anti-Markovnikov addition

Anti-Markovnikov addition is any addition to an alkene where the pieces end up the 'wrong way round' compared with the Markovnikov prediction — the hydrogen lands on the more-substituted carbon and the other group on the less-substituted one. It is not a violation of chemistry; it simply happens when the reaction does not go through the usual carbocation, so a different rule decides the orientation.

There are two famous routes. The first is radical hydrobromination: HBr in the presence of peroxides (or light) adds via a chain of radicals rather than ions. A bromine radical adds first, and it adds to whichever carbon leaves the more stable carbon radical (the more substituted one) — which is the opposite carbon from where Br would land ionically, so Br ends up on the less-substituted carbon. The second is hydroboration-oxidation, where boron (the electron-poor atom) attaches to the less-hindered, less-substituted carbon and is later replaced by OH, putting the hydroxyl anti-Markovnikov.

Anti-Markovnikov methods are prized precisely because they reach products the standard ionic additions cannot. Want the hydroxyl on the terminal carbon of a 1-alkene? Markovnikov hydration gives the internal alcohol, but hydroboration-oxidation gives the terminal one. Having both orientations available — pick your conditions, pick your regiochemistry — is what makes alkenes such versatile synthetic starting points.