Alkanes and free radicals
Alkanes are saturated hydrocarbons, fairly unreactive because C–C and C–H bonds are strong and non-polar. They're used as fuels: complete combustion gives CO₂ and H₂O; incomplete gives CO and soot. Cracking breaks long chains into shorter alkanes and alkenes (thermal: high T and p; catalytic: zeolite, lower T). With Cl₂ or Br₂ in UV light, alkanes undergo free-radical substitution: initiation (), propagation (; ), termination (two radicals join). The mixture of products is a disadvantage. Cl• radicals from CFCs destroy ozone catalytically.
≈ 45 minAlkenes and electrophilic addition
The C=C bond has a σ bond and a weaker π bond with high electron density, which attracts electrophiles. In electrophilic addition, the π electrons attack the electrophile (e.g. H in HBr, or an induced-dipole Br₂), forming a carbocation; the anion then bonds to it. Alkenes add H₂ (Ni, 150 °C), steam (H₃PO₄, 300 °C, 60 atm), HX and X₂. Markovnikov: with unsymmetrical alkenes, the more stable carbocation (tertiary > secondary > primary, because alkyl groups are electron-donating) forms the major product. Cold dilute acidified KMnO₄ gives a diol; hot concentrated KMnO₄ cleaves the C=C.
≈ 50 min