Addition of Hydrogen Halides

Markovnikov's Rule: Electrophile ($$\text{H}^+$$) adds to carbon with more hydrogens to form the more stable carbocation.

Peroxide Effect / Anti-Markovnikov:

$$\begin{aligned}\text{R--CH=CH}_2 + \text{HBr} \xrightarrow{\text{R}_2\text{O}_2} \text{R--CH}_2\text{--CH}_2\text{Br} \quad (\text{Applies only to HBr})\end{aligned}$$