Required Practical 6 - Organic Tests (AQA A-Level Chemistry): Revision Notes

4.2.4 Required Practical 6 - Organic Tests

Aim

To use chemical tests to identify the presence of different functional groups in organic compounds, including alcohols, aldehydes, alkenes, and carboxylic acids.

Equipment

Chemicals

• Alcohol sample (e.g., ethanol, propan-2-ol)
• Metallic sodium
• Acidified potassium dichromate
• Fehling's solution (A and B)
• Bromine water
• Dilute ethanoic acid
• Solid sodium carbonate
• 1-bromobutane (or other halogenoalkane)
• Sodium hydroxide solution (NaOH)
• Silver nitrate solution (AgNO₃), dilute nitric acid
• Calcium hydroxide solution (limewater)

Apparatus

• Test tubes (clean and dry)
• Test tube rack
• Teat pipettes
• Measuring cylinders (for accurate volumes)
• Beakers (for water baths)
• Anti-bumping granules
• Thermometer
• Water bath (for gentle heating)
• Filter paper
• Spatula

Risk Assessment

• Sodium is highly reactive with water:
  • Handle sodium carefully with dry tweezers.
  • Dispose of excess sodium in ethanol to avoid water contact.
• Potassium dichromate is a strong oxidizer and toxic:
  • Use safety goggles, gloves, and a lab coat.
• Fehling's solution contains alkalis and copper compounds:
  • Avoid skin contact and wear protective gear.
• Halogenoalkanes are volatile and can be toxic:
  • Use in a well-ventilated area or fume cupboard.
• Nitric acid and silver nitrate are irritants:
  • Handle with care, wearing gloves and eye protection.

Method

Test for Alcohols

1. Metal Test:

• Add a small piece of metallic sodium to the alcohol in a test tube.
• Observe the release of hydrogen gas (test with a lit splint for a "squeaky pop").
• Dispose of excess sodium in ethanol.

2. Potassium Dichromate Test:

• Add a few drops of acidified potassium dichromate to the alcohol.
• Gently warm and observe the colour change:
• Orange to green indicates a primary or secondary alcohol.
• No change (remains orange) indicates a tertiary alcohol.

Test for Aldehydes

1. Fehling's Solution:

• Mix equal volumes of Fehling's A and B to prepare the test reagent.
• Add 5 drops of the reagent and a few anti-bumping granules to a test tube containing the aldehyde.
• Warm gently in a hot water bath and maintain for a few minutes.
• A brick-red precipitate confirms the presence of an aldehyde.

Test for Alkenes

1. Bromine Water:

• Add 1 cm³ of bromine water to 2 drops of the suspected alkene.
• Shake vigorously.
• If the solution decolourises from orange to colourless, an alkene is present.

Test for Carboxylic Acids

1. Sodium Carbonate:

• Add a small spatula of sodium carbonate to a test tube.
• Carefully add 2 cm³ of dilute ethanoic acid using a pipette.
• Collect the gas produced and bubble through limewater.
• If limewater turns cloudy, a carboxylic acid is confirmed (due to $CO₂$ production).

Test for Halogenoalkanes

1. Nucleophilic Substitution:

• Add 5 drops of 1-bromobutane to 1 cm³ of $NaOH$ in a test tube.
• Warm in a water bath at approximately 60°C.
• Acidify with 2 cm³ of dilute nitric acid and add 1 cm³ of $AgNO₃$.
• Observe the precipitate:
• White precipitate for chloride ions.
• Cream precipitate for bromide ions.
• Yellow precipitate for iodide ions.

Results

Test	Reagent Used	Observation	Conclusion
Alcohol (Sodium Test)	Metallic sodium	Bubbles of hydrogen gas; "squeaky pop" with a lit splint	Presence of an alcohol confirmed
Alcohol (Dichromate)	Acidified potassium dichromate	Orange to green for primary/secondary alcohol; remains orange for tertiary	Identifies primary/secondary or tertiary alcohol
Aldehyde	Fehling's solution	Formation of a brick-red precipitate	Presence of an aldehyde confirmed
Alkene	Bromine water	Decolourises from orange to colourless	Presence of an alkene confirmed
Carboxylic Acid	Sodium carbonate	Effervescence, limewater turns cloudy	Presence of a carboxylic acid confirmed
Halogenoalkane	NaOH, dilute nitric acid, silver nitrate	White, cream, or yellow precipitate	Identifies $Cl⁻, Br⁻, \text{or}\ I⁻$ ions based on precipitate colour

Errors and Improvements

• Errors:
  • Misinterpreting subtle colour changes.
  • Cross-contamination between test tubes affecting results.
• Improvements:
  • Use new test tubes for each test to avoid contamination.
  • Perform colour tests in well-lit conditions to improve observation accuracy.