Ethanoyl chloride and ethanoic anhydride are both acylating agents that can react with ethanol to form an ester. Explain why ethanoyl chloride is described as the more vigorous acylating agent, and describe one advantage of using ethanoic anhydride over ethanoyl chloride in industrial ester synthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Acylating agents are widely used in the chemical industry to introduce acyl groups into organic molecules. Ethanoyl chloride reacts violently with ethanol at room temperature, producing misty fumes of hydrogen chloride gas. Ethanoic anhydride reacts more slowly with ethanol but still produces the same ester product.
Model answer (5 marks)
Ethanoyl chloride reacts much faster with ethanol than ethanoic anhydride because the C–Cl bond is highly polarised and easily cleaved, giving a highly reactive acyl chloride intermediate. The chloride ion is a good leaving group, so the acylation proceeds rapidly.
In contrast, ethanoic anhydride has a C–O bond that is less polarised; the acylation is slower because the leaving group (acetate) is less reactive.
An advantage of ethanoic anhydride is that the by‑product is acetic acid, which is non‑corrosive and can be recycled, whereas ethanoyl chloride releases corrosive HCl gas. This makes ethanoic anhydride safer to handle and cheaper to store on an industrial scale.
In contrast, ethanoic anhydride has a C–O bond that is less polarised; the acylation is slower because the leaving group (acetate) is less reactive.
An advantage of ethanoic anhydride is that the by‑product is acetic acid, which is non‑corrosive and can be recycled, whereas ethanoyl chloride releases corrosive HCl gas. This makes ethanoic anhydride safer to handle and cheaper to store on an industrial scale.
Examiner tips
- Mention the polarity of the C–Cl bond and the good leaving ability of Cl⁻ to justify the faster reaction.
- Explain that HCl is hazardous and that acetic acid is safer and can be reused.
- Show the difference in reaction rates and link to the bond strengths.
- Use the exact terminology: acyl chloride, acylating agent, leaving group.
Mark scheme (5 marks)
- Ethanoyl chloride reacts faster / more readily with ethanol than ethanoic anhydride
- The C–Cl bond is more polar / more easily broken than the C–O bond in ethanoic anhydride, making ethanoyl chloride a more effective acylating agent
- Ethanoyl chloride produces HCl (hydrogen chloride) as a by-product, which is corrosive / toxic / hazardous
- Ethanoic anhydride does not produce HCl / produces ethanoic acid as a by-product, which is less hazardous
- Ethanoic anhydride is cheaper / more economical / easier to store and handle safely at industrial scale
Key terms in this question
acylating agent · ethanoyl chloride · ethanoic anhydride
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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