Theory: Organic Chemistry
1. Phthalein Dye Test Details
Reaction: phenol + phthalic anhydride →(H₂SO₄, heat) colourless product. Add NaOH → ring opens → pink/red fluorescent dye (fluorescein). The fluorescent pink colour is characteristic. Only aromatic phenols react — the acidic phenolic OH is needed for the condensation. Aliphatic alcohols, aldehydes, ketones, carboxylic acids: no reaction.
2. FeCl₃ Test for Phenols
Neutral FeCl₃ + phenol → violet/purple colour (ferric phenoxide complex). Different phenols give different colours: phenol=violet, cresols=green, catechol=green, resorcinol=violet, salicylic acid=violet. Enols also give positive FeCl₃ test. Alcohols, aldehydes: no colour with FeCl₃.
3. Distinguishing Phenol from Alcohol
Phenol tests: FeCl₃ (violet), phthalein dye (pink fluorescence), bromine water (white ppt 2,4,6-tribromophenol instantly). Alcohol tests: Lucas test (ZnCl₂/HCl), esterification. Phenols dissolve in NaOH but not Na₂CO₃. Carboxylic acids dissolve in both. Na metal: both phenol and alcohol evolve H₂ (but phenol faster).
4. Tollens Test for Aldehydes
Aldehydes reduce Tollens' reagent [Ag(NH₃)₂]⁺ → silver mirror. RCHO + 2[Ag(NH₃)₂]⁺ → RCOO⁻ + 2Ag↓. Ketones do not react. Reducing sugars (glucose, fructose, maltose): positive. Sucrose: negative (non-reducing). Formaldehyde especially good (gives Ag mirror quickly).
5. Fehling's Test for Aldehydes
Fehling's solution = Cu²⁺ complex (blue). Aldehyde reduces → Cu⁺ (red Cu₂O precipitate). Aliphatic aldehydes: positive. Aromatic aldehydes (benzaldehyde): NEGATIVE (cannot reduce Fehling's). Ketones: negative. Glucose, fructose: positive. Sucrose: negative.
6. Iodoform Test
CH₃COR (methyl ketone) + I₂/NaOH → CHI₃ (iodoform, yellow crystals) + RCOONa. Positive for: CH₃CHO, CH₃COR, CH₃CH₂OH (ethanol), CH₃CHOH−R (secondary methyl carbinol). Negative for: HCHO, most aldehydes, ketones without CH₃CO− group.
7. Carbylamine Test for Primary Amines
RNH₂ + CHCl₃ + 3KOH(alc.) → RNC (isocyanide, foul smell). ONLY primary amines give this test. Secondary and tertiary amines: negative. Distinguish 1°, 2°, 3° amines this way.
8. Lucas Test for Alcohols
3° alcohol → immediate turbidity. 2° → turbidity in 5 min. 1° → no turbidity at RT. Works for ≤6 carbon alcohols (must be soluble in Lucas reagent). Phenols: no reaction with Lucas reagent at all.
Frequently Asked Questions
1. Why phthalein dye test is specific to phenols? ⌄
The reaction requires the acidic aromatic −OH group. The −OH of phenol is more reactive than aliphatic −OH because benzene ring delocalisation makes phenol more acidic (pKa~10 vs ~16 for alcohols). The condensation with phthalic anhydride requires this specific reactivity. Aliphatic −OH is too weakly acidic to undergo this condensation under these conditions.
2. What is fluorescein and what gives it colour? ⌄
Fluorescein is the dye formed in the phthalein dye test: phenol + phthalic anhydride → fluorescein. Its structure contains an extended π system with multiple conjugated rings including a xanthene core. In alkaline solution, the phenoxide form has a large conjugated system that absorbs blue light (~490 nm) and fluoresces strongly green/yellow-green. The fluorescence is bright even at very low concentrations — making this a sensitive test.
3. Can the phthalein dye test be used for quantitative analysis? ⌄
No — it's a qualitative identification test only. For quantitative analysis of phenols: spectrophotometric methods (UV absorption at 270 nm), colorimetric methods (Folin-Ciocalteu reagent), or HPLC are used. The phthalein dye test just confirms the presence of phenolic −OH group.
4. What is the structure of phthalic anhydride? ⌄
Phthalic anhydride = 1,2-benzenedicarboxylic anhydride. Structure: benzene ring with two −CO− groups forming a cyclic anhydride ring at ortho positions. Molecular formula C₈H₄O₃. In the phthalein dye test, 2 moles of phenol react with 1 mole of phthalic anhydride. The reaction is a condensation (acid-catalysed by conc. H₂SO₄) to form the colourless compound, which then opens to give fluorescein in base.
5. Why does bromine water decolourise with phenol instantly? ⌄
Phenol undergoes electrophilic aromatic substitution rapidly with Br₂(aq) at ALL ortho and para positions simultaneously — no catalyst needed (unlike benzene). This is because the −OH group is strongly activating (donates electrons via resonance to ortho/para). Product: 2,4,6-tribromophenol (white precipitate, insoluble in water). This instant decolourisation of bromine water with white precipitate formation is the most rapid test for phenol.
6. How do you distinguish salicylic acid from benzoic acid? ⌄
Both are aromatic carboxylic acids. Salicylic acid has an extra −OH group (ortho to COOH). Distinguishing tests: 1. FeCl₃: salicylic acid gives violet colour (phenolic OH); benzoic acid gives no/yellowish colour. 2. Na₂CO₃ + heat: salicylic acid decomposes to phenol + CO₂ (decarboxylation); benzoic acid does not. 3. Phthalein dye test: salicylic acid positive (has phenolic OH); benzoic acid negative. 4. Melting point: salicylic acid (159°C) vs benzoic acid (122°C).
7. What functional groups are present in aspirin? ⌄
Aspirin = acetylsalicylic acid = 2-acetoxybenzoic acid. Structure: benzene ring with −COOH (ortho) and −OCOCH₃ groups. Functional groups: carboxylic acid (−COOH) and ester (−OCOCH₃). Note: the phenolic −OH of salicylic acid has been acetylated → aspirin does NOT give FeCl₃ test (no free phenolic OH). If aspirin hydrolyses (old/hydrated tablets): free salicylic acid formed → FeCl₃ gives violet colour. This is used to test aspirin quality.
8. Why is phenol more acidic than alcohol? ⌄
Phenol (ArOH): after losing H⁺, the phenoxide ion (ArO⁻) is stabilised by resonance — the negative charge delocalises into the benzene ring (ortho/para positions). This extra stability makes phenol easier to ionise (pKa ~10). Alcohol (ROH): after losing H⁺, alkoxide ion (RO⁻) has no resonance stabilisation — localised negative charge is less stable → harder to ionise (pKa ~16-18). More stable conjugate base → stronger acid. Phenol is ~10⁶ times more acidic than typical alcohols.