Markovnikov's rule
/ mar-KOV-ni-kov /
Markovnikov's rule is the old rhyme that, when an acid like HX adds across an alkene, the hydrogen goes to the carbon that already has more hydrogens, and the X goes to the carbon with fewer — 'the rich get richer'. So adding HBr to propene puts Br on the middle carbon, not the end one. As a memory aid it works, but the rule as originally stated is just a description; the real reason lives one layer deeper.
What actually happens is electrophilic addition through a carbocation. When H+ adds, it can land on either carbon, and the choice is decided by which leftover carbocation is more stable. Adding H to the less-substituted carbon leaves the positive charge on the more-substituted carbon, which is more stable (more alkyl groups donate electron density to soothe the charge). The nucleophile then bonds to that more-substituted carbon. So Markovnikov's rule is best restated as: the electrophile adds to give the more stable carbocation.
Restating it mechanistically is not pedantry — it is what lets you predict the unexpected. If a more stable carbocation can form by a hydride or methyl shift, the product can come from the rearranged cation, breaking the naive 'count the hydrogens' version. And it explains anti-Markovnikov outcomes: under radical conditions (HBr with peroxides) the intermediate is a radical, not a cation, and stability of the radical, not a cation, now sets the regiochemistry, reversing the orientation.
Adding HCl to 2-methylpropene ((CH3)2C=CH2): H goes to the CH2 end, leaving a tertiary carbocation (CH3)3C(+), and Cl bonds there, giving 2-chloro-2-methylpropane (tert-butyl chloride), not the primary isomer.
The proton adds wherever it leaves the more stable carbocation behind.
Markovnikov's rule is not a law of nature but a consequence of carbocation stability; memorizing 'H to the carbon with more H' fails the moment a rearrangement or a radical mechanism changes which intermediate is most stable.