Hydrocarbons and Halohydrocarbons — A/L Chemistry

What must I remember about Hydrocarbons and Halohydrocarbons?

• Alkanes are unreactive non-polar hydrocarbons that undergo free radical substitution with halogens under UV light (initiation, propagation, termination steps).
• Alkenes undergo electrophilic addition across the C=C double bond following Markovnikov's rule (electrophile adds to form the more stable carbocation intermediate).
• Benzene possesses special aromatic stability due to delocalized π electrons and undergoes electrophilic aromatic substitution rather than addition.
• Haloalkanes undergo nucleophilic substitution (SN1 vs SN2) and base-catalyzed elimination to form alkenes.
• Mechanistic contrast: SN1 is a two-step process via a planar carbocation intermediate (favored by 3° halides), while SN2 is a single-step concerted backside attack with inversion of configuration (favored by 1° halides).

Test yourself (1 minute)

1. What intermediate does electrophilic substitution on benzene pass through?

   Benzene electrophilic substitution mechanism proceeds via a non-aromatic arenium ion (sigma complex) intermediate before losing a proton to restore aromaticity.

2. Reagent distinction

   aqueous KOH promotes nucleophilic substitution to alcohols, whereas hot ethanolic KOH promotes elimination to alkenes.

3. Unsaturation tests

   alkenes decolorize bromine water (Br₂/H₂O or Br₂/CCl₄) without light and turn cold alkaline KMnO₄ brown (MnO₂ precipitate).

Practise Hydrocarbons and Halohydrocarbons questions today

Source: NIE G.C.E. (A/L) Chemistry Syllabus (Unit 8)

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Grade 12 · Term 3