Use phosphorus(III) chloride to make an acyl chloride
Recall that a carboxylic acid reacts with PCl₃ and heat to replace its –OH group with chlorine and form an acyl chloride.
Section 33 · Carboxylic acids and derivatives
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 condition is specified when PCl₃ converts a carboxylic acid to an acyl chloride?
What you will learn
- Recall PCl₃ and heat as an acyl-chloride preparation route
- Identify replacement of the acid –OH by chlorine
- Recognise phosphorous acid as the balanced-equation by-product
Replace the carboxyl –OH
A carboxylic acid reacts with phosphorus(III) chloride, PCl₃, on heating to form an acyl chloride. The carbon skeleton stays the same; the –OH in –COOH is replaced by Cl.
For ethanoic acid: 3CH₃COOH + PCl₃ → 3CH₃COCl + H₃PO₃. The equation shows three acid molecules provide the three chlorine atoms in PCl₃.
Use the specified condition
The syllabus explicitly pairs PCl₃ with heat. Name the reagent and condition when describing the route.
The reaction produces an acyl chloride, not a halogenoalkane: chlorine is attached to the carbonyl carbon in RCOCl.
Pause and explain
Which bond changes when RCOOH forms RCOCl?
Worked example
Give the reagent and condition for converting ethanoic acid to ethanoyl chloride using PCl₃.
Show the worked solution
- Choose phosphorus(III) chloride.
- Apply heat, as stated for this reagent.
- Replace the carboxyl –OH with Cl.
Answer Heat ethanoic acid with PCl₃ to form ethanoyl chloride.
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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.
Complete: 3CH₃COOH + PCl₃ → …
Give me a hint
Each PCl₃ molecule contains three chlorine atoms.
Compare my reasoning
- Form ethanoyl chloride from each acid molecule.
- Use three CH₃COCl molecules.
- Balance the phosphorus-containing by-product as H₃PO₃.
3CH₃COOH + PCl₃ → 3CH₃COCl + H₃PO₃.
Look for these in your work
- Acyl chloride formula
- Chlorine count balanced
- By-product balanced
Compare the structures of ethanoic acid and ethanoyl chloride.
Give me a hint
Focus on the group attached to the carbonyl carbon.
Compare my reasoning
- Ethanoic acid contains –C(=O)OH.
- Ethanoyl chloride contains –C(=O)Cl.
- The methyl group is unchanged.
The –OH group is replaced by Cl; CH₃COOH becomes CH₃COCl.
Look for these in your work
- Functional groups identified
- Substitution stated
- Carbon skeleton preserved
A response says ‘ethanoic acid reacts with PCl₃ under UV to form chloroethane’. Correct the reagent condition and product class.
Give me a hint
Check both the stated condition and which carbon bears chlorine.
Compare my reasoning
- PCl₃ is used with heat.
- The carboxyl –OH is replaced by Cl.
- The product is an acyl chloride, ethanoyl chloride.
Use PCl₃ and heat to make ethanoyl chloride, CH₃COCl; it is not chloroethane and UV is not the stated condition.
Look for these in your work
- Condition corrected
- Product class corrected
- Structure distinguished
Common mistakes
- Writing PCl₅ when the question specifies PCl₃
- Omitting heat
- Replacing a hydrogen on the alkyl chain
- Calling the product chloroethane
What can you explain now?
What condition is specified when PCl₃ converts a carboxylic acid to an acyl chloride?
Compare with the explanation
Heat
The syllabus names PCl₃ and heat.
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Make it stick
What does PCl₃ do to a carboxylic acid, and what condition accompanies it?
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