Hydrocarbons and Halohydrocarbons — A/L Chemistry

How do I practise Hydrocarbons and Halohydrocarbons?

1 question with a worked solution

Check before you answer: Markovnikov Addition: Mechanistic explanation of electrophilic addition to unsymmetrical alkenes via the more stable carbocation intermediate.

1. In the electrophilic aromatic substitution reaction between methylbenzene (toluene) and a nitration mixture (conc. HNO₃ + conc. H₂SO₄) at 50 °C, what is the attacking electrophile and the directing effect of the methyl group?
   1. A) Electrophile is NO₂⁻; methyl group is deactivating and meta-directing
   2. B) Electrophile is NO₃⁻; methyl group is activating and meta-directing
   3. C) Electrophile is NO₂⁺; methyl group is activating and ortho/para-directing
   4. D) Electrophile is HNO₂; methyl group is deactivating and ortho/para-directing
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   Correct answer: C) Electrophile is NO₂⁺; methyl group is activating and ortho/para-directing

   Concentrated H₂SO₄ protonates concentrated HNO₃ to generate the reactive nitronium ion electrophile: HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2HSO₄⁻. The methyl (-CH₃) substituent is electron-donating via inductive (+I) and hyperconjugation effects, which selectively stabilizes carbocation intermediates at the ortho and para positions, directing substitution to ortho-nitrotoluene and para-nitrotoluene.

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   Step 1: Identify generation of active electrophile: HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2HSO₄⁻ The electrophile is the nitronium ion (NO₂⁺). Step 2: Assess electronic effect of -CH₃ group on benzene ring: - Alkyl groups release electrons through positive inductive effect (+I) and C-H hyperconjugation. - Electron density increases especially at positions 2, 4, 6 (ortho and para). Step 3: Evaluate Wheland intermediate stability: The carbocation formed upon attack at ortho/para positions has an especially stable tertiary-like resonance contributor where the positive charge is directly on the carbon bearing the methyl group. Step 4: Conclude: Electrophile is NO₂⁺; methyl group is activating and ortho/para-directing. Answer: Electrophile is NO₂⁺; methyl group is activating and ortho/para-directing.
   Modelled on: the A/L Chemistry examination pattern

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Where does Hydrocarbons and Halohydrocarbons fit in the syllabus?

Grade 12 · Term 3