SolutionsDehydrogenation
1. Complete the reactions
Write the organic product of the dehydrogenation of:
- Ethane (loses one molecule of H2)
- Cyclohexane (loses three molecules of H2)
- Cyclooctane (loses one molecule of H2)
- Ethene (ethylene), CH2=CH2.
- Benzene, C6H6.
- Cyclooctene, C8H14.
2. Oxidation or reduction?
Classify each transformation as an oxidation or a reduction:
- Propane → propene
- Ethene → ethane
- Cyclohexane → benzene
- Oxidation (loses hydrogen).
- Reduction (gains hydrogen).
- Oxidation.
3. The hydrogen acceptor
In the iridium catalytic cycle, tert-butylethene is 'sacrificed'. What does it become, and what is it for?
It accepts the hydrogen lost by cyclooctane and becomes 2,2-dimethylbutane. The catalyst is thus freed for a new cycle and the equilibrium is shifted towards cyclooctene.
4. Energy
Is the dehydrogenation of an alkane exothermic or endothermic? Why does it need high temperatures and catalysts?
Endothermic: two strong C–H bonds are broken and only a π bond and an H–H bond are formed. That is why it needs an energy input (high temperature) and catalysts that lower the activation energy.