will be lost as hold-up in the apparatus.
Hold-up would result in a reduced yield of product. To overcome this problem and to ensure that a
maximum amount of product is distilled, a higher boiling "chaser" solvent is added to the distillation
flask and the distillation is continued until the temperature rises well over the BP of cyclohexene. At this
point it can be assumed that all product has distilled into the collection vial, along with some chaser.
2 The collection vial now contains cyclohexene, water, toluene, and small amounts of other impurities.
Because any water present will interfere with the distillation (water will co-distill and will not separate),
prior to a final distillation, to obtain pure product, all water must be removed. This is done in two steps.
First, the sample is mixed well with an aqueous saturated sodium chloride solution (sat’d salt) and the
lower aqueous layer is removed and discarded. This is a pre-drying step that removes most of the water.
Next, anhydrous calcium chloride pellets are added. Anhydrous CaCl is an i2organic drying agent that
binds strongly with water and thus removes any traces of water from the solution. After about five
minutes, the solution is separated from the pellets and transferred to the clean and dry fractional
distillation apparatus. The dried solution is then fractionally distilled to produce purified cyclohexene.
To achieve optimal separation, the distillation must be done at a slow steady rate. To ensure that the
fraction collected as product is relatively pure cyclohexene, this fraction is collected over a narrow range
at the boiling point of cyclohexene. The purity of the cyclohexene is determined by gas chromatographic
(GC) analysis and a % yield calculated.
3 Chemical Tests. The presence of the alkene functional group can be indicated by carrying out simple
reactions in which a color change can be observed. One such reaction is with bromine in
dichloromethane. The bromine reagent is a reddish-orange color. When it is dripped slowly into a
sample of alkene, the bromine reacts with the alkene to form a nearly colorless dibromide. Because the
bromine is consumed, the mixture loses the reddish-orange color. If a sample decolorizes bromine under
these conditions it can be inferred that an alkene functional group is likely present.
A second color test is the reaction with potassium permanganate. An alkene reacts with potassium
permanganate to form a colorless diol. Under acid conditions the diol reacts further to form a mixture of
colorless carboxylic acids or in the case of a cyclic alkene, a dicarboxylic acid. As the permanganate is
consumed, the mixture loses its deep purple color. At the same time manganese dioxide, a brown
precipitate, forms. If a sample decolorizes permanganate and forms a brown precipitate it can be inferred
that an alkene functional group is present.
KMnO 4 OH
Procedure. (rev 6/10)
During Weeks 2 and 3 of lab, you will do two experiments - Cyclohexene and Melting Points (MPs).
Because of a limited number of melting point devices and gas chromatographs, one half of the class will
do Cyclohexene during the 2nd week and the other half will do MPs. During Week 3, you will switch
and do the experiment that you did not do during Week 2. The experiment that you will do during Week
2 will be posted on the web. Be sure to prepare for the correct experiment. Melting Points are
described in a separate handout.
The " Things to Watch Out For In Distillations" listed for the distillation experiment are common to this
distillation as well - review them carefully. As always, use care when inserting the thermometer into the
thermometer adaptor. Hold the thermometer close to the adaptor and push and twist gently away from
you. Breakage could result in a serious injury. Check for frayed connectors and cracked flasks.
Cyclohexene has a disagreeable odor, characteristic of volatile alkenes. Allow the apparatus to cool
before disassembling it in the fume hood, and dispose of the wastes in the hood in the ORGANIC
LIQUID WASTE CONTAINER. Rinse the apparatus in the waste hood with a SMALL AMOUNT of
acetone and dispose of that in the ORGANIC LIQUID WASTE CONTAINER.
CAUTION: cyclohexene is very flammable. Handle phosphoric acid with care. It is corrosive to tissue.
If your skin comes into contact with phosphoric acid, wash the contaminated area immediately with
water, then soap and water. Clean up spills immediately using the sodium bicarbonate in the hood.
Prepare a flow diagram including all steps in the reaction, work-up, and identification. Prepare a table of
reagents, products, and byproducts. (Sample of each on Chem 269 website – handouts page.)
Preparation of Cyclohexene. As always, if the sand bath is being used in an experiment, turn it on
right away to a setting of just under 40. This way it will reach operating temperature by the time it is
needed. In this experiment the phosphoric acid need not be measured very accurately. However, the
4 amount of cyclohexanol determines the theoretical amount of product so it must be measured accurately.
