Warm an alcohol with acidified dichromate and watch the colour. Primary and secondary alcohols turn it green, tertiary alcohols leave it orange, and how you set up the apparatus decides whether you collect an aldehyde or an acid.
Acidified potassium dichromate(VI) is a mild oxidising agent; when it oxidises an alcohol, orange Cr2O72− is reduced to green Cr3+. A primary alcohol is oxidised to an aldehyde, which must be distilled off as it forms; with excess oxidant heated under reflux it goes on to a carboxylic acid. A secondary alcohol gives a ketone. A tertiary alcohol is not oxidised, because the C–OH carbon has no hydrogen. The tri-iodomethane test (alkaline iodine, warm) gives a pale yellow precipitate of CHI3 with alcohols containing CH3CH(OH)–.
Warm each class of alcohol with acidified dichromate in the simulator. Switch between distillation and reflux for the primary alcohol and compare the products.
| Alcohol | Conditions | Product | Equation using [O] |
|---|---|---|---|
| ethanol (primary) | distil off as it forms | ethanal | CH3CH2OH + [O] → CH3CHO + H2O |
| ethanol (primary) | excess oxidant, heat under reflux | ethanoic acid | CH3CH2OH + 2[O] → CH3COOH + H2O |
| propan-2-ol (secondary) | heat | propanone | CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O |
| 2-methylpropan-2-ol (tertiary) | heat | no reaction | stays orange |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What colour change is seen when acidified potassium dichromate(VI) oxidises an alcohol?
What is the product when propan-2-ol is oxidised by acidified dichromate(VI)?
Why is 2-methylpropan-2-ol not oxidised by acidified dichromate(VI)?
Which alcohol gives a yellow precipitate with alkaline iodine?
Add alkaline iodine solution to several alcohols in the simulator and warm. Note which give a yellow precipitate and look for the shared structural feature.
| Compound | Contains CH3CH(OH)– or CH3CO–? | Result |
|---|---|---|
| ethanol | yes | pale yellow CHI3 |
| propan-2-ol, butan-2-ol | yes | pale yellow CHI3 |
| ethanal, propanone | yes (CH3CO–) | pale yellow CHI3 |
| methanol, propan-1-ol | no | no precipitate |
| 2-methylpropan-2-ol | no | no precipitate |
Distillation gives the aldehyde; reflux gives the carboxylic acid. The colour change is orange to green. Tertiary alcohols are not oxidised because there is no hydrogen on the carbon bearing the OH. The tri-iodomethane test needs a CH3 group next to the CH(OH), so propan-1-ol is negative.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Ethanol is heated under reflux with excess acidified dichromate(VI). What is the main organic product?
How is ethanal obtained rather than ethanoic acid when ethanol is oxidised?
Which compound gives BOTH a green colour with acidified dichromate(VI) AND a yellow precipitate with alkaline iodine?
Which equation shows the oxidation of propan-1-ol to propanoic acid?
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