Preparation of cyclohexene from cyclohexanol.

EXPERIMENT 8

Preparation of cyclohexene from cyclohexanol.
In this experiment an alkene (cyclohexene) will be prepared by dehydration of an alcohol (cyclohexanol) using an acid catalyst such as phosphoric acid. This is one of the most common methods of preparing alkenes.

The crude product is contaminated with water, unreacted alcohol, phosphoric acid and some side products. Washing with water removes most of the impurities. Treatment with sodium carbonate solution removes traces of acid and a final wash with water removes any remaining carbonate.

The mechanism of the dehydration of cyclohexanol probably involves the formation of a carbocation.

This carbocation can react in any o fthe ways shown below:

1. With water to yield cyclohexanol - the starting material. (Note that all the steps in this reaction are reversible);

2. by losing a proton to yield cyclohexene;

3. with cyclohexanol to yield dicyclohexyl ether.

Dicyclohexyl ether then is a probable side product of the dehydration of cyclohexanol. It is immiscible with water is likely to co-distill and may therefore be present in the first distillate. To remove dicylcohexy ether completely a second distillation of the product is usually carried out.

Reference:

T.W.G. Solomons and C. Fryhle, Organic Chemistry, Chapter 7.7, Dehydration of Alcohols.

Safety Features - CAUTION!

1. Phosphoric acid (85%) can cause severe burns. Wash all spills on the skin with cold water for 15 min. See your demonstrator.

2. Bromine causes severe burns. Keep bromine away from your skin. Do not breathe the vapours. Use only in the FUME HOOD.

3. Cyclohexanol can be irritating to the respiratory system and skin. Do not breathe vapours and prevent contact with skin.

4. Potassium permanganate is a strong oxidizing agent. Handle with care.

5. Cyclohexene has an unpleasant smell. Cover all containers of this compound and do not leave drying agent, glass wool or towels coated with the compound lying on the bench.

Procedure:

Pour cyclohexanol (10.0 g, 10.6 mL, b.p. 161°) into a 50 mL round bottom flask (small neck) and cautiously add 85% phosphoric acid (3 mL). Add 3 boiling chips and arrange for a distillation using a cooled 10 mL graduated cylinder as a receiver. (Cool the cylinder by standing it in a beaker of ice and water).

Heat the round bottom flask slowly with a small flame. When white fumes appear in the round bottom flask, and about 10 mL of distillate have been collected, discontinue the distillation.
Transfer the distillate to a separatory funnel (60 mL) (Check first to ensure that the stopcock of the separatory flask is closed!). The mixture will separate into two layers.
Run off the lower layer into a conical flask and set this aside.

Add water (10mL) to the liquid in the separatory funnel, stopper the funnel and shake to allow thorough mixing of the liquids. (This exercise is termed "washing the cyclohexene with water".

Return the funnel to the clamp, loosen the stopper and allow the layers to separate. Cyclohexene (density 0.81 g cm^-3) will separate out from the water. Run off the aqueous layer. (Make sure that it is aqueous by adding a drop of water to the flask in which you have collected it!!)

No wash the cyclohexene (where is it?) with 10% sodium carbonate solution (10 mL). Allow the layers to separat and run off the aqueous layer.

Repeat the washing with water (10 mL).

Transfer the cyclohexene into a clean dry conical flask and add a spatula full of calcium chloride. (Please remember to quickly cover the bottle of calcium carbonate after use since it is hygroscopic and will soon pick up moisture rendering it useless for the rest of the class).

Stopper you conical flask, swirl the mixture and allow to stand. If the mixture is cloudy you will need to add another spatula full of calcium carbonate. Allow the solution to stand over the drying agent for about fifteen minutes then, by gravity filtration using cotton wool, filter the cyclohexene into a dried pre-weighed test-tube.
Immediately stopper the tube with a cork and discard the drying agent and cotton wool into the waste containers which are in the fume hood.

If the liquid is cloudy indicating the presence of moisture, add a few lumps of calcium chloride to the test tube and allow the mixture to stand until clear. Decant the liquid to a clean, dry pre-weighed vial that has been labelled in the usual way.

If the liquid is clear, place the sample in a suitably labelled vial. Calculate the determine the yield and percentage yield of cyclohexene. Carry out the bromine/dichloromethane and permanganate tests (see Appendix I on Tests for functional groups) on your product. Have the remainder of your sample available for grading.

Infrared Spectroscopy
In this experiment an alkene has been prepared from an alcohol. Functional group absorptions can be used to assist in the identification of reactants and products. (See Appendix 3). The region shown below is where the O-H and C-H stretches are found and if you ran a sample of your own product it should be possible to determine whether the conversion was successful.

Note on your worksheet the value of the O-H absorption of cyclohexanol and the value of the C=C absorption of cyclohexene.

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Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.

Created Oct 2002. Links checked and/or last modified 13th October 2006.
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