Experiment 11: Dehydration of Cyclohexanol
INTRODUCTION
In this experiment, cyclohexanol is dehydrated by aqueous sulfuric acid to produce
cyclohexene as the sole product [equation (1)], and no rearrangement is possible in this
reaction.
OH H2SO4
heat + H2O(1)
cyclohexanol cyclohexene
The mechanism of this reaction is shown in equation 2. The hydroxyl group of an alcohol
is a poor leaving group but is converted to a much better one by protonation of oxygen
to give an oxonium ion. These types of proton transfers are typically very fast reactions
and can be reversible. Next, in the rate-determining step, loss of a molecule of water
gives the carbocation intermediate. Finally, deprotonation gives the desired alkene
product and regenerates the acid catalyst. Note that even weakly basic compounds, such
as water, can deprotonate a carbocation to form an alkene. Overall, this mechanism is
referred to as an E1 elimination reaction
C C
+ H+
C C
OH
H
fast C C
O
H
H
H
slow C C
H
carbocation
intermediate
+ H2O
C C
H
fast
+ H+
(2)
The mechanism for this reaction depends on the class of alcohol being dehydrated, and
the one shown in equation (2) is for 2º and 3º alcohols. The –OH group in an alcohol is a
poor leaving group but is converted to a better one by protonation of oxygen to given an
oxonium ion. The oxonium ion undergoes decomposition by loss of a molecule of water to
give a carbocation intermediate whose formation is the rate-determining step. This
mechanism is referred to as an E1 mechanism. Careful examination of the mechanism
reveals that hydrogen ion (the proton) is a catalyst in the reaction; it is consumed at the
beginning and regenerated at the end.
For this alcohol, cyclohexanol, carbocation rearrangements to form a more stable
carbocation are not possible and so cyclohexene is observed as the only product.
