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Markovnikov’s Rule
Introduction to Markovnikov’s Rule
In organic chemistry, Markovnikov's rule or Markownikoff's rule?describes the results of an addition reaction, also known as a halogenation reaction (under standard conditions), carried out after the addition of the protonic acid HX or other polar reagents to an asymmetric olefin. It was first proposed by Russian chemist Vladimr Markovnikov in 1865.[1]
Explanation of Markovnikov’s Rule
When the proton acid HX (X=Cl, Br, I) is added to asymmetric substituted alkenes, the addition of acidic hydrogen occurs on the less substituted carbon atom?of?the double bond, while halide X is added to ?the more alkyl substituted carbon atom. In other words, hydrogen is added to the carbon atoms with more number of hydrogen atoms, while halides are added to the carbon atoms with the least number of hydrogen atoms. [1, 2]
The driving force behind the reaction is the Carbocation formed when H is added to alkene?in the first step of the reaction .[3]?When H is added to carbon with more hydrogen atoms connected, it forms the most due to factors such as induction and hyperconjugation, while when Br is added to carbon with less hydrogen atoms, the main product is obtained. [1]
Less?stable carbocation (H added to less hydrogen-rich carbon) also exist in small quantities. The products generated through them do not conform to the Markovnikov’s?rule, and are usually by-products of the reaction .[1]
Markovnikov’s?rule?also applies to alkynes, following the same steps as olefins, as shown in the following example:
Application of Markovnikov’s Rule
1,Halohydrin formation (in alcohol and water)
2,Oxymercuration?and Demercuration
3,Acid catalyzed hydration [5]
These reactions are the laboratory use of Markovian and are used in various chemical process.
Introduction to Anti-Markovnikov’s Rule
The Anti-Markovnikov’s rule describes regional chemistry that does not involve a mechanism for the formation of carbon positive ions. In this case, the halogen adds to the less substituted carbon rather than to the more substituted carbon as in the case of the Markovnikov’s rule. [1,4]
Explanation of Anti-Markovnikov’s Rule
The most common reaction for the Anti-Markovnikov’s Rule is the radical addition reaction. This reaction mechanism only applies to HBr in the presence of benzoyl peroxide (C14H10O4) or hydrogen peroxide (H2O2), not to HCl or HI.[4]?Where peroxide is essential because it acts as a catalyst to decompose HBr into Br and H radicals (a radical is any chemical with one unpaired electron). [4]
The bromine radical attacks the alkene?first. It will attack the less substituted carbon in order to form a more stable carbon?radical. The stability of the carbon?radical is similar to that of the positive charged species, the more substituted the radical is the more stable it is. The formed radical then seizes hydrogen from another molecule of HBr, releasing another Br radical, so the reaction can continue and the overall result of the reaction is the addition of hydrogen to the more substituted carbon atom. [1,4]?
As in the case of reactions conforming to Markovnikov’s?rules, Br attacks the substituted more carbon, producing a small amount of by-products formed from less stable carbon radical. [4]
Application of Anti-Markovnikov’s Rule
1,Hydroboration-Oxidation
2,Radical halogenation [5]
Again, the mechanism is applied in these reactions for a number of laboratory uses.
Reference
1.?W.Markownikoff (1870). "Ueber die Abh?ngigkeit der verschiedenen Vertretbarkeit des Radical wasserstoffs in den isomeren Butters?uren" [On the dependence of the different substitutions of the radical hydrogen in the isomeric butyric acids]. Annalen der Chemie. 153 (1): 228–259.?https://doi.org/10.1002/jlac.18701530204
2.McMurry, John. "Section 7.8: Orientation of Electrophilic Reactions: Markovnikov's Rule". Organic Chemistry (8th ed.). p. 240. ISBN 9780840054548
3.Clayden, Jonathan (2012). Organic Chemistry. Oxford University Press. pp. 977, 985.
4.Lewis, David E. (2021). "The Logic Behind Markovnikov's Rule: Was It an Inspired Guess? …No!". Angewandte Chemie International Edition. 60 (9): 4412–4421. https://doi.org/10.1002/anie.202008228
5.Nishizawa, Mugio; Asai, Yumiko; Imagawa, Hiroshi (2006). "TiCl4 Induced Anti-Markovnikov Rearrangement". Organic Letters. 8 (25): 5793–6.?https://doi.org/10.1021/ol062337x
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