The shape of the lesson
By the end of the lesson, learners can
- Making acyl chlorides, and hydrolysis: Compare the hydrolysis of different chloro compounds in the simulator, watching how fast a precipitate forms with silver nitrate.
- Esters, amides and the mechanism: Step through the mechanism in the simulator with water, ethanol and ammonia, then use the table to see that every nucleophile follows the same addition-elimination pattern as water in Fig. 1.
- Make: RCOOH + PCl5, PCl3 (heat) or SOCl2.
- Hydrolysis: acyl chloride >> chloroalkane > chlorobenzene.
- Products: water to acid, alcohol or phenol to ester, NH3 or amine to amide.
- Mechanism: addition (lone pair to C) then elimination (Cl− leaves).
One picture every idea returns to
An acyl chloride, RCOCl, is made from a carboxylic acid with PCl5 (room temperature), PCl3 and heat, or SOCl2. Its carbonyl carbon is bonded to two electronegative atoms, O and Cl, so it is strongly δ+ and planar, open to attack by nucleophiles. Acyl chlorides react by nucleophilic addition-elimination: a lone pair adds to the C=O carbon, then the C=O reforms and Cl− leaves with H+ as HCl. With water they give the carboxylic acid (vigorous, steamy HCl fumes); with alcohols and phenols, esters; with ammonia and amines, amides. Ease of hydrolysis: acyl chloride >> chloroalkane > chlorobenzene.
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 most reactive acid derivative.. Swap the OH of a carboxylic acid for Cl and you get an acyl chloride, which fumes in moist air. 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: Making acyl chlorides, and hydrolysis | Compare the hydrolysis of different chloro compounds in the simulator, watching how fast a precipitate forms with silver nitrate. Project tab 4 of the lab (Carboxylic acids and derivatives (A Level) lab, hydrolysis race) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Esters, amides and the mechanism | Step through the mechanism in the simulator with water, ethanol and ammonia, then use the table to see that every nucleophile follows the same addition-elimination pattern as water in Fig. 1. Project tab 5 of the lab (Carboxylic acids and derivatives (A Level) lab, addition-elimination) and let a student drive it while the class predicts. | Anticipation / Reaction Guide |
| 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, 16 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, amides and the mechanism, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| Show the mechanism in two steps: addition (lone pair to C, C=O π electrons to O) then elimination (C=O reforms, Cl − leaves). | Start every curly arrow from a lone pair or a bond, never from an atom or a charge sign. The by-product is always HCl (or its ammonium salt), not water. Chlorobenzene is the hardest to hydrolyse, not the easiest: its C–Cl bond has partial double-bond character. |
Acyl chlorides: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Write equations for making propanoyl chloride from propanoic acid using (a) PCl5, (b) SOCl2, (c) PCl3.
Explain why ethanoyl chloride is hydrolysed much more easily than chloroethane, and why chlorobenzene is not hydrolysed at all under the same conditions.
Describe, with curly arrows, the mechanism for the reaction of ethanoyl chloride with ethanol.
6.00 g of ethanoic acid reacts with excess SOCl2 and 5.89 g of ethanoyl chloride (Mr 78.5) is collected. Calculate the percentage yield.
Give the products when ethanoyl chloride reacts with (a) phenol, (b) excess methylamine.
1.57 g of ethanoyl chloride reacts with excess ammonia. Calculate the maximum mass of ethanamide (Mr 59.0) formed.
Answer key and marking notes
- (a) CH3CH2COOH + PCl5 → CH3CH2COCl + POCl3 + HCl.
- (b) CH3CH2COOH + SOCl2 → CH3CH2COCl + SO2 + HCl.
- (c) 3CH3CH2COOH + PCl3 → 3CH3CH2COCl + H3PO3 (heat).
- In ethanoyl chloride the carbonyl carbon is bonded to both O and Cl, so it is very δ+; it is also planar and unhindered, so water attacks it readily.
- In chloroethane the carbon is bonded to only one electronegative atom, so it is less δ+ and needs OH− and heat.
- In chlorobenzene a Cl lone pair overlaps with the ring, strengthening the C–Cl bond, and the electron-rich ring repels nucleophiles.
- Show the C=O carbon as δ+ and O, Cl as δ−.
- Addition: an arrow from a lone pair on the ethanol O to the carbonyl C, and an arrow from the C=O π bond to the carbonyl O, giving a tetrahedral intermediate with O− and a positive O.
- Elimination: an arrow from the O− lone pair to reform C=O, and an arrow from the C–Cl bond to Cl, which leaves as Cl−.
- H+ is lost from the positive O and combines with Cl−: products ethyl ethanoate and HCl.
- n(ethanoic acid) = 6.00 ÷ 60.0 = 0.100 mol, so the maximum is 0.100 × 78.5 = 7.85 g.
- Yield = 5.89 ÷ 7.85 × 100 = 75.0%.
- (a) phenyl ethanoate, CH3COOC6H5, and HCl.
- (b) N-methylethanamide, CH3CONHCH3, and methylammonium chloride, CH3NH3+Cl−.
- n(CH3COCl) = 1.57 ÷ 78.5 = 0.0200 mol; the ratio is 1 : 1.
- Mass of ethanamide = 0.0200 × 59.0 = 1.18 g.
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.
Which reagent does NOT convert ethanoic acid into ethanoyl chloride?
- Adilute hydrochloric acid
- Bphosphorus(V) chloride, PCl5
- Cphosphorus(III) chloride, PCl3, with heat
- Dsulfur dichloride oxide, SOCl2
Silver nitrate solution is added to three chloro compounds at 50 °C. The results are shown. Which row identifies X, Y and Z?
