What is a characteristic reaction of alkenes?

Alkenes are characterized by their carbon-carbon double bonds, which make them more reactive than alkanes. One of the hallmark reactions of alkenes is hydrogenation. This process involves the addition of hydrogen (H₂) across the double bond, effectively converting the alkene into an alkane. The reaction typically requires a catalyst, such as palladium, platinum, or nickel, to proceed efficiently.

Hydrogenation is significant in organic chemistry since it can be used to reduce unsaturated fats in the food industry or modify various compounds for synthetic purposes. The nature of the double bond in alkenes makes them particularly susceptible to undergo this addition reaction, highlighting their reactivity compared to saturated compounds.

The other options, while they are chemical reactions, do not specifically characterize alkenes in the same direct way that hydrogenation does. Reduction refers to a broader category of reactions that involves the gain of electrons or atoms, while hydrolysis typically refers to a reaction with water that breaks chemical bonds, and substitution involves replacing an atom or group in a compound, which is more typical of alkanes or aromatic compounds rather than alkenes specifically. Therefore, hydrogenation stands out as the most characteristic reaction of alkenes.