Lesson 4: Alcohols — Classification, Oxidation, Substitution and Dehydration
Classify primary, secondary and tertiary alcohols, then trace how structure controls oxidation, substitution and dehydration products under specified conditions.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Primary, secondary and tertiary alcohol structures; oxidation using acidified potassium permanganate (MnO4-/H+) and acidified potassium dichromate (Cr2O72-/H+); substitution of the hydroxyl group to a haloalkane using HX, PCl3, PCl5 or SOCl2; elimination of water with concentrated H2SO4 to give an alkene.
Classify alcohols using structural formulae, observe and record colour changes in lab oxidations (purple to colourless, orange to green), write oxidation pathways for primary and secondary alcohols, and predict the major product when an asymmetric alcohol is dehydrated.
🎥 Media Anchor & Pedagogical Scaffold
Alcohol Oxidation & Functional Group Conversion
Video Clip: GCSE Chemistry — Alcohols — structure, properties, classification, and reactions | Cognito (Runtime: 4m 07s).
🧠 1. Before Viewing (Activate & Predict)
How do breathalysers detect ethanol in human breath using dichromate oxidation chemistry?
👁️ 2. During Viewing (Watch With a Job)
- Functional group: draw the hydroxyl (-OH) group and explain what makes it the characteristic group of all alcohols.
- Oxidation: the video shows what happens when a primary alcohol is oxidised. Write the type of product formed and name one reagent used for the oxidation.
- Fermentation vs synthesis: name TWO ways ethanol can be produced — state ONE advantage of each method.
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Conduct the microscale alcohol oxidation practical. Discuss safety when handling concentrated acid catalysts.
Immediate Task: Complete Section 4 of your Organic Portfolio: Alcohol Classification & Oxidation Colour Change Diagnostic Matrix.
⚡ Whakaoho | Do Now: Organic Recall Challenge (10 mins)
Orange to green. Early breathalysers worked by blowing through a tube of acidified potassium dichromate. If the crystals turned from orange to green, the driver had been drinking.
Two minutes: dichromate is an oxidising agent and ethanol is an alcohol. Predict what the ethanol was turned into, and say what happened to the chromium to make it change colour. You have met both halves before — put them together.
⚠️ Hazard
Acidified potassium dichromate is toxic, a known human carcinogen, and an oxidiser; the concentrated acid catalyst used with it is corrosive. Wear safety glasses, avoid skin contact, and never pour dichromate waste down the sink — dispose of it only as your kaiako directs.
📖 Activity 1: Core Reaction Mechanism & Structure Analysis (25 mins)
Classify first (10 min). You cannot predict an oxidation product until you know which kind of alcohol you have. Sort these six into primary, secondary and tertiary by counting what is attached to the carbon bearing the –OH: propan-1-ol, propan-2-ol, 2-methylpropan-2-ol, butan-1-ol, butan-2-ol, 2-methylbutan-1-ol. Then predict each one's oxidation product with warm acidified dichromate. One will resist oxidation entirely — find it and explain what it is missing.
Kaiako answer key — Classify first. Primary: propan-1-ol, butan-1-ol, 2-methylbutan-1-ol (the branch is on the next carbon along, not on the carbon holding the –OH — that is the one students misclassify). Secondary: propan-2-ol, butan-2-ol. Tertiary: 2-methylpropan-2-ol. Oxidation with warm acidified dichromate: the three primary alcohols go to an aldehyde and on to the acid (propanoic, butanoic, 2-methylbutanoic); the two secondary alcohols go to a ketone (propanone, butanone); 2-methylpropan-2-ol does nothing. What it is missing is a hydrogen on the carbon carrying the –OH. Oxidation has to remove that C–H, and a tertiary alcohol has three carbon groups there instead — so there is no bond to break without breaking the carbon skeleton.
Scope note for kaiako. The standard's reaction list names oxidation of primary alcohols to carboxylic acids with MnO4−/H+ or Cr2O72−/H+, and oxidation of alkenes with MnO4−. Secondary alcohols going to ketones is not on that list. Teach it anyway — it is what makes the classification mean something, and a student who cannot say what a secondary alcohol does cannot explain why a tertiary one does nothing — but tell your class which of the three is the assessable one.
Two steps, not one (15 min). A primary alcohol oxidises twice. Draw ethanol → ethanal → ethanoic acid and label the reagent and condition driving each step. Then answer: if you wanted to stop at the aldehyde, what would you change about the apparatus, and why does distilling it off as it forms work?
