14.2.4—Mechanism: electrophilic addition in alkenes,

The π bond donates an electron pair to an electrophile. In Br₂ addition, a bromine electrophile is attacked and Br⁻ completes the addition; in HBr addition, H⁺ adds first and Br⁻ attacks the carbocation-like intermediate.

Curly arrows begin at the π bond or a lone pair and end at the electron-poor atom. The intermediate and its stability help determine which product predominates.

Ethene gives 1,2-dibromoethane. Propene gives mainly 2-bromopropane with HBr because the more stable secondary carbocation pathway is favoured.

Do not draw a radical mechanism for HBr addition, and do not start a curly arrow at a positively charged atom.