IB Chemistry HLTopic 6 — Electron TransferPaper 1 & 2Organic~11 min read
Oxidation of Alcohols
Whether an alcohol can be oxidised, and how far, comes down to one thing you can spot in two seconds: how many carbon atoms are attached to the carbon holding the OH group. Count them, and you already know the answer.
📚 What you need to know
Alcohols are primary, secondary or tertiary depending on how many carbons are joined to the C—OH carbon.
The oxidising agent is acidified potassium dichromate(VI), written as [O] in equations.
The colour change is orange to green as Cr2O72− is reduced to Cr3+.
Distil to collect the aldehyde as it forms; reflux to push all the way to the carboxylic acid.
Tertiary alcohols resist because there is no hydrogen on the C—OH carbon to remove.
Classifying an alcohol
Propan-1-ol is primary, propan-2-ol is secondary, and 2-methylpropan-2-ol is tertiary. Same three carbons, completely different chemistry.
The real reason tertiary alcohols will not oxidise is worth understanding rather than memorising. Oxidation here means removing a hydrogen from the C—OH carbon along with the one on the oxygen. A tertiary carbon has three carbons and the OH taking up all four bonds — there is no hydrogen left on it to take. Breaking a C—C bond instead would need far harsher conditions.
Where each alcohol can go
Because tertiary alcohols leave the mixture orange, this is a quick test: orange after warming means tertiary, green means primary or secondary.
Distil or reflux?
With a primary alcohol you get to choose the product, and the choice is made by the glassware.
🧩 Picking the apparatus
Want the aldehyde? Use distillation. The aldehyde has no hydrogen bonding between its molecules, so it boils below the alcohol and can be removed the moment it forms.
Add the oxidising agent slowly and keep the mixture only warm, so the aldehyde escapes before it is attacked again.
Want the carboxylic acid? Use reflux with excess oxidising agent.
In reflux the vapour condenses and drips back, so nothing escapes until it has been fully oxidised.
Then distil off the carboxylic acid at the end.
Why the aldehyde boils lowest: an alcohol has an O—H group and hydrogen bonds to its neighbours. An aldehyde has a C=O but no O—H, so it cannot hydrogen bond to itself. Weaker forces between molecules means a lower boiling point — and that is what lets you distil it away.
Primary, all the way (reflux)
CH3CH2OH + 2[O] → CH3COOH + H2O
Secondary
CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O
Worked examples
WORKED EXAMPLE
Butan-2-ol is warmed with acidified potassium dichromate(VI) under reflux. Name the organic product, write the equation and give the colour change.
Step 1: Classify the alcoholThe OH is on carbon 2, which is joined to two other carbons. Secondary.Step 2: Secondary alcohols stop at the ketoneReflux makes no difference here — there is nowhere further to go.CH3CH(OH)CH2CH3 + [O] → CH3COCH2CH3 + H2OStep 3: The dichromate is reducedButanone; orange to greenthe green is Cr3+, formed as Cr goes from +6 down to +3
WORKED EXAMPLE
A student wants ethanal, not ethanoic acid, from ethanol. Describe how they should set the experiment up and explain why it works.
Step 1: Choose the apparatusDistillation, not reflux, so the product leaves the flask.Step 2: Control the conditionsWarm gently and add the dichromate dropwise, keeping the alcohol in excess.Step 3: Explain why it worksEthanal has no O—H group, so no hydrogen bonding between its molecules and a lower boiling point than ethanol.Distil the ethanal off as soon as it forms, before it can be oxidised againthe boiling point argument is the part that earns the explanation mark
WORKED EXAMPLE
Three unlabelled alcohols are warmed with acidified dichromate(VI). Sample A stays orange, B turns green and gives a product with no O—H in its infrared spectrum, C turns green and gives a broad O—H peak. Identify each.
Step 1: Sample ANo colour change means no oxidation at all.A is tertiaryStep 2: Sample BIt oxidised, but the product has a C=O and no O—H, so it is a ketone.B is secondaryStep 3: Sample CA broad O—H peak with a C=O means a carboxylic acid, which only comes from a primary alcohol.A tertiary, B secondary, C primarythe colour change alone cannot separate primary from secondary — you need the product
💡 Exam tip
Use [O] in equations, and check the equation still balances for atoms afterwards.
Give the colour change as orange to green, and name the ions if the question is worth two marks.
Name the apparatus precisely: distillation for the aldehyde, reflux for the acid.
Explain tertiary alcohols with “no hydrogen on the carbon bearing the OH“, not just “it is tertiary”.
Classify from the structure, not the name. Some names look primary and are not.
If asked for the reagent in full, write acidified potassium dichromate(VI) and specify dilute sulfuric acid.
⚠ Common mix-up
Counting hydrogens instead of carbons. The classification is about carbons attached to the C—OH carbon.
Thinking reflux turns a secondary alcohol into an acid. It stops at the ketone whatever you do.
Forgetting the water on the right of the equation.
Saying the colour change is green to orange. It is orange to green — dichromate is the orange one.
Mixing up distillation and reflux. Distil to remove a product, reflux to keep it in.
Assuming methanol behaves differently. It has no carbons attached, but it still behaves as a primary alcohol.
Up next: Reducing Carboxylic Acids, Aldehydes and Ketones — everything on this page, run in reverse. Same ladder, opposite direction, different reagents.
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