Part 2: A connected strategy for organic qualitative analysis

Choose tests that distinguish competing structures and combine observations without claiming more than the evidence supports.

Ask what each test can actually establish

Use separate portions of the sample for different tests so one reagent does not contaminate the next. Begin with plausible candidate functional groups from the structure, formula or other evidence. Select a test that separates those candidates rather than repeating several non-discriminating observations.

Record an observation first—decolourisation, precipitate, silver deposit or effervescence—then explain the reaction and inference. “Carbon dioxide” is an inferred identity, while bubbling is the initial observation. Confirmatory evidence can identify the gas where needed.

One molecule can contain several groups and give several positive tests. Conversely, different groups may respond to the same reagent. Build a consistent overall structure instead of assuming each test identifies one unique molecule.

The required functional-group tests in context

The table summarises the OCR test families. Reagents, conditions and physical observations belong together. A negative result is meaningful only if the test was carried out appropriately and the sample could contact the reagent.

The specified acidity distinction for phenol is reaction with NaOH but no reaction with carbonate. C₆H₅OH + OH⁻ → C₆H₅O⁻ + H₂O shows neutralisation. This is weaker acidity than a carboxylic acid in the required comparison. The bromine test is useful additional evidence, but does not replace learning the acidity behaviour.

Worked identification: combine positive and negative results

An original unknown has molecular formula C₄H₈O. It gives a DNPH precipitate, no silver mirror with Tollens’ reagent and no carbonate effervescence. DNPH supports a carbonyl; the negative Tollens’ result favours a ketone within the simple candidates; the formula and carbonate result are inconsistent with assigning COOH.

Butan-2-one is a suitable structure. The conclusion comes from combining the carbonyl class with the four-carbon formula. A DNPH precipitate alone would also fit several aldehydes and would not determine that structure.

Suppose a different unknown both effervesces with carbonate and gives a DNPH precipitate. Do not discard one result: a molecule can contain COOH and a separate aldehyde or ketone. Use further evidence to distinguish and locate them.

Avoid turning a useful test into an absolute rule

Bromine-water decolourisation is not unique to alkenes because phenol reacts readily too; the white precipitate and other structural evidence help distinguish the cases. Dichromate oxidation is not unique to alcohols because aldehydes also reduce it.

Silver nitrate tests require release of halide ions from haloalkanes. Aryl halides do not undergo ordinary hydrolysis in the same way as typical haloalkanes, so absence of a precipitate under those conditions is not a universal proof that a molecule contains no halogen.

Some hydroxyketones can reduce Tollens’ reagent through chemistry beyond the simple aldehyde/ketone comparison. The core rule is useful within its intended scope; supplied unfamiliar structures or data can require a more careful interpretation.

Turn the observations into a defensible answer

For each test write reagent and relevant conditions, visible observation, then the functional-group inference. Check whether another candidate would give the same observation. Choose a second test or an analytical method when the first is ambiguous.

In a practical plan include separate aliquots, suitable controls or reference samples where useful, and safe handling appropriate to the actual reagents. Do not propose mixing every reagent in one tube. Module 1 also expects interpretation of limitations and the quality of evidence.

A strong identification can combine a functional-group test with IR, molecular mass and NMR. Chemical tests establish reactive features; spectroscopy can resolve the detailed connectivity that a colour change cannot.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources