A2 · Practice questions · Acyl chlorides

Add, then eliminate.

Six original Cambridge-style questions on acyl chlorides and the addition-elimination mechanism.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Very δ+ carbon, HCl every time.

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).

Recall

[3 marks]

Write equations for making propanoyl chloride from propanoic acid using (a) PCl₅, (b) SOCl₂, (c) PCl₃.

(a) CH₃CH₂COOH + PCl₅ → CH₃CH₂COCl + POCl₃ + HCl. ✓
(b) CH₃CH₂COOH + SOCl₂ → CH₃CH₂COCl + SO₂ + HCl. ✓
(c) 3CH₃CH₂COOH + PCl₃ → 3CH₃CH₂COCl + H₃PO₃ (heat). ✓

Explain

[3 marks]

Explain why ethanoyl chloride is hydrolysed much more easily than chloroethane, and why chlorobenzene is not hydrolysed at all under the same conditions.

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. ✓

Explain

[4 marks]

Describe, with curly arrows, the mechanism for the reaction of ethanoyl chloride with ethanol.

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. ✓

Calculation

[2 marks]

6.00 g of ethanoic acid reacts with excess SOCl₂ and 5.89 g of ethanoyl chloride (M_r 78.5) is collected. Calculate the percentage yield.

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%. ✓

Recall

[2 marks]

Give the products when ethanoyl chloride reacts with (a) phenol, (b) excess methylamine.

(a) phenyl ethanoate, CH₃COOC₆H₅, and HCl. ✓
(b) N-methylethanamide, CH₃CONHCH₃, and methylammonium chloride, CH₃NH₃⁺Cl⁻. ✓

Calculation

[2 marks]

1.57 g of ethanoyl chloride reacts with excess ammonia. Calculate the maximum mass of ethanamide (M_r 59.0) formed.

n(CH₃COCl) = 1.57 ÷ 78.5 = 0.0200 mol; the ratio is 1 : 1. ✓
Mass of ethanamide = 0.0200 × 59.0 = 1.18 g. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.