Experiment: Oxidation of alcohol to carboxylic acid

The oxidation of an alcohol to a carboxylic acid is a Form 5 practical in the Carbon Compounds chapter. It shows how ethanol is converted into ethanoic acid, links the alcohol and carboxylic acid homologous series, and gives the acid that is later used to make an ester. This guide sets out the aim, the apparatus, a numbered procedure, the observations to expect, and the science process skills the practical papers reward.

Aim

To oxidise ethanol to ethanoic acid using an acidified oxidising agent, and to identify the products from the colour change of the oxidising agent and the properties of the product.

Apparatus and materials

• Boiling tube or round-bottomed flask fitted for gentle heating
• Reflux or a simple condenser (or a boiling tube stood in a warm water bath)
• Bunsen burner, tripod, gauze and water bath (or a beaker of hot water)
• Dropper, measuring cylinder and test tubes
• Ethanol
• Acidified potassium dichromate(VI) solution (orange) OR acidified potassium manganate(VII) solution (purple)
• Dilute sulfuric acid (to acidify the oxidising agent)
• Blue litmus paper and universal indicator to test the product

Procedure

1. Measure a small volume of ethanol into the boiling tube.
2. Add dilute sulfuric acid, then add the oxidising agent (potassium dichromate(VI) or potassium manganate(VII)) a little at a time; the mixture must be acidified.
3. Fit a condenser or stand the tube in a hot water bath, and warm the mixture gently; do not heat it with a strong direct flame.
4. Observe and record the colour change of the oxidising agent as the mixture is warmed.
5. Continue warming until the colour change is complete, then allow the mixture to cool.
6. Test the product by its smell (waft gently towards you) and by touching a drop to damp blue litmus paper and to universal indicator.
7. Record all observations, including the final colour of the mixture and the effect on the indicators.

Expected observations

When acidified potassium dichromate(VI) is used, the orange colour of the mixture changes to green as the ethanol is oxidised. When acidified potassium manganate(VII) is used instead, its purple colour fades and is decolourised. On warming, a sharp, sour, vinegar-like smell develops, which is the smell of the carboxylic acid formed. The cooled product turns damp blue litmus paper red and gives an acidic colour with universal indicator, showing the product is acidic.

Inference and conclusion

The alcohol is oxidised by the acidified oxidising agent: ethanol is converted first to an aldehyde and then to ethanoic acid, a carboxylic acid. The orange dichromate(VI) is reduced to green chromium(III) ions, or the purple manganate(VII) is reduced and decolourised; these colour changes are the sign that oxidation has taken place. The vinegar-like smell and the acidic response to litmus and universal indicator confirm that a carboxylic acid has formed. The conclusion is that a primary alcohol can be oxidised to a carboxylic acid by warming it with an acidified oxidising agent.

Science process skills (Paper 3 style)

Making an inference. From the observation that the orange dichromate turns green, infer that the ethanol has been oxidised, because the dichromate(VI) has itself been reduced.

Identifying variables. If the investigation compared the colour change with and without acid, the manipulated variable is whether acid is added, the responding variable is whether the oxidising agent changes colour, and the controlled variables are the volume of ethanol, the amount of oxidising agent and the heating.

Operational definition. Define the end of the oxidation operationally: the reaction is judged complete when the colour of the oxidising agent stops changing on further warming.

Communicating. Record the colour before and after for the oxidising agent, and the results of the smell and indicator tests, so the identity of the product as a carboxylic acid is fully justified.

Safety precautions

• Wear safety goggles, because dilute sulfuric acid and the oxidising agents are corrosive and can irritate the skin and eyes.
• Warm the mixture gently using a water bath rather than a strong flame, because ethanol is flammable and could catch fire over a naked flame.
• Waft the smell towards you rather than inhaling directly, so that the sharp acid vapour is not breathed in.
• Handle potassium dichromate(VI) with particular care and dispose of it as instructed, because it is toxic and harmful.
• Point the mouth of the tube away from people while heating, so nobody is hit if the liquid spits.

Common errors

• Forgetting to acidify. Without added acid the oxidation does not proceed properly and the colour change is incomplete; add dilute sulfuric acid before the oxidising agent.
• Heating with a strong flame. A direct strong flame can ignite the flammable ethanol; warm gently in a water bath or under reflux.
• Boiling off the product. Heating too hard without a condenser drives off volatile ethanol and product before they react; use reflux or gentle warming.
• Misreading the colour change. State it exactly, orange to green for dichromate(VI), purple to colourless for manganate(VII), not just “it changed colour”.
• Testing the smell unsafely. Never put the tube to your nose; waft the vapour towards you so the sharp acid is not inhaled.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can state the colour change for each oxidising agent, explain why the mixture must be acidified, and justify that a carboxylic acid has formed from the smell and indicator tests. Oxidation of alcohols is a core reaction across SPM Chemistry because it connects the alcohol and carboxylic acid series and supplies the ethanoic acid used in esterification, and the same reasoning about colour changes and product tests is rewarded in the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).