IB Chemistry SLTopic 6 — Electron TransferPaper 1 & 2Organic~10 min read
Reducing Unsaturated Compounds
This is the reduction that actually uses hydrogen gas. It turns a double bond into a single one, and it is the reason a bottle of sunflower oil and a tub of margarine can start out as the same substance.
📚 What you need to know
Adding hydrogen across a C=C bond is called hydrogenation, and it is a reduction.
Conditions: a nickel catalyst, about 200 °C and a pressure of roughly 1000 kPa.
alkene + H2 → alkane, and alkyne + 2H2 → alkane.
Each multiple bond needs one molecule of H2; a triple bond counts as two.
Carbon’s oxidation state falls, which is what makes this a reduction rather than just an addition.
Hydrogenating vegetable oils raises their melting point, turning liquid oils into solid fats.
The reason is packing: straight saturated chains sit closer together, so dispersion forces are stronger.
Opening the double bond
A C=C bond is two bonds between the same pair of carbons, and one of them is much easier to break than the other. Supply hydrogen and a catalyst, and that weaker bond gives way: one hydrogen atom attaches to each carbon and the molecule becomes saturated.
Nothing leaves the molecule. Every atom that was in the alkene is still there, with two more attached — which is what makes this an addition reaction as well as a reduction.
Hydrogenation of an alkene
C2H4(g) + H2(g) → C2H6(g) nickel catalyst, about 200 °C and 1000 kPa
Counting the hydrogen
Alkynes have a triple bond, which is one sigma bond and two weaker ones. Both of the weak ones can be broken, so an alkyne can take up either one molecule of hydrogen or two, depending on how far you let the reaction go.
The same counting works on a large molecule. A fatty acid chain with three C=C bonds needs three molecules of hydrogen to be fully saturated.
Why this counts as reduction
It is easy to file hydrogenation under “addition” and forget it belongs in a redox topic. Assign oxidation states and the reason becomes clear.
Carbon’s oxidation state
in ethene, each carbon is –2 → in ethane, each carbon is –3
Each carbon has gained a bond to hydrogen, and hydrogen is the less electronegative partner, so the carbon takes the shared pair in our bookkeeping. Its oxidation number falls, which is the definition of reduction. The old shorthand agrees: gain of hydrogen is reduction.
From oil to margarine
Vegetable oils are esters of long-chain fatty acids, and those chains contain C=C bonds. Each double bond puts a permanent kink in the chain, and kinked chains cannot lie neatly against one another.
Remove the kinks by hydrogenation and the chains lie flat against each other. More contact means more London dispersion forces, and more energy needed to melt the solid.
Margarine is made by partial hydrogenation. Convert every double bond and you get something hard and waxy; stop part way and you can tune the melting point until the product is solid in the fridge but spreads straight from it.
WORKED EXAMPLE
Write an equation for the reduction of propene, state the conditions, and name the product.
The equationC₃H₆ + H₂ → C₃H₈The conditionsnickel catalyst, about 200 °C, about 1000 kPaThe productpropaneOne C=C bond, so one molecule of hydrogen. Check the formula: an alkane fits CₙH₂ₙ₂, and 3 × 2 + 2 = 8.
WORKED EXAMPLE
A fatty acid chain contains two C=C bonds and one C≡C bond. Deduce how many molecules of hydrogen are needed to saturate it completely, and explain your reasoning.
Count the double bonds2 × C=C = 2 moleculesCount the triple bondA triple bond has two bonds that can be broken by addition, so it takes up two molecules on its own.1 × C≡C = 2 molecules4 molecules of H₂ in totalAdd up the bonds that can be opened, not the number of unsaturated sites.
WORKED EXAMPLE
Explain why hydrogenating a vegetable oil converts it from a liquid into a solid at room temperature.
BeforeThe chains contain C=C bonds, and each one puts a kink in the chain so the molecules cannot pack closely.The changeHydrogenation removes the double bonds, leaving straight saturated chains.AfterThe chains lie closer together, so there is more surface contact and the London dispersion forces between them are stronger.more energy is needed to separate them, so the melting point risesMention packing, surface contact and London dispersion forces by name. “The molecules are closer” on its own is not an explanation.
💡 Exam tip
Give all three conditions: nickel catalyst, raised temperature, raised pressure.
Count one H2 per multiple bond, remembering a triple bond is worth two.
If asked why it is a reduction, quote the fall in the oxidation state of carbon or the gain of hydrogen.
Check your product formula against CnH2n+2 for an alkane.
For margarine, build the answer from kinks → packing → dispersion forces → melting point.
Say partial hydrogenation when explaining how a spreadable texture is achieved.
⚠️ Common mix-up
Calling hydrogenation an oxidation because something is being added.
Using one H2 for a triple bond when full saturation is asked for.
Saying the double bond “breaks” completely. Only one of the two bonds opens; the carbons stay joined.
Explaining the melting point with hydrogen bonding. Hydrocarbon chains have only dispersion forces.
Forgetting the catalyst in the conditions, or naming an acid catalyst instead of nickel.
Confusing this with reducing a carbonyl: LiAlH4 and NaBH4 do not reduce isolated C=C bonds.
That completes electron transfer. Up next: Electron-Pair Sharing Reactions — the third great family of mechanisms, where nothing is fully given or taken and a lone pair simply attacks.
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