The shape of the lesson
By the end of the lesson, learners can
- Classes and hydrogen bonds: Sort alcohols into primary, secondary and tertiary in the simulator, then switch to the hydrogen bonding view and compare boiling points with alkanes.
- Four key reactions: Run each reaction of ethanol in the simulator and note the observations.
- Hydrogen bonding: high b.p., soluble in water.
- Na: alkoxide + H2.
- PCl5: steamy HCl fumes.
- Esterification: acid + alcohol, conc. H2SO4, reversible.
One picture every idea returns to
Alcohols contain the –OH group. Hydrogen bonding between molecules gives them much higher boiling points than alkanes of similar Mr, and lets short-chain alcohols mix with water. They are made by hydrating alkenes, hydrolysing halogenoalkanes, reducing carbonyl compounds or hydrolysing esters. They react with sodium to give an alkoxide and hydrogen (more slowly than water, as alcohols are weaker acids); with HX, PCl5 or SOCl2 to give halogenoalkanes; with hot concentrated acid or Al2O3 by dehydration to alkenes; and with carboxylic acids, using a concentrated H2SO4 catalyst, to form esters.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, The –OH group at work.. An OH group makes alcohols hydrogen-bond, so they boil higher and mix with water. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Classes and hydrogen bonds | Sort alcohols into primary, secondary and tertiary in the simulator, then switch to the hydrogen bonding view and compare boiling points with alkanes. Project tab 2 of the lab (Hydroxy compounds lab, hydrogen bonding) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Four key reactions | Run each reaction of ethanol in the simulator and note the observations. Project tab 4 of the lab (Hydroxy compounds lab, reactions of ethanol) and let a student drive it while the class predicts. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Four key reactions, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Alcohols react with sodium more slowly than water does, because they are weaker acids: the ethyl group pushes electron density towards O, making O–H harder to break. | In esterification, the concentrated H 2 SO 4 is a catalyst and the reaction is reversible, so yields are well below 100%. |
Reactions of alcohols: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why propan-1-ol (Mr 60) has a much higher boiling point than butane (Mr 58).
Write equations for the reactions of ethanol with (a) sodium, (b) PCl5, (c) ethanoic acid.
Butan-2-ol is heated with concentrated phosphoric acid. Name all the organic products and explain why there is more than one.
4.60 g of ethanol reacts with excess propanoic acid to give 6.12 g of ethyl propanoate (Mr 102.0). Calculate the percentage yield.
Give two methods of making ethanol, with reagents and conditions.
Explain why the solubility of alcohols in water decreases as the carbon chain gets longer.
Answer key and marking notes
- Both have similar numbers of electrons, so similar id-id forces.
- Propan-1-ol also forms hydrogen bonds between –OH groups (H on O, lone pairs on O).
- Hydrogen bonds are much stronger, so more energy is needed to separate the molecules.
- (a) 2C2H5OH + 2Na → 2C2H5ONa + H2.
- (b) C2H5OH + PCl5 → C2H5Cl + POCl3 + HCl.
- (c) CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O.
- But-1-ene, cis-but-2-ene and trans-but-2-ene.
- The H can be removed from C1 or C3 next to the C–OH carbon, giving different positions of C=C.
- But-2-ene has restricted rotation with two different groups on each carbon, so it exists as cis and trans isomers.
- n(ethanol) = 4.60 ÷ 46.0 = 0.100 mol, so the maximum is 0.100 mol = 10.2 g ester.
- Yield = 6.12 ÷ 10.2 × 100 = 60.0%.
- Hydration of ethene: steam, H3PO4 catalyst, 300 °C, 6 MPa.
- Hydrolysis of bromoethane: NaOH(aq), heat under reflux (or reduction of ethanal with NaBH4).
- The –OH group can hydrogen bond to water.
- A longer non-polar hydrocarbon chain cannot, and disrupts water's hydrogen bonding, so solubility falls.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The chart compares the boiling points of two alcohols with alkanes of similar relative molecular mass. Why does ethanol (78 °C) boil so much higher than propane (−42 °C)?
