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Alkene reactions include hydrogenation, halogenation, hydrohalogenation, hydration, dihydroxylation, epoxidation, and cleavage. Their outcomes depend on the reagents and pathway: additions may be syn or anti, and hydration can follow Markovnikov or anti-Markovnikov orientation. Some pathways can rearrange, while oxymercuration–reduction avoids rearrangement. Alkynes can be made by double elimination from dihalides and undergo hydrogenation, halogenation, hydrohalogenation, hydration, and oxidative cleavage. Partial hydrogenation distinguishes cis-alkene formation with Lindlar catalyst from trans-alkene formation with sodium in liquid ammonia. Terminal alkynes also form alkynyl anions that can participate in SN2 alkylation. Together, these reactions connect mechanism and reagent selection to product structure.