What you'll learn

• How to carry out the core test-tube reactions for organic analysis (Required Practical 6).
• The correct reagents, conditions, and observations for identifying alkenes, carboxylic acids, alcohols, and aldehydes.
• How to deduce an unknown organic structure by piecing together multiple test results.

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What makes a good chemical test?

Chemists often need to identify an unknown liquid or check if a reaction has successfully produced a new functional group. To do this, we rely on qualitative tests.

For a test to be useful in a test tube, the result must be blindingly obvious to the naked eye. We are looking for:

• A distinct colour change.
• The formation of a solid precipitate.
• Effervescence (fizzing or bubbling).

In Required Practical 6, you are expected to confidently test for four functional groups: alkenes, carboxylic acids, alcohols, and aldehydes.

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1. Testing for Alkenes

Alkenes contain a carbon–carbon double bond ($C=C$). Because this double bond is electron-rich, it readily undergoes electrophilic addition reactions.

The Bromine Water Test

• Reagent: Bromine water, $B{r}_2(aq)$.
• Procedure: Add a few drops of bromine water to the unknown sample and shake the test tube gently.
• Observation if positive: The solution changes from orange to colourless.
• Explanation: The bromine adds across the double bond to form a dihalogenoalkane. As the $B{r}_2$ molecules are consumed, their characteristic orange colour disappears.

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2. Testing for Carboxylic Acids

Carboxylic acids contain the $-COOH$ functional group. They are weak acids, but they are still acidic enough to react with carbonates to produce carbon dioxide gas.

The Carbonate Test

• Reagent: Aqueous sodium carbonate, $N{a}_{2}C{O}_3(aq)$, or solid sodium hydrogencarbonate, $NaHC{O}_3(s)$.
• Procedure: Add a small spatula measure of sodium hydrogencarbonate (or a few $c{m}^3$ of sodium carbonate solution) to your sample.
• Observation if positive: Vigorous effervescence (fizzing) as carbon dioxide gas is evolved.
• Confirmation: To be absolutely certain the gas is $C{O}_2$, you can bubble it through limewater. The limewater will turn cloudy.

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3. Testing for Alcohols

Alcohols contain the hydroxyl group ($-OH$). We classify them as primary ($1^{\circ }$), secondary ($2^{\circ }$), or tertiary ($3^{\circ }$) depending on how many carbon atoms are attached to the carbon holding the $-OH$ group.

We can identify primary and secondary alcohols because they can be oxidised. Tertiary alcohols cannot be easily oxidised.

The Potassium Dichromate(VI) Test

• Reagent: Acidified potassium dichromate(VI) solution, written as $K_{2}C{r}_2{O}_7/H^{+}$. (Usually acidified with dilute sulfuric acid, $H_{2}S{O}_4$).
• Procedure: Add the sample to the acidified dichromate solution and warm gently in a hot water bath.
• Observation if positive: The solution turns from orange to green.
• Explanation: Primary and secondary alcohols reduce the orange dichromate(VI) ions ($C{r}_2{O}_7^{2-}$) to green chromium(III) ions ($C{r}^{3+}$).

Primary alcohols are oxidised first to aldehydes, and then to carboxylic acids. Secondary alcohols are oxidised to ketones. Tertiary alcohols will show no visible change (the solution stays orange) because they have no hydrogen atom on the carbon bearing the $-OH$ group to be removed during oxidation.

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4. Distinguishing Aldehydes from Ketones

Aldehydes and ketones both contain the carbonyl group ($C=O$). However, aldehydes have a hydrogen atom attached to the carbonyl carbon, meaning they can be oxidised further to form carboxylic acids. Ketones cannot be oxidised further under mild conditions.

We exploit this difference using mild oxidising agents.

Tollens' Reagent (The Silver Mirror Test)

• Reagent: Ammoniacal silver nitrate (made by adding aqueous ammonia to silver nitrate until the initial brown precipitate just dissolves).
• Procedure: Add a few drops of the unknown sample to Tollens' reagent in a clean test tube and warm gently in a hot water bath.
• Observation if positive: A metallic silver mirror forms on the inside of the test tube.
• Explanation: The aldehyde is oxidised to a carboxylic acid. At the same time, the colourless silver(I) complex ions, $[Ag(N{H}_3{)}_2{]}^{+}$, are reduced to solid silver atoms ($Ag$). Ketones cause no change.

Fehling's Solution

• Reagent: A blue solution containing copper(II) ions ($C{u}^{2+}$) in an alkaline environment.
• Procedure: Add the unknown to Fehling's solution and warm in a hot water bath.
• Observation if positive: The blue solution produces a brick-red precipitate.
• Explanation: The aldehyde is oxidised, reducing the blue copper(II) ions to copper(I) oxide ($C{u}_{2}O$), which is an insoluble brick-red solid. Ketones cause no change.

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Putting it all together

In exams, you are rarely asked about just one test in isolation. You will usually be given molecular formulas and a sequence of test results, and asked to deduce the identity of the molecule.