Anti-Markovnikov Addition
Addition in which the hydrogen attaches to the carbon bearing fewer hydrogens, reversing the normal regiochemistry.
Unit
Dimensionless
In depth
The reversal occurs when peroxides are present, because the mechanism switches from ionic to free radical. A bromine radical adds first to give the more stable carbon radical, which then abstracts hydrogen — placing the two atoms the opposite way round. The effect is specific to HBr.
Examples in the real world
Propene with HBr and peroxide giving 1-bromopropane; hydroboration–oxidation giving the less substituted alcohol.