Q: Why do organic functional group tests need a separate Paper 4 study route? A: The current 9476 syllabus includes qualitative organic analysis, but it also says candidates are not required to carry out tests involving 2,4-DNPH, phosphorus(V) chloride, or phenol in the practical examination. Use this guide to understand functional-group evidence and avoid over-training tests that are outside the official carried-out Paper 4 requirement.
TL;DR Organic QA still needs precise observation and inference: bromine water, acidified KMnO₄, carbonate reactions, Tollens' and Fehling's logic, and supplied QA Notes can all appear in interpretation. Do not assume every named reagent below is a carried-out Paper 4 requirement. Treat 2,4-DNPH, phosphorus(V) chloride, and phenol tests as syllabus-limit examples to recognise, not as required practical procedures for 9476 Paper 4. Read this alongside the H2 Chemistry Experiments hub and the Qualitative Analysis Workflow for inorganic counterparts.
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9476 Paper 4 does not require candidates to carry out 2,4-DNPH tests, but the chemistry can still matter for interpretation and written organic reasoning.
organic functional group tests
The quick-reference matrix below
Use it to compare observations and avoid treating one positive result as a complete identity.
Use the format guide for official timing, marks, and the Paper 4 skill strands.
Quick functional-test map
Observation before identity: Colour, precipitate, gas, or mirror.
One positive test may not be enough: Use the next test to narrow the group.
A strong answer says what is supported and what is ruled out: Link each result to a structural feature.
Concrete example: 2,4-DNPH positive tells you a carbonyl is present. Tollens' positive then points to an aldehyde. Fehling's negative but Tollens' positive suggests an aromatic aldehyde rather than an aliphatic aldehyde.
1 | Why organic functional group tests need a 9476 limit check
In H2 Chemistry Paper 4, candidates receive Qualitative Analysis Notes in the question paper. The Chemistry Data Booklet is for written papers, not the practical examination.
This distinction matters practically. The current SEAB 9476 syllabus includes qualitative organic analysis and expects students to be familiar with tests for organic compounds as detailed in the Qualitative Analysis Notes. It also states that candidates are not required to carry out tests involving 2,4-DNPH, phosphorus(V) chloride, or phenol in the practical examination.
For organic QA interpretation you still need to know:
Which reagent or combination of reagents to use.
The precise observation - colour, physical state of precipitate, whether warming is required.
What the observation does and does not confirm - specifically, which structural features are consistent with the result and which are ruled out.
A common examiner comment is that candidates describe an observation vaguely ("colour change") without specifying what colour or whether a precipitate forms. In organic functional group testing, the physical form of the product - silver mirror on glass, brick-red solid settling through solution, white crystalline precipitate - is part of the mark-scoring observation.
The safe study rule is: use official QA Notes and the question instructions as the boundary for what to perform in Paper 4; use broader organic test knowledge to interpret results and avoid false inferences. [1]
2 | Quick-reference matrix
The table below gives the expected outcome (positive or negative) for each combination of functional group and reagent. "Positive" means the named observation occurs; "negative" means no visible reaction. One or two words describe the key positive observation where space allows.
Functional group
2,4-DNPH
Tollens'
Fehling's
Br₂ water
Acid. KMnO₄
Lucas
NaHCO₃
FeCl₃
Aldehyde (aliphatic)
yellow/orange ppt
silver mirror
brick-red ppt
decolourises
decolourises
no reaction (cold)
negative
negative
Aldehyde (aromatic)
yellow/orange ppt
silver mirror
negative
decolourises
decolourises
no reaction
negative
negative
Ketone
yellow/orange ppt
negative
negative
negative
negative (cold)
no reaction
negative
negative
Primary alcohol
negative
negative
negative
negative
decolourises (warm)
negative (cold)
negative
negative
Secondary alcohol
negative
negative
negative
negative
decolourises (warm)
slow cloudiness
negative
negative
Tertiary alcohol
negative
negative
negative
negative
negative
immediate cloudiness
negative
negative
Alkene
negative
negative
negative
decolourises
decolourises
-
negative
negative
Carboxylic acid
negative
negative
negative
negative
negative
-
effervescence
negative
Phenol
negative
negative
negative
white ppt
negative
-
negative
purple/violet
Notes on the matrix: Lucas reagent applies only to alcohols. Acidified KMnO₄ requires heat for primary and secondary alcohol oxidation; tertiary alcohols are resistant. Aromatic aldehydes (e.g., benzaldehyde) give a positive Tollens' result but a negative Fehling's result - this distinction is a frequent exam point. The bromine water result for phenol includes both decolourisation and a white precipitate (2,4,6-tribromophenol).
