Cyclohexene from Cylohexanol

 

Prior reading: Bruice, Chapter 12.5, p.445

 

 

Theory

One of the most useful and general methods of preparing alkenes or olefins is based on the dehydration of alcohols with acids.  Strong acids such as sulfuric and phosphoric acids are required to form the oxonium ion from which the hydronium ion is eliminated.

 

In this lab experiment, you will carry out the acid catalyzed dehydration of cyclohexanol.  No regio- or stereoisomers are possible; cyclohexene is the only alkene that can be formed under these conditions.  Water will be removed via azeotropic distillation to drive the equilibrium to product.  The purity of the product will be determined by IR analysis.  Both yield and product purity are important in this experiment.

 

Another broad class of organic transformations are elimination reactions, and within that class, the E1 elimination reaction constitutes another of the cornerstone reactions of organic chemistry.  Mechanistically related to the SN1 reaction, the E1 often competes with attempted substitution.  Through proper choice of reaction conditions, it can usually be made the predominant or exclusive pathway.  For example, one of the principal ways to prepare alkenes is via the acid catalyzed dehydration of an alcohol.

 

In this experiment you will learn about the acid catalysed dehydration of cyclohexanol to form an cyclohexene according to the equation:

 

 

You will use the practical techniques of refluxing and separating two immiscible liquids using a separating funnel and use your results to calculate the percentage yield of the cyclohexene product you obtain.  You will need to know the boiling points of cyclohexanol and cyclohexene to properly carry out the distillations described below. 

 

For those of you with strong interests in biochemistry or biology, equilibrium reactions are of particular significance since they are the rule for the organic chemistry carried out by a cell.  While glucose metabolism provides the cell with a quick energy source, long-term energy reserves are stored as water insoluble glycogen and fat (lipids).  You might, for example, think about the advantage for living systems to have water insoluble energy reserves. 

 

Cyclohexene can be prepared from cyclohexanol by dehydration with potassium bisulfate, oxalic acid, sulfuric acid, p-toluenesulfonic acid, phosphoric acid, iodine, and at elevated temperatures over various catalysts..  Cyclohexene has also been prepared from cyclohexyl chloride by passage with steam over activated carbon at 260300, and from benzene by treatment with the substance Ca(NH3)6.  The procedure described is a slight modification of that described by Senderens and later modified by Osterberg and Kendall.

This preparation is referenced from:

Org. Syn. Coll. Vol. 1, 158

Org. Syn. Coll. Vol. 2, 102

Org. Syn. Coll. Vol. 2, 171

Org. Syn. Coll. Vol. 3, 217


 

Procedure.

Add 0.3 mL water into a 5 mL long neck round bottom flask.  To this, add 0.4 mL of 85% phosphoric acid (Use Caution!  Can cause severe burns!), followed by 2.0 g (weigh accurately to 0.001 g) of cyclohexanol and a boiling chip.  Set up a distillation apparatus as you did for the fractional distillation experiment (see demonstration set-up in the lab).  Add copper wire to the neck of the flask.  Note that the bulb of the thermometer must be completely below the side arm of the distillation head.  If necessary, wrap the distillation head and fractionating column with cotton or glass wool.

 

Heat the mixture gently (refluxing) on the sand bath.  After a few minutes increase the temperature by pushing hot sand against the flask to begin the distillation.  Distill until the mixture remaining in the flask has a volume of about 0.5 mL and very little material is distilling.  The distillate should be biphasic (i.e., two phases) and is a mixture of product and water.  Collect until the drops are no longer biphasic. 

 

Wash the distillate with saturated aqueous NaCl solution and dry the organic layer with anhydrous calcium chloride.  Remove your product with a pipette and purify it by redistillation in a dry and clean fractional distillation set up, recording the boiling point range of your cyclohexene.  You can dry your original fractional distillation equipment by first washing it with water, then ethanol, and finally a little acetone.  Weigh your product (to 0.001 g) and analyze it by FTIR. 

 

Safety 

Phosphoric acid is a strong acid capable of producing severe burns to skin or eyes. If some is spilled on you, remove contaminated clothes and rinse skin thoroughly.  If splashed in eyes, rinse for five to ten minutes with eyewash or sink nozzle; remember to rinse under your eyelids.  Cyclohexene and toluene are not particularly dangerous but are highly flammable.  Both are quite painful if splashed in the eyes and must be removed by extensive eye washing.

 

 

Disposal

The aqueous solutions (pot residues and washes) should be diluted with water and neutralized before flushing down the drain.  The ethanol wash and sodium chloride solution also can be flushed down the drain, while the acetone wash should be placed in the acetone waste container.  Once free of solvent, the anhydrous calcium chloride can be placed in the nonhazardous solid waste container.

 

 

Analysis

Comment on your percent yield and purity of your product. Compare your spectrum with a reference spectrum from the web or from the library.  Look for peaks due to starting material in your spectrum.

 

 

Questions

1. Provide a detailed mechanism for the dehydration of 3-methyl-2-butanol. 

2. What is the function of the anhydrous calcium chloride?  Of the saturated sodium chloride solution?

 

 

 

References

Check out the set-up for the distillation apparatus on this website

http://courses.chem.psu.edu/chem35/HTML/Experiments/Exp72.pdf

http://www.bradley.edu/las/chm/Course/250/Cyclohexene.pdf