Lesson 2: Structural & Geometric (Cis-Trans) Isomerism
Compare structural and cis-trans isomerism by building alkene models, testing the conditions for restricted rotation, and relating molecular arrangement to physical properties.
🎯 Ngā Whāinga Akoranga | Learning Intentions
The difference between structural isomers (same molecular formula, different bonding connectivity) and geometric cis-trans isomers (restricted rotation around C=C double bond with two different groups on each double-bonded carbon).
Build and compare physical cis-trans alkene models, explain why alkanes cannot form cis-trans isomers, and link isomerism to physical properties (melting/boiling points).
🎥 Media Anchor & Pedagogical Scaffold
Understanding Structural vs Geometric Isomers
Video Clip: Organic Chemistry: Isomerism — structural, stereoisomers, E/Z, and cis-trans | iGCSE/GCSE (Glecta) (Runtime: 13m 27s).
🧠 1. Before Viewing (Activate & Predict)
How can two chemical compounds with the exact same molecular formula have completely different melting points, boiling points, and biological activities?
👁️ 2. During Viewing (Watch With a Job)
- Define structural isomers: two molecules are structural isomers if they have the same _______ formula but different _______ formulas. Fill the blanks from the video.
- Cis/trans: explain in your own words what makes a molecule capable of cis-trans (geometric) isomerism. What structural feature is required?
- Draw: sketch cis-but-2-ene and trans-but-2-ene side-by-side. Label the groups on each carbon of the double bond.
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Emphasise the 2-part NCEA Excellence rule for geometric isomerism: restricted C=C rotation AND non-identical groups on each double-bonded carbon.
Immediate Task: Complete Section 2 of your Organic Portfolio: Isomer Classification & Cis-Trans Proof Justifications.
⚡ Whakaoho | Do Now: Organic Recall Challenge (10 mins)
Two fats, one formula. Oleic acid and elaidic acid are both C18H34O2. Both are octadec-9-enoic acid — same chain length, same acid group, double bond in the same place. Oleic acid melts at about 16 °C, so it is liquid in the olive oil in your kitchen. Elaidic acid melts at 42–44 °C, so it is solid at room temperature.
On your own, in two minutes: what is left that could possibly be different between them? Write your guess before anyone says the word isomer. Then predict — which one do you think packs together more tightly, and how did the melting points tell you? (Melting points: PubChem, CID 445639 and 637517.)
📖 Activity 1: Core Reaction Mechanism & Structure Analysis (25 mins)
You will need: one Molymod kit per pair.
Build it (10 min). In pairs, build but-2-ene twice — once with both methyl groups on the same side of the double bond, once on opposite sides. Now try to turn one into the other without breaking a bond. You cannot. That is the whole idea: the C=C double bond will not rotate. Then build butane and try the same thing with the single bond. It spins freely. Write one sentence explaining, in your own words, why the double bond is the thing that makes cis-trans isomers possible.
Sort it (10 min). Sort these six: but-2-ene, but-1-ene, pent-2-ene, 2-methylbut-2-ene, 1,2-dichloroethene, 1,1-dichloroethene. For each one, decide: can this show geometric isomerism — yes or no? You must give a reason, and the reason has to name BOTH conditions: (1) restricted rotation, and (2) two different groups on each carbon of the double bond. Watch for the trap — but-1-ene has a double bond and still cannot do it. Work out why before you are told.
Kaiako answer key — Sort it. Yes: but-2-ene, pent-2-ene, 1,2-dichloroethene — each alkene carbon carries two different groups. No: but-1-ene (the terminal carbon carries two hydrogens), 2-methylbut-2-ene (one alkene carbon carries two identical methyls), 1,1-dichloroethene (one carbon carries two identical chlorines). Every “no” here has restricted rotation and still fails — which is the point: restricted rotation alone is not enough.
Explain it back (5 min). Return to the Do Now. You now know what differs between oleic and elaidic acid. Using your models, explain to your partner why the trans isomer melts 26 °C higher. Point at the shape of the chain when you say it — the cis kink is the answer, not a fact to memorise.
📝 Activity 2: Level 2 Chemistry Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Organic Portfolio — Section 2: Isomer Classification. Three things to submit:
- Classification table. Six structures from Activity 1. For each: molecular formula, whether it shows structural isomerism, whether it shows geometric isomerism, and the reason. A reason that only says "it has a double bond" is an Achieved answer at best.
- Annotated pair. Draw cis-but-2-ene and trans-but-2-ene side by side and label, on the drawing: the restricted bond, the two different groups on each double-bonded carbon, and the kink.
- Excellence paragraph. Explain the oleic/elaidic melting point difference by linking geometry → how closely the molecules pack → strength of the intermolecular forces → energy needed to melt. Merit stops at "the trans one is straighter". Excellence carries the chain all the way to the melting point and says why each step follows.
🏫 Kaiako Planning & Pedagogy Notes
Where this sits in the standard. Constitutional and geometric (cis and trans) isomers are named explicitly in the standard's property list, alongside solubility, melting and boiling points. This lesson is where those two lists meet: geometry is used to explain a physical property.
Exit ticket (3 min, on a slip, collected). But-1-ene has a C=C and cannot show cis-trans isomerism. In one sentence, say why.
What to watch for while they work. In the sort task, a reason that stops at "it has a double bond" is an Achieved answer. Push every pair to name both conditions every time — restricted rotation and two different groups on each doubly bonded carbon — until it is automatic.
Differentiation. Support: Keep the physical models on the desk for the whole lesson. Students who cannot see the kink in a drawing can feel it in a model, and the melting-point explanation follows from the shape, not from the words. Extension: Ask for a compound that has structural isomers but no geometric ones, and one that has both. Or ask why cis-trans isomerism does not need a chiral centre.
The error to head off. Students say the trans isomer is "stronger". It is not — the molecules are identical in bonding. It packs more closely, so more energy is needed to separate the molecules, so the melting point is higher. Insist on that chain, not the shortcut.
Other teaching approach — Portfolio Mastery Course: Geometric (cis-trans) Isomerism in Alkenes →