Connect acid preparation, esterification and reduction
Track a carboxylic acid through oxidation, ester formation and LiAlH₄ reduction without losing its carbon skeleton.
Section 18 · Carboxylic acids and derivatives
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Track the carbon skeleton, name reagents and conditions, and show products and observations. Written practice is self-checked
Before you begin
- Use alcohol oxidation and carbonyl chemistry from Topics 16–17, functional-group names, acid–base equations and balanced reaction formulae.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reagent changes a carboxylic acid back to a primary alcohol?
What you will learn
- Plan a multi-step route
- Match reagents to functional groups
- Check acid/alcohol and carbon counts
Connect oxidation and reduction
Primary alcohol → aldehyde → carboxylic acid by acidified dichromate(VI)/manganate(VII) and reflux. LiAlH₄ reduces the acid back to the primary alcohol.
The carbon chain is retained in both directions; distil if the aldehyde is the target rather than continuing to the acid.
Branch into ester routes
An acid combines with an alcohol under concentrated H₂SO₄ catalysis to form an ester and water; hydrolysis reverses the ester into acid/alcohol.
Nitrile and ester hydrolysis offer additional acid-preparation routes. Include acidification after alkaline hydrolysis.
Pause and explain
What happens to the carbon skeleton during these oxidation and reduction routes?
Worked example
Plan a route from ethanol to ethyl ethanoate, then state how to obtain ethanoic acid from the ester.
Show the worked solution
- Oxidise ethanol fully under reflux to ethanoic acid.
- React the acid with ethanol using concentrated H₂SO₄ catalyst.
- Hydrolyse the ester with dilute acid and heat.
Answer Ethanol → ethanoic acid (acidified oxidant, reflux); ethanoic acid + ethanol → ethyl ethanoate + water (conc. H₂SO₄ catalyst); hydrolysis with dilute acid and heat returns ethanoic acid and ethanol.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Plan from butan-1-ol to butanoic acid and then back to an alcohol.
Give me a hint
Use oxidation under reflux, followed by LiAlH₄ reduction.
Compare my reasoning
- Primary alcohol oxidation gives an aldehyde intermediate.
- Reflux with acidified oxidant continues to butanoic acid.
- LiAlH₄ reduces it to butan-1-ol after work-up.
Butan-1-ol → butanoic acid (acidified K₂Cr₂O₇/KMnO₄, reflux) → butan-1-ol (LiAlH₄, work-up).
Look for these in your work
- Oxidation conditions correct
- Acid product correct
- Reduction product correct
A three-carbon acid is esterified with propanol. Name the ester and hydrolysis products.
Give me a hint
The alcohol contributes propyl; the acid contributes propanoate.
Compare my reasoning
- The ester is propyl propanoate.
- Hydrolysis regenerates propanoic acid and propanol.
- Alkaline hydrolysis first gives propanoate, then acidification gives acid.
Propyl propanoate; hydrolysis gives propanoic acid and propanol (or propanoate before acidification).
Look for these in your work
- Ester named
- Products recovered
- Alkaline work-up noted
Which route is suitable for converting a nitrile to an acid, and what happens to the nitrile carbon?
Give me a hint
Use hydrolysis and count the –CN carbon.
Compare my reasoning
- Heat with dilute acid or dilute alkali.
- Acidify after the alkaline route.
- The nitrile carbon becomes the carboxyl carbon.
Nitrile hydrolysis gives the corresponding carboxylic acid; after alkaline hydrolysis acidify, and retain the –CN carbon as –COOH.
Look for these in your work
- Hydrolysis conditions stated
- Acidification included
- Carbon tracked
Common mistakes
- Distilling when the acid is the oxidation target
- Using LiAlH₄ to make an acid
- Confusing esterification and hydrolysis conditions
What can you explain now?
Which reagent changes a carboxylic acid back to a primary alcohol?
Compare with the explanation
LiAlH₄
It reduces –COOH to terminal –CH₂OH.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do oxidation, hydrolysis, esterification and reduction connect the acid functional group to other compounds?
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.