Lesson 3: Alkenes, Addition Reactions & Markovnikov's Rule
Demonstrating understanding of organic chemical structure, properties, and reactions for NCEA Level 2 Chemistry.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Addition reactions of alkenes with H2 (catalytic hydrogenation), X2 (halogenation with Br2/Cl2), HX (hydrohalogenation), and H2O/H+ (hydration to form alcohols). Markovnikov's rule ('rich get richer') for asymmetrical addition.
Predict major and minor products in asymmetrical alkene addition reactions, write balanced structural equations, and explain bromine water decolorization lab observations.
🎥 Media Anchor & Pedagogical Scaffold
Alkene Addition Mechanisms & Markovnikov's Rule
Video Clip: GCSE Chemistry — Addition Reactions of Alkenes — hydration, halogenation, and hydrogenation | Cognito (Runtime: 5m 45s).
🧠 1. Before Viewing (Activate & Predict)
Why is the C=C double bond in alkenes significantly more reactive than the single C-C bond in alkanes?
👁️ 2. During Viewing (Watch With a Job)
- List the 3 addition reactions: which reagents react with alkenes in each of these reactions — hydrogenation, hydration, and halogenation?
- Test for alkenes: how does bromine water behave when shaken with (a) an alkane, and (b) an alkene? Why is this a useful test?
- Conditions: for the hydration of ethene to ethanol, what conditions does the video state are required? (catalyst, temperature, pressure?)
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Guide students through the Bromine Water practical in the fume hood, emphasising qualitative test observations for unsaturation.
Immediate Task: Complete Section 3 of your Organic Portfolio: Addition Reaction Pathways & Major/Minor Product Justification.
⚡ Whakaoho | Do Now: Organic Recall Challenge (10 mins)
The orange that disappears. Add bromine water to hexane and it stays orange. Add it to hex-1-ene and the colour vanishes in seconds.
Predict before you are told: the bromine has not evaporated and it has not been diluted. Where has it gone, and what does that tell you is present in one molecule and absent in the other? Write your answer, then keep it — you will test it in Activity 1.
📖 Activity 1: Core Reaction Mechanism & Structure Analysis (25 mins)
Run the test (15 min). Following your kaiako's safety brief, add bromine water to the samples provided. Record what you see for each, not what you expect. Then draw the product of the reaction with hex-1-ene and show where both bromine atoms ended up. Addition, not substitution — no atoms leave.
Predict the major product (10 min). Add HBr to propene. Two products are possible; one dominates. Draw both, then use Markovnikov's rule to say which is major and — this is the Excellence part — why, in terms of which carbon is more stable when it carries the positive charge. If you can only state the rule, you have Merit at best.
📝 Activity 2: Level 2 Chemistry Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Organic Portfolio — Section 3. Submit: (1) your bromine-water observations table with a structural explanation for each result; (2) equations for hydrogenation, halogenation, hydrohalogenation and hydration of one named alkene, with reagents and conditions; (3) a Markovnikov justification that explains the stability reasoning, not just the outcome.
🏫 Kaiako Planning & Pedagogy Notes
NCEA Level 2 Alignment: Direct preparation for NCEA Level 2 Chemistry (External) (Demonstrate understanding of the properties of selected organic compounds). Emphasise linking observations and reagents to molecular structure for Merit/Excellence grades.