Lesson at a Glance | He Tirohanga Whakamua
Ngā Whāinga Ako | Learning Intentions
Students will know
- The 2 mandatory conditions for cis-trans (geometric) isomerism:
1) A carbon-carbon double bond () that prevents rotation.
2) Two different groups attached to each carbon of the double bond. - Cis-isomer: Identical groups on the SAME side of the double bond.
- Trans-isomer: Identical groups on OPPOSITE sides of the double bond.
Students will demonstrate
- By drawing 3D-angled structural diagrams of cis-but-2-ene and trans-but-2-ene.
- By completing Section 3 of their Level 2 Organic Chemistry Mastery Portfolio.
Curriculum alignment
- NZC (2007) · Science · Level 7 · Material World: “Relate properties of matter to structure and bonding.”
Do Now | Tīmatanga Whakaaro (10 min)
Restricted Rotation Prompt:
"If you hold two pencils joined by a single piece of string, you can spin one pencil without moving the other. What happens if you join the two pencils using TWO parallel rigid wooden rods?"
Unpack: The pencils can no longer rotate independently! In an alkene, the -bond overlapping above and below the -bond forms a rigid planar system that prevents rotation around the double bond, locking atoms into fixed 3D geometric positions.
Cis vs Trans Geometry (15 min)
Both methyl () groups are on the same side of the double bond. Polar bond dipoles add together, giving a higher boiling point.
Methyl () groups are on opposite sides of the double bond. Symmetric structure allows tighter crystal lattice packing, giving a higher melting point.
The Two Rules (15 min)
Where this usually goes wrong: students apply cis/trans to anything containing a double bond. Both rules must hold, and the second one is the one that gets skipped.
A double bond cannot rotate, so the groups either side are locked in place. A single bond rotates freely at room temperature, which is why alkanes have no geometric isomers no matter how they are drawn.
If either double-bonded carbon carries two identical groups, flipping the molecule gives you back the same compound. This is the rule that decides borderline cases, and it is worth testing explicitly rather than by eye.
Do this now: test 1,1-dichloroethene and 1,2-dichloroethene against BOTH rules. Only one has geometric isomers — name which rule the other one fails.
📁 Organic Chemistry Portfolio — Section 3: Cis-Trans Geometric Isomerism
Students open their Level 2 Chemistry Portfolio and complete Section 3:
Section 3 Requirements:
1. Cis-Trans Diagram Pair: Draw and label clear 3D-angled structures for cis-but-2-ene and trans-but-2-ene.
2. 2-Rules Evaluation Checklist: Apply the 2 rules to 4 molecules (but-1-ene, but-2-ene, 1,2-dichloroethene, 2-methylbut-2-ene) to justify which exhibit geometric isomerism.
3. Excellence Non-Existence Rationale: 1 paragraph explaining why but-1-ene CANNOT form geometric isomers (C1 is attached to two identical Hydrogen atoms, so swapping them yields the exact same molecule).
Exit Verification | Ka Mutu Hoki (5 min)
Exit Check:
"My Section 3 explicitly states both mandatory rules and explains why C1 in but-1-ene having 2 hydrogens prevents cis-trans isomerism."
Teacher Planning & NCEA Alignment
NCEA Level 2 Chemistry Alignment (4 Credits External):
- Geometric Isomerism: Explain cis-trans isomerism in alkenes using restricted rotation and non-identical groups on double-bonded carbons.
- Property Variations: Compare physical properties (boiling point, melting point) between cis and trans isomers.
Vocabulary: Geometric isomerism, cis-isomer, trans-isomer, stereoisomerism, restricted rotation, double bond (), dipole moment.
Other teaching approach: Guided Viewing & Practical Chemistry →