Distinguish alcohol reactions in a mixed pathway
Use reagents, conditions and observations to separate oxidation, dehydration, substitution, esterification and iodoform reactions.
Section 16 · Hydroxy compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked
Before you begin
- Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which observation indicates oxidation by acidified dichromate?
What you will learn
- Identify the reaction from its reagents
- Predict a product and observation
- Explain a multi-step route without mixing conditions
Reagent clues identify the transformation
Acidified dichromate or manganate(VII) oxidises alcohols; distillation favors aldehydes from primary alcohols while reflux favors carboxylic acids. Secondary alcohols give ketones; tertiary alcohols resist oxidation in the syllabus model.
A heated catalyst or concentrated acid dehydrates an alcohol to an alkene. Concentrated H₂SO₄ catalyses esterification with a carboxylic acid. HX or chlorinating agents substitute –OH to make a halogenoalkane.
Use the observation only for its intended inference
Acidified dichromate changes orange to green when a primary or secondary alcohol is oxidised. Alkaline iodine gives yellow CHI₃ from CH₃CH(OH)–R.
Read all reagents and conditions before predicting. Orange-to-green shows oxidation but does not by itself tell primary from secondary; yellow iodoform supports one specific structure.
Pause and explain
Which test gives yellow triiodomethane from a methyl carbinol structure?
Worked example
An unknown alcohol is warmed with acidified dichromate, which changes orange to green; the product distils as an aldehyde. What class and apparatus choice are consistent?
Show the worked solution
- A positive colour change indicates oxidation.
- An aldehyde from oxidation is consistent with a primary alcohol.
- Distillation removes aldehyde before further oxidation.
Answer A primary alcohol is consistent; distillation is used to obtain its aldehyde and limit further oxidation.
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.
A primary alcohol is to be converted first to an aldehyde and then to a carboxylic acid. State the two apparatus choices.
Give me a hint
Control how long the aldehyde stays with oxidant.
Compare my reasoning
- Distil to remove aldehyde for the first product.
- Reflux for continued oxidation to the acid.
- Use an acidified oxidising agent in both steps.
Distil to obtain aldehyde; reflux to obtain carboxylic acid.
Look for these in your work
- First condition correct
- Second condition correct
- Reason linked
An alcohol gives orange-to-green dichromate and yellow CHI₃ with alkaline iodine. What can each result indicate?
Give me a hint
The two reagents test different features.
Compare my reasoning
- Dichromate indicates oxidation of a primary or secondary alcohol.
- Iodoform indicates CH₃CH(OH)–R.
- Neither observation alone gives every structural detail.
It is oxidisable (primary or secondary) and contains CH₃CH(OH)–R; the results test different properties.
Look for these in your work
- Tests kept separate
- Structures inferred
- Inference limits stated
A route begins with a tertiary alcohol and proposes a carboxylic acid using mild acidified dichromate. Explain the problem.
Give me a hint
Check whether a tertiary alcohol is oxidised in this syllabus model.
Compare my reasoning
- Tertiary alcohols cannot be oxidised under the stated model.
- The proposed reagent does not change that class-specific outcome.
- Choose a different starting material or route for a carboxylic acid.
The route is inconsistent: a tertiary alcohol cannot be oxidised to a carboxylic acid under the syllabus conditions.
Look for these in your work
- Class recognized
- Oxidation outcome corrected
- Route constraint stated
Common mistakes
- Using one observation to claim more than it proves
- Confusing a yellow iodoform precipitate with a silver halide
- Ignoring whether the reaction is distilled or refluxed
What can you explain now?
Which observation indicates oxidation by acidified dichromate?
Compare with the explanation
Orange to green
Dichromate(VI) is reduced during oxidation of primary or secondary alcohols.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do you distinguish oxidation, dehydration, esterification and the iodoform reaction from their reagents and observations?
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.