Specimen-style evidence to identity decision table
Paper 4 answers should move from evidence to identity, not from memorised test names to guesses. Use phrasing like "This supports..." and "This rules out..." before naming the functional group.
Observation set
Evidence statement
Likely identity or next check
Acidified manganate(VII) is decolourised on warming, but 2,4-DNPH is negative
Oxidisable group present, but no carbonyl group detected before oxidation
Primary or secondary alcohol; use Lucas reagent or oxidation product tests
Alkaline iodine gives a yellow precipitate
Iodoform-positive group present
Methyl ketone, ethanal, ethanol, or a secondary alcohol with CH3CH(OH)-
Fehling's reagents give a brick-red precipitate after warming
Aliphatic aldehyde reduces Cu(II) to Cu(I) oxide
Aliphatic aldehyde; confirm with 2,4-DNPH and Tollens'
Tollens' positive but Fehling's negative
Aldehyde present, but not reducing Fehling's under these conditions
Aromatic aldehyde is likely
Phosphorus(V) chloride gives steamy white fumes
O-H group is present
Alcohol, phenol, or carboxylic acid; use carbonate and FeCl3 tests to separate
Sodium carbonate gives effervescence
Acidic group releases CO2 from carbonate
Carboxylic acid if bicarbonate also gives effervescence
Avoid writing "compound is X because reagent Y was added." The observation is the evidence; the reagent only creates the test condition.
3 | Test 1: 2,4-DNPH (Brady's reagent) - detecting carbonyls
What it detects: The carbonyl group (C=O) in both aldehydes and ketones.
Reagent: 2,4-dinitrophenylhydrazine dissolved in dilute sulfuric acid. This is Brady's reagent and is usually supplied as a ready-made orange solution in school labs.
Procedure: Add a few drops of the unknown compound to approximately 1 cm³ of Brady's reagent in a test tube. Shake gently.
Positive observation: A yellow, orange, or red crystalline precipitate forms. The colour of the precipitate corresponds loosely to the degree of conjugation in the product (2,4-dinitrophenylhydrazone), though for exam purposes "yellow/orange precipitate" is the standard safe answer for any carbonyl compound.
What it confirms: The presence of a carbonyl functional group (C=O). It does not distinguish between an aldehyde and a ketone.
What it rules out: If Brady's reagent gives no precipitate, neither an aldehyde nor a ketone is present.
9476 Paper 4 limit: The current syllabus says candidates are not required to carry out tests involving 2,4-DNPH in the practical examination. Keep this reaction in your organic reasoning toolkit, but do not build your Paper 4 lab plan around performing it unless the question explicitly provides the relevant context.
4 | Test 2: Tollens' reagent - the silver mirror test
What it detects: Aldehydes specifically. Ketones give no reaction.
Reagent preparation: This is the most technically demanding of the eight tests to prepare correctly, and incorrect preparation is a frequent source of error.
Dissolve a small quantity of silver nitrate (AgNO3) in water to give a clear solution.
Add a few drops of sodium hydroxide solution. A dark brown precipitate of silver(I) oxide forms.
Add aqueous ammonia (NH3(aq)) dropwise, swirling between drops, until the precipitate just dissolves and the solution becomes clear again. This produces the diamminesilver(I) complex, [Ag(NH3)2]+.
The reagent must be prepared fresh immediately before use. Do not store Tollens' reagent. If left to stand - particularly if any silver has deposited - explosive silver nitride (Ag3N) can form. Dispose of any unused reagent immediately by washing down the sink with plenty of water.
Procedure: Add a few drops of the unknown compound to the freshly prepared Tollens' reagent. Place the test tube in a water bath at approximately 60 °C. Do not heat directly over a flame.
Positive observation: A bright silver mirror forms on the interior of the glass test tube. The solution may also become grey if the silver deposits as a dispersed precipitate rather than a coherent mirror, but the mirror is the canonical Paper 4 observation.
The half-equation for the oxidation of an aldehyde:
RCHO+2[Ag(NH3)2]++3OH−→RCOO−+2Ag(s)+4NH3+2H2O
The aldehyde is oxidised to a carboxylate; the silver(I) ion is reduced to silver metal.
