hydrohalogenation
Hydrohalogenation is the addition of a hydrogen halide — HCl, HBr, or HI — across a carbon-carbon double or triple bond. The H and the X simply add to opposite ends of the pi bond, turning an alkene into an alkyl halide. It is the textbook example of electrophilic addition, the one usually met first, because its two-step ionic mechanism is the clean template for so much that follows.
Step one: the alkene's pi electrons reach out and pluck the proton off HX, forming a new C-H bond and a carbocation on the other carbon, while the halide drifts off as an anion. Step two: that halide anion bonds to the positive carbon. Because a carbocation forms, the proton adds to give the more stable cation, which is the mechanistic basis of Markovnikov's rule — H goes to the carbon with more hydrogens, X to the more substituted carbon. Reactivity follows HI > HBr > HCl, mirroring how easily each H-X bond breaks.
Hydrohalogenation matters as both a real transformation and a teaching tool. Practically it converts cheap alkenes into alkyl halides, which are themselves springboards for substitution and elimination. Conceptually it crystallizes the big ideas: a nucleophilic pi bond, an electrophile, a carbocation intermediate, Markovnikov regiochemistry, the possibility of rearrangement, and even the radical anti-Markovnikov twist for HBr with peroxides. Master this one and the rest of electrophilic addition feels familiar.
HCl adds to 2-methyl-2-butene to give, after a possible hydride shift consideration, the chloride on the more substituted carbon; with a simple alkene like cyclohexene, HBr gives bromocyclohexane cleanly.
H and X add across the double bond; Markovnikov orientation follows from the carbocation.
Because the intermediate is a free carbocation, hydrohalogenation can give rearranged products (via hydride or alkyl shifts); if a more stable cation is one shift away, expect it to form.