Which reaction adds a group across a carbon-carbon multiple bond?

• A. Addition reaction
• B. Substitution reaction
• C. Combustion reaction
• D. Esterification

Answer: (A)

Adding across a carbon–carbon multiple bond means the pi bond is opened and two new bonds are formed to the atoms or groups being added, saturating the unsaturated bond. This is what an addition reaction does: the reagents come in and attach to each carbon of the double (or triple) bond, converting the double or triple bond into single bonds while attaching new groups. For example, hydrogenation adds H2 across a C=C to give an alkane, or halogenation adds halogen across the bond to give a dihalide. The other processes don’t involve attaching new groups across an unsaturated bond: substitution replaces a substituent on a saturated carbon without breaking the pi bond, combustion is an oxidation reaction, and esterification forms an ester through a condensation that does not add across a carbon–carbon multiple bond.