Dehydration of Alcohols
Alcohols Worksheet
1. Dehydration of Secondary Alcohols
Pentan-2-ol can be dehydrated to form pent-1-ene.
a) Write an equation for the reaction.
C5H11OH → C5H10 + H2O
b) Sulfuric acid can be used in this reaction. What is its purpose?
It acts as a catalyst.
c) Sulfuric acid not a good choice for this reaction in some respects. What alternative reaction could the sulfuric acid cause to occur? Write the equation for this reaction.
Sulfuric acid is an oxidising agent. It could oxidise the alcohol to a ketone.
C5H11OH + [O] → C5H10O + H2O
d) Outline the mechanism for the dehydration of pentan-2-ol to pent-1-ene. Use H₂SO₄ in the mechanism rather than H⁺ ions.
e) Pentan-2-ol can also be dehydrated to form two other isomers of pent-1-ene. Draw their structures and name them.
E-pent-2-ene (Trans) and Z-pent-2-ene (Cis)
2. Dehydration of Tertiary Alcohols
Methylpropan-2-ol can also be dehydrated.
a) State the name of the product that will be formed in this reaction.
Methylpropene
b) Outline the mechanism for this reaction.
c) Explain why this reaction is faster than the reaction in question 1.
The tertiary carbocation formed during the dehydration of methylpropan-2-ol is more stable than the secondary carbocation formed during the dehydration of pentan-2-ol, lowering the activation energy.
3. E2 Mechanisms (Extension)
This question is for students that have learnt about E2 mechanisms.
a) Outline the mechanism for the dehydration of ethanol with sulfuric acid.
b) Suggest why the other mechanism (E1) is not feasible here.
Primary carbocations (which would be formed in an E1 mechanism for ethanol) are not stable enough for the reaction to progress via that route.