# Explain why an acyl chloride, such as ethanoyl chloride, is a more reactive acylating agent than a carboxylic acid anhydride, such as ethanoic anhydride, when reacting with a nucleophile such as water.

> Edexcel A-Level Chemistry (9CH0) — 17.3 Carboxylic acids and derivatives · Explain · 5 marks

> Acyl chlorides and carboxylic acid anhydrides are both used as acylating agents in organic synthesis. Chemists often choose between them based on reactivity and the by-products formed.

## Mark scheme (5 marks)

1. The C–Cl bond in an acyl chloride is more polar than the C–O bond in an anhydride
2. The carbonyl carbon in an acyl chloride is therefore a stronger electrophile
3. The C–Cl bond is weaker than the C–O bond in an anhydride, so it is easier to break during the reaction
4. The by-product of an acyl chloride reacting with water is HCl, which is a strong acid and fully dissociates, whereas an anhydride produces a carboxylic acid (ethanoic acid), which is a weak acid
5. Acyl chlorides react vigorously (even at room temperature / with cold water), whereas anhydrides react more slowly, illustrating the greater reactivity of acyl chlorides

## Key terms

- [acylating agent](https://www.gradenine.co.uk/glossary/acylating-agent)

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