Flashcards · 7405 · AS Topic 22

Alkenes (AS Level) flashcards

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Why do alkenes react with electrophiles?: The C=C bond has two shared pairs between the same two carbons, so it is a centre of high electron density that attracts electron-deficient species.
Electrophile: An electron pair acceptor, e.g. H⁺, Hδ+ in HBr, or Brδ+ in Br₂.
Test for a C=C bond: Shake with bromine water at room temperature: orange → colourless for an alkene. An alkane stays orange.
Carbocation stability order, and why: Tertiary > secondary > primary. More alkyl groups release electron density and spread the positive charge.
Major product of 2-methylpropene + HBr: 2-bromo-2-methylpropane, formed via the more stable tertiary carbocation (CH₃)₃C⁺.
Product of ethene with cold concentrated sulfuric acid: Ethyl hydrogensulfate, CH₃CH₂OSO₂OH. The hydrogensulfate bonds to carbon through its negative O atom.
Why are polyalkenes unreactive?: The chains are saturated, with strong, non-polar C–C and C–H bonds and no C=C for electrophiles to attack.
How does a plasticiser make PVC flexible?: Its small molecules get between the polymer chains, push them apart and weaken the intermolecular forces, so the chains slide more easily.