Introduction to Required Practical 5

Welcome to Required Practical 5! In this practical, we are moving from the world of test tubes and titrations into the world of organic synthesis. The goal of this practical is to learn how to use distillation to separate a specific product from a reaction mixture as it forms.

The most common example you will encounter in AQA Chemistry is the oxidation of a primary alcohol (like ethanol) to produce an aldehyde (like ethanal). If we don't remove the aldehyde quickly, it will further oxidize into a carboxylic acid. Distillation is our "escape route" for the aldehyde, allowing us to collect it before it reacts further.

Note: For more information on identifying the functional groups you make here, see Required Practical 6: Tests for alcohol, aldehyde, alkene and carboxylic acid.

The Distillation Setup

Distillation works by taking advantage of different boiling points. When we heat a mixture, the substance with the lowest boiling point turns into a gas first, travels through a cooling system, and turns back into a liquid in a separate container.

Essential Apparatus

  • Round-bottom or Pear-shaped flask: This holds our reaction mixture (e.g., ethanol and acidified potassium dichromate).
  • Heat source: We usually use an electric heater, water bath, or sand bath. We avoid Bunsen burners because many organic chemicals (like alcohols and aldehydes) are highly flammable.
  • Liebig Condenser: A glass tube surrounded by a "water jacket" that stays cold.
  • Thermometer: To monitor the temperature of the vapor.
  • Anti-bumping granules: Small stones added to the flask.

How to set it up correctly (Common Exam Questions!)

There are a few "golden rules" for setting up distillation that examiners love to test:

  1. The Thermometer Bulb: This must be placed exactly at the T-junction of the glass adapter. Why? Because we need to measure the temperature of the vapor as it enters the condenser to identify exactly what is being distilled.
  2. Water Flow: In a Liebig condenser, the cold water must enter at the bottom (the end furthest from the heat) and exit at the top. This ensures the condenser is always full of water and provides the most efficient cooling.
  3. Anti-bumping granules: These provide a "surface" for small bubbles to form. Without them, the liquid might boil violently and "bump" up into the condenser, which is dangerous and ruins your purity.
  4. Keep it open: Never seal the end of the condenser where the product comes out! If you create a closed system and heat it, the pressure will build up and the apparatus might explode.

Step-by-Step: Distilling Ethanal from Ethanol

Don't worry if this seems like a lot of steps; the process is actually very logical once you see it in action.

The Reaction:
\(CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O\)
(Ethanol + Oxidizing Agent \(\rightarrow\) Ethanal + Water)

  1. Prepare the Oxidizing Agent: Add a limited amount of acidified potassium dichromate(VI) (\(K_2Cr_2O_7\)) to a flask. The "acidified" part usually means it is mixed with dilute sulfuric acid (\(H_2SO_4\)).
  2. Add the Alcohol: Gently add your ethanol to the flask with some anti-bumping granules.
  3. Heat Gently: Use an electric heater or water bath. The ethanol will react with the dichromate.
  4. Collect the Product: Ethanal has a much lower boiling point than ethanol or water. It will vaporize, travel into the condenser, turn back into a liquid, and drip into your collection flask (which should be kept in an ice bath to prevent the volatile ethanal from evaporating).

Quick Review: Why do we use distillation here instead of reflux? Distillation removes the ethanal as soon as it forms. If we used reflux (which keeps the vapors in the flask), the ethanal would stay in contact with the oxidizing agent and turn into ethanoic acid.

Safety and Hazards

Working with organic chemicals requires care. Always refer to AT k (safe handling of substances):

  • Potassium Dichromate(VI): This is toxic and a carcinogen. Wear gloves and avoid skin contact.
  • Concentrated Sulfuric Acid: Highly corrosive. Handle with extreme care.
  • Flammability: Organic liquids like ethanol are flammable. This is why we use electric heaters or water baths (AT b) instead of open flames.

Key Takeaways for Your Exam

Distillation vs. Reflux: Use distillation to get an aldehyde from a primary alcohol. Use reflux to get a carboxylic acid.

The Color Change: During the reaction, the orange dichromate(VI) ions (\(Cr_2O_7^{2-}\)) are reduced to green chromium(III) ions (\(Cr^{3+}\)). If the solution turns green, oxidation has happened!

Efficiency: To ensure you get the best yield, ensure all joints in the glassware are tight (to prevent vapor escaping) and the cooling water is flowing correctly.

Common Mistake to Avoid: Don't forget that secondary alcohols can also be distilled to produce ketones. However, ketones cannot be oxidized further, so the choice between distillation and reflux is less critical for them than it is for aldehydes.