$word_{count}$: 220

Understanding stereochemistry of addition is critical for JEE problems involving product prediction. Three categories exist: Anti addition (groups from opposite faces): Br2/Cl2 via cyclic halonium ion, and epoxide ring-opening under acidic conditions. Syn addition (groups from same face): H2/Pd or Pt or Ni (surface delivery), BH3 (concerted 4-center TS), OsO4 (cyclic osmate ester), and cold dilute KMnO4 (cyclic manganate ester). Non-stereospecific (either face): HX via planar carbocation intermediate, H2O/H+ via carbocation. The stereochemical outcome depends on the starting alkene geometry and must be worked out in 3D. For example: cis-2-butene + Br2 (anti) → racemic $\frac{R,R}{(S,S)}$ mixture; trans-2-butene + Br2 (anti) → meso compound. OsO4 gives the opposite pairing: cis-2-butene → meso diol; trans-2-butene → racemic diol. In JEE, draw the alkene in 3D, apply the stereochemistry rule, and determine R/S at each new stereocenter. Remember that achiral starting materials always give racemic products (or meso), never a single enantiomer.