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6.3.3Organic Chemistry and Analysis

Acyl Chlorides — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

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Section 1 of 5Structure and Reactivity

6.3.3 Acyl Chlorides

Acyl chlorides have the general formula RCOCl and are named from the parent acid, for example ethanoyl chloride, CH₃COCl.

Electron deficiency
  • The carbonyl carbon is bonded to both oxygen and chlorine, so it carries a large δ+ charge and is easily attacked.
Leaving group
  • The chloride ion leaves readily, which makes substitution rapid and usually complete at room temperature.
Fumes
  • Reactions release hydrogen chloride, which fumes in moist air and appears as steamy white fumes.
Storage
  • They react with moisture in the air, so they must be kept dry and handled in a fume cupboard.

Section 2 of 5Reactions of Ethanoyl Chloride

Each reaction replaces the Cl atom by the group from the nucleophile.

Reactions of Ethanoyl Chloride
Water
  • CH₃COCl + H₂O → CH₃COOH + HCl, giving a carboxylic acid.
Alcohols
  • CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl, giving an ester and no reversible equilibrium.
Ammonia
  • CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl, giving a primary amide; the second NH₃ takes up the HCl.
Primary amines
  • CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃Cl, giving an N-substituted amide.
NucleophileOrganic productInorganic product
H₂OCarboxylic acidHCl
ROHEsterHCl
NH₃Primary amideNH₄Cl
RNH₂N-substituted amideRNH₃Cl

Section 3 of 5Addition–Elimination Mechanism

1
The nucleophile uses its lone pair to attack the δ+ carbonyl carbon, breaking the C=O π bond.
2
An unstable intermediate forms with a negatively charged oxygen and a positively charged nucleophile atom.
3
The C=O double bond reforms and the chloride ion leaves as the leaving group.
4
A proton is lost from the nucleophile atom, producing HCl or a chloride salt.
5
Because HCl is lost and not recycled, the reaction goes to completion, unlike esterification from a carboxylic acid.

Section 4 of 5Worked Example

Question

  • 7.85 g of ethanoyl chloride (M = 78.5 g mol⁻¹) reacts with excess ethanol.
  • 6.50 g of ethyl ethanoate (M = 88.1 g mol⁻¹) is collected.
  • Calculate the percentage yield.

  1. Moles of ethanoyl chloride = 7.85 ÷ 78.5 = 0.100 mol.
  2. The equation is 1 : 1, so theoretical moles of ester = 0.100 mol.
  3. Theoretical mass = 0.100 × 88.1 = 8.81 g.
  4. Percentage yield = 6.50 ÷ 8.81 × 100 = 73.8%.

Answer Percentage yield = 73.8%

Section 5 of 5Exam Checklist & Tips

6.3.3 – Key Points for the Exam

  • Acyl chloride A compound RCOCl derived from a carboxylic acid by replacing –OH with –Cl.
  • Addition–elimination A two-stage mechanism in which a nucleophile adds, then a leaving group is lost.
  • Amide A compound containing the –CONH– group.
  • Ester A compound RCOOR' formed from an acid derivative and an alcohol.

Common Mistakes

  • Writing only one mole of NH₃ in the amide equation and forgetting the second that forms NH₄Cl.
  • Describing the mechanism as nucleophilic substitution in a single step, with no intermediate.
  • Saying the reaction with alcohols is reversible; it goes to completion.
  • Giving the by-product of the water reaction as H₂, when it is HCl.

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