What it confirms: An aldehyde is present.
What it rules out: Ketones do not reduce Tollens' reagent under normal lab conditions. A negative result with Tollens' - after confirming a positive with 2,4-DNPH - points to a ketone.
5 | Test 3: Fehling's solution - detecting aliphatic aldehydes
What it detects: Aliphatic aldehydes. Aromatic aldehydes such as benzaldehyde give a negative result. Ketones give a negative result.
Reagent preparation: Fehling's solution is made by mixing equal volumes of two separate solutions immediately before use.
Fehling's A: a blue solution of copper(II) sulfate (CuSO4).
Fehling's B: a colourless alkaline solution of sodium potassium tartrate (Rochelle salt) in sodium hydroxide. The tartrate acts as a chelating ligand to keep copper(II) in solution under alkaline conditions.
When mixed, the two solutions give a deep blue solution containing the copper(II)-tartrate complex.
Procedure: Add the unknown compound to freshly mixed Fehling's solution. Warm gently in a water bath at approximately 60 °C. Do not boil.
Positive observation: A brick-red precipitate of copper(I) oxide (Cu2O) forms. The solution changes from blue to colourless (or pale blue) as the copper(II) is reduced.
Why aromatic aldehydes do not react: Benzaldehyde fails to reduce Fehling's solution because the electron-withdrawing effect of the benzene ring through resonance lowers the reducing power of the aldehyde group. The copper(II) complex, which is a weaker oxidising agent than the diamminesilver(I) complex in Tollens', cannot oxidise benzaldehyde under these mild conditions.
This is the key distinction: if a compound gives a positive Tollens' result but a negative Fehling's result, the aldehyde is aromatic.
6 | Test 4: Bromine water - alkenes, phenol, and activated aromatics
What it detects: Carbon-carbon double bonds (alkenes), phenol, and activated aromatic rings.
Reagent: Aqueous bromine solution (orange-brown).
Procedure: Add the unknown compound to bromine water in a test tube. Observe immediately and after gentle shaking. No heat is required for a true alkene or phenol; warming may be needed for some activated aromatic compounds.
Positive observations - three distinct cases:
Alkene: The orange-brown bromine water is decolourised rapidly. No precipitate forms. The reaction is electrophilic addition of bromine across the double bond.
Phenol: The orange-brown colour is decolourised and a white precipitate forms. The precipitate is 2,4,6-tribromophenol, formed by electrophilic substitution at the three positions activated by the hydroxyl group.
Simple benzene ring (not activated): No reaction at room temperature without a catalyst.
Common student mistake: Any decolourisation of bromine water is not automatically evidence of an alkene. Aldehydes can also decolourise bromine water by an oxidation mechanism (not addition). The presence of a white precipitate alongside decolourisation points specifically to phenol. If decolourisation occurs without precipitate in a compound already shown to contain no alkene by other methods, consider an aldehyde.
7 | Test 5: Acidified KMnO₄ - a broad oxidising test
What it detects: A wide range of oxidisable functional groups: alkenes, primary and secondary alcohols, aldehydes, and some aromatic side-chains with benzylic hydrogen atoms.
Reagent: Dilute potassium permanganate solution acidified with dilute sulfuric acid. The solution is purple.
Procedure: Add the unknown compound to acidified KMnO₄. Shake. For alcohols and some other groups, warming may be required before the colour change is observed.
Positive observation: The purple colour of permanganate is discharged. The solution becomes colourless (under acidic conditions, Mn2+ is the reduced product). Under neutral or alkaline conditions, a brown precipitate of MnO2 would form instead - but the acidified version is standard for Paper 4.
What it confirms: An oxidisable functional group is present. Acidified KMnO₄ is less selective than the other tests. A positive result narrows the field significantly but does not identify the group on its own.
Selectivity note: Tertiary alcohols do not react with acidified KMnO₄ at room temperature because they have no hydrogen on the carbon bearing the -OH group. This selectivity is exploited in multi-test identification: if bromine water and acidified KMnO₄ both give negative results for a compound known to be an alcohol, it is likely tertiary.
8 | Test 6: Lucas reagent - distinguishing alcohol classes
What it detects: The class of an alcohol (primary, secondary, or tertiary) by its rate of reaction with a halogenating agent.
