Functional Group Identification Revision Notes | A Level AQA Chemistry

1. Why Functional Group Identification Matters

Imagine you are handed an unknown colourless liquid in a laboratory. How do you find out what it is? The answer lies in its functional groups — the reactive parts of the molecule that behave in predictable, testable ways.

At A Level, AQA expects you to carry out a systematic series of chemical tests and interpret the results to identify which functional group is present. This is not guesswork — each test exploits a specific reaction that only certain functional groups undergo, producing a clear, observable change.

The four functional groups you must be able to test for are: alkenes (C=C double bond), alcohols (–OH), aldehydes (–CHO), and carboxylic acids (–COOH). Each has at least one dedicated test, and some tests allow you to distinguish between groups that might otherwise look similar.

2. Testing for Alkenes: Bromine Water

The alkene functional group contains a carbon-carbon double bond (C=C). The pi bond in this double bond is electron-rich and relatively weak, making it highly reactive towards electrophiles — species that are attracted to electron density.

When bromine water (orange-brown) is added to an alkene, the bromine molecule acts as an electrophile. The pi electrons of the C=C bond attack the bromine, breaking the Br–Br bond and adding a bromine atom to each carbon. The product is a dibromoalkane, which is colourless.

The observable result: the orange-brown colour of bromine water is discharged to give a colourless solution. This is the positive test for an alkene. If the bromine water remains orange-brown, no alkene is present.

3. Testing for Alcohols: Acidified Potassium Dichromate(VI)

Alcohols contain the hydroxyl group (–OH) attached to a carbon chain. On their own, alcohols do not react with bromine water or give a silver mirror with Tollens' reagent — so those tests are negative. The key test for an alcohol uses an oxidising agent.

When a primary alcohol is warmed with acidified potassium dichromate(VI) solution ($\text{K}_2\text{Cr}_2\text{O}_7$ in dilute $\text{H}_2\text{SO}_4$), the alcohol is oxidised. The dichromate(VI) ion ($\text{Cr}_2\text{O}_7^{2-}$, orange) is reduced to the chromium(III) ion ($\text{Cr}^{3+}$, green). The observable change is a colour change from orange to green.

Primary alcohols are oxidised first to aldehydes, and then — if excess oxidising agent is present and the mixture is heated under reflux — to carboxylic acids. Secondary alcohols are oxidised to ketones and stop there. Tertiary alcohols are not oxidised by acidified dichromate(VI) under these conditions, so the solution remains orange.

Notice that both primary and secondary alcohols give an orange-to-green colour change. To distinguish between them — and to identify whether the product is an aldehyde or a ketone — you need to follow up with Tollens' reagent or Fehling's solution (covered in the next section).

4. Testing for Aldehydes: Tollens' Reagent and Fehling's Solution

Aldehydes and ketones both contain the carbonyl group (C=O). This makes them look similar on paper, but their reactivity is very different. Aldehydes have a hydrogen atom attached to the carbonyl carbon (–CHO), which makes them readily oxidised. Ketones have two carbon groups on either side of the carbonyl and are not oxidised by mild oxidising agents.

This difference in oxidisability is the basis for two classic tests that distinguish aldehydes from ketones: Tollens' reagent and Fehling's solution.

The half-equation for the reduction of silver ions is: $\text{Ag}^+(\text{aq}) + e^- \rightarrow \text{Ag}(\text{s})$. The aldehyde is simultaneously oxidised to a carboxylate ion. The overall result is a silver mirror on the glass — one of the most visually striking positive tests in A Level chemistry.

Fehling's solution works on the same principle but uses copper(II) ions instead of silver ions. The deep blue solution (containing $\text{Cu}^{2+}$ complexed with tartrate) is reduced by the aldehyde to give a brick-red precipitate of copper(I) oxide ($\text{Cu}_2\text{O}$). Ketones give no reaction with either Tollens' or Fehling's.

5. Testing for Carboxylic Acids: Sodium Carbonate and pH

Carboxylic acids contain the –COOH group. They are weak acids, meaning they only partially dissociate in water, but they are acidic enough to react with carbonates and hydrogencarbonates — a reaction that produces carbon dioxide gas.

The standard test is to add sodium carbonate solution ($\text{Na}_2\text{CO}_3$(aq)) or solid sodium hydrogencarbonate ($\text{NaHCO}_3$) to the unknown compound. A carboxylic acid reacts to produce effervescence (bubbling) as carbon dioxide gas is released. The gas can be confirmed by passing it through limewater, which turns milky.

The ionic equation for the reaction with carbonate is: $2\text{RCOOH}(\text{aq}) + \text{CO}_3^{2-}(\text{aq}) \rightarrow 2\text{RCOO}^-(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$. The effervescence is the key observable — not just a colour change.

