Reactions

First, the alkene reacts with BH₃ through a concerted four-center transition state. The B-H bond adds across the carbon-carbon double bond through syn addition, i.e. both the hydrogen atom and the boron atom are delivered to the same face of the alkene. Because boron is sterically hindered, it preferentially bonds to the less substituted carbon atom, while hydrogen is transferred to the more substituted carbon atom. This regioselectivity results in the anti-Markovnikov placement of boron. The organoborane intermediate is then oxidized with the addition of H₂O₂ under basic conditions (typically, using sodium hydroxide). Hydroperoxide anion (HOO^-) attacks the boron center, generating a peroxyborate intermediate. Migration of the C-B bond to the α-oxygen atom generates a borate ester intermediate. Finally, hydrolysis of the latter intermediate yields the corresponding alcohol. Overall, hydroboration installs a hydroxyl group on the less substituted carbon of the original double bond and a hydrogen atom on the more substituted carbon, giving the anti-Markovnikov alcohol.