- AX ethanoyl chloride, Y 1-chlorobutane, Z chlorobenzene
- BX chlorobenzene, Y 1-chlorobutane, Z ethanoyl chloride
- CX 1-chlorobutane, Y ethanoyl chloride, Z chlorobenzene
- DX ethanoyl chloride, Y chlorobenzene, Z 1-chlorobutane
What is the organic product when ethanoyl chloride reacts with phenol?
- Aphenyl ethanoate, CH3COOC6H5
- Bethyl benzoate, C6H5COOC2H5
- Cchlorobenzene, C6H5Cl
- Dethanoic acid, CH3COOH
Which equation shows the reaction of ethanoyl chloride with excess ammonia?
- ACH3COCl + 2NH3 → CH3CONH2 + NH4Cl
- BCH3COCl + NH3 → CH3CH2NH2 + HCl
- CCH3COCl + NH3 → CH3COONH4 + Cl
- DCH3COCl + 2NH3 → CH3CONH2 + NH3 + HCl
Why does ethanoyl chloride react with water much more readily than chloroethane does?
- Aits carbonyl carbon is bonded to both O and Cl, so it is strongly δ+ and open to attack
- Bits C–Cl bond is stronger than in chloroethane
- Cethanoyl chloride is ionic
- Dchloroethane has no δ+ carbon atom
In the addition-elimination mechanism for ethanoyl chloride and methanol, which statement about the first step is correct?
- Aa lone pair on the methanol O attacks the carbonyl C, and the C=O π electrons move onto the carbonyl O
- Bthe C–Cl bond breaks first to give a carbocation, which methanol then attacks
- Cthe carbonyl O attacks the H of methanol
- DCl− attacks the methanol, releasing CH3+
1.57 g of ethanoyl chloride (Mr 78.5) is added to water and excess aqueous silver nitrate is then added. What mass of silver chloride (Mr 143.4) precipitates?
- A2.87 g
- B1.43 g
- C5.74 g
- D1.57 g
A student wants to make N-ethylpropanamide, CH3CH2CONHCH2CH3, in good yield from a carboxylic acid in two steps. Which route works?
- Apropanoic acid + SOCl2, then add excess ethylamine at room temperature
- Bpropanoic acid + SOCl2, then add excess ethanol
- Cethanoic acid + PCl5, then add excess propylamine
- Dpropanoic acid + dilute HCl, then add ethylamine
Quiz key
Dilute HCl does not replace the OH of a carboxylic acid; in water any acyl chloride would be hydrolysed straight back anyway. PCl5 gives CH3COCl + POCl3 + HCl; PCl3 with heat gives 3CH3COCl + H3PO3; SOCl2 gives CH3COCl + SO2 + HCl. All three are valid methods.
Ethanoyl chloride is hydrolysed instantly (very δ+ carbonyl carbon), so X gives AgCl at once. 1-chlorobutane is hydrolysed slowly by water at 50 °C, so Y gives a precipitate after some minutes. Chlorobenzene is not hydrolysed, because its C–Cl bond is strengthened by overlap of a Cl lone pair with the ring, so Z gives none. The other rows reverse the order, put the chloroalkane first, or swap the chloroalkane and chlorobenzene.
CH3COCl + C6H5OH → CH3COOC6H5 + HCl. Ethyl benzoate would need benzoyl chloride and ethanol. The Cl does not move onto the ring, so chlorobenzene does not form. Ethanoic acid forms only if water is the nucleophile.
NH3 attacks the carbonyl carbon to give ethanamide; the HCl formed is neutralised by excess NH3 to give NH4Cl. Ethylamine would need a reduction; the C=O is kept in the amide. The third equation is unbalanced and gives an ammonium salt of the acid instead of an amide. The last leaves NH3 and HCl side by side, which would react.
Two electronegative atoms (O and Cl) pull electron density from the carbonyl carbon, and the planar C=O carbon is easy to approach, so even water reacts at room temperature. A stronger C–Cl bond would make hydrolysis slower, not faster. Ethanoyl chloride is a covalent liquid, not an ionic solid. Chloroethane does have a δ+ carbon, but less so, with only one electronegative atom, so it needs OH− and heat.
In the addition step the nucleophile's lone pair adds to the δ+ carbon and the π bond breaks, giving a tetrahedral intermediate with O−; only then is Cl− eliminated as C=O reforms. Losing Cl− first to form a carbocation describes an SN1 pathway, not addition-elimination. The carbonyl O is not the nucleophile here. Cl− is the leaving group, formed in the second step, and it does not attack methanol.
n(CH3COCl) = 1.57 ÷ 78.5 = 0.0200 mol, giving 0.0200 mol Cl− and so 0.0200 × 143.4 = 2.87 g AgCl. 1.43 g halves the moles. 5.74 g doubles them, as if both the acid and HCl gave Cl−; the ethanoic acid contains no Cl. 1.57 g just repeats the mass of acyl chloride.
SOCl2 turns propanoic acid into propanoyl chloride, which reacts with ethylamine by addition-elimination to give CH3CH2CONHC2H5; the excess amine removes the HCl. Ethanol would give the ester ethyl propanoate, not an amide. Ethanoyl chloride with propylamine gives N-propylethanamide, CH3CONHCH2CH2CH3, an isomer. Dilute HCl does not make an acyl chloride, and an acid with an amine simply forms a salt.
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.