Predict reactions of molecules with several functional groups
Use the stated reagent and conditions to predict which group reacts and what remains in the product.
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
What should guide the predicted reaction of a molecule with several groups?
What you will learn
- Predict reactions from functional-group evidence
- Apply conditions to select the intended transformation
- Track unchanged groups and limitations
Match conditions to the target group
A molecule can contain several functional groups, but a question gives reagents and conditions to identify a reaction in the syllabus. For a primary alcohol, distillation can give an aldehyde while reflux can allow oxidation to continue to a carboxylic acid.
A carboxylic acid can react with an alcohol using concentrated sulfuric acid as catalyst to form an ester. A nitrile can be hydrolysed to a carboxylic acid or reduced to an amine; these are different transformations with different reagents.
Track the rest of the molecule
Change the bond or group targeted by the stated reaction and keep the other parts of the structure unless another specified step changes them. For example, hydrolysing a nitrile in a molecule that also has a separate alcohol group does not automatically remove that alcohol.
Do not assume perfect selectivity beyond the syllabus statement. If conditions could affect another group, describe the intended syllabus transformation and flag any selectivity the question does not establish.
Pause and explain
During nitrile hydrolysis, what happens to a separate alcohol group not targeted by the stated reaction?
Worked example
Predict the product when CH₃CH(OH)CN is hydrolysed with dilute acid.
Show the worked solution
- Identify –CN as the group being hydrolysed.
- The nitrile carbon becomes the carboxyl carbon.
- Retain the existing –OH group and three-carbon total.
Answer 2-hydroxypropanoic acid, CH₃CH(OH)COOH.
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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.
Predict the product when CH₃CH₂CN is hydrolysed with dilute acid.
Give me a hint
Convert –CN to –COOH and retain the nitrile carbon.
Compare my reasoning
- The two-carbon ethyl fragment remains.
- The nitrile carbon becomes the acid carbon.
- Name the three-carbon carboxylic acid.
Propanoic acid, CH₃CH₂COOH.
Look for these in your work
- Nitrile conversion correct
- Carbon count retained
- Product named
A primary alcohol is oxidised with acidified dichromate under reflux. What product class is targeted?
Give me a hint
Reflux favours further oxidation of the aldehyde intermediate.
Compare my reasoning
- A primary alcohol first forms an aldehyde.
- Under reflux oxidation can continue.
- The target is a carboxylic acid.
A carboxylic acid is the target product under reflux.
Look for these in your work
- Primary alcohol recognized
- Condition interpreted
- Product class correct
A molecule contains both an alkene and a carboxylic acid. Hydrogen and nickel are supplied with heat. Which change is supported by the syllabus reaction?
Give me a hint
Identify the group targeted by hydrogenation.
Compare my reasoning
- Hydrogenation adds H₂ across C=C.
- The alkene becomes a saturated C–C bond.
- Do not remove the separate –COOH group without a stated reaction for it.
The alkene C=C is hydrogenated; the separate carboxylic acid group is retained in the predicted syllabus transformation.
Look for these in your work
- Target group identified
- Hydrogenation described
- Other group retained
Common mistakes
- Changing an unrelated group
- Ignoring the specified oxidation condition
- Dropping the nitrile carbon
- Claiming more selectivity than the route supports
What can you explain now?
What should guide the predicted reaction of a molecule with several groups?
Compare with the explanation
The specified reagent and conditions
They identify the syllabus transformation being asked about.
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Make it stick
How do reagent, conditions and the untargeted groups constrain a product prediction?
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.