Acidified potassium dichromate

Acidified potassium dichromate(VI) is potassium dichromate, K₂Cr₂O₇, dissolved with dilute sulfuric acid. It is an oxidising agent used to oxidise primary and secondary alcohols and aldehydes. When it reacts, it changes colour from orange to green, as the orange dichromate(VI) ions are reduced to green chromium(III) ions. Tertiary alcohols and ketones are not oxidised, so the solution stays orange.

What is acidified potassium dichromate?

Potassium dichromate is an orange solid. In solution it gives the dichromate(VI) ion, Cr₂O₇²⁻, in which chromium has an oxidation number of +6. That is what the (VI) in the name means: see what is oxidation for how to work it out.

It is acidified because the reaction needs hydrogen ions: you can see 14 H⁺ in the half equation below. Dilute sulfuric acid is used to supply them. Chromium(VI) compounds are toxic and are classed as carcinogens, so they are handled with gloves and eye protection, in small amounts.

The colour change: orange to green

When dichromate(VI) oxidises something, it gains electrons and is itself reduced. Each chromium falls from +6 to +3, so the pair of chromium atoms gains 6 electrons:

Check the balance: there are 7 oxygens on each side, 14 hydrogens on each side, and the charge is (−2) + 14 + (−6) = +6 on the left and 2 × (+3) = +6 on the right. How to build half equations like this one is covered in half equations.

Oxidising alcohols with acidified potassium dichromate

AQA A-level Chemistry names acidified potassium dichromate(VI) as a suitable oxidising agent for alcohols. What you get depends on the type of alcohol: how many carbon atoms are bonded to the carbon holding the OH group.

In equations, the oxidising agent is written as [O], which stands for an oxygen atom from the dichromate. For ethanol:

For the secondary alcohol pentan-3-ol, the product is the ketone pentan-3-one:

A tertiary alcohol has no hydrogen atom on the carbon bonded to the OH group, so it cannot lose one to form a C=O bond. It is not easily oxidised, and the dichromate stays orange.

Aldehyde or carboxylic acid: distillation or reflux

A primary alcohol is oxidised in two steps, so the method decides where it stops.

To get the aldehyde: distil it off as it forms

Use excess alcohol and only a little dichromate, warm gently, and distil the product out of the flask straight away. Aldehydes have much lower boiling points than the alcohols they come from, because there are no hydrogen bonds between aldehyde molecules, only weaker forces. Ethanal boils at about 20 °C and ethanol at about 78 °C, so the ethanal leaves as a vapour before it can be oxidised further.

To get the carboxylic acid: heat under reflux

Use excess acidified dichromate and heat the mixture under reflux, with a condenser fitted vertically above the flask. Any aldehyde that boils off condenses and drips back into the flask, where it keeps meeting the oxidising agent until it is all turned into the carboxylic acid. The acid can then be distilled off at the end. Overall:

Using dichromate to identify an alcohol

Warming an unknown alcohol with acidified potassium dichromate is a quick test:

1. Warm a few drops of the alcohol with acidified potassium dichromate(VI) in a water bath.
2. If it stays orange, the alcohol is tertiary.
3. If it turns green, it is primary or secondary. Distil off the product and test it with Tollens' reagent or Fehling's solution.
4. An aldehyde gives a silver mirror with Tollens' reagent, or a brick-red precipitate with Fehling's solution, so the alcohol was primary. A ketone gives no change, so the alcohol was secondary.

Acidified dichromate also oxidises aldehydes to carboxylic acids, turning green, while ketones leave it orange. That gives another way to tell aldehydes and ketones apart.

Check your understanding.

Worked example: planning a synthesis

You want to make butanoic acid, CH₃CH₂CH₂COOH, from an alcohol. Which alcohol, and which conditions?

• A carboxylic acid comes from oxidising a primary alcohol with the same carbon chain, so use butan-1-ol, CH₃CH₂CH₂CH₂OH.
• You want the full oxidation, so use excess acidified potassium dichromate(VI) and heat under reflux.
• Equation: CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O.
• Observation: orange to green.

Acidified dichromate in breathalysers

The original Breathalyzer, invented by Robert Borkenstein of the Indiana State Police in 1954, used this reaction. Breath was bubbled through a solution containing potassium dichromate; any ethanol reduced the orange dichromate to green chromium(III), and the amount of colour change showed how much alcohol was present. Modern breath testers use infrared spectroscopy or electrochemical sensors instead.

Common mistakes

• Getting the colours backwards. It starts orange and ends green when an oxidation happens.
• Saying the dichromate is oxidised. It is the oxidising agent, so it is reduced; the alcohol is oxidised.
• Mixing up reflux and distillation. Distil immediately for the aldehyde; reflux for the carboxylic acid.
• Expecting a ketone to oxidise further. Ketones are not oxidised by acidified dichromate.
• Forgetting water in [O] equations for alcohol oxidation. Each alcohol to aldehyde or ketone step makes H₂O.
• Writing Cr²⁺ or Cr⁶⁺ for the products and reactants. It is Cr₂O₇²⁻ going to Cr³⁺.

Practise with a short timed paper

Oxidation of alcohols is in the first year of A-level Chemistry. The daily A-level Chemistry: Year 1 paper is a short timed paper of 8 questions, with a new one every day. To check the balance of an equation, try the Equation Balancer.

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