Introduction: Exploring the World of Organic Practical Work

Welcome! In this chapter, we are moving away from theoretical equations and into the lab. We will focus on two essential Required Practicals (RP4 and RP5) for your International AS Level Chemistry. These practicals are all about separating substances and identifying what they are using chemical "detective work."

Don't worry if organic chemistry feels like a lot of symbols and structures right now. These practicals are very visual and logical. By the end of these notes, you'll know exactly how to set up a distillation and how to use simple test-tube reactions to tell the difference between various organic molecules.

Required Practical 4: Distilling a Product from a Reaction

Distillation is a technique used to separate a liquid from a mixture based on its boiling point. In your AS studies, this is most commonly used when oxidizing a primary alcohol to an aldehyde. Because the aldehyde has a lower boiling point than the alcohol, we can "distil it off" as soon as it forms to prevent it from being further oxidized into a carboxylic acid.

The Setup: Step-by-Step

1. The Reaction Mixture: You place your reactants (e.g., ethanol and acidified potassium dichromate(VI)) into a round-bottom flask.
2. Anti-bumping Granules: Before heating, you add small, stone-like "anti-bumping granules." These provide a surface for small bubbles to form, preventing the liquid from splashing violently or "bumping."
3. The Heat Source: You heat the flask (usually with a water bath or electric heater for flammable organic liquids).
4. The Thermometer: A thermometer is placed at the top of the still head. Crucial Tip: The bulb of the thermometer must be level with the opening to the condenser to accurately measure the temperature of the vapor entering the condenser.
5. The Liebig Condenser: This is a glass tube surrounded by a "water jacket." Cold water enters at the bottom and leaves at the top. This ensures the jacket is always full of cold water to maximize cooling.
6. Collection: The vapor cools, turns back into a liquid (condenses), and drips into a collection flask (the receiver).

Quick Review: Why use Distillation?

  • It separates substances with different boiling points.
  • It allows us to remove a product (like an aldehyde) from the reaction mixture before it reacts further.
  • It helps in purifying a liquid product.

Common Mistake to Avoid: Never seal the end of the distillation apparatus. If gas cannot escape, pressure will build up and the glassware could explode!

Required Practical 5: Organic Functional Group Tests

How do you know what’s inside a clear liquid in a test tube? We use specific chemical tests that give a visible result (like a color change or bubbles) for specific functional groups. Here are the tests you must know for your OxfordAQA AS exams:

1. Testing for Alkenes (The Carbon-Carbon Double Bond)

Alkenes are unsaturated hydrocarbons containing a \(C=C\) bond. They react quickly with bromine.

  • Reagent: Bromine water (\(Br_{2(aq)}\)).
  • Initial Color: Orange/Yellow.
  • Positive Result: The solution turns colorless (it decolourizes).

2. Testing for Alcohols (Primary and Secondary)

Primary and secondary alcohols can be oxidized, while tertiary alcohols cannot.

  • Reagent: Acidified potassium dichromate(VI) (\(K_{2}Cr_{2}O_{7}\) acidified with \(H_{2}SO_{4}\)).
  • Initial Color: Orange.
  • Positive Result: The solution turns green.
  • Note: This test works for primary and secondary alcohols because they are oxidized. It stays orange for tertiary alcohols.

3. Distinguishing Aldehydes from Ketones

Both contain a carbonyl group (\(C=O\)), but aldehydes are easily oxidized, whereas ketones are not. We use two main tests for this:

A. Tollens' Reagent (The Silver Mirror Test)
  • What it is: A colorless solution containing the complex ion \([Ag(NH_{3})_{2}]^{+}\).
  • Positive Result (Aldehyde): A silver mirror forms on the inside of the test tube.
  • Result for Ketone: No change (stays colorless).
B. Fehling’s Solution
  • Initial Appearance: A blue solution.
  • Positive Result (Aldehyde): A brick-red precipitate forms upon warming.
  • Result for Ketone: No change (stays blue).

4. Testing for Carboxylic Acids

Carboxylic acids are, as the name suggests, acidic! They react with carbonates to produce gas.

  • Reagent: Sodium hydrogencarbonate (\(NaHCO_{3}\)) or Sodium carbonate (\(Na_{2}CO_{3}\)).
  • Positive Result: Effervescence (fizzing) as carbon dioxide (\(CO_{2}\)) gas is released.
  • Confirmation: You can bubble the gas through limewater; it will turn cloudy.

Summary Table of Organic Tests

Use this table as a quick reference guide for your revision:

Functional Group Reagent Observation (Positive)
Alkene (\(C=C\)) Bromine water Orange to colorless
Alcohol (\(1^{\circ}\) or \(2^{\circ}\)) Acidified \(K_{2}Cr_{2}O_{7}\) Orange to green
Aldehyde Tollens' Reagent Silver mirror forms
Aldehyde Fehling's Solution Blue solution to brick-red ppt
Carboxylic Acid \(NaHCO_{3(aq)}\) Effervescence (\(CO_{2}\))

Key Takeaways for Your Exam

  • Distillation separates liquids based on boiling point. Remember the thermometer placement and the water-in-at-the-bottom rule for the condenser.
  • Anti-bumping granules are essential for smooth boiling.
  • Tollens' Reagent involves the specific complex ion \([Ag(NH_{3})_{2}]^{+}\). This is a frequent exam question!
  • Ketones do not react with Tollens' or Fehling's because they cannot be easily oxidized.
  • Effervescence is the fancy chemistry word for "fizzing" or "bubbles"—use it in your exam answers!

Don't worry if you forget which test is which at first. Just remember: Alkenes need Bromine, Acids need Carbonates, and Aldehydes love to make Silver mirrors!

For more details on identifying ions or doing titrations, please refer to the chapter on "Required practicals 1-3: titration, enthalpy and ion tests (AS)".