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JEE Main levelSingle correct optionDifficulty 2 of 3nm018

An open-chain (acyclic) hydrocarbon has the molecular formula . Which of the following CANNOT be its IUPAC name?

  1. AHexa-1,5-diyne
  2. BHex-1-en-3-yne
  3. CHex-1-en-5-yne
  4. DHexa-1,3,5-triene

Answer: A

C6H8 has DU = (12 + 2 - 8)/2 = 3, and with no ring there must be three pi bonds. A triene (3 pi bonds) and an ene-yne (1 + 2 = 3 pi bonds) both fit. Hexa-1,5-diyne has 4 pi bonds and the formula C6H6, so it cannot be C6H8.
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