7.4 - Preparation and Properties of Ethanal

The overall reaction is:

$$\text{C}_2\text{H}_5\text{OH} + \text{Na}_2\text{Cr}_2\text{O}_7/H_2SO_4 \rightarrow \text{CH}_3\text{CHO} + \text{Cr}^{3+} + H_2\text{O}$$

Materials and Apparatus Required

Chemicals

• Ethanol ($C₂H₅OH$)
• Sodium dichromate ($Na₂Cr₂O₇$)
• Concentrated sulfuric acid ($H₂SO₄$)
• Potassium manganate (VII) solution
• Fehling's solution (No. 1 and No. 2)
• Silver nitrate solution
• Sodium hydroxide solution
• Aqueous ammonia solution
• Dilute nitric acid
• Ice-cold water

Apparatus

• Quickfit distillation apparatus
• Anti-bumping granules
• Beakers, test tubes, and droppers
• Graduated cylinder
• Retort stands and clamps
• Bunsen burner or heating mantle
• Dropping funnel
• Conical flask
• Thermometer (-10°C to 100°C)
• Water bath

Safety Precautions

• Wear safety glasses and gloves throughout the experiment.
• Sodium dichromate is a strong oxidising agent and toxic. Handle carefully, using a fume cupboard if possible.
• Sulfuric acid is corrosive and can cause severe burns. Always add acid to water slowly when diluting.
• Ethanol and ethanal are highly flammable. Keep away from open flames.
• Dispose of chemicals and waste safely as per local regulations.

Method

Preparation of Ethanal

1. Set up the distillation apparatus as shown in the diagram for distillation with addition.
2. Add anti-bumping granules and 12 cm³ of water to the round-bottom flask.
3. Slowly, with swirling, add 4 cm³ of concentrated sulfuric acid to the water.
4. Dissolve 10 g of sodium dichromate in 10 cm³ of water in a beaker and add 10 cm³ of ethanol.
5. Transfer this solution to the dropping funnel.
6. Surround the conical flask (for collecting ethanal) with a beaker of ice water to prevent ethanal loss due to its low boiling point (21°C).
7. Heat the dilute acid mixture in the flask until it begins to boil, then turn off the heat.
8. Slowly add the ethanol mixture from the dropping funnel to the boiling acid, maintaining gentle boiling. Ethanal distils off as it is formed and is collected in the conical flask.
9. Redistil the impure ethanal, collecting the fraction boiling between 20°C and 23°C.

Investigating the Properties of Ethanal

Oxidation by Potassium Manganate (VII):

10. Add 2 cm³ of ethanal to a test tube with 1 cm³ of potassium manganate (VII) solution and 4 cm³ of dilute sulfuric acid.
11. Warm the mixture in a water bath and observe the colour change.

Fehling's Test:

12. Mix 1 cm³ of Fehling's solution No. 1 with 1 cm³ of Fehling's solution No. 2 in a test tube.
13. Add 1 cm³ of ethanal, heat gently, and observe any colour change.

Tollen's Test:

14. Add 3 cm³ of silver nitrate to a clean test tube, followed by 1 cm³ of sodium hydroxide.
15. Add aqueous ammonia drop by drop until the precipitate dissolves.
16. Add a few drops of ethanal, shake, and warm in a water bath.
17. Observe if a silver mirror forms.

Results

Test	Observation	Conclusion
Potassium Manganate (VII) Test	Solution decolourises	Ethanal is oxidised
Fehling's Test	Red-brown precipitate forms	Ethanal is oxidised
Tollen's Test	Silver mirror forms	Ethanal is an aldehyde

Example Questions with Answers