6.2.10Additional lessons
Reduction of Unsaturated Compounds — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 141 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Catalytic Hydrogenation
Reduction of an unsaturated compound adds hydrogen across multiple bonds.
Alkene to alkane
- Alkene to alkane C₂H₄ + H₂ → C₂H₆ using finely divided nickel at about 150 °C turns a double bond into a single bond.
Alkyne to alkane
- Alkyne to alkane With excess hydrogen, C₂H₂ + 2H₂ → C₂H₆ because the triple bond is reduced in two stages.
Catalyst
- Catalyst Nickel, platinum or palladium provides a surface on which hydrogen and the unsaturated compound react.
Industrial use
- Industrial use Hydrogenation hardens vegetable oils by converting C=C bonds into C–C bonds, raising the melting point.
Section 2 of 4Other Reductions
1
Benzene needs harsher conditions than alkenes: C₆H₆ + 3H₂ → C₆H₁₂ with nickel at high temperature and pressure.
2
The delocalised ring makes benzene more stable, so addition of hydrogen needs a higher energy input.
3
A nitrile is reduced by LiAlH₄ to a primary amine: CH₃CN + 4[H] → CH₃CH₂NH₂.
4
Nitrobenzene is reduced with tin and concentrated hydrochloric acid to phenylamine: C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O.
5
A final alkali treatment frees the amine from its salt, because it forms in acid as a phenylammonium ion.
Section 3 of 4Comparing Reduction Methods
Comparing Reduction Methods
Conditions
- Conditions Alkene hydrogenation is mild, while benzene hydrogenation needs high temperature and pressure.
Reagents
- Reagents Hydrogen gas with a metal catalyst is used for C=C, whereas LiAlH₄ is used for polar groups like nitriles.
Functional group change
- Functional group change Each reduction replaces a multiple bond, such as C≡N or N=O, with more bonds to hydrogen.
Usefulness
- Usefulness These reductions make amines and saturated compounds that are starting materials for synthesis routes.
Section 4 of 4Exam Checklist & Tips
6.2.10 – Key Points for the Exam
- Hydrogenation The addition of hydrogen across a multiple bond, usually with a metal catalyst.
- Unsaturated compound A molecule containing at least one carbon-carbon double or triple bond.
- Nitrile An organic compound containing the –C≡N functional group.
- Phenylamine An aromatic amine, C₆H₅NH₂, made by reducing nitrobenzene.
Common Mistakes
- Writing hydrogenation of an alkene as giving an alcohol; it gives an alkane.
- Assuming benzene hydrogenates as easily as an alkene, which ignores delocalisation.
- Forgetting that reducing nitrobenzene produces water as well as the amine.
- Using LiAlH₄ in aqueous solution, where it would be destroyed.
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