The double bond in an alkene consists of a sigma bond and a pi bond. Although the double bond, in total, is stronger than a single bond, the pi bond itself is weaker and can react with electron-deficient species such as a hydrogen ion. Hydroiodic acid, HI, is a strong acid and therefore a source of H+ ions that will react with 2-butene, forming a carbocation. The positively charged carbocation will react with the negatively charged iodide ion from the hydroiodic acid to form the corresponding iodine-substituted alkane.
Draw the structure for the product of the reaction between 2-butene and hydroiodic acid, HI.