Alkenes undergo which type of reaction across the C=C bond?

• A. Substitution
• B. Addition
• C. Elimination
• D. Combustion

Answer: (B)

Across the C=C bond, alkenes react by adding atoms or groups to each carbon, breaking the pi bond in the process. The pi bond is relatively exposed and electron-rich, so reagents can attack it and form two new single bonds, yielding a saturated product. This is why addition reactions are characteristic for alkenes: you typically see hydrogen added to make alkanes, halogens added to give dihalides, or water added to form alcohols, all by attaching across the double bond.

Substitution involves replacing an atom on a saturated carbon and isn’t centered on breaking the C=C pi bond. Elimination tends to remove atoms to form or strengthen a multiple bond elsewhere, rather than adding across the existing double bond. Combustion is a general oxidation process with oxygen, not specifically about adding across the C=C bond.