Choose PCl₅ or SOCl₂ to form an acyl chloride
Recognise PCl₅ or SOCl₂ as alternative reagents for converting a carboxylic acid into 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
Which pair are alternative reagents for forming an acyl chloride from a carboxylic acid?
What you will learn
- Recall PCl₅ and SOCl₂ as acyl-chloride preparation reagents
- Write the products for each reagent route
- Distinguish these routes from the PCl₃-and-heat route
Two alternative reagents
A carboxylic acid can form an acyl chloride with PCl₅ or with thionyl chloride, SOCl₂. Both replace the acid –OH with Cl.
RCOOH + PCl₅ → RCOCl + POCl₃ + HCl. With SOCl₂: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
Keep reagent-specific products distinct
The syllabus names PCl₅ or SOCl₂ as alternatives; it does not pair these with the PCl₃-specific heat condition.
When asked for all routes, state PCl₃ and heat, PCl₅, or SOCl₂. Check atom balance and name the acyl chloride product rather than an alkyl chloride.
Pause and explain
What gaseous by-products accompany the SOCl₂ route?
Worked example
Convert propanoic acid to propanoyl chloride using SOCl₂ and give the by-products.
Show the worked solution
- Keep the propanoyl skeleton intact.
- Replace –OH by Cl.
- Account for the sulfur reagent as SO₂ and HCl.
Answer CH₃CH₂COOH + SOCl₂ → CH₃CH₂COCl + SO₂ + HCl.
Try a topic test
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Test this topic →From guided practice to a new situation
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 the SOCl₂ reaction with ethanoic acid.
Give me a hint
Replace –OH by Cl and balance the sulfur-containing products.
Compare my reasoning
- The organic product is CH₃COCl.
- SOCl₂ contributes S and O to SO₂.
- The removed acid –OH and reagent chlorine account for HCl overall.
CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl.
Look for these in your work
- Acyl chloride correct
- SO₂ included
- HCl included
List the syllabus routes from a carboxylic acid to an acyl chloride.
Give me a hint
State each reagent and the one explicitly paired condition.
Compare my reasoning
- PCl₃ with heat.
- PCl₅ as an alternative.
- SOCl₂ as another alternative.
Use PCl₃ and heat, or use PCl₅, or use SOCl₂.
Look for these in your work
- PCl₃ route
- Heat paired correctly
- Both alternatives stated
A learner gives RCOOH + SOCl₂ → RCOCl + SO₃ + HCl. Identify the atom-balance problem.
Give me a hint
Count oxygen atoms on both sides.
Compare my reasoning
- The reactants contain two oxygen atoms total.
- RCOCl contains one oxygen, leaving one oxygen for the sulfur oxide.
- The balanced by-product is SO₂, not SO₃.
Use SO₂: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
Look for these in your work
- Oxygen counted
- Correct oxide named
- Equation balanced
Common mistakes
- Mixing the PCl₃ and PCl₅ by-products
- Omitting SO₂ or HCl from the SOCl₂ equation
- Writing an alkyl chloride
- Treating PCl₅ and SOCl₂ as sequential reagents
What can you explain now?
Which pair are alternative reagents for forming an acyl chloride from a carboxylic acid?
Compare with the explanation
PCl₅ or SOCl₂
Either reagent can replace the carboxyl –OH with chlorine.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What three reagent options form acyl chlorides from carboxylic acids, and which one is heated?
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.