Step 1 — Hydroboration (BH3 or 9-BBN + alkene):

• Concerted 4-center transition state (no carbocation intermediate)
• B adds to less substituted C (anti-Markovnikov — steric control)
• H adds to more substituted C
• Syn addition (both from same face)
• Occurs 3 times: alkene + BH3 → R3B (trialkylborane)

Step 2 — Oxidation $\frac{H2O2}{NaOH}$:

• Replaces B with OH (retention of configuration)
• Net result: Anti-Markovnikov, syn addition of H and OH

JEE importance: This is the key reagent for converting terminal alkenes to primary alcohols (anti-Markovnikov). Compare with acid-catalyzed hydration (Markovnikov → secondary/tertiary alcohol).