Conversions and distinguishing tests
Board conversions take two or three steps, each with a named reagent. Distinguishing tests rely on Tollens', Fehling's, iodoform and NaHCO₃.
Conversions the board likes
| From → to | Steps, with reagents |
|---|---|
| Ethanol → 3-hydroxybutanal | PCC (or Cu, 573 K) → ethanal; dil. NaOH → CH₃CH(OH)CH₂CHO (aldol) |
| Ethanal → but-2-enal | dil. NaOH → 3-hydroxybutanal; heat → CH₃CH=CHCHO + H₂O |
| Propanone → propene | NaBH₄ → propan-2-ol; 85% H₃PO₄, 440 K → CH₃CH=CH₂ |
| Benzoic acid → benzaldehyde | SOCl₂ → benzoyl chloride; H₂, Pd-BaSO₄ (Rosenmund) → C₆H₅CHO |
| Toluene → benzoic acid | Alkaline KMnO₄, heat → potassium benzoate; H₃O⁺ → C₆H₅COOH |
| Benzene → m-nitroacetophenone | CH₃COCl, anhydrous AlCl₃ → acetophenone; conc. HNO₃ + conc. H₂SO₄ → 3-nitroacetophenone (-COCH₃ is meta-directing) |
| Bromobenzene → 1-phenylethanol | Mg, dry ether → C₆H₅MgBr; CH₃CHO, then H₃O⁺ → C₆H₅CH(OH)CH₃ |
| Ethanoic acid → methane | NaOH → CH₃COONa; soda lime, heat → CH₄ |
| Ethanoic acid → 2-chloroethanoic acid | Cl₂, red phosphorus (HVZ) → ClCH₂COOH |
| Propanoic acid → propenoic acid | Br₂, red P → CH₃CH(Br)COOH; alcoholic KOH, heat, then H₃O⁺ → CH₂=CHCOOH |
| Benzoic acid → 3-nitrobenzyl alcohol | conc. HNO₃ + conc. H₂SO₄ → 3-nitrobenzoic acid; B₂H₆, then H₃O⁺ → m-O₂NC₆H₄CH₂OH |
| 4-Methylacetophenone → benzene-1,4-dicarboxylic acid | Alkaline KMnO₄, heat; then H₃O⁺ (both side chains → -COOH) |
Distinguishing tests
| Pair | Test | What you see |
|---|---|---|
| Propanal and propanone | Tollens' reagent, warm | Propanal: silver mirror; propanone: no change |
| Ethanal and benzaldehyde | Fehling's solution, heat | Ethanal: red-brown precipitate; benzaldehyde: no change |
| Benzaldehyde and acetophenone | Tollens' reagent (or I₂ + NaOH) | Benzaldehyde: silver mirror; acetophenone: no mirror, but yellow CHI₃ with I₂ + NaOH |
| Acetophenone and benzophenone | I₂ + NaOH, warm | Acetophenone: yellow precipitate; benzophenone: none |
| Pentan-2-one and pentan-3-one | I₂ + NaOH, warm | Pentan-2-one: yellow precipitate; pentan-3-one: none |
| Ethanal and propanal | I₂ + NaOH, warm | Ethanal: yellow precipitate; propanal: none |
| Propanal and propan-1-ol | 2,4-DNP | Propanal: orange precipitate; propan-1-ol: none |
| Phenol and benzoic acid | NaHCO₃ solution (or neutral FeCl₃) | Benzoic acid: CO₂ effervescence; phenol: none. FeCl₃: phenol violet, benzoic acid buff precipitate |
| Benzoic acid and ethyl benzoate | NaHCO₃ solution | Acid: effervescence; ester: none |
| Methanoic acid and ethanoic acid | Tollens' reagent, warm | Methanoic acid: silver mirror (its C=O carbon carries an H, like an aldehyde); ethanoic acid: none |
Activity: tests for the aldehyde and ketone groups
- 1★2,4-DNP test (both): take 2 to 3 drops of the liquid, or a few crystals dissolved in 2 to 3 mL of ethanol. Add a few drops of alcoholic 2,4-DNP reagent and shake.A yellow, orange or orange-red precipitate shows a C=O group. If none forms, warm in a water bath for a few minutes and cool
- 2Schiff's test (aldehyde): to 3 to 4 drops of the compound, add 2 to 3 drops of Schiff's reagent (a magenta dye decolourised by SO₂).A pink colour appears with an aldehyde; ketones do not respond
- 3Tollens' test (aldehyde): to 1 mL of about 2% AgNO₃ solution, add 1 to 2 drops of NaOH; brown Ag₂O forms. Add ammonia solution drop by drop until the precipitate dissolves. Add the compound and heat in a water bath for about 5 minutes.A silver mirror forms on the inside of the test tube
- 4Fehling's test (aliphatic aldehyde): mix 1 mL each of Fehling's A and B. Add 2 to 3 drops of the compound and heat in a water bath for about 2 minutes.A red-brown (brick red) precipitate of Cu₂O
