Integrate a multi-step route to a hydroxycarboxylic acid
Combine carbonyl addition and nitrile hydrolysis to plan and explain a route with a one-carbon extension.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 36. Routes use reactions listed in the syllabus and state the relevant reagents and conditions. Check every intermediate and possible by-product; no route implies a practical procedure. Written practice is self-checked
Before you begin
- Use the functional-group reactions from Topics 13–19 and 29–35, including their reagents, conditions, products and limitations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which two functional groups are present in 2-hydroxypropanoic acid?
What you will learn
- Identify the groups in intermediates
- Plan a two-step route to a multi-functional target
- Analyse reaction types, conditions and by-products
Plan from the target groups
A hydroxycarboxylic acid contains both an alcohol –OH and a carboxylic acid –COOH. One route is to form a hydroxynitrile from an aldehyde or ketone using HCN with KCN catalyst and heat, then hydrolyse its –CN group.
The first step is nucleophilic addition to the carbonyl; the second is nitrile hydrolysis. The carbon from HCN becomes the carboxyl carbon, while the –OH formed in the first step remains in the target.
Explain each arrow separately
For ethanal → 2-hydroxypropanenitrile, HCN adds across C=O and introduces a carbon; KCN is a catalyst and heat is specified. Hydrolysis with dilute acid then converts the nitrile into 2-hydroxypropanoic acid.
A route analysis should also note that acid hydrolysis gives an ammonium salt by-product; alkaline hydrolysis requires acidification if the final product requested is the carboxylic acid. Confirm the carbon count before accepting the target name.
Pause and explain
Which reaction type is the first step from ethanal to a hydroxynitrile?
Worked example
Plan and analyse a route from ethanal to 2-hydroxypropanoic acid.
Show the worked solution
- Add HCN with KCN catalyst and heat to make 2-hydroxypropanenitrile.
- Hydrolyse the nitrile with dilute acid.
- Identify the added carbon and retain the hydroxy group.
Answer Step 1: nucleophilic addition, HCN/KCN catalyst/heat, gives 2-hydroxypropanenitrile. Step 2: nitrile hydrolysis with dilute acid gives 2-hydroxypropanoic acid and an ammonium salt.
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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.
Give the structures and reaction types for ethanal → 2-hydroxypropanenitrile → 2-hydroxypropanoic acid.
Give me a hint
Name the intermediate and label addition before hydrolysis.
Compare my reasoning
- HCN/KCN catalyst/heat adds across ethanal C=O.
- The product is CH₃CH(OH)CN.
- Hydrolyse –CN to obtain CH₃CH(OH)COOH.
Ethanal —nucleophilic addition (HCN, KCN catalyst, heat)→ CH₃CH(OH)CN —hydrolysis→ CH₃CH(OH)COOH.
Look for these in your work
- Correct intermediate
- Reaction types ordered
- Target structure correct
What happens to the carbon atoms during this route from ethanal to 2-hydroxypropanoic acid?
Give me a hint
Count the carbonyl substrate and HCN contribution.
Compare my reasoning
- Ethanal has two carbons.
- HCN contributes one carbon to the nitrile.
- Hydrolysis keeps that carbon as the carboxyl carbon.
The route increases the carbon count from two to three; HCN supplies the carbon that becomes the carboxyl carbon.
Look for these in your work
- Starting count correct
- Added carbon identified
- Hydrolysis retention stated
Analyse the likely by-product class from acid hydrolysis of 2-hydroxypropanenitrile.
Give me a hint
Account for the nitrile nitrogen under acidic conditions.
Compare my reasoning
- The organic product is a hydroxycarboxylic acid.
- The nitrile nitrogen is protonated during acid hydrolysis.
- An ammonium salt forms in the acidic mixture.
2-hydroxypropanoic acid forms with an ammonium salt as a nitrogen-containing by-product; the exact counterion depends on the acid used.
Look for these in your work
- Organic target identified
- Nitrogen accounted
- Counterion qualified
Common mistakes
- Calling the first step hydrolysis
- Skipping the hydroxynitrile intermediate
- Losing the HCN carbon
- Changing the existing hydroxy group during nitrile hydrolysis
What can you explain now?
Which two functional groups are present in 2-hydroxypropanoic acid?
Compare with the explanation
Alcohol –OH and carboxylic acid –COOH
The target contains both hydroxy and acid groups.
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Make it stick
How do you connect the target groups to route steps and account for carbon and nitrogen atoms?
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.