Explain addition–elimination at an acyl chloride
Follow nucleophilic addition to the carbonyl and chloride elimination to explain acyl-chloride reactions with water, alcohols, phenol, ammonia and amines.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 33. Focus on the named compounds, reagents and conditions in each outcome; apply the specified reaction type rather than extending it to unlisted cases. Written practice is self-checked
Before you begin
- Recall carboxylic acids, esters, phenols, alcohols, halogenoalkanes and electrophilic/nucleophilic reaction ideas from earlier topics.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens in the first step of acyl-chloride addition–elimination?
What you will learn
- Describe the two stages of acyl-chloride addition–elimination
- Track the tetrahedral intermediate
- Relate the mechanism to the listed nucleophiles
Add the nucleophile to the carbonyl
The carbonyl carbon is electron-deficient. A nucleophile such as H₂O, ROH, phenol, NH₃ or an amine donates a lone pair to this carbon; the C=O π pair moves onto oxygen to form a tetrahedral intermediate.
This first step is nucleophilic addition. It temporarily changes the carbonyl carbon from trigonal planar to tetrahedral.
Eliminate chloride and adjust protons
The oxygen lone pair reforms the C=O bond and chloride leaves. This is elimination, giving the substitution product after any necessary proton transfer.
For the listed overall reactions, the outcomes are a carboxylic acid, ester or amide plus HCl. With excess ammonia or amine, a second base molecule accepts the HCl. The overall pathway is addition–elimination, not a one-step displacement at an sp³ carbon.
Pause and explain
What happens after the tetrahedral intermediate forms?
Worked example
Outline the mechanism when an alcohol reacts with an acyl chloride.
Show the worked solution
- The alcohol oxygen lone pair attacks the carbonyl carbon.
- The C=O π bond moves to oxygen, forming a tetrahedral intermediate.
- The carbonyl reforms as chloride leaves; proton transfer gives ester and HCl.
Answer Nucleophilic addition forms a tetrahedral intermediate; elimination restores C=O and expels Cl⁻, then proton transfer gives the ester and HCl.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Put the key steps for water reacting with an acyl chloride in order.
Give me a hint
Follow the carbonyl π bond and then the leaving group.
Compare my reasoning
- Water attacks the carbonyl carbon.
- The π pair moves to oxygen, forming a tetrahedral intermediate.
- C=O reforms and Cl⁻ leaves; proton transfer gives acid and HCl.
Nucleophilic addition → tetrahedral intermediate → elimination of chloride → proton transfer.
Look for these in your work
- Nucleophilic attack
- Intermediate
- Leaving group and product
Explain why the same broad mechanism applies to alcohol, phenol, ammonia and amine reactions.
Give me a hint
Identify what those reagents have in common at the reacting atom.
Compare my reasoning
- Each has a lone pair on O or N.
- That lone pair attacks the acyl carbon.
- The carbonyl reforms as chloride leaves.
Each reagent provides an O or N lone pair nucleophile; addition to the carbonyl followed by chloride elimination produces substitution.
Look for these in your work
- Lone pair
- Acyl carbon
- Addition–elimination
A student draws direct attack at chlorine followed by loss of the whole carbonyl group. Correct the mechanism’s reaction centre.
Give me a hint
The acyl carbonyl is the electrophile.
Compare my reasoning
- The nucleophile attacks the carbonyl carbon.
- The C=O π electrons move to oxygen.
- Chloride leaves when the carbonyl reforms.
Attack occurs at the carbonyl carbon, producing a tetrahedral intermediate; chloride is expelled during elimination while the carbonyl is restored.
Look for these in your work
- Attack site
- Intermediate
- Correct leaving group
Common mistakes
- Calling the first step electrophilic addition
- Forgetting the C=O π bond moves onto oxygen
- Skipping the tetrahedral intermediate
- Showing chloride attack as the nucleophile
What can you explain now?
What happens in the first step of acyl-chloride addition–elimination?
Compare with the explanation
A nucleophile attacks the carbonyl carbon
The carbonyl carbon is the electrophilic site.
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Make it stick
Which bond changes show the addition and elimination stages in an acyl-chloride mechanism?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.