- Aethanol molecules form hydrogen bonds between their O-H groups, which need much more energy to overcome than the instantaneous dipole-induced dipole forces in propane
- Bethanol has a much larger Mr, so its instantaneous dipole-induced dipole forces are much stronger
- Cthe covalent O-H bond in ethanol must be broken when it boils
- Dpropane is a gas because its molecules are linear, so they cannot pack together
Ethanol is manufactured from ethene. What are the reagent and catalyst?
- Asteam, with a phosphoric(V) acid (H3PO4) catalyst, at about 300 °C and high pressure
- Baqueous sodium hydroxide, heated under reflux
- Csodium tetrahydridoborate(III), NaBH4, in water
- Dhot concentrated sulfuric acid acting as a dehydrating agent
A small piece of sodium is added to dry ethanol. Which statement is correct?
- Abubbles of hydrogen form and sodium ethoxide is produced, more slowly than with water, because ethanol is a weaker acid than water
- Bhydrogen forms faster than with water, because ethanol is a stronger acid than water
- Csodium hydroxide and ethane are formed
- Dno reaction occurs, because ethanol is a neutral covalent compound
Phosphorus(V) chloride is added to a dry organic liquid at room temperature and steamy fumes are given off. What do the fumes show?
- Athe liquid contains an O-H group; the fumes are hydrogen chloride
- Bthe liquid contains a C=C double bond; the fumes are chlorine
- Cthe liquid is an alkane; the fumes are hydrogen gas
- Dthe liquid contains a C=O group; the fumes are water vapour
Ethanol is warmed with ethanoic acid and a few drops of concentrated sulfuric acid. Which description of the reaction is correct?
- Aa reversible esterification giving ethyl ethanoate and water, with H2SO4 as a catalyst
- Ba complete reaction giving ethyl ethanoate and hydrogen, with H2SO4 as a reactant
- Ca reversible reaction giving ethanoyl ethanol and water, with no catalyst needed
- Da dehydration of ethanol to ethene, with ethanoic acid as the catalyst
Butan-2-ol, CH3CH(OH)CH2CH3, is passed over heated aluminium oxide. How many different alkenes (including stereoisomers) can form?
- Athree: but-1-ene, cis-but-2-ene and trans-but-2-ene
- Bone: but-2-ene only, because H is always removed from the more substituted carbon
- Ctwo: but-1-ene and but-2-ene, because cis and trans forms are the same compound
- Dfour: but-1-ene, cis- and trans-but-2-ene, and 2-methylpropene
4.60 g of ethanol is heated with excess ethanoic acid and concentrated H2SO4. The yield of pure ethyl ethanoate, CH3COOC2H5, is 5.28 g. What is the percentage yield? (Mr: ethanol 46.0, ethyl ethanoate 88.0)
- A60.0%
- B115%
- C87.1%
- D88.0%
Which reaction produces propan-2-ol, CH3CH(OH)CH3?
- Areduction of propanone, CH3COCH3, with NaBH4
- Breduction of propanal, CH3CH2CHO, with NaBH4
- Cheating 1-bromopropane under reflux with NaOH(aq)
- Dheating propyl ethanoate, CH3COOCH2CH2CH3, under reflux with dilute acid
Quiz key
The O-H bond is highly polar, and the H of one ethanol molecule is attracted to a lone pair on the O of another: a hydrogen bond. Alkanes have only instantaneous dipole-induced dipole forces, so far less energy is needed to separate their molecules. The same O-H group lets small alcohols hydrogen bond with water, which is why they are soluble, although solubility falls as the non-polar hydrocarbon chain gets longer. The Mr values are 46 and 44, almost identical, so differences in instantaneous dipole forces cannot explain a 120 ° gap. Boiling separates molecules; no covalent bond breaks. Propane is not linear (its C-C-C angle is about 109.5°), and shape is a minor factor here.
