markovnikovregel

Markovnikov's rule, also known in German as Markovnikov-Regel, is a guideline in organic chemistry that predicts the regioselectivity of certain additions to asymmetric alkenes. In the typical electrophilic addition of a protic acid (such as H-X) or water under acidic conditions, the proton adds to the carbon of the double bond that bears more hydrogen atoms. This generates the more stable carbocation on the more substituted carbon, and the nucleophile attaches to that carbon, yielding the more substituted product. Consequently, in the addition of HCl to propene, chloride adds to the middle carbon to form 2-chloropropane; hydration of propene gives isopropanol.

Exceptions and variations exist. In radical additions, such as HBr in the presence of peroxides, the rule is reversed, producing anti-Markovnikov products where bromine attaches to the less substituted carbon. Additionally, some reactions proceed via non-carbocation pathways, such as oxymercuration-demercuration, which can give Markovnikov products without rearrangement, while carbocation rearrangements can alter the outcome in other cases.

The rule is widely used as a heuristic to predict products of electrophilic additions to alkenes, including hydrohalogenation and hydration, but it is not universal. Reactions may follow different mechanisms or exhibit rearrangements that override simple expectations.

Historically, the rule was proposed by Vladimir Markovnikov in the late 19th century and has since become a fundamental concept in teaching and practicing organic synthesis. It provides a practical framework for understanding how substituent patterns influence the outcome of addition reactions to double bonds.