The shape of the lesson
By the end of the lesson, learners can
- Ranking the O–H acids: Add sodium to water and to ethanol in the simulator and compare how fast hydrogen is given off.
- Esters from acyl chlorides: Make an ester in the simulator both ways, from an acid and from an acyl chloride, then compare the conditions in the table.
- Order: ethanol < water < phenol < ethanoic acid.
- Alkyl group: electron-donating, destabilises RO−.
- Ka = 10−pKa: one pKa unit is a factor of 10.
- RCOCl + R'OH: ester + HCl, room temperature, not reversible.
One picture every idea returns to
An acid HA is strong when its anion A− is stable, so the order of acidity follows the stability of the anion. In an alkoxide ion, RO−, the alkyl group is electron-donating: it pushes extra electron density onto the already negative oxygen, which destabilises the ion. So alcohols (pKa about 16) are slightly weaker acids than water (pKa 15.7), and much weaker than phenol (pKa 10.0), whose charge is delocalised into the ring. The order is ethanol < water < phenol < ethanoic acid. Alcohols react with acyl chlorides at room temperature to give esters and HCl: unlike esterification with a carboxylic acid, this needs no catalyst and is not reversible.
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, Why alcohols are the weakest acids.. Water, alcohols and phenol all have an O–H bond that can lose H +, but they do it to very different extents. 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: Ranking the O–H acids | Add sodium to water and to ethanol in the simulator and compare how fast hydrogen is given off. Project tab 2 of the lab (Alcohols and phenol lab, sodium bench) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Esters from acyl chlorides | Make an ester in the simulator both ways, from an acid and from an acyl chloride, then compare the conditions in the table. Project tab 3 of the lab (Alcohols and phenol lab, ester two ways) 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, 14 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 Esters from acyl chlorides, 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 are weaker acids than water, not stronger: the alkyl group is electron-donating and destabilises the alkoxide ion. | Do not say the alkyl group ‘strengthens the O–H bond’ as the whole explanation; the mark is for the anion stability. With an acyl chloride the by-product is HCl, not water, and the reaction is not reversible and needs no acid catalyst. |
Alcohols: further reactions: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why ethanol is a weaker acid than water.
The pKa of water is 15.7 and that of phenol is 10.0. Calculate Ka for each and the ratio Ka(phenol) : Ka(water).
Write an equation for the reaction of propan-2-ol with ethanoyl chloride and name the organic product.
Give two advantages of making ethyl ethanoate from ethanoyl chloride rather than ethanoic acid, and one disadvantage.
Sodium ethoxide solution is a stronger base than sodium hydroxide solution of the same concentration. Explain this using the acidities of ethanol and water.
An ester C6H5COOCH3 is needed. Name the acyl chloride and alcohol used, and write the equation.
Answer key and marking notes
- Both lose H+ from an O–H bond, forming C2H5O− and OH−.
- The ethyl group is electron-donating, so it increases the negative charge density on the O of the ethoxide ion.
- This makes C2H5O− less stable (more ready to accept H+ back) than OH−, so ethanol ionises less.
- Ka(water) = 10−15.7 = 2.00 × 10−16 mol dm−3; Ka(phenol) = 10−10.0 = 1.00 × 10−10 mol dm−3.
- Ratio = 105.7 = 5.01 × 105: phenol is about half a million times more acidic.
- CH3COCl + CH3CH(OH)CH3 → CH3COOCH(CH3)2 + HCl.
- The product is 1-methylethyl ethanoate (isopropyl ethanoate).
- Advantage: the reaction is fast at room temperature, with no catalyst or heating.
- Advantage: it is not reversible, so the yield is much higher.
- Disadvantage: ethanoyl chloride is expensive, reacts violently with water and releases corrosive HCl fumes, so it must be handled in a fume cupboard.
- Ethanol is a weaker acid than water, so its conjugate base (ethoxide) holds on to H+ more strongly.
- So C2H5O− is a stronger base than OH− (the weaker the acid, the stronger its conjugate base).
- Benzoyl chloride and methanol.
- C6H5COCl + CH3OH → C6H5COOCH3 + HCl (methyl benzoate).
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 shows the pKa values of ethanol, water, phenol and ethanoic acid, labelled P to S. Which compound is phenol?
- AQ
- BP
- CR
- DS
Why is ethanol a slightly weaker acid than water?
- Athe ethyl group is electron-donating, increasing the negative charge density on the O of C2H5O− and destabilising it
- Bthe ethyl group is electron-withdrawing, stabilising C2H5O−
- Cthe charge on C2H5O− is delocalised over the ethyl group
- Dethanol has two O–H bonds and water has one
Which equation shows the reaction of ethanol with ethanoyl chloride?