This is electrophilic addition of water (hydration): C2H4(g) + H2O(g) ⇌ C2H5OH(g), using steam and an H3PO4 catalyst at about 300 °C and 6 to 7 MPa. NaOH(aq) makes alcohols by hydrolysing halogenoalkanes, but it does not react with alkenes. NaBH4 is a reducing agent that turns aldehydes and ketones into alcohols; ethene has no C=O group. Hot concentrated H2SO4 does the reverse: it dehydrates ethanol to ethene.
2C2H5OH + 2Na → 2C2H5O−Na+ + H2. The sodium fizzes gently and dissolves. The reaction is slower than with water because the ethyl group pushes electron density towards the O-H oxygen, making the O-H bond less easy to break and the ethoxide ion less stable than OH−; ethanol is a weaker acid than water. Saying alcohols are more acidic than water is the classic error. No NaOH or ethane forms: the C-O bond is untouched and only the O-H hydrogen is replaced. Ethanol is neutral to indicators, but its O-H hydrogen still reacts with a reactive metal like sodium.
C2H5OH + PCl5 → C2H5Cl + POCl3 + HCl. The OH group is replaced by Cl, a substitution reaction, and the HCl gas forms steamy (misty) fumes in moist air, turning blue litmus red. The test works for O-H groups in both alcohols and carboxylic acids. Alkenes are tested with bromine water, not PCl5, and no chlorine is released. Alkanes do not react with PCl5 and produce no hydrogen. Aldehydes and ketones do not give steamy fumes with PCl5 in this test. Other routes to halogenoalkanes are SOCl2 (giving SO2 and HCl) or HX made from KBr and concentrated H2SO4.
C2H5OH + CH3COOH ⇌ CH3COOC2H5 + H2O. The product is the ester ethyl ethanoate (fruity smell), named with the alcohol part first. The reaction is slow and reaches equilibrium, so the concentrated H2SO4 is a catalyst (it also absorbs some water, which shifts the position towards the ester). The small molecule lost is water, not hydrogen, and the reaction does not go to completion. The ester is not called ethanoyl ethanol, and without an acid catalyst the reaction is far too slow. Dehydration to ethene needs hot excess concentrated acid or Al2O3; a few drops of acid with a carboxylic acid present gives the ester.
In dehydration the OH and an H from an adjacent carbon are lost as water. Removing H from C1 gives CH2=CHCH2CH3 (but-1-ene); removing H from C3 gives CH3CH=CHCH3 (but-2-ene). Each carbon of the C=C in but-2-ene carries two different groups (H and CH3), so it exists as cis and trans isomers. That makes three alkenes. Claiming only one alkene forms is the classic error: both adjacent carbons have hydrogens. Cis and trans but-2-ene are different compounds, because rotation about C=C is restricted. 2-methylpropene would need a rearrangement of the carbon skeleton, which dehydration does not do.
Moles of ethanol = 4.60 / 46.0 = 0.100 mol. The ratio is 1:1, so the theoretical mass of ester = 0.100 × 88.0 = 8.80 g. Percentage yield = 5.28 / 8.80 × 100 = 60.0%, which is typical for a reversible reaction that reaches equilibrium. 115% divides the mass of ester by the mass of ethanol, ignoring the different Mr values; a yield above 100% is impossible. 87.1% divides the mass of ethanol by the mass of ester, which compares different substances. 88.0% uses the Mr of ethanoic acid (60.0) instead of the ester to get a theoretical mass of 6.00 g.
NaBH4 supplies H− ions that add to the C=O carbon; the ketone carbon is bonded to two other carbons, so reduction gives the secondary alcohol: CH3COCH3 + 2[H] → CH3CH(OH)CH3. Reducing an aldehyde gives a primary alcohol, here propan-1-ol. Hydrolysis of 1-bromopropane puts OH on carbon 1, again giving propan-1-ol. Acid hydrolysis of propyl ethanoate (a reversible reaction; alkaline hydrolysis goes to completion) gives ethanoic acid and propan-1-ol, because the propyl group is attached through carbon 1.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.