MarkovnikovHydratisierung

Markovnikov's Rule is an empirical rule in organic chemistry that describes the regioselectivity of electrophilic addition reactions of alkenes and alkynes. Specifically, when a protic acid HX (where X is a halogen or other electronegative group) is added to an asymmetric alkene or alkyne, the hydrogen atom from the acid adds to the carbon atom that already has the greater number of hydrogen atoms, and the halide or other group adds to the carbon atom with fewer hydrogen atoms. This means the addition is not random, but follows a predictable pattern.

The rule was first formulated by the Russian chemist Vladimir Markovnikov in 1869. It is explained by the stability of the intermediate carbocation formed during the reaction mechanism. The more substituted carbocation is generally more stable due to hyperconjugation and inductive effects. Therefore, the electrophile (the hydrogen ion, H+) will add to the carbon that leads to the formation of the most stable carbocation, which is the one with more existing hydrogen atoms. Subsequently, the nucleophile (the halide ion or other group) attacks this more stable carbocation at the more substituted carbon. While Markovnikov's Rule is a powerful predictive tool, there are exceptions, particularly in reactions involving radicals or certain organometallic reagents, which may lead to anti-Markovnikov addition.