CP5: Oxidation of Ethanol
This practical shows how the oxidation of ethanol can be controlled. Ethanol is a primary alcohol, so it can be oxidised to ethanal if the aldehyde is distilled off as it forms, or to ethanoic acid if the reaction mixture is heated under reflux before distillation.
The decision point: distillation or reflux?
The most important idea in this practical is that conditions control the product. Ethanol is a primary alcohol, so it can be oxidised first to an aldehyde and then further to a carboxylic acid.
To make the aldehyde, the product must be removed from the oxidising mixture quickly. To make the acid, the reaction mixture must be heated for longer under reflux so that the aldehyde remains in the flask and is oxidised further.
| Target product | Apparatus | Key condition | Reason |
|---|---|---|---|
| Ethanal | Distillation | Gently heat and distil the aldehyde as it forms | Prevents further oxidation to ethanoic acid |
| Ethanoic acid | Reflux, then distillation | Heat under reflux for about 30 minutes | Keeps volatile material in the flask so oxidation can continue |
Core practical logic: the product is not decided by the starting alcohol. It is decided by whether the aldehyde is removed or returned to the reaction flask.
Partial oxidation: making ethanal
Ethanal is made by partial oxidation of ethanol. The reaction mixture contains acidified potassium dichromate(VI), but the aldehyde is distilled off as soon as it forms.
This prevents the ethanal from remaining in contact with the oxidising agent long enough to be converted into ethanoic acid.
Add acidified potassium dichromate(VI) to a pear-shaped flask and cool it in an ice-water bath.
Set up the flask for distillation and add a few anti-bumping granules.
Prepare a diluted sample of ethanol and add it slowly, a few drops at a time, allowing the reaction to subside after each addition.
Remove the ice-water bath and allow the flask to warm to room temperature.
Heat very gently and collect 3-4 cm3 of clear, colourless distillate in a test tube surrounded by cold water.
Full oxidation: making ethanoic acid
Ethanoic acid is made when ethanol is oxidised more fully. The mixture is heated under reflux so that vapours condense and return to the flask instead of escaping.
During reflux, any ethanal formed remains in the oxidising mixture and is further oxidised to ethanoic acid. After reflux, the product is collected by distillation.
Add acidified potassium dichromate(VI) to the flask and cool the flask in an ice-water bath.
Set up the apparatus for reflux and add anti-bumping granules.
Add diluted ethanol slowly down the reflux condenser, allowing the reaction to subside after each addition.
Remove the ice-water bath and allow the flask to warm to room temperature.
Heat gently under reflux for about 30 minutes, then rearrange the apparatus for distillation.
Distil 3-4 cm3 of clear, colourless liquid into a test tube surrounded by cold water.
Apparatus focus: what examiners look for
CP5 often tests apparatus understanding. Students must know why the condenser is positioned differently in distillation and reflux, and why the water supply enters at the bottom.
Distillation setup
- Condenser slopes downwards from the flask to the receiver.
- The thermometer bulb should sit at the T-junction.
- Water enters the condenser at the bottom and leaves at the top.
- The receiving test tube is cooled in cold water because ethanal is volatile.
- For Quickfit apparatus, do not draw extra lines between the flask, adaptor and condenser.
Reflux setup
- The condenser is vertical above the flask.
- The top of the condenser must be open, not sealed.
- Water enters at the bottom and leaves at the top.
- Vapours condense and return to the reaction flask.
- Anti-bumping granules promote smooth, even boiling.
Why water enters at the bottom: this fills the condenser jacket completely, giving efficient cooling and preventing air gaps.
Interactive: choose the product
You are oxidising ethanol using acidified potassium dichromate(VI). Select the product you want, then check the apparatus, equation and test results.
Product selected: ethanal
Use distillation. Heat gently and distil off the aldehyde as it forms so that it does not remain in the oxidising mixture.
Tollens’ reagent
Silver mirror forms.
Fehling’s solution
Blue solution changes to reddish-brown.
Magnesium ribbon
No visible reaction.
Sodium hydrogencarbonate
No effervescence.
Product selected: ethanoic acid
Use reflux, then distillation. Reflux keeps volatile material in the flask, allowing the aldehyde intermediate to oxidise further to the carboxylic acid.
Tollens’ reagent
No silver mirror.
Fehling’s solution
No colour change.
Magnesium ribbon
Effervescence occurs.
Sodium hydrogencarbonate
Effervescence occurs.
