Alkenes from Alcohol
Aim
The aim of this experiment is to prepare cyclohexene and compare that starting weight of
cyclohexanol to the yield weight percentage of cyclohexene.
Introduction
One way to synthesize alkenes is by dehydration of alcohols, a process in which alcohols
undergo E2 or E2 mechanisms to lose water and form a double bond.
The dehydration reaction of alcohols to generate alkene proceeds by heating the alcohols in the
presence of a strong acid such as Sulphuric or Phosphoric acid, at high temperatures.
The required range of reaction temperature decreases with increasing substitution of
hydroxy-containing carbon;
● 1O alcohols: 170OC -180OC
● 2O alcohols: 100OC -140OC
● 3O alcohols: 25OC - 80OC
If the reaction is not sufficiently heated, the alcohols do not dehydrate to form alkenes, but react
with one another to form esters;
Apparatus & Reagents
Apparatus
● 5 ml round bottomed long necked flask
● Cupper sponge
● Fractionating column
● Sand bath
● 1 ml flasks
● Pasteur pipette
● Thermometer
Reagents
● Filtering cyclohexanol
● 5ml of 85% phosphoric acid
Procedure
2.0g of cyclohexanol and 0.5ml of 85% phosphoric acid and a boiling chip were introduced into
a 5ml round bottomed long necked flask. Copper sponge was added to the neck of the flask and
the mixture shaken. The mixture was transferred into a fractional distillation apparatus. This was
followed by heating the mixture gently on a sand bath, then distilled until the residue in the flask
was a volume of about 0.5ml to 1 ml. The assembly is allowed to cool slightly by removing it
from the sand, the thermometer is removed briefly and 2ml of toluene is added into the top of the
column using a Pasteur pipette. The distillation was continued until the added layer distilled by
half. The contents of the vial were transferred into a reaction tube and vial rinsed with toluene
subsequently. The mixture was washed with an equal volume of saturated sodium chloride to
remove the aqueous layer. Sufficient anhydrous calcium chloride was added to the reaction tube
so that clumping together is prevented. Dry cyclohexene solution was transferred to the distilling
flask and using a boiling chip the product was distilled.