🔁 Activity 1b: The Other Two Things You Can Do to an –OH (20 mins)
⚠️ Paper work. Everything in this activity is written, not run. PCl5 and SOCl2 react violently with water and are not school-laboratory reagents; hot concentrated sulfuric acid is severely corrosive. The bench work in this lesson is the oxidation above, under its own hazard panel.
Swap it out — substitution (10 min). Oxidation is not the only thing that happens to a hydroxyl group. Four reagents replace an –OH with a chlorine or bromine and leave the carbon skeleton alone: a hydrogen halide (HCl or HBr), PCl3, PCl5, and SOCl2. Write the equation for propan-1-ol with each of the four, and name the organic product each time. Then answer: the organic product is the same every time, so why would a chemist choose one reagent over another? Look at what leaves alongside it.
Take the water out — elimination (10 min). Warm an alcohol with concentrated sulfuric acid and it loses a molecule of water: the –OH goes, a hydrogen from a neighbouring carbon goes with it, and the two carbons close up into a double bond. Draw the dehydration of ethanol. Then try butan-2-ol, where the –OH has a neighbouring carbon on each side: draw both possible alkenes, decide which is the major product, and justify your choice by relating the two structures to each other. Finally, compare this reaction with the hydration you met in Lesson 3 and say what the relationship between them is.
Kaiako answer key — Swap it out. With propan-1-ol, CH3CH2CH2OH:
• + HCl → 1-chloropropane + H2O
• 3 CH3CH2CH2OH + PCl3 → 3 CH3CH2CH2Cl + H3PO3
• + PCl5 → 1-chloropropane + POCl3 + HCl
• + SOCl2 → 1-chloropropane + SO2 + HCl
The choice is about the by-products, which is the point of the question. SOCl2 is the tidiest: both by-products are gases, so they leave the mixture on their own and the haloalkane is left behind clean. PCl5 gives steamy HCl fumes the moment it touches an alcohol, which is why it doubles as a test for a hydroxyl group. HX is the mildest and slowest.
Kaiako answer key — Take the water out. Ethanol, heated with concentrated H2SO4, gives ethene + H2O; only one alkene is possible. Butan-2-ol can lose its water two ways: towards C1 giving but-1-ene, or towards C3 giving but-2-ene. But-2-ene is the major product (Saytzeff), and the justification the standard is looking for is a structural comparison: the hydrogen is taken from the neighbouring carbon that has fewer hydrogens on it, which leaves the double bond carrying more carbon groups. Students who only name the rule have not justified anything. The relationship with Lesson 3 is that dehydration and hydration are the same reaction run in opposite directions — concentrated acid and heat drive water out, dilute acid and water drive it back in — and noticing that is worth more than either equation on its own.
📝 Activity 2: Level 2 Chemistry Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Organic Portfolio — Section 4. Submit: (1) six alcohols classified with reasons; (2) oxidation equations with reagents, conditions and the observed colour change; (3) an explanation of why tertiary alcohols resist oxidation, written in terms of bonds rather than 'because they are tertiary'; (4) the four substitution equations for propan-1-ol with the by-product named in each; (5) the dehydration of butan-2-ol with both alkenes drawn and the major product justified by comparing the two structures.
🏫 Kaiako Planning & Pedagogy Notes
Where this sits in the standard. Classification of alcohols as primary, secondary or tertiary is in the standard's property list. This lesson also covers three of its reactions: oxidation of primary alcohols to carboxylic acids with MnO4−/H+ or Cr2O72−/H+, substitution of alcohols with hydrogen halides, PCl3, PCl5 and SOCl2, and elimination of water from alcohols.
Exit ticket (3 min, on a slip, collected). 2-methylbutan-1-ol: primary, secondary or tertiary, and what is your reason? Then name its oxidation product.
What to watch for while they work. In the classification task, watch for students counting the branches on the whole molecule rather than the carbons attached to the one carbon carrying the –OH. 2-methylbutan-1-ol catches most of a class the first time.
Differentiation. Support: Give the classification as a physical action — put a finger on the carbon holding the –OH, then count the carbons touching that finger. Students who can do that can classify haloalkanes in Lesson 7 with no new teaching. Extension: Ask why the same alcohol can be pushed towards an alkene or towards a haloalkane depending on the reagent, and what that says about which part of the molecule is actually reacting.
The error to head off. Students explain the tertiary alcohol's resistance by saying "it is tertiary". That is the observation restated. The answer is that there is no hydrogen on the carbon carrying the –OH for the oxidant to remove.