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In alkene halogenation, Cl₂ or Br₂ forms a cyclic halonium ion that is opened by anti addition, producing a vicinal dihalide without carbocation rearrangement. In water or alcohol, solvent participation instead gives an anti halohydrin, with OH directed to the more substituted carbon. Alkynes can add one or two equivalents of halogen. Hydrogenation adds hydrogen syn across an alkene using a metal catalyst; alkynes can be reduced fully to alkanes or selectively to cis- or trans-alkenes. Lindlar catalyst favors the cis product, while dissolving-metal reduction favors the trans product. The lesson also connects hydrogenation to reaction energetics and stereochemistry.