Mini challenge
A student obtains no colour change with Fehling’s solution and clear effervescence with sodium hydrogencarbonate. What product did they probably make, and what was the likely practical error?
They probably made ethanoic acid. Effervescence with sodium hydrogencarbonate indicates a carboxylic acid, and no Fehling’s colour change means the product is not an aldehyde. The likely error was using reflux or failing to distil ethanal away as it formed.
Product testing: use the pattern of results
The practical does not rely on one test alone. Students should use the full pattern of test results to decide whether the product is an aldehyde or a carboxylic acid.
| Test | Ethanal result | Ethanoic acid result | Interpretation |
|---|---|---|---|
| Tollens’ reagent | Silver mirror forms on the inside of the test tube | No silver mirror | Positive result supports the presence of an aldehyde |
| Fehling’s solution | Blue solution changes to reddish-brown when warmed | No colour change | Positive result supports the presence of an aldehyde |
| Magnesium ribbon | No visible reaction | Effervescence | Effervescence supports the presence of an acid |
| Sodium hydrogencarbonate | No effervescence | Effervescence | Carbon dioxide forms when a carboxylic acid reacts with hydrogencarbonate |
Exam focus: sodium hydrogencarbonate effervescence points towards ethanoic acid, not ethanal.
Safety and observations
Safety
- Wear goggles and chemical-resistant gloves.
- Ethanol and ethanal are flammable.
- Avoid skin contact with reactants and products.
- Heat very gently to reduce bumping.
- Use anti-bumping granules for smoother boiling.
- Never seal the top of a condenser.
Observation
Acidified potassium dichromate(VI) changes from orange to green because dichromate(VI) ions, Cr2O72−, are reduced to Cr3+ ions.
Core practical questions and answers
Use these as revision prompts after studying the method. They target the reasoning behind the apparatus, conditions and product tests.
1. What is meant by refluxing?
Refluxing means heating a reaction mixture while vapours are condensed and returned to the flask. It allows prolonged heating at a steady reaction temperature without the flask boiling dry.
2. Why do some experiments require refluxing?
Some organic reactions are slow and need prolonged heating. Reflux allows the mixture to be heated for longer without losing volatile reactants or products.
3. Why must the ethanol solution be added slowly?
The oxidation reaction is exothermic. Slow addition reduces the risk of dangerous splashing and helps keep the reaction controlled.
4. Why are anti-bumping granules added?
Anti-bumping granules provide nucleation sites so many small bubbles form. This promotes smooth boiling and reduces sudden vigorous bumping.
5. Why should water enter the condenser at the bottom?
Water entering at the bottom fills the condenser jacket fully. This gives efficient cooling because the condenser remains surrounded by cold water.
6. Why is the distillate collected in a test tube surrounded by cold water?
Ethanal has a low boiling point and is volatile. Cooling the receiver reduces evaporation of the distillate after it has condensed.
7. Write the equation for oxidation of ethanol to ethanal.
CH3CH2OH + [O] → CH3CHO + H2O
8. Write the equation for oxidation of ethanol to ethanoic acid.
CH3CH2OH + 2[O] → CH3COOH + H2O
9. Which of ethanol, ethanal and ethanoic acid can form hydrogen bonds?
Ethanol and ethanoic acid can form hydrogen bonds because each contains hydrogen directly bonded to oxygen. Ethanal contains a polar C=O bond but does not have hydrogen directly bonded to oxygen, nitrogen or fluorine, so it cannot hydrogen bond to itself in the same way.
10. Why can ethanal only be obtained without the reflux step?
During reflux, ethanal is returned to the reaction flask and remains in contact with acidified potassium dichromate(VI). It is then further oxidised to ethanoic acid.
11. What happens when secondary and tertiary alcohols are oxidised?
Secondary alcohols are oxidised to ketones. Ketones are not easily oxidised further under these conditions because they do not have a hydrogen atom attached to the carbonyl carbon. Tertiary alcohols are not easily oxidised because they do not have a hydrogen atom attached to the carbon bearing the OH group.
Exam-style reasoning
Scenario: A student was told to make ethanal but produced a solution that gave no Fehling’s colour change and effervescence with sodium hydrogencarbonate.
Conclusion: the student produced ethanoic acid, not ethanal. The likely error was that the apparatus was set up for reflux, or the aldehyde was not distilled off quickly enough. Ethanal must be removed as soon as it forms to prevent further oxidation.
If a secondary alcohol such as propan-2-ol was oxidised instead, the product would be propanone. Propanone is a ketone, so prolonged reflux would not produce a carboxylic acid under these conditions.