Lesson 3: Alkene Reactions — Addition, Oxidation and the Alkane Contrast
Predict alkene addition and oxidation products, apply Markovnikov’s rule to asymmetric reactants, and contrast these pathways with UV substitution in alkanes.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Addition reactions of alkenes with H2/Pt, Cl2, Br2, HX and H2O/H+ (conc. H2SO4/H2O). Markovnikov's rule for identifying major and minor products from asymmetric alkenes. Oxidation of an alkene with cold dilute MnO4- to a diol. Substitution of an alkane with a halogen in UV light, limited to monosubstitution.
Predict major and minor products in asymmetric alkene addition, write balanced structural equations, distinguish addition from substitution by their products, and explain bromine water decolourisation 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.
⚠️ Hazard
Bromine water is toxic and corrosive, and its vapour irritates eyes and airways. Work in the fume hood as briefed, wear safety glasses, and tell your kaiako immediately if any contacts skin — rinse with water for at least 10 minutes.
📖 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 two samples provided — the hexane and the hex-1-ene from the Do Now — and to a third bottle your kaiako labels only as "unknown". 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 then justify it from this molecule: which of the two doubly bonded carbons already carries more hydrogens, and where does the H from HBr therefore add? Relating the rule back to the structure in front of you is what the standard means by justifying. Naming the rule and stopping is not.
🧭 Activity 1b: Two Reactions the Do Now Left Hanging (15 mins)
(a) The alkane is slow, not inert. In the Do Now the hexane stayed orange. Leave that same mixture in bright UV light and the colour does eventually fade — but by a different reaction. An alkane and a halogen react by substitution: one hydrogen is swapped for one halogen atom, and the hydrogen leaves as a hydrogen halide. Write the equation for methane with Cl2 in UV light, stopping at monosubstitution. Then answer in one sentence: if you were handed only the products of each reaction, how would you tell a substitution from an addition?
(b) A second oxidant for the C=C. Bromine water is not the only reagent that reports on a double bond. Cold, dilute permanganate (MnO4−) oxidises an alkene to a diol — an –OH on each of the two carbons that used to share the double bond. Draw the product of cold dilute MnO4− with ethene and name it. Then state what a chemist would see, and why an alkane in the same tube shows nothing.
Kaiako answer key — Activity 1b. (a) CH4 + Cl2 ⟶ CH3Cl + HCl, UV light. The tell is the second product: substitution always ejects something (here HCl), so the organic product has the same number of carbons but has lost an atom and gained another. Addition ejects nothing — every atom of both reactants ends up in one larger molecule. The second tell is the condition: the alkene reacts with bromine in the dark and in seconds; the alkane needs UV and is slow.
(b) Ethene + cold dilute MnO4− ⟶ HOCH2CH2OH, ethane-1,2-diol. Under neutral or alkaline conditions the purple MnO4− is reduced to brown MnO2, so students see the purple go and a brown solid appear; if the permanganate is acidified the purple simply fades to colourless as Mn2+ forms. An alkane has no C=C to oxidise, so nothing happens either way. Watch for: the standard limits alkene oxidation to MnO4−, and limits alkane substitution to monosubstitution — students who write CH2Cl2 have gone past what is being asked for.
Watch for on Markovnikov: carbocation stability explains the rule beautifully and is worth saying aloud to a class that is ready for it, but it is not required by this standard, which asks students to relate structure and functional groups to chemical properties. A student who says "the H adds to the carbon that already has more hydrogens, so the Br ends up on carbon 2, giving 2-bromopropane as the major product" has justified it in the terms the standard uses.
📝 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 relates the structure of your named alkene to the major product, not just the outcome; (4) the UV monosubstitution equation and the diol from cold dilute MnO4−, each with the observation a chemist would record.
🏫 Kaiako Planning & Pedagogy Notes
Where this sits in the standard. This lesson covers the whole of the standard's addition list for alkenes — H2/Pt, Cl2, Br2, H2O/H+ and the hydrogen halides, including major and minor products from asymmetric alkenes — plus two more items from the reaction list: oxidation of alkenes with MnO4−, and substitution of alkanes with halogens limited to monosubstitution.
Exit ticket (3 min, on a slip, collected). Hexane and hex-1-ene both eventually decolourise bromine. Give the one condition that tells the two reactions apart, and the one product difference.
What to watch for while they work. At the bench, listen for students recording what they expected rather than what they saw. In the Markovnikov task, a pair who can only name the rule has stopped at Merit — ask them which carbon carries more hydrogens and make them say the consequence out loud.
Differentiation. Support: Give the four addition reagents as a table with a worked ethene example already filled in, and have students complete the same table for propene. The pattern transfers; the blank page does not. Extension: Ask them to predict the products of adding HBr to 2-methylbut-2-ene, where both carbons of the double bond carry carbon groups, and to say why the rule is harder to apply there.
The error to head off. Students write CH2Cl2 for the alkane substitution. The standard limits it to monosubstitution — one hydrogen, one halogen, one HX leaving. Stop it in the first equation they write.