What type of reaction do alkynes undergo across the triple bond?

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Alkynes react by addition across the triple bond. The C≡C bond has two pi electrons exposed, making it highly receptive to reagents that can add to both carbon atoms at once. When a reagent adds across the bond, each carbon gains part of the reagent, breaking pi bonds and forming new sigma bonds. After the first addition, the triple bond becomes a double bond, and with more reagent, it can be reduced further to a single bond. This is why many classic alkyne reactions—hydrogenation, halogenation, hydration, etc.—proceed by adding across the bond rather than removing or substituting atoms from the carbon framework. Substitution would replace existing substituents, elimination would form a multiple bond from a saturated system in a different context, and isomerization would rearrange atoms or double bonds without adding across the pi system.