Lesson 1: Alkanes, Alkenes, Alkynes & IUPAC Nomenclature
Use formulae, models and IUPAC rules to distinguish saturated and unsaturated hydrocarbons and name straight-chain, branched and halo-substituted structures unambiguously.
🎯 Ngā Whāinga Akoranga | Learning Intentions
IUPAC naming conventions for alkanes, alkenes, alkynes, haloalkanes and branched hydrocarbons, up to eight carbons in the longest chain. The three hydrocarbon general formulae: alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n−2.
Construct 3D molecular models, draw structural and condensed formulae, name branched hydrocarbons systematically, and decide from a molecular formula alone whether a compound can be saturated.
🎥 Media Anchor & Pedagogical Scaffold
IUPAC Nomenclature & Hydrocarbon Architecture
Video Clip: GCSE Chemistry — Hydrocarbons, alkanes and the homologous series (2026/27 exams) | Cognito (Runtime: 5m 05s).
🧠 1. Before Viewing (Activate & Predict)
Why is a standardised global naming system (IUPAC) essential for chemists, medical researchers, and chemical engineers worldwide?
👁️ 2. During Viewing (Watch With a Job)
- Define: what is the general formula for alkanes? What does the suffix '-ane' tell you about a molecule's structure?
- Homologous series: the video explains what makes a series 'homologous'. List THREE properties that members of the alkane homologous series share.
- Name: using IUPAC rules from the video, name a straight-chain alkane with 4 carbons and draw its structural formula.
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Use physical Molymod kit sets to demonstrate structural flexibility of single C-C bonds vs rigid C=C double bonds.
Immediate Task: Complete Section 1 of your Level 2 Chemistry Portfolio: IUPAC Naming & Structural Drawing Mastery Sheet.
⚡ Whakaoho | Do Now: Organic Recall Challenge (10 mins)
Why bother with a long name? A chemist in Hamilton emails a chemist in Berlin the words amyl alcohol. Berlin draws a molecule. Hamilton meant a different one. Both were reasonable readings — the old name covers eight different C5H11OH molecules and does not say which.
Two minutes, on your own: write down what a chemical name has to do that a common name does not. Then look at 3-methylbutan-1-ol — longer, uglier, and it can only be drawn one way. Which parts of that name are carrying the information?
📖 Activity 1: Core Reaction Mechanism & Structure Analysis (25 mins)
Name it, then break it (15 min). Working in pairs, name the six structures below. For each, write the three decisions in order: longest chain, position numbers, substituent names. Then swap with another pair and draw from their names only — no peeking at the original. Any molecule you cannot draw unambiguously means the name failed, not you. Find the naming rule that would have saved it.
The six structures.
- CH3CH2CH2CH2CH3
- CH3CH(CH3)CH2CH3
- CH3CH2CH=CHCH3
- CH3CH2CHBrCH3
- CH3C≡CCH2CH3
- CH2ClCH2CH(CH3)CH3
Alkane, alkene or alkyne? (10 min). Sort these eight formulae into saturated and unsaturated without drawing them: C3H8, C4H8, C5H8, C6H14, C2H2, C7H16, C5H10, C6H12. Work out the rule that lets you do it from the formula alone, and write it down. Then push it: two of these are traps, because a formula tells you how much unsaturation there is, not what shape it takes. Find both and say what else each formula could be.
Kaiako answer key — Name it, then break it. (1) pentane. (2) 2-methylbutane. (3) pent-2-ene. (4) 2-bromobutane. (5) pent-2-yne. (6) 1-chloro-3-methylbutane — number from the chlorine end, because the halogen and the methyl are both substituents and the first point of difference rule gives 1,3 rather than 2,4.
Kaiako answer key — Alkane, alkene or alkyne? Compare the hydrogen count with 2n+2 for that many carbons. Every two hydrogens short of CnH2n+2 is one degree of unsaturation, and each degree is one double bond or one ring. Saturated (CnH2n+2): C3H8, C6H14, C7H16. One degree (CnH2n): C4H8, C5H10, C6H12. Two degrees (CnH2n−2): C5H8, C2H2. The two traps: C6H12 is one degree short, so it is an alkene or a cycloalkane — cyclohexane fits the formula perfectly. C5H8 is two degrees short, so it is an alkyne or a diene or a cyclopentene. C2H2 is the one unambiguous case in the list: with only two carbons there is no room for a ring, so it can only be ethyne. The formula counts the unsaturation; only a structure tells you where it is.
📝 Activity 2: Level 2 Chemistry Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Organic Portfolio — Section 1. Submit: (1) the six structures named, with the three naming decisions shown for each; (2) three names you were given, drawn correctly; (3) one short paragraph on why C6H12 could be either a cycloalkane or an alkene, and what extra information settles it; (4) one sentence saying what C5H8 could be, and why C2H2 cannot be anything but ethyne.
🏫 Kaiako Planning & Pedagogy Notes
Where this sits in the standard. The standard limits the compounds to alkanes, alkenes, alkynes, haloalkanes, primary amines, alcohols and carboxylic acids, and limits naming and drawing to eight carbons in the longest chain. This lesson establishes the naming for the three hydrocarbon families and the haloalkanes; the functional groups arrive from Lesson 4 onwards.
Exit ticket (3 min, on a slip, collected). Name CH3CH2CHBrCH3, and say what C4H6 tells you about the molecule and what it does not.
What to watch for while they work. In the swap step, the pair that cannot draw a partner's molecule has found a naming failure — go to that pair first, because the diagnosis is on their desk already. Listen for students numbering the chain from the wrong end; that is the single most common lost mark in this whole standard.
Differentiation. Support: Give the naming rules as a three-step checklist on a card (longest chain, lowest locants, alphabetical substituents) and let students keep it for the whole unit. Model the first structure aloud before they start. Extension: Ask for a molecule whose name changes depending on which end you number from, and have them state the rule that settles it. Or give C5H8 and ask for three structurally different compounds that fit.
The error to head off. Students read a molecular formula as if it named a structure. C6H12 is an alkene or a cycloalkane, C5H8 is an alkyne or a diene or a cycloalkene. The formula counts unsaturation; only a structure locates it.