Which statement about addition reactions to alkenes is true?

• A. A reaction that removes a molecule to form a double bond
• B. A reaction that adds across a double bond to give a single product
• C. They create two products
• D. A reaction that replaces one group with another

Answer: (B)

Addition reactions to alkenes involve breaking the carbon–carbon pi bond and forming two new bonds to the added atoms or groups. This addition converts the double bond into a single bond and typically gives one main product for that set of reactants. The idea is that both atoms of the added reagent attach across the two carbons, so you don’t end up with two separate products in a simple addition.

For example, adding hydrogen bromide across ethene attaches H to one carbon and Br to the other, producing bromoethane as a single product. This contrasts with elimination, which would reform a double bond, or substitution, which would replace a group on a saturated carbon. The statement that addition across the double bond yields a single product best describes what happens in these reactions.