Which type of reaction do alkenes readily undergo due to the C=C bond?

• A. Substitution
• B. Addition reactions
• C. Combustion only
• D. Polymerization only

Answer: (B)

The key idea is that the carbon–carbon double bond in alkenes is reactive and opens up to form new bonds. The pi bond is high in energy and can break to let atoms or groups add across the two carbons, turning the double bond into two new single bonds. That’s why alkenes readily undergo addition reactions—hydrogenation adds H–H across the bond, halogenation adds halogen atoms, and hydration adds water across the bond, all giving saturated products. Substitution isn’t driven by the double bond itself, and while combustion and polymerization can involve alkenes, they’re not the typical reactions prompted specifically by the C=C bond in the same direct way addition reactions are.