Place the round-bottomed (rb) flask into a 30 mL beaker to hold it upright, set this onto the balance and
press zero (this will set the weight of the container to zero – this is called taring the flask), and measure
in about 2.0 g (± 0.05 g, but weigh it exactly, e.g., 1.964 g or 2.033 g) of cyclohexanol by dropping it in
with a pipet. Using a buret in the hood, add about 0.5 mL of 85% phosphoric acid to the rb flask. Note
that the rb flask becomes warm - addition of acid to alcohol is exothermic. Set up a fractional
distillation apparatus in the same way as in the distillation experiment (remember the boiling chips). Be
sure to use the distillation column (in some rare cases a slightly longer narrower glass tube that looks a
lot like the distillation column may have inadvertently gotten mixed in to some of the lockers. Be sure to
not use this tube - this tube, a chromatography column used in another course, does not connect tightly.)
Use the black plastic connector to connect the flask to the distilling head. This is more resistant to the
reactants than is the white connector. Insulate the column by loosely wrapping it with a piece of
aluminum foil. Be sure to use an ice-cooled receiver. Otherwise product will evaporate and the odor
will be disagreeable.
Heat the mixture sufficiently to distill product over into the collection vial (remember to make a small
depression in the sand bath so that the flask may be lowered into it for effective heating). Unlike in a
normal distillation this first distillation can and should be done fairly rapidly, not at the usual rate of one
drop per 20-30 seconds, but instead at a rate of about one drop per second or two. (If liquid is not
collecting in the vial within about 20 minutes, something is wrong – there may not be enough heat, the
column may be packed too tightly, you may have measured the quantities incorrectly, or there may be a
leak. Check with your TA.) Note the temperature range over which distillate is collected. Continue
distilling until distillation slows dramatically and most of the liquid has distilled (there will be about 0.5
mL of liquid remaining in the flask). Lift the apparatus out of the hot sand and allow it to cool for a
couple of minutes. At this point, most of the product mixture has been collected in the collection vial.
There should be two phases (layers) of liquid in the vial. Water, being one of the products, is insoluble
in cyclohexene so forms a second layer. The density of water is greater than that of cyclohexene so the
water layer would be expected to form the lower layer. This may not be obvious for two reasons: on
this small scale of reaction there is such a small amount of water formed it may simply cause the liquid
to appear cloudy at first and not form a second layer immediately; also, water tends to adhere to glass so
instead of actually forming a second lower layer it may just appear as small droplets sticking to the side
of the vial.
If one were to stop at this point, a significant amount of product would be left behind in the distilling
apparatus (recall “HOLDUP”) and the yield of product would suffer. A method to ensure that all
desired product distills is to add a higher-boiling liquid (called a chaser) to the distilling flask and to then
continue the distillation. To add chaser, slide the sand bath to one side so that the chaser, if spilled, will
not fall into the hot sand. Measure about 2 mL of toluene into a 10 mL graduated cylinder, carefully
remove the thermometer and adaptor, and pipet the toluene onto the top of the copper-packed column.
The toluene will drip down into the rb flask. Replace the thermometer and continue the distillation.
When the temperature reaches 90°, continue distilling until about 25 drops (about 1 mL, very
approximate) of toluene has been collected then lift the apparatus up from the hot sand to stop the
distillation. Remove and cap the vial. For safe keeping place it into a small beaker. Allow the
apparatus to cool before trying to clean it.
Pipet the contents of the vial into a reaction tube. To minimize loss of product rinse the vial with a small
amount (< 10 drops) of fresh toluene and pipet this into the reaction tube. Rinsing in this way is a
general technique used to minimize loss of product and should be used whenever a material is being
transferred from one container to another (call this a transfer rinse). At this point, water, toluene, and
5 small amounts of other impurities are mixed in with the cyclohexene product. Water will interfere with
the final distillation so it must be removed completely. To do this, add saturated aqueous sodium
chloride solution to the reaction tube, the volume being about equal to the volume of liquid already in
the tube. Mix the two layers thoroughly by drawing some of the lower up into the pipet and expelling it
back through the solution repeatedly for a couple of minutes. This procedure is called washing or
extracting and will be used in several future experiments. The technique will be demonstrated by your
TA. Washing with saturated sodium chloride solution is a pre-drying step that removes most of the
water from the organic phase. Remove the lower aqueous (NaCl) layer with a pipet and temporarily
place it into another container, being very careful to remove all of the lower layer but none of the upper
organic layer. Using a clean dry pipet transfer the organic layer to a clean dry vial (do a transfer rinse
using a few drops of toluene). There should be no droplets of water present at this point. If there are
carefully remove all visible droplets of water. Add several spheres of calcium chloride drying agent to
the vial, cap the vial, and swirl the contents well to be sure that all of the liquid comes into contact with
the drying agent. Notice that the spheres clump together. This indicates that the drying agent is doing
its job and has absorbed water. To be sure that enough drying agent has been added, add an additional
few spheres of calcium chloride, swirl, observe, and continue this until the newly added spheres no
longer clump together. The danger of adding too much drying agent is that it also absorbs the product
and can lead to loss. (If all the visible droplets of water were not completely removed first,