A-Level Chemistry May/June 2025 Q3(e): Alkene Z contains two C=C bonds. Z reacts with an excess of hot concentrated acidified…

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  The reaction involves the oxidative cleavage of a diene (an alkene with two C=C bonds) by hot, concentrated, acidified KMnO₄. We can deduce the structure of the original alkene, Z, by working backwards from the oxidation products.

  Step 1: Analyse the oxidation products to identify the fragments of Z.

  • Product: CH₃COCH₃ (propanone) A ketone is formed from the oxidation of a disubstituted carbon atom in a double bond (R₂C=). This means one of the C=C bonds in Z was of the form ...=C(CH₃)₂.

  • Product: CO₂ and H₂O Carbon dioxide and water are formed from the complete oxidation of a terminal, unsubstituted CH₂= group. This means Z has a H₂C=... group at one end of the molecule.

  • Product: HOOCCH₂COOH (propanedioic acid) A dicarboxylic acid is formed from the oxidation of a chain segment between two double bonds. The two -COOH groups originate from two =CH- groups, and the -CH₂- group between them was not part of a double bond. This means there was a ...=CH-CH₂-CH=... fragment in the middle of Z.

  Step 2: Assemble the fragments to construct Z.

  We have three fragments:

  1. H₂C= (from CO₂)
  2. ...=C(CH₃)₂ (from CH₃COCH₃)
  3. ...=CH-CH₂-CH=... (from HOOCCH₂COOH)

  These fragments must be joined together at the double bonds to form the original diene. The only way to connect these three pieces is:

  H₂C= + CH-CH₂-CH + =C(CH₃)₂

  This gives the final structure for Z.

  Structure of Z:

  H₂C=CH-CH₂-CH=C(CH₃)₂

  Alternatively, a skeletal formula can be drawn:

        CH₃
       /
  //\/=
      \
       CH₃
  

  (5-methylhepta-1,4-diene)

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  How the marks are awarded

  • M1 — Correctly deducing any one of the structural features of Z from its oxidation products. For example, stating that the formation of CH₃COCH₃ implies a ...=C(CH₃)₂ group, OR that CO₂ implies a H₂C=... group, OR that HOOCCH₂COOH implies a ...=CHCH₂CH=... group.
  • M2 — Correctly assembling all the deduced fragments to give the full, correct structure of Z, which is H₂C=CHCH₂CH=C(CH₃)₂.

  Common mistakes

  • Incorrectly assembling the fragments, for example putting the C(CH₃)₂ group in the middle of the chain.
  • Misinterpreting the product propanedioic acid (HOOCCH₂COOH), leading to an incorrect central fragment, e.g., using a -CH₂CH₂- link instead of -CH₂-.
  • Confusing the oxidation rules, e.g., thinking a ketone comes from a monosubstituted C=C bond.
  • Drawing a structure with only one double bond or an incorrect number of carbon atoms.

  Examiner tip: Master the rules for oxidative cleavage of alkenes, as working backwards from the products is a reliable method to determine the original structure.

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