The rule in plain words
Markovnikov's original statement: when HX adds across a double bond, the hydrogen ends up on the carbon that already has more hydrogens. The modern version: the H+ adds to give the most stable carbocation intermediate. Both wordings give the same answer for simple alkenes.
Carbocation logic
H+ adds first, giving one of two possible carbocations. Mark which side of the alkene becomes more substituted — that's where the H went. The more substituted carbocation is more stable (3° > 2° > 1° > methyl), so it forms preferentially. The nucleophilic X then attacks that cation. Net result: the X attached to the more substituted carbon.
Anti-Markovnikov (radical HBr)
When peroxides are added (a radical initiator), HBr adds by a radical chain. Br• adds first to the alkene, giving the more stable radical (3° > 2° > 1°). H then adds to whatever's left. The Br ends up on the LESS substituted carbon — the opposite of the ionic outcome. Memorize: "peroxides flip the bromide."
Other anti-Markovnikov methods
Hydroboration-oxidation (BH₃ then H₂O₂/NaOH) gives an anti-Markovnikov alcohol — the boron goes to the less hindered carbon (sterics), then oxidation puts OH where B was. Oxymercuration-demercuration gives the Markovnikov alcohol cleanly because no rearrangement happens.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.