How does bromine react with unsaturated hydrocarbons?

Bromine reacts with unsaturated hydrocarbons, such as alkenes and alkynes, through an addition reaction that breaks the carbon-to-carbon multiple bonds to accommodate the bromine atoms.12

• Alkenes: Bromine adds to alkenes to yield dihaloalkanes. This reaction is rapid even at room temperature and does not require a catalyst.2
• Alkynes: Alkynes react with bromine by first adding one molecule of the halogen to form a dihaloalkene, followed by the addition of a second molecule to yield a tetrahaloalkane.2

These reactions are characteristic of unsaturated compounds, as they cause the carbon atoms to become saturated with the maximum number of attached groups. While electrophilic addition is the standard mechanism for these reactions, it is worth noting that the addition of hydrogen bromide (HBr) to olefins can also occur via a free-radical mechanism, which may result in a different orientation of substituents compared to electrophilic addition.134