Oxidation of alcohols and preparative techniquesEdexcel International A Level Chemistry: Revision notes

Acidified potassium dichromate(VI), K₂Cr₂O₇ in dilute sulfuric acid, is an oxidising agent. Orange Cr₂O₇²⁻ is reduced to green Cr³⁺, so a colour change from orange to green shows an alcohol (or aldehyde) was oxidised. In equations the oxidising agent can be written as [O].

The product depends on the type of alcohol:

• Primary alcohol: aldehyde, then carboxylic acid
• Secondary alcohol: ketone (no further oxidation)
• Tertiary alcohol: no reaction (no H on the C–OH carbon); the solution stays orange

For a primary alcohol:

• Distil the product as it forms to get the aldehyde: CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O. The aldehyde has a lower boiling temperature (no hydrogen bonding between its molecules), so it leaves before it can be oxidised further.
• Heat under reflux with excess oxidising agent to get the carboxylic acid: CH₃CH₂CHO + [O] → CH₃CH₂COOH.

Secondary alcohol: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O (propanone).

• Aldehyde versus ketone: warm with Fehling's or Benedict's solution. An aldehyde gives a brick-red precipitate (blue solution changes to red); a ketone gives no change.
• Carboxylic acid: add sodium carbonate or sodium hydrogencarbonate. Acids give effervescence of CO₂.

Together with the dichromate colour change, these tests let you tell primary, secondary and tertiary alcohols apart.

Techniques for an organic liquid:

1. Heating under reflux: for slow reactions, without losing volatile reactants.
2. Extraction with a solvent in a separating funnel: shake, allow layers to separate, run off the lower layer.
3. Drying with an anhydrous salt (anhydrous sodium sulfate, magnesium sulfate or calcium chloride) to remove water; decant or filter.
4. Distillation: collect the fraction at the expected boiling temperature.
5. Boiling temperature determination: a pure liquid boils at a constant temperature close to the data value; impurities lower or widen the range.

Propanal (bp 49 °C): add propan-1-ol dropwise to warm acidified dichromate(VI); distil off propanal as it forms into a cooled receiver. Test it with Fehling's solution.

Propanoic acid (bp 141 °C): heat propan-1-ol with excess acidified dichromate(VI) under reflux (anti-bumping granules), then distil and collect the fraction around 141 °C. Test with sodium carbonate.

Safety: dichromate(VI) is toxic and a carcinogen, propan-1-ol and propanal are flammable, so use a heating mantle or water bath rather than a naked flame.