You can also use universal indicator or a pH metre to confirm acidity. Carboxylic acids give a pH typically in the range 3–5 in dilute aqueous solution. However, pH alone cannot distinguish a carboxylic acid from a mineral acid or other acidic species, so the carbonate test is more specific.

6. Required Practical 6: Identifying Organic Functional Groups

Required Practical 6 brings together all four functional group tests in a systematic investigation. You will be given unknown organic compounds and must apply the tests in a logical order to identify which functional group each compound contains. The practical also develops your ability to evaluate results and justify conclusions.

One critical practical point: Tollens' reagent must be freshly prepared for each experiment. If left to stand, it can form silver nitride ($\text{Ag}_3\text{N}$), which is explosive. This is not just a safety rule — aged reagent also gives unreliable results, which would undermine your conclusions.

Worked example: Systematic Identification of an Unknown Organic Compound

An unknown colourless liquid gives the following results: bromine water remains orange-brown; acidified potassium dichromate(VI) turns green on warming; Tollens' reagent gives no silver mirror; sodium carbonate solution gives no effervescence. Identify the functional group present and name the class of compound.

1. Bromine water remains orange-brown → negative result — No decolourisation means no alkene (C=C) is present. The compound does not undergo electrophilic addition with bromine.
2. Orange → green on warming → positive result — The compound is oxidised by acidified potassium dichromate(VI). This indicates either a primary or secondary alcohol is present. Tertiary alcohols do not react and would leave the solution orange.
3. No silver mirror → negative result — No aldehyde is present. If the compound were an aldehyde, it would have been oxidised by Tollens' reagent to give a silver mirror. The compound is therefore not an aldehyde.
4. No effervescence → negative result — No carboxylic acid is present. A carboxylic acid would react with sodium carbonate to release CO₂ gas.
5. Negative: alkene, aldehyde, carboxylic acid. Positive: oxidised by dichromate. — The only functional group consistent with all four results is an alcohol. Since the compound is oxidised (positive dichromate test) but gives no silver mirror (negative Tollens'), it is either a primary alcohol (which would give an aldehyde on oxidation, but the aldehyde test was run on the original compound, not the oxidation product) or a secondary alcohol (oxidised to a ketone, which does not react with Tollens'). The most definitive conclusion from these results alone is that the compound is a secondary alcohol.

Answer: The compound is a secondary alcohol. It is oxidised by acidified potassium dichromate(VI) (orange to green), but does not decolourise bromine water, does not give a silver mirror with Tollens' reagent, and does not effervesce with sodium carbonate.

7. Putting It All Together: A Systematic Decision Framework

In an exam, you may be asked to identify an unknown compound from a set of test results, or to plan a sequence of tests. The key is to work through the tests in a logical order, using each result to narrow down the possibilities before moving to the next test.

• Test 1 — Bromine water: decolourisation → alkene present; no change → no alkene
• Test 2 — Acidified potassium dichromate(VI), warm: orange to green → primary or secondary alcohol (or aldehyde); remains orange → tertiary alcohol, alkene, or no oxidisable group
• Test 3 — Tollens' reagent, warm gently: silver mirror → aldehyde; no change → ketone or non-carbonyl compound
• Test 4 — Fehling's solution, warm: brick-red precipitate → aldehyde; remains blue → ketone or non-carbonyl compound
• Test 5 — Sodium carbonate solution: effervescence (CO₂) → carboxylic acid; no effervescence → no carboxylic acid

Notice that Tests 3 and 4 are both aldehyde tests — you do not need to use both in every situation, but knowing both is required by the specification. In practice, Tollens' reagent is more visually dramatic and is more commonly asked about in AQA exam questions.

The structure above shows ethanal (acetaldehyde), a simple aldehyde. The –CHO group is clearly visible: a carbonyl carbon with one hydrogen directly attached. This hydrogen is what makes aldehydes oxidisable — and what distinguishes them from ketones, where both substituents on the carbonyl carbon are carbon groups.

8. Exam Technique and Common Pitfalls

Functional group identification questions appear regularly in AQA Paper 2 and in synoptic questions. The marks are often straightforward to earn — but only if you use precise language. Vague answers such as 'the solution changes colour' will not score without specifying the exact colour change.

Across this lesson, every test you have studied exploits a specific chemical reaction unique to — or characteristic of — a particular functional group. Bromine water tests for the reactive pi bond of alkenes; dichromate tests for oxidisable –OH groups; Tollens' and Fehling's distinguish the oxidisable –CHO of aldehydes from the non-oxidisable ketone carbonyl; and sodium carbonate detects the acidic –COOH of carboxylic acids. Mastering the exact observations and the chemistry behind them is what separates a full-mark answer from a partial one.

Move from recall to application: identification questions are strongest when you link each observation to a specific functional group and then rule out alternatives using a second test.