In chemical reactions at the site of the destruction of the double bond in alkenes and the triple in alkynes, various particles can join. What laws obey this process? The Russian scientist VV Markovnikov studied the behavior of asymmetric ethylene homologs during hydrohalogenation and hydration. He found that the reaction mechanism depends on how much hydrogen is attached to the carbon in the double bond. The hypothesis put forward by the scientist was confirmed after discoveries in the field of atomic structure. Markovnikov’s rule laid the foundation for the creation of a scientific theory that has practical application. It allows you to more rationally organize the production of polymers, lubricants, alcohols.
Markovnikov Rule
The Russian scientist in his work devoted a lot of time to studying the mechanism of addition of asymmetric reagents to unsaturated hydrocarbons. In his article published in German in 1870, V.V. Markovnikov drew the attention of the scientific community to the selectivity of the interaction of hydrogen halides with carbon atoms located in a double bond in asymmetric alkenes. The Russian researcher cited the data that he obtained experimentally in his laboratory. Markovnikov wrote that halogen necessarily joins the carbon atom that contains the smallest number of hydrogen atoms. The work of the scientist gained great popularity at the beginning of the 20th century. His proposed hypothesis of the mechanism of interaction was called the “Markovnikov rule”.
The life and work of an organic scientist
Vladimir Vasilievich Markovnikov was born on December 25 (13 according to the Art. Style) December 1837. He studied at the University of Kazan, later taught at this educational institution and at Moscow University. Markovnikov studied the behavior of unsaturated hydrocarbons in interaction with hydrogen halides since 1864. Until 1899, scientists from other countries did not attach importance to the conclusions of the
Russian chemist. Markovnikov, in addition to the rule named in his honor, made a number of other discoveries:
- received cyclobutanedicarboxylic acid;
- he studied the oil of the Caucasus and discovered in it organic substances of a special composition - naphthenes;
- established a difference in the melting temperatures of compounds with branched and straight chains;
- proved isomerism of fatty acids.
The works of the scientist contributed significantly to the development of domestic chemical science and industry.
The essence of the hypothesis put forward by Markovnikov
The scientist spent many years studying the reactions of addition of reagents to unsaturated hydrocarbons with one double bond (alkenes). He noticed that if hydrogen is present in the compounds, then it goes to that carbon atom, which contains more particles of this kind. Anion joins the adjacent carbon. This is Markovnikov’s rule, its essence. The scientist brilliantly predicted the behavior of particles, the structure of which at that time still had not very clear ideas. In accordance with the rule, complex substances having the composition of HX are attached to ethylene hydrocarbons, where X:
- halogen;
- hydroxyl;
- acid residue of sulfuric acid;
- other particles.
The modern sounding of the Markovnikov rule differs from the scientist's formulations: a hydrogen atom from an HX molecule attached by an alkene is sent to that carbon with a double bond that already contains more hydrogen, and particle X goes to the least hydrogenated atom.
The mechanism of attachment of electrophilic particles
Let us consider the types of chemical transformations in which the Markovnikov rule is applied. Examples:
- The reaction of addition of hydrogen chloride to propene. During the interaction between particles, the double bond is destroyed. The chlorine anion is directed to the less hydrogenated carbon, which was in the double bond. Hydrogen interacts with the most hydrogenated of these atoms. 2-chloro propane is formed.
- In the reaction of the addition of a water molecule, the hydroxyl from its composition approaches less hydrogenated carbon. Hydrogen binds to the most hydrogenated atom in a double bond.
There are exceptions to the rule proposed by Markovnikov in those reactions where alkanes are the reactants, in which the carbon at the double bond already has an electronegative group nearby. It partially selects the electron density, to which positively charged hydrogen is usually attracted. The rule is not observed in reactions proceeding by a radical rather than electrophilic mechanism (Harish effect). These exceptions do not detract from the merits of the rule deduced by the outstanding Russian organic chemist V.V. Markovnikov.