In chemical reactions, various particles can be added at the site of the destruction of the double bond in alkenes and the triple bond in alkynes. What are the laws governing this process? The behavior of asymmetric ethylene homologues during hydrohalogenation and hydration was studied by the Russian scientist VV Markovnikov. He found that the mechanism of the reaction depends on the number of hydrogen carbon is bonded to in the double bond. The hypothesis put forward by the scientist was confirmed after discoveries in the field of the structure of the atom. 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, lubricating oils, alcohols.
Markovnikov's rule
The Russian scientist spent a lot of time in his work studying the mechanism of addition of asymmetric reagents to unsaturated hydrocarbons. In his article published in Germanin 1870, V. V. Markovnikov drew the attention of the scientific community to the selectivity of the interaction of hydrogen halides with carbon atoms that are in a double bond in unsymmetrical alkenes. The Russian researcher cited data that he obtained empirically in his laboratory. Markovnikov wrote that the halogen necessarily attaches to the carbon atom that contains the smallest number of hydrogen atoms. The works of the scientist gained great popularity at the beginning of the 20th century. The hypothesis of the interaction mechanism proposed by him was called "Markovnikov's rule".
Life and work of an organic scientist
Vladimir Vasilievich Markovnikov was born on December 25 (13 according to the old style) December 1837. He studied at the University of Kazan, later taught at this educational institution and at Moscow University. Markovnikov has been studying the behavior of unsaturated hydrocarbons when interacting with hydrogen halides since 1864. Until 1899, scientists from other countries did not attach any importance to the conclusions of the Russian chemist. Markovnikov, in addition to the rule named after him, made a number of other discoveries:
- obtained cyclobutanedicarboxylic acid;
- explored the oil of the Caucasus and discovered in it organic substances of a special composition - naphthenes;
- established the difference in melting temperatures of compounds with branched and straight chains;
- proved the isomerism of fatty acids.
The works of the scientist greatly contributed to the development of domestic chemical science and industry.
The essencehypothesis put forward by Markovnikov
The scientist devoted many years to studying 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 the carbon atom that contains more particles of this type. The anion attaches itself to the neighboring carbon. This is Markovnikov's rule, its essence. The scientist ingeniously 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 HX are added to ethylene hydrocarbons, where X:
- halogen;
- hydroxyl;
- acid residue of sulfuric acid;
- other particles.
The modern sound of Markovnikov's rule differs from the scientist's formulations: the hydrogen atom from the HX molecule attached by the alkene goes to the carbon in the double bond that already contains more hydrogen, and the X particle goes to the least hydrogenated atom.
Mechanism for attachment of electrophilic particles
Let's consider the types of chemical transformations in which Markovnikov's rule is applied. Examples:
- The reaction of addition of hydrogen chloride to propene. In the course of interaction between particles, the destruction of the double bond occurs. The chlorine anion goes to the less hydrogenated carbon that was in the double bond. Hydrogen interacts with the most hydrogenated of these atoms. 2-chlorine is formedpropane.
- In the addition reaction of a water molecule, the hydroxyl from its composition approaches the less hydrogenated carbon. Hydrogen attaches to the most hydrogenated atom at the double bond.
There are exceptions to the rule proposed by Markovnikov in those reactions where the reactants are alkenes, in which the carbon in the double bond already has an electronegative group nearby. It partially takes away the electron density, to which positively charged hydrogen is usually attracted. The rule is also not observed in reactions proceeding according to a radical rather than an electrophilic mechanism (the Harish effect). These exceptions do not detract from the merits of the rule developed by the outstanding Russian organic chemist V. V. Markovnikov.
