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Alkenes are electron-rich due to their pi bond and undergo electrophilic addition as their characteristic reaction. Markovnikov's rule governs regiochemistry: the electrophile (H+) adds to the less substituted carbon, generating the more stable carbocation intermediate. Anti-Markovnikov addition occurs only with HBr/peroxide (Kharasch effect) via radical mechanism, and with hydroboration-oxidation (concerted, no carbocation). Stereochemistry depends on mechanism: anti addition via cyclic intermediates (Br2/bromonium ion, mCPBA/epoxide then ring-opening), syn addition via surface or cyclic mechanisms (H2/catalyst, BH3, OsO4), and non-stereospecific via planar carbocation (HX, H2O/H+). Key reagent-product mapping: HBr → Markovnikov bromide; HBr/ROOR → anti-Markovnikov bromide; H2O/H+ → Markovnikov alcohol; BH3 then H2O2/NaOH → anti-Markovnikov primary alcohol (syn); OsO4 → syn-diol; Br2 → anti-1,2-dibromide; mCPBA → epoxide; O3/Zn → aldehydes/ketones; KMnO4 (cold) → syn-diol. Ozonolysis is a critical structure-determination tool — cleaves C=C into C=O fragments. Reverse ozonolysis (given products, deduce alkene) is frequently tested. Zaitsev's rule governs elimination: strong non-bulky base → more substituted alkene (major); bulky base (KOtBu) → less substituted (Hofmann product). Oxymercuration-demercuration gives Markovnikov alcohol without rearrangement, solving the major problem of acid-catalyzed hydration. Conjugated dienes undergo both 1,2 and 1,4 addition; kinetic control (low T) favors 1,2, thermodynamic control (high T) favors 1,4.