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A Levels Chemistry (9701)•9701/12/M/J/25
Question 36 from 9701/12/M/J/25

Explanation

Dehydration of propan-1-ol yields propene, which undergoes electrophilic addition with Br2.

Steps:

  • Propan-1-ol (CH3CH2CH2OH) dehydrates over Al2O3 to form propene (CH3CH=CH2) via loss of water.
  • Propene reacts with Br2 in anti-Markovnikov fashion? No, it's symmetrical addition across the double bond.
  • The product is 1,2-dibromopropane (CH3CHBrCH2Br), a vicinal dibromide.
  • Match the structural formula showing Br on adjacent carbons in a three-carbon chain.

Why D is correct:

  • D depicts CH3-CHBr-CH2Br, matching the addition product formula from alkene + Br2, following Markovnikov's rule for unsymmetrical alkenes.

Why the others are wrong:

  • A shows monobromopropane (CH3CH2CH2Br), a substitution product, not addition.
  • B shows 2-bromopropane (CH3CHBrCH3), incorrect regiochemistry for propene addition.
  • C shows a geminal dibromide (CH3CBr2CH3), violating anti addition mechanism.

Final answer: D

Topic: Hydrocarbons

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