A student proposes the following first step in a synthesis: HO \/\ H2SO4 Only a small
The Correct Answer and Explanation is:
Correct Answer
The reaction of cyclohexanol with a catalytic amount of sulfuric acid (
H2SO4H2SO4) results in an elimination reaction, specifically a dehydration, to produce cyclohexene and water.
Reaction:
Explanation
This transformation is a classic example of an acid-catalyzed dehydration of a secondary alcohol. The role of sulfuric acid is not to act as a reagent but as a catalyst, which is why only a small amount is needed. The mechanism proceeds via an E1 (Elimination, Unimolecular) pathway.
Step 1: Protonation of the Hydroxyl Group
The reaction begins with the protonation of the alcohol’s hydroxyl (-OH) group by the strong acid. The oxygen atom of the hydroxyl group acts as a Lewis base, donating a pair of electrons to a proton from
H2SO4H2SO4. This step is a fast and reversible equilibrium that converts the poor leaving group, hydroxide (-OH), into an excellent leaving group, water (
H2OH2O).
Step 2: Formation of a Carbocation
The protonated alcohol loses a molecule of water to form a secondary carbocation intermediate. This step is the slow, rate-determining step of the E1 mechanism. The departure of the neutral water molecule is energetically favorable.
Step 3: Deprotonation to Form the Alkene
In the final step, a weak base, such as a water molecule or the bisulfate ion (
HSO4−HSO4−), removes a proton from a carbon atom adjacent to the positively charged carbon. The electrons from this carbon-hydrogen bond then move to form a new pi bond between the two carbon atoms, resulting in the formation of the alkene, cyclohexene. This step also regenerates the acid catalyst (H+), allowing it to participate in the reaction again.
Because the starting material, cyclohexanol, is symmetrical, the removal of a proton from either adjacent carbon leads to the same product. This reaction is typically performed with heat, which favors elimination over the competing substitution reaction (ether formation) and helps to distill the lower-boiling cyclohexene product from the reaction mixture, driving the equilibrium forward.
