A Levels Chemistry (9701)•9701/12/M/J/20
Explanation
Periodic acid cleavage of vicinal diols Steps:
- Ethane-1,2-diol (HOCH₂CH₂OH) cleaves to 2 HCHO with periodic acid.
- Propane-1,2-diol (CH₃CH(OH)CH₂OH) cleaves to HCHO + CH₃CHO.
- Alkaline aqueous iodine(VII) provides HIO₄ for oxidative cleavage.
- Acetaldehyde from propane-1,2-diol gives iodoform test; formaldehyde from ethane-1,2-diol does not. Why A is correct:
- Periodic acid (from iodine(VII)) cleaves 1,2-diols to distinct aldehydes per the Malaprade reaction, enabling product-based distinction. Why the others are wrong:
- B: Both diols dehydrate similarly with ethanol/H₂SO₄ to form ethers, showing no difference.
- C: Both reduce acidified dichromate (orange to green) via alcohol oxidation.
- D: Both evolve H₂ with sodium due to acidic OH groups.
Final answer: A
Topic: Hydroxy compounds
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