Intermediate Topic 778 of 1102 Organic Chemistry
Hydrogenation
Addition of H2 across multiple bonds using metal catalysts.
In-depth explanation of Hydrogenation
Hydrogenation is the addition of hydrogen, H2, across a carbon-carbon multiple bond, catalysed by finely divided metals such as platinum, palladium or nickel. The hydrogen atoms add to the same face of the alkene, which is called syn addition, giving an alkane. The reaction is exothermic, releasing about 120 kJ/mol for a simple alkene. Measured heats of hydrogenation show that benzene is considerably more stable than expected, which provides evidence for its aromatic stabilization. Industrially, the process is used to harden vegetable oils into margarine.
Going deeper: Hydrogenation
Key ideas: Alkene plus H2 over Pt, Pd or Ni (Raney nickel at 150 C, Pd-C at room temperature) gives alkane by syn addition on the catalyst surface. Heat of hydrogenation is about 120 kJ/mol for a typical alkene, lower for more stable alkenes; for benzene it is only 208 kJ/mol rather than the 360 expected for three isolated double bonds. Lindlar catalyst stops alkynes at the cis alkene.
Common mistakes and connections: Students forget the catalyst or that syn addition matters for stereochemistry. Another mistake is thinking benzene reduces as easily as alkenes; it needs high pressure. Links to alkene stability measurements, hardening of vegetable oil (margarine) and aromatic stabilisation.
Visualization of Hydrogenation
Adding H2 across C=C releases about 125 kJ/mol; the metal catalyst lowers the barrier but does not change the overall energy. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Partly hydrogenating vegetable oil converts some of its C=C bonds to C-C bonds, producing a solid fat used in margarine.
Frequently asked questions about Hydrogenation
What is Hydrogenation?
Addition of H2 across multiple bonds using metal catalysts.
Is Hydrogenation a beginner, intermediate or advanced topic?
Hydrogenation is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Hydrogenation?
Study these earlier topics in Organic Chemistry first: Electrophilic Addition, Markovnikov's Rule, Anti-Markovnikov Addition.
What comes after Hydrogenation?
The next topics in Organic Chemistry are Hydration of Alkenes and Ozonolysis.
Can I practise Hydrogenation with quizzes?
Yes. The interactive lesson includes a self-check question on Hydrogenation, and the Organic Chemistry quiz draws on every topic in this area.
Where Hydrogenation fits in Organic Chemistry
Hydrogenation is topic 50 of 148 in Organic Chemistry.