Required practical 6: Tests for alcohol, aldehyde, alkene and carboxylic acid

Introduction to Required Practical 6

Welcome to the world of chemical detective work! In Required Practical 6, you will learn how to use simple "test-tube" reactions to identify the functional groups in organic molecules. Whether you are dealing with an alcohol, an aldehyde, an alkene, or a carboxylic acid, each has a unique chemical "fingerprint" that we can reveal using specific reagents.

By the end of these notes, you will know exactly which chemicals to add and what observations to look for to solve these organic puzzles. This knowledge is essential for AQA Paper 2 and Paper 3.

Safety and Technique (AT b, d, k)

Before we start the tests, remember these core practical skills:

1. Safe Handling (AT k): Organic chemicals are often flammable (especially alcohols and aldehydes) and can be corrosive or toxic. Always wear eye protection and avoid skin contact.

2. Heating (AT b): Because organic liquids are flammable, you should never heat them directly over a Bunsen burner flame. Instead, use an electric heater or a water bath.

3. Test-tube Reactions (AT d): These are qualitative tests. We aren't measuring amounts; we are looking for a clear change, like a colour shift or fizzing.

1. Testing for Alkenes

Alkenes are unsaturated hydrocarbons containing a \(C=C\) double bond. This double bond is a region of high electron density that reacts readily with halogens.

The Test: Add Bromine Water (\(Br_{2(aq)}\)) to the sample and shake.

Observations:
- Positive result: The orange solution decolourises (turns from orange to colourless).
- Negative result: The solution remains orange.

How it works: The bromine atoms add across the double bond in an electrophilic addition reaction, forming a colourless dibromoalkane.

Common Mistake: Don't say the solution turns "clear." Clear just means you can see through it. Use the word "colourless."

2. Testing for Alcohols

Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°). We use an oxidising agent to tell them apart.

The Test: Add acidified potassium dichromate(VI) (\(K_{2}Cr_{2}O_{7}\) acidified with dilute \(H_{2}SO_{4}\)) and warm the mixture in a water bath.

Observations:
- Primary and Secondary Alcohols: The solution changes from orange to green.
- Tertiary Alcohols: No colour change (remains orange).

Why the change?
Primary and secondary alcohols are easily oxidised. The orange dichromate(VI) ions (\(Cr_{2}O_{7}^{2-}\)) are reduced to green chromium(III) ions (\(Cr^{3+}\)). Tertiary alcohols cannot be oxidised easily because there is no hydrogen atom on the carbon bonded to the \(-OH\) group.

3. Testing for Aldehydes and Ketones

Both aldehydes and ketones contain a \(C=O\) carbonyl group, but aldehydes can be further oxidised into carboxylic acids, while ketones cannot. We use this difference to distinguish them.

A. Tollens' Reagent (The Silver Mirror Test)

The Test: Add Tollens' reagent (ammoniacal silver nitrate) to the sample and warm gently in a water bath.

Observations:
- Aldehyde: A silver mirror forms on the inside of the test tube.
- Ketone: No change (solution remains colourless).

The Chemistry: The aldehyde reduces the silver(I) complex ion \([Ag(NH_{3})_{2}]^{+}\) to metallic silver \(Ag_{(s)}\).

B. Fehling's Solution

The Test: Add Fehling's solution (a blue solution containing \(Cu^{2+}\) ions) and warm in a water bath.

Observations:
- Aldehyde: Blue solution forms a brick-red precipitate.
- Ketone: No change (remains blue).

The Chemistry: The aldehyde reduces blue \(Cu^{2+}\) ions to a red precipitate of copper(I) oxide (\(Cu_{2}O\)).

4. Testing for Carboxylic Acids

Carboxylic acids are weak acids, but they are strong enough to react with carbonates.

The Test: Add a small amount of sodium hydrogencarbonate (\(NaHCO_{3}\)) or sodium carbonate (\(Na_{2}CO_{3}\)) to the solution.

Observations:
- Positive result: Effervescence (fizzing/bubbles) as carbon dioxide gas is produced.
- Note: You can prove the gas is \(CO_{2}\) by bubbling it through limewater, which will turn cloudy.

The Equation:
\(RCOOH + NaHCO_{3} \rightarrow RCOONa + H_{2}O + CO_{2}\)

Quick Review Summary Table

Alkene: Bromine Water \(\rightarrow\) Orange to Colourless
1°/2° Alcohol: Acidified \(K_{2}Cr_{2}O_{7}\) \(\rightarrow\) Orange to Green
Aldehyde: Tollens' Reagent \(\rightarrow\) Silver Mirror
Aldehyde: Fehling's Solution \(\rightarrow\) Blue to Brick-Red ppt
Carboxylic Acid: \(NaHCO_{3}\) \(\rightarrow\) Effervescence (\(CO_{2}\))

Exam Tip: "Distinguish between..."

If an exam question asks you to "distinguish between two compounds," you must provide:

1. The reagent you would add.
2. The observation for compound A.
3. The observation for compound B (even if the observation is "no visible change").

Example: To distinguish propanal (aldehyde) and propanone (ketone), add Tollens' reagent. Propanal forms a silver mirror; propanone shows no visible change.

Check your understanding

Q1: Why is acidified potassium dichromate not useful for distinguishing between a primary and a secondary alcohol?
Answer: Both will give a positive result (orange to green) because both can be oxidised.

Q2: Why do we use a water bath instead of a Bunsen burner for these tests?
Answer: Most organic compounds, like alcohols and aldehydes, are highly flammable.

Q3: What would you see if you added bromine water to cyclohexane?
Answer: No visible change (remains orange) because cyclohexane is an alkane, not an alkene.

Key Takeaway: Practical 6 is all about matching the right reagent to the right functional group. Memorise the colour changes and always remember your safety protocols!