- ACH3COCl + C2H5OH → CH3COOC2H5 + HCl
- BCH3COCl + C2H5OH ⇌ CH3COOC2H5 + H2O
- CCH3COCl + C2H5OH → CH3COOH + C2H5Cl
- DCH3COCl + C2H5OH → CH3CONHC2H5 + HCl
What are the conditions for making an ester from an alcohol and an acyl chloride?
- Aroom temperature, no catalyst needed
- Bheat under reflux with concentrated H2SO4 catalyst
- Cheat with aqueous NaOH
- DUV light
The pKa of phenol is 10.0 and the pKa of ethanol is about 16. Approximately how many times larger is the Ka of phenol than that of ethanol?
- A1 × 106
- B6
- C1.6
- D1 × 10−6
Sodium ethoxide is added to water. Which statement best describes what happens?
- Aethoxide ions remove H+ from water, forming ethanol and OH−, because ethanol is a weaker acid than water
- Bnothing happens, because sodium ethoxide is a neutral salt
- Cethanol releases H+ to water, giving an acidic solution
- Dethoxide ions are oxidised to ethanal by water
An ester with formula CH3CH2COOCH(CH3)2 is to be made in high yield. Which pair of reagents should be used?
- Apropanoyl chloride and propan-2-ol
- Bpropanoic acid and propan-2-ol, with a few drops of concentrated H2SO4
- Cpropanoyl chloride and propan-1-ol
- Dpropan-2-yl chloride and propanoic acid
Which statement correctly explains the order of acidity ethanol < water < phenol < ethanoic acid?
- Athe negative charge of the anion is increasingly spread out: intensified by an alkyl group, on one O, partly into a ring, shared equally over two O atoms
- Bthe O–H bond gets longer from ethanol to ethanoic acid
- Cthe number of hydrogen atoms in the molecule decreases
- Dthe anions become increasingly basic from ethoxide to ethanoate
Quiz key
Phenol has pKa 10.0, between the carboxylic acid and water, so it is Q. P (4.76) is the strongest acid, ethanoic acid, whose anion shares its charge over two O atoms. R (15.7) is water. S (about 16) is ethanol, the weakest acid of the four, because the ethyl group destabilises the ethoxide ion. Choosing S confuses a large pKa with a strong acid.
An alkyl group pushes electron density towards the O, so in C2H5O− the negative charge is concentrated even more; the ion is less stable than OH− and forms less readily. Alkyl groups donate, not withdraw, electrons, so the second option has the direction wrong. There is no π system in an ethyl group to delocalise the charge. Ethanol has only one O–H bond and water has two, so the last option is simply false.
The alcohol O attacks the carbonyl carbon and HCl is eliminated, giving ethyl ethanoate; the reaction goes to completion. Water is the by-product only when a carboxylic acid is used, and that reaction is the reversible one. The Cl is not transferred to the ethyl group, so chloroethane does not form. An amide (C–N) needs an amine, not an alcohol.
The very δ+ carbonyl carbon of an acyl chloride reacts with alcohols readily at room temperature, with no catalyst. Reflux with concentrated H2SO4 is needed for the slower carboxylic acid route. Aqueous NaOH would hydrolyse the acyl chloride (and any ester formed). UV light is for free-radical substitution of alkanes.
Ka(phenol) / Ka(ethanol) = 10−10 / 10−16 = 106. 6 is only the difference in pKa: each pKa unit is a factor of 10. 1.6 is the ratio of the pKa values, which has no meaning. 10−6 inverts the ratio and would make phenol the weaker acid.
Since ethanol (pKa about 16) is a weaker acid than water (15.7), C2H5O− is a stronger base than OH−: C2H5O− + H2O → C2H5OH + OH−, so the solution is strongly alkaline. A salt of a very weak acid is not neutral in water. No ethanol is present at the start to release H+, and ethanol would not make water acidic anyway. Water is not an oxidising agent here.
CH3CH2COCl + (CH3)2CHOH → CH3CH2COOCH(CH3)2 + HCl goes to completion, giving a high yield. The acid-catalysed route gives the right ester but is reversible, so the yield is limited. Propan-1-ol would give the propyl ester, CH3CH2COOCH2CH2CH3, an isomer. 2-chloropropane is a halogenoalkane, not an acyl chloride, and does not esterify a carboxylic acid under normal conditions.
Acid strength follows anion stability. Ethoxide has extra electron density from the ethyl group; hydroxide has its charge on one O; phenoxide spreads some charge into the ring; ethanoate shares it equally between two electronegative O atoms. O–H bond length is almost the same in all four and is not the explanation. Counting H atoms is irrelevant (water has two, phenol six). The conjugate bases become less basic, not more, as the acids get stronger.
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.