the law defining the order of addition of water and hydrogen halides to asymmetrical olefins, established empirically by V. V. Markovnikov and formulated in 1869.
The Markovnikov rule states that the hydrogen atom adds to the more hydrogenated carbon atom, and the hydroxyl group or halogen atom adds to the less hydrogenated carbon atom of an olefin. According to modern concepts, this orientation is determined by the distribution of electron density in the reagent molecules and by the stability of the intermediate carbonium ions.
where δ+ and δ-designate partial electric charges.
The addition, in accordance with the Markovnikov rule, of HC1 to vinyl chloride to yield ethylidene chloride is explained by the conjugation of an unshared electron pair of the chloride atom with a ダ bond:
The modern electronic interpretation of the Markovnikov rule also makes it possible to explain a number of cases of addition that are contrary to the rule. For example, the addition of electrophilic and nucleophilic agents to compounds with conjugated bonds containing an electronegative group at the double-bonded carbon atom occurs in accordance with a shift of electron density to the most electronegative atoms. For example,
Another violation of the Markovnikov rule occurs in reactions involving the addition of HBr to olefins in the presence of peroxides (the Kharasch effect), since the reaction mechanism here is radical. Atomic bromine is the attacking particle, and the orientation of the addition is determined by the stability of the intermediate free carbon radical:
The term “Markovnikov rule” is often used in a broader sense to include the reaction of olefins and acetylenes with reagents of the type
in which the positively polarized particle X takes the role of the hydrogen atom. Thorough consideration of the electronic and spatial factors makes it possible to predict the orientation of addition in practically all cases.
B. L. DIATKIN