6.2.10Additional lessons
Reduction of Unsaturated Compounds — IB Chemistry SL Revision Notes & Topical Questions
5 min read 7 parts Lesson 93 of 98
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.
Part 1 of 7Hydrogenation
Hydrogen gas is passed over a heated metal catalyst along with the unsaturated compound.
Definition
- Definition Hydrogenation is the addition of hydrogen across a C=C or C≡C bond, which is a reduction.
Alkenes
- Alkenes C₂H₄ + H₂ → C₂H₆; the alkene becomes the corresponding alkane.
Alkynes
- Alkynes Alkynes add hydrogen twice: HC≡CH + H₂ → H₂C=CH₂ and then H₂C=CH₂ + H₂ → C₂H₆.
Catalyst
- Catalyst Finely divided nickel at about 150 °C is typical, or platinum or palladium at lower temperatures.
Part 2 of 7Applications
Applications
Margarine
- Margarine Vegetable oils contain C=C bonds;
- hydrogenation raises the melting point so they become semi-solid fats.
Partial hydrogenation
- Partial hydrogenation Only some C=C bonds are saturated, but it may produce trans isomers that raise health concerns.
Test for unsaturation
- Test for unsaturation Bromine water is decolourised by alkenes before hydrogenation but not by the alkane product.
Atom economy
- Atom economy Hydrogenation is an addition reaction with a single product, so its atom economy is 100%.
Part 3 of 7Predicting Products
1
Count the double and triple bonds in the starting compound.
2
Add one H₂ for each C=C and two H₂ for each C≡C.
3
Keep the carbon skeleton the same, and write the saturated product.
4
Check the product no longer decolourises bromine water.
Part 4 of 7Evidence and Extra Detail
Evidence and Extra Detail
Bromine water
- Bromine water An alkene decolourises bromine water by addition, while the hydrogenated alkane does not.
Degree of unsaturation
- Degree of unsaturation The more C=C bonds a fat contains, the more hydrogen it can take up per mole.
Melting point
- Melting point Saturated chains pack together more closely, so hydrogenated fats have higher melting points.
Choice of catalyst
- Choice of catalyst The metal surface adsorbs hydrogen and the alkene, weakening bonds and allowing hydrogen to add.
Part 5 of 7Industrial Hydrogenation of Oils
1
Hot liquid vegetable oil is mixed with hydrogen under pressure with a nickel catalyst.
2
Hydrogen adds across some of the C=C bonds, converting them to single bonds.
3
The product has a higher melting point, so it becomes a spreadable solid at room temperature.
4
The catalyst is filtered off and the product is purified for food use.
Part 6 of 7Worked Example
Question
- Calculate the volume of hydrogen at STP needed to hydrogenate 2.10 g of propene, C₃H₆ (M = 42.09 g mol⁻¹).
- Molar volume at STP is 22.7 dm³ mol⁻¹.
- C₃H₆ + H₂ → C₃H₈, a 1:1 mole ratio.
- n(C₃H₆) = 2.10 / 42.09 = 0.0499 mol, so n(H₂) = 0.0499 mol.
- V = 0.0499 × 22.7 = 1.13 dm³.
Answer 1.13 dm³ of hydrogen.
Part 7 of 7Exam Checklist & Tips
6.2.10 – Key Points for the Exam
- Hydrogenation Addition of hydrogen to an unsaturated compound.
- Unsaturated Containing at least one C=C or C≡C bond.
- Catalyst Nickel, platinum or palladium that provides a surface for the reaction.
- Addition reaction A reaction in which two reactants combine into one product.
Common Mistakes
- Thinking hydrogenation breaks the carbon chain.
- Forgetting that alkynes need two moles of hydrogen to become alkanes.
- Giving the catalyst as an acid.
- Describing hydrogenation as oxidation; adding hydrogen to a compound is reduction.
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