Reagent: Zinc chloride (ZnCl2) dissolved in concentrated hydrochloric acid. The reagent is a Lewis acid catalyst system that converts the -OH group to -Cl via an SN1 mechanism (for secondary and tertiary alcohols) or SN2 (for primary).
Procedure: Add the unknown alcohol (a few drops or a small amount) to Lucas reagent in a test tube at room temperature. Observe for cloudiness (the alkyl chloride product is insoluble in the Lucas reagent).
Positive observations by alcohol class:
Tertiary alcohol: Immediate cloudiness (within seconds to 1 minute) due to rapid SN1 reaction stabilised by a tertiary carbocation.
Secondary alcohol: Cloudiness appears after approximately 5 to 10 minutes at room temperature.
Primary alcohol: No cloudiness at room temperature. A positive reaction requires heating.
What it confirms: The class of the alcohol. It does not identify the specific compound or the chain length.
Important limitation: Lucas reagent is only applicable to alcohols with fewer than approximately six carbon atoms. Longer-chain alcohols are not sufficiently soluble in the aqueous reagent for the test to work reliably.
9 | Test 7: Sodium carbonate and sodium bicarbonate - carboxylic acid vs phenol
What it detects: Acidic functional groups: carboxylic acids and phenols. The two reagents allow them to be distinguished.
Reagents and outcomes:
Na2CO3 (sodium carbonate): reacts with both carboxylic acids and phenols to produce effervescence (CO2) from carboxylic acids; phenols react to form sodium phenoxide but do not produce a gas.
NaHCO3 (sodium bicarbonate): reacts with carboxylic acids to produce brisk effervescence (CO2); does not react with phenols.
Key distinction: If the compound effervesces with NaHCO3, it is a carboxylic acid. If it does not effervesce with NaHCO3 but dissolves in Na2CO3 solution, phenol is more likely.
What this rules out: Alcohols and esters do not react with either sodium carbonate or sodium bicarbonate.
10 | Test 8: Neutral FeCl₃ - identifying phenol
What it detects: Phenols specifically, through formation of a coloured complex with iron(III) ions.
Procedure: Add a few drops of neutral FeCl₃ solution to the unknown compound in aqueous solution or dissolved in a small volume of water.
Positive observation: A purple or violet complex forms immediately. The colour arises from coordination of the phenoxide oxygen to the iron(III) centre.
What it confirms: A phenol group is present.
What it rules out: Simple aliphatic alcohols, carboxylic acids, and aldehydes do not give a purple complex with neutral FeCl₃. This test in combination with the NaHCO₃ test gives high confidence in distinguishing phenol from carboxylic acid: phenol gives purple with FeCl₃ and no effervescence with NaHCO₃; carboxylic acid gives neither purple complex nor the FeCl₃ colour, but does give effervescence with NaHCO₃.
9476 Paper 4 limit: The current syllabus says candidates are not required to carry out tests involving phenol in the practical examination. Use this section for interpretation and theory linkage only.
11 | Worked scenario: identifying an unknown compound
An unknown compound X gives the following test results:
2,4-DNPH: yellow-orange crystalline precipitate forms within 1 minute.
Tollens' reagent: no silver mirror after 5 minutes in a 60 °C water bath.
Fehling's solution: no brick-red precipitate after warming.
Acidified KMnO₄: purple colour is discharged on warming.
Lucas reagent: slow cloudiness after approximately 8 minutes at room temperature.
Interpreting the results step by step:
Step 1 - 2,4-DNPH positive confirms a carbonyl group (C=O). X is an aldehyde or a ketone.
Step 2 - Tollens' negative rules out an aldehyde (both aliphatic and aromatic). X is therefore a ketone.
Step 3 - Acidified KMnO₄ positive (on warming) seems initially inconsistent: ketones are not normally oxidised by KMnO₄. However, if X is a mixture or if the structure contains an additional secondary alcohol group elsewhere in the molecule, the KMnO₄ result is explained by that secondary alcohol.
Step 4 - Lucas test gives slow cloudiness (approximately 8 minutes) is consistent with a secondary alcohol, confirming that X contains a secondary -OH group.
