Addition of Hydrogen Halides

Alkenes react with the hydrogen halides HCl, HBr and HI at room temperature. The H atom adds to one carbon of the C=C and the halogen atom adds to the other, giving a halogenoalkane.CH₂=CH₂ + HBr → CH₃CH₂BrThe product here is bromoethane. The hydrogen halide is used as a gas, or as a concentrated solution. Cyclohexene reacts with HCl in the same way to give chlorocyclohexane.Reactivity: HI reacts fastest and HCl slowest:HI > HBr > HClThe H-X bond has to break during the reaction. Going down the group the halogen atom gets larger, so the H-X bond gets longer and weaker (bond enthalpies: H-Cl 432, H-Br 366, H-I 298 kJ mol⁻¹). The H-I bond is the weakest, so HI reacts most readily.Symmetrical alkenes such as ethene, but-2-ene and cyclohexene give only one product, because it does not matter which carbon the H atom joins. For example, but-2-ene + HBr gives only 2-bromobutane.Unsymmetrical alkenes such as propene can give two products: CH₃CH=CH₂ + HBr can form 1-bromopropane or 2-bromopropane, depending on which carbon the H adds to. Which one forms more is explained by the mechanism in later lessons.This reaction is a useful way to make halogenoalkanes, which are starting materials for many other compounds.