MARKOVNIKOV RULE Meaning and
Definition
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The Markovnikov rule is a fundamental concept in organic chemistry that predicts the regioselectivity of electrophilic addition reactions of unsymmetrical alkenes. According to this rule, when a hydrogen halide (such as HCl or HBr) adds to an unsymmetrical alkene, the hydrogen atom of the acid attaches itself to the carbon atom of the double bond that already has the greater number of hydrogen atoms. Simultaneously, the halide atom attaches itself to the other carbon atom of the double bond. This rule is named after the Russian chemist Vladimir Markovnikov, who formulated it in the late 19th century.
The underlying principle of the Markovnikov rule is the greater stability of the carbocation intermediate formed during the reaction. The carbon atom of the alkene that already has more hydrogen atoms is more likely to form a stable carbocation because it can distribute the positive charge more efficiently, due to its greater number of electron-donating hydrogens. In contrast, the carbon atom with fewer hydrogens would form a less stable carbocation, as it would have a larger positive charge density.
The Markovnikov rule is a valuable tool that allows chemists to predict the major product of electrophilic addition reactions of unsymmetrical alkenes, providing crucial information for synthetic applications and understanding reaction mechanisms. However, there are exceptions to this rule in certain cases where the formation of a more stable carbocation is disfavored or other factors influence the regioselectivity, but these cases are less common.
Etymology of MARKOVNIKOV RULE
The term "Markovnikov rule" is named after Russian chemist Vladimir Markovnikov (1837-1904). It refers to a rule in organic chemistry that describes the regioselectivity of some addition reactions. Vladimir Markovnikov proposed this rule in 1870 while studying the addition of hydrogen halides to alkenes. The rule states that in the addition of a protic acid to an asymmetric alkene, the hydrogen atom of the acid adds to the carbon with fewer alkyl substituents, while the halide group adds to the carbon with more alkyl substituents. The term "Markovnikov rule" was coined in honor of Vladimir Markovnikov's contribution to this concept in organic chemistry.