Conclusion: X is most likely a hydroxy-ketone - a compound containing both a ketone carbonyl and a secondary alcohol group. A structure such as CH3CO-CH(OH)-CH3 (3-hydroxybutanone) would be consistent with all four results: positive 2,4-DNPH (ketone carbonyl), negative Tollens'/Fehling's (no aldehyde), positive acidified KMnO₄ on warming (secondary alcohol), and secondary Lucas rate.
This scenario illustrates the core strategy: each test result eliminates functional groups from the list of candidates. The conclusion is the intersection of what each positive confirms and each negative rules out.
12 | Common student errors
Using Fehling's on an aromatic aldehyde. Benzaldehyde gives a positive Tollens' result (silver mirror) but a negative Fehling's result (no brick-red precipitate). Students who treat these tests as interchangeable miss the aromaticity distinction. If the question gives a compound with a benzene ring and an aldehyde group, do not expect a Fehling's positive.
Assuming bromine water decolourisation equals alkene. Aldehydes also decolourise bromine water by oxidation, not addition. If the compound has already been shown to contain an aldehyde (positive 2,4-DNPH and Tollens'), bromine water decolourisation does not add evidence for a separate alkene group - it may simply be due to the aldehyde.
Skipping the water bath for Tollens' and Fehling's. Both tests require gentle heating (approximately 60 °C water bath) for a reliable result. Cold conditions with short contact time give false negatives. Never heat over a direct flame - bumping and spattering make the result uninterpretable and create safety issues.
Forgetting to acidify KMnO₄. Using neutral or alkaline KMnO₄ gives a brown precipitate (MnO2) as the reduced product, not a colourless solution. The observation must specify "acidified" and the endpoint must be reported as "purple discharged / colourless" - not "brown ppt forms".
Preparing Tollens' reagent in advance. Stale Tollens' reagent loses activity and may form explosive silver nitride. Prepare it immediately before use and dispose of the remainder before the end of the practical session.
13 | ACE evaluation: what examiners want
ACE (Accuracy, Completeness, Evaluation) marks in Paper 4 require more than stating a limitation - they require a quantified or mechanistic improvement strategy.
Tollens' reagent freshness. Tollens' reagent prepared more than a few minutes before use begins to deposit silver, reducing the concentration of the active [Ag(NH3)2]+ complex. This leads to a fainter silver mirror or a longer time to develop, potentially causing a false negative for a weakly reducing aldehyde. The qualitative limitation becomes quantifiable as follows: prepare Tollens' reagent in batches of no more than 2 cm³, use within 5 minutes, and standardise by checking that freshly prepared reagent gives a full silver mirror with a known 1% ethanal solution within 3 minutes at 60 °C before applying it to the unknown. This converts qualitative limitation into a quantitative fix - a target time and a reference compound set a measurable performance standard.
Temperature control in Fehling's and Tollens'. A water bath at 60 °C gives consistent mild heating across all replicates. Heating directly on a hot plate produces variable temperatures - the solution near the bottom of the tube may exceed 80 °C while the top remains cooler. This introduces variability in reaction rate and may cause charring or spattering that obscures the observation. Specification: use a water bath thermostatted to 60±2 °C and verify with a thermometer in the bath (not the tube) before beginning the series.
Contamination between tests. Rinsing test tubes with distilled water between tests can leave trace amounts of previous reagents, causing carry-over. For example, trace Tollens' reagent left in a tube can give a faint silver deposit with the next compound even if it is a ketone, leading to a false positive. Fix: rinse each tube three times with distilled water, then once with the new reagent before adding the sample; alternatively, use a fresh tube for each test.
Lucas reagent temperature sensitivity. The rate of cloudiness in the Lucas test is strongly temperature-dependent - a tertiary alcohol at 20 °C gives an immediate result, but at 15 °C the same reaction may take 2-3 minutes, potentially being misclassified as secondary. Control the ambient temperature by conducting all Lucas tests in a water bath at a fixed temperature of 25±1 °C, and compare results within a single session.
14 | Next steps
Work through these tests in combination with your broader Paper 4 preparation:
H2 Chemistry Qualitative Analysis Workflow - the inorganic counterpart, covering cation and anion identification with the QA Notes printed in the Paper 4 question paper.
H2 Chemistry notes hub - topic-by-topic 9476 notes for the organic-chemistry theory (Topic 11) behind these functional-group tests.
Need a supervised lab session? We run H2 Chemistry Paper 4 practical workshops with full equipment access, trained supervisors, and SEAB-aligned marking rubrics. Contact us to find out more →
