Distinguishing Tests, Uses & Important Compounds

The Distinguishing-Tests Toolbox

This section collects the chapter's laboratory tests so you can tell the related compounds apart at a glance.

Carbonyl present (aldehyde or ketone)? → 2,4-DNP test gives an orange-yellow precipitate for any aldehyde or ketone (not for acids/alcohols).

Aldehyde vs ketone? → Tollens' reagent (silver mirror) and Fehling's solution (red Cu$_2$O) are positive for aldehydes only. (Fehling's responds only to aliphatic aldehydes; benzaldehyde does not.)

Methyl ketone / $\mathrm{CH_3CO-}$ group? → iodoform test (yellow $\mathrm{CHI_3}$ with $\mathrm{I_2/NaOH}$).

Carboxylic acid? → $\mathrm{NaHCO_3}$ gives brisk effervescence ($\mathrm{CO_2}$) — and only carboxylic acids do this among common -OH-containing organic compounds.

Phenol? → neutral $\mathrm{FeCl_3}$ gives a violet colour (acids and alcohols do not).

Key Point: 2,4-DNP = any carbonyl; Tollens/Fehling = aldehydes; iodoform = $\mathrm{CH_3CO-}$/$\mathrm{CH_3CH(OH)-}$; $\mathrm{NaHCO_3}$ = carboxylic acid; neutral $\mathrm{FeCl_3}$ = phenol.

Worked Distinctions (the exam classics)

Acetaldehyde vs acetone: both give 2,4-DNP and iodoform; but only acetaldehyde gives a Tollens' silver mirror and Fehling's red precipitate (it is the aldehyde).

Acetic acid vs phenol: acetic acid → effervescence with $\mathrm{NaHCO_3}$; phenol → violet with neutral $\mathrm{FeCl_3}$.

Benzaldehyde vs acetaldehyde: both give Tollens' mirror; but only acetaldehyde gives a Fehling's precipitate and a positive iodoform test; benzaldehyde (aromatic, no $\mathrm{CH_3CO-}$ group) gives neither.

Pentan-2-one vs pentan-3-one: both are ketones (2,4-DNP positive), but only pentan-2-one has a $\mathrm{CH_3CO-}$ group → positive iodoform; pentan-3-one is iodoform-negative.

Key Point: combine tests — 2,4-DNP first (is it a carbonyl?), then Tollens/Fehling (aldehyde?), then iodoform (methyl ketone?), and $\mathrm{NaHCO_3}$ / $\mathrm{FeCl_3}$ to spot acids vs phenols.

Commercially & Biologically Important Compounds

Methanal (formaldehyde): sold as formalin (about 40% aqueous solution); used as a disinfectant/preservative of biological specimens and to make bakelite and other resins.

Ethanal (acetaldehyde): an intermediate in making acetic acid, ethyl acetate and many chemicals.

Propanone (acetone): an important solvent (for paints, varnishes, nail-polish remover) and chemical feedstock; made industrially by the cumene process (along with phenol).

Benzaldehyde: "oil of bitter almonds", used in perfumes and flavourings and as a synthetic intermediate.

Methanoic acid (formic acid): used in the leather (tanning) and rubber industries and as a reducing agent.

Ethanoic acid (acetic acid): the acid of vinegar (about 5–8% solution); a major industrial solvent and feedstock.

Benzoic acid and its sodium salt (sodium benzoate): common food preservatives. Higher fatty acids (stearic, palmitic) form soaps (their sodium salts).

Key Point: formalin (formaldehyde) = preservative/bakelite; acetone = solvent (cumene process); acetic acid = vinegar; sodium benzoate = food preservative; fatty-acid salts = soaps.

Solved Examples

Example 1: Full identification

A liquid gives an orange 2,4-DNP precipitate, a Tollens' silver mirror, but no iodoform. Suggest its identity class.

Solution: 2,4-DNP positive ⇒ carbonyl; Tollens positive ⇒ aldehyde; iodoform negative ⇒ not ethanal or a methyl ketone. So it is an aldehyde other than ethanal (e.g. propanal, benzaldehyde).

Example 2: Acid or phenol?

An unknown gives effervescence with $\mathrm{NaHCO_3}$. Acid or phenol?

Solution: Carboxylic acid — only acids are strong enough to liberate $\mathrm{CO_2}$ from $\mathrm{NaHCO_3}$.

Example 3: Two ketones

Distinguish pentan-2-one from pentan-3-one.

Solution: Iodoform test: pentan-2-one has a $\mathrm{CH_3CO-}$ group → yellow $\mathrm{CHI_3}$ (positive); pentan-3-one has no $\mathrm{CH_3CO-}$ → negative.

Example 4: Identify formalin

What is formalin and one of its uses?

Solution: Formalin is a 40% aqueous solution of methanal (formaldehyde); it is used as a disinfectant and a preservative for biological specimens (and to make bakelite).

Example 5: Benzaldehyde vs acetaldehyde

Give one test that distinguishes benzaldehyde from acetaldehyde.

Solution: The iodoform test (or Fehling's): acetaldehyde is positive (iodoform and Fehling's); benzaldehyde is negative for both. (Both, however, give a Tollens' mirror.)

Example 6: Acetone uses

Give two important uses of acetone.

Solution: As an important organic solvent (paints, varnishes, nail-polish remover) and as a chemical feedstock (it is co-produced with phenol in the cumene process).

Example 7: Vinegar

What is the active component of vinegar and its approximate concentration?

Solution: Ethanoic (acetic) acid, present as roughly a 5–8% aqueous solution in vinegar.

Example 8: Aldehyde vs ketone in one reagent

Which single reagent distinguishes an aldehyde from a ketone?

Solution: Tollens' reagent — the aldehyde gives a silver mirror; the ketone gives no reaction.

Example 9: Sodium benzoate

Why is sodium benzoate added to foods?

Solution: It is a food preservative — it inhibits the growth of moulds and bacteria, extending shelf life.

Example 10: Choosing tests in sequence

You have four bottles: an aldehyde, a methyl ketone, a carboxylic acid and a phenol. Outline a test scheme.

Solution: (1) $\mathrm{NaHCO_3}$ → effervescence identifies the carboxylic acid. (2) Neutral $\mathrm{FeCl_3}$ → violet identifies the phenol. (3) Tollens' reagent → silver mirror identifies the aldehyde. (4) The remaining one (no Tollens', positive iodoform) is the methyl ketone.