Anti-Markovnikov rule describes the regiochemistry where the substituent is bonded to a less substituted carbon, rather than the more substitued carbon. This process is quite unusual, as carboncations which are commonly formed during alkene, or alkyne reactions tend to favor the more substitued carbon.

According to this rule, in an unsymmetrical alkene when the addition of halogen acid (HX) takes place, then the negative part of the halogen acid is attached to that carbon atom which is double bonded as well have the least number of hydrogen atoms.

The Anti-Markovnikov rule ,in which the substituent is attached to a less substituted carbon instead of the more substituted carbon. This is because substituted carbocation allows for more hyperconjugation and induction, resulting in a more stable carbocation.