IB Chemistry HLTopic 6 — Electron TransferPaper 1 & 2Organic~8 min read
Reducing Unsaturated Compounds
Adding hydrogen across a double or triple bond is called hydrogenation, and it is a reduction. It is also the reaction that turns runny vegetable oil into margarine you can spread — one of the few bits of organic chemistry sitting in most people’s fridges.
📚 What you need to know
Hydrogenation adds H2 across a multiple bond. It is a reduction and an addition reaction.
Conditions: a nickel catalyst, about 200 °C and around 1000 kPa.
One H2 per double bond. An alkyne needs two to reach the alkane.
Saturating a fat raises its melting point, because the straighter chains pack closer and the London forces between them get stronger.
Partial hydrogenation is used to get the right texture for margarine.
One hydrogen molecule per multiple bond
Count the multiple bonds and you have counted the moles of hydrogen. That single idea answers almost every calculation on this topic.
Ethene to ethane
CH2=CH2 + H2 → CH3CH3
Ethyne all the way to ethane
CH≡CH + 2H2 → CH3CH3
Why is this a reduction when nothing obviously loses electrons? Check the oxidation numbers on the carbons. In ethene each carbon is −2; in ethane each is −3. The number falls, so the carbons have been reduced. The old rule “reduction is gain of hydrogen” is really just this in disguise.
From oil to margarine
Vegetable oils are unsaturated: their long fatty acid chains contain C=C double bonds. Those double bonds put permanent kinks in the chains, so the molecules cannot pack together neatly. Weak packing means weak London forces, a low melting point, and an oil that is liquid at room temperature.
Hydrogenate some of those double bonds and the chains straighten out:
Straighter chains lie closer together, so the surface contact between molecules increases.
More contact means stronger London dispersion forces.
Stronger forces need more energy to break, so the melting point rises and the fat becomes solid.
Why only partly hydrogenate? Fully saturating the oil gives something hard and waxy that will not spread. Manufacturers stop part way, leaving some C=C bonds behind, to land on a texture that is solid in the fridge but soft enough to use straight away.
There is a real trade-off here worth knowing for an evaluation question. Partial hydrogenation can convert some natural cis double bonds into trans ones, and trans fats are linked to heart disease. Many manufacturers now use other methods for exactly this reason.
Worked examples
WORKED EXAMPLE
0.200 mol of an oil molecule containing three C=C bonds is fully hydrogenated. Calculate the volume of hydrogen needed at STP. (Molar volume = 22.7 dm3 mol−1)
Step 1: One H2 per double bondThree double bonds per molecule means three H2 per molecule.n(H2) = 0.200 × 3 = 0.600 molStep 2: Convert moles to volumeV = 0.600 × 22.7 = 13.62 dm313.6 dm3 of hydrogen“fully” is the key word — it means every multiple bond reacts
WORKED EXAMPLE
But-2-yne is reacted with one mole equivalent of hydrogen over a nickel catalyst. Give the product and its molecular formula.
Step 1: Start from the alkyneBut-2-yne is C4H6, with a triple bond between carbons 2 and 3.Step 2: One H2 takes the triple bond down to a double bondC4H6 + H2 → C4H8Step 3: Name itThe double bond is still between carbons 2 and 3.But-2-ene, C4H8a second mole of H2 would take it on to butane, C4H10
WORKED EXAMPLE
Explain, in terms of intermolecular forces, why a hydrogenated vegetable oil is solid at room temperature while the original oil is liquid.
Step 1: Describe the unsaturated moleculesC=C bonds put kinks in the chains, so the molecules cannot pack closely.Step 2: Link packing to the forcesLess surface contact means weaker London dispersion forces between molecules.Step 3: Describe the saturated productRemoving the C=C bonds straightens the chains, so they pack closer, contact more and attract more strongly.Stronger London forces need more energy to overcome, so the melting point risesthe chain of reasoning is shape, then packing, then forces, then melting point
💡 Exam tip
Quote the conditions in full: nickel catalyst, about 200 °C, 1000 kPa.
Call it both an addition and a reduction if the question asks for the type of reaction.
For calculations, count the multiple bonds first. That gives you the mole ratio directly.
Melting point explanations must name London dispersion forces and mention surface contact or packing.
Say the catalyst lowers the activation energy and is not used up. It does not shift the position of equilibrium.
Mention trans fats if a question asks you to evaluate the process.
⚠ Common mix-up
Using one H2 for an alkyne. A triple bond needs two to reach the alkane.
Calling it a substitution. Nothing leaves the molecule — it is an addition.
Saying hydrogen bonding holds the fat molecules together. These are hydrocarbon chains, so it is London forces.
Thinking the catalyst changes the amount of product. It changes the rate only.
Confusing hydrogenation with hydration. Hydrogenation adds H2; hydration adds water to give an alcohol.
Forgetting to double the moles of H2 when a molecule has more than one multiple bond.
Up next: The Hydrogen Electrode — back to electrochemistry for the HL material, starting with the reference point that gives every half-cell its number.
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