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Dehydration of an Alcohol: Cyclohexene from Cyclohexanol

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A secondary alcohol (cyclohexanol) is used to obtain an olefin (cyclohexene) by using the dehydration of alcohol method. The dehydration reaction can be carried out in either of these two method: (a) -heating a mixture of alcohol and dehydrating agent usually H2SO4 in a distilling flask and collect the olefin as it is distills from the mixture.

- preferred in the laboratory as elaborate equipment is not required.

(b) - passing alcohol vapor through heated tube (350o)packed with dehydrating agent which is usually activated by Al 2O3 and collecting the olefin as it emerges from the tube.

- broadly used in industry since it lends itself continuous operation.

In the preparation of olefins, the dehydration reaction is very important. Besides that, olefins are the raw materials that are widely used in plastic industry and it is used in laboratory to prepare important compounds as it is use as intermediates. The dehydration of cyclohexanol is carried out in such a way that the product, cyclohexene, distills from the reaction mixture as it is formed. To eliminate the olefin from contact, this distillation method is used with sulfuric acid before polymerization can set in. The distillation technique is also use to serve as a first stage in the eventual purification of the olefin. The products and side products falls under three categories: (a) gases (SO2 and CO2)

(b) distillate (cyclohexene, unreacted cyclohexonal, H2O, traces of H2SO4 )

(c) residue, composed of high-boiling or non-volatile substances (dicyclohexyl ether, mono-and dicyclohexy sulfate, polymer and carbon)

Pure cyclohexene is obtained from the crude distillate by the following procedure:

1. Treatment with aqueous sodium carbonate solution, o remove sulfurous acid

2. Addition of calcium chloride, to remove all of the water and part of the cyclohexanol

3. Distillation to separate the remainder of the cyclohexanol.

The dehydration of an alcohol with phosphoric acid instead of sulfuric acid has two distinct advantages:

1. Very little organic material is lost through oxidation by the acid and

2. The product is not contaminated with volatile decomposition products

Phosphoric acid is not an oxidizing agent and therefore provides a higher yield of olefin.



Theoretical yield

Weight of cyclohexanol used: 1 ml = 0.95 g

10 ml = 9.5 g

Molecular weight of cyclohexanol used: 100.16 g mol-1

Number of mole of cyclohexanol used: 9.5 g / 100.16 g mol-1 = 0.0948 mol

1 mol Cyclohexanol : 1 mol Cyclohexene

Thus, number



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