- 5Sodium nitroprusside test (ketone): add a few drops of freshly made sodium nitroprusside solution to the compound, then NaOH solution drop by drop.A red colour shows a ketone with a CH₃CO- group, such as acetone. Many school labs use this test, but your lab manual may not list it
★ key step
Activity: tests for the carboxyl group
- 1Litmus test: put a drop of the compound (or its solution) on moist blue litmus paper.It turns red. Phenols also do this, so confirm with the next test
- 2★Sodium hydrogencarbonate test: add the compound to about 2 mL of saturated NaHCO₃ solution.Brisk effervescence; the gas (CO₂) turns lime water milky
- 3Ester test: heat about 0.1 g of the compound with 1 mL of ethanol and 2 to 3 drops of conc. H₂SO₄ in a water bath at about 50 °C for 10 to 15 minutes. Pour the mixture into sodium carbonate solution.A sweet, fruity smell of an ester. The Na₂CO₃ neutralises the leftover acids so the smell is clear
- 4Neutral FeCl₃ test: add neutral ferric chloride solution to a neutral solution of the acid.Ethanoate gives a deep red colour; benzoate gives a buff precipitate
★ key step
Worked example: identify the compound
Compound A (C₈H₈O) gives an orange precipitate with 2,4-DNP and a yellow precipitate with I₂ and NaOH. It does not reduce Tollens' reagent. Vigorous oxidation of A with alkaline KMnO₄ gives benzoic acid. Identify A and write its iodoform reaction.
- 2,4-DNP positive, so A has a C=O group.
- No Tollens' reaction, so A is a ketone.
- Iodoform positive, so A has a CH₃CO- group.
- Oxidation gives benzoic acid, so A has one side chain on a benzene ring.
- C₆H₅- (6 C) + -COCH₃ (2 C) = 8 C. So A is C₆H₅COCH₃, acetophenone (C₈H₈O).
- C₆H₅COCH₃ + 3I₂ + 4NaOH → C₆H₅COONa + CHI₃ + 3NaI + 3H₂O
A is acetophenone (1-phenylethanone), C₆H₅COCH₃.
How to answer a conversion question
- Count the carbons first. Same number: only the functional group changes. One more: use KCN, or a Grignard reagent with CO₂. One fewer: use soda lime (decarboxylation).
- Write each step as its own arrow, with the reagent above and the condition below.
- Name the intermediate at each step, and the named reaction where there is one (Rosenmund, HVZ, Etard).
- Check ring directing effects: -CHO, -COR and -COOH are meta-directing, so put them in first if you want a meta product.
- Prefer selective reagents: PCC to stop at an aldehyde, B₂H₆ to reduce -COOH but not -NO₂.
Food labs find the reducing sugars in honey and jaggery by titrating them with Fehling's solution. They find the acid content of vinegar by titrating it with a base.
2 marks: “How will you convert: (i) propan-1-ol to 3-hydroxy-2-methylpentanal (ii) ethanoic acid to ethanal?” (i) CH₃CH₂CH₂OH → (PCC) CH₃CH₂CHO → (dil. NaOH) CH₃CH₂CH(OH)CH(CH₃)CHO. (ii) CH₃COOH → (SOCl₂) CH₃COCl → (H₂, Pd-BaSO₄) CH₃CHO.
3 marks: “Give a simple chemical test to distinguish: (i) phenol and propanoic acid (ii) hexan-2-one and hexan-3-one (iii) methanoic acid and ethanoic acid.” (i) NaHCO₃: CO₂ effervescence with propanoic acid only. (ii) I₂ + NaOH: yellow CHI₃ with hexan-2-one only. (iii) Tollens' reagent: silver mirror with methanoic acid only.
3 marks: “How will you bring about: (i) butanone to but-2-ene (ii) bromobenzene to 2-phenylpropan-2-ol (iii) benzene to 3-bromoacetophenone?” (i) NaBH₄ → butan-2-ol → (85% H₃PO₄, 440 K) but-2-ene (major). (ii) Mg/dry ether → C₆H₅MgBr → CH₃COCH₃, then H₃O⁺ → C₆H₅C(OH)(CH₃)₂. (iii) CH₃COCl/anhydrous AlCl₃ → acetophenone → Br₂/FeBr₃ → 3-bromoacetophenone, as -COCH₃ is meta-directing.
Count carbons first. +1: KCN or CO₂. −1: soda lime. Then test by group: CHO → Tollens', CH₃CO → iodoform, COOH → NaHCO₃.