Flashcards · 9620 · AS Topic 22

Alkenes (AS Level) flashcards

19 cards on AS Topic 22, subtopics 3.3.4.1 to 3.3.4.4. Pick a subtopic or study them all.

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Why are alkenes attacked by electrophiles?: The C=C double bond (four shared electrons) is a centre of high electron density, which attracts electrophiles.
Electrophile: An electron pair acceptor: a positive ion or a species with a δ+ atom.
Unsaturated: Contains a C=C double bond, so more atoms can add on.

Electrophilic addition of HBr to an alkene: the three curly arrows: From the middle of the C=C to the Hδ+ of HBr; from the H–Br bond to Br; then from a lone pair on Br⁻ to the carbocation C⁺.
Why can non-polar Br₂ act as an electrophile?: The electron density of the C=C repels the Br–Br bonding electrons, inducing a dipole: the nearer Br becomes δ+.
Test for a C=C double bond: Shake with bromine water: orange to colourless if C=C is present. A saturated compound leaves it orange.
Why does propene + HBr give mainly 2-bromopropane?: It forms via the secondary carbocation, which is more stable than the primary one, because alkyl groups release electrons and spread the positive charge. The more stable carbocation forms more readily.
Order of stability of carbocations: Primary < secondary < tertiary.
Reagent and conditions for adding sulfuric acid to an alkene: Cold, concentrated H₂SO₄. The H of H₂SO₄ is the electrophile; the hydrogensulfate ion bonds to C⁺ through its negatively charged O.

How do you draw the repeating unit of an addition polymer from its monomer?: Turn the C=C into C–C, keep every group on its own carbon, add extension bonds, brackets and n.
Monomer of the chain –CH₂–CF₂–CH₂–CF₂–: CH₂=CF₂ (1,1-difluoroethene).
Why are poly(alkenes) unreactive?: They are saturated (no C=C left) and contain strong, non-polar C–C and C–H bonds.
How does a plasticiser make PVC flexible?: Small molecules lodge between the polymer chains and push them apart, weakening the intermolecular forces so the chains can slide over each other.
Uses of unplasticised and plasticised PVC: Unplasticised (rigid): water and sewage pipes, doors, bank cards. Plasticised (flexible): cable insulation, inflatable products, flooring.

How is epoxyethane made industrially?: 2CH₂=CH₂ + O₂ → 2C₂H₄O, silver catalyst (about 220–280 °C, 1–3 MPa). Atom economy 100%.
Why is epoxyethane so reactive?: Its three-membered ring has bond angles of about 60° (not 109.5°), so it is strained with weak C–O bonds; the polar Cδ+–Oδ− bonds attract nucleophiles.
Hazards of making and using epoxyethane: The competing combustion of ethene is far more exothermic; epoxyethane is extremely flammable, explosive with air, toxic and carcinogenic.
Products of epoxyethane with water and with ethanol: Water: ethane-1,2-diol, HOCH₂CH₂OH (antifreeze, coolant, making PET). Ethanol: 2-ethoxyethanol, CH₃CH₂OCH₂CH₂OH (a glycol ether solvent).
How are non-ionic surfactants made from epoxyethane?: A long-chain alcohol reacts with several epoxyethane molecules: ROH + nC₂H₄O → R(OCH₂CH₂)ₙOH, giving a non-polar tail and an uncharged polar head.