Identify the addition reaction which is not undergone by the alkenes.

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Alkenes readily undergo straightforward addition reactions that add two groups across the double bond in one go: hydration adds an H and an OH, hydrogenation adds H2 to form an alkane, and halogenation adds two halogen atoms to give a vicinal dihalide. Mercuration, however, isn’t a simple, single-step addition to the alkene. In the typical mercuration process (oxymercuration-demercuration), mercury adds across the double bond to form a mercurinium ion, water then attacks to give a mercurial alcohol, and finally mercury is removed to yield the alcohol. The overall transformation is hydration, but the mercuration step itself is part of a two-step sequence rather than an isolated direct addition product. That’s why mercuration is not considered a standard direct addition reaction of alkenes.