Oxygen-Containing Organic Compounds — A/L Chemistry

How do I practise Oxygen-Containing Organic Compounds?

1 question with a worked solution

Check before you answer: Confirmatory Test Observations: Precise recorded observations for identification reagents: 2,4-DNP (orange/yellow precipitate for C=O), Tollens' (silver mirror for aldehydes), Lucas reagent (turbidity timing for 1°/2°/3° alcohols), and neutral FeCl3 (violet complex for phenols).

1. An unknown neutral organic compound X (C₃H₆O) forms an orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNP) and produces a shiny silver mirror with Tollens' reagent ([Ag(NH₃)₂]⁺). Identify compound X and the organic product formed when X is reduced by sodium borohydride (NaBH₄).
   1. A) Compound X is propanal; reduction product is propan-1-ol
   2. B) Compound X is prop-2-en-1-ol; reduction product is propan-1-ol
   3. C) Compound X is propanone; reduction product is propan-2-ol
   4. D) Compound X is methyl ethanoate; reduction product is methanol
   Check my answer

   Correct answer: A) Compound X is propanal; reduction product is propan-1-ol

   Reaction with 2,4-DNP indicates a carbonyl compound (aldehyde or ketone). Reaction with Tollens' reagent confirms that X is an aldehyde (ketones like propanone cannot be oxidized by Tollens' reagent). For C₃H₆O, the only aldehyde is propanal (CH₃CH₂CHO). Reduction of propanal with NaBH₄ converts the -CHO group to a primary alcohol, yielding propan-1-ol (CH₃CH₂CH₂OH).

   Show worked solution

   Step 1: Test 1 with 2,4-DNP: Formation of 2,4-dinitrophenylhydrazone indicates carbonyl functional group (C=O), so X is an aldehyde or a ketone. Step 2: Test 2 with Tollens' reagent ([Ag(NH₃)₂]⁺): Reduction of Ag⁺ to metallic silver Ag(s) indicates an aldehyde group (-CHO), as ketones do not react under mild oxidation. Step 3: Formula deduction: Molecular formula C₃H₆O with an aldehyde group must be propanal: CH₃CH₂CHO. Step 4: Reduction with NaBH₄: CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH (propan-1-ol). Answer: Compound X is propanal; reduction product is propan-1-ol.
   Modelled on: the A/L Chemistry examination pattern

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Where does Oxygen-Containing Organic Compounds fit in the syllabus?

Grade 12 · Term 3