This MedLibrary.org supplementary page on Markovnikov's rule is provided directly from the open source Wikipedia as a service to our readers. Please see the note below on authorship of this content, as well as the Wikipedia usage guidelines. To search for other content from our encyclopedia supplement, please use the form below:
Related Sponsors
In chemistry, Markovnikov's rule or Markownikoff's rule is an observation based on Zaitsev's rule. It was formulated by the Russian chemist Vladimir Vasilevich Markovnikov in 1870 [1][2]. In chemical reactions found particularly in organic chemistry, the rule states that with the addition of H-X to an alkene, the acid hydrogen (H) becomes attached to the carbon with the greatest number of hydrogens, and the halide (X) group becomes attached to the carbon with the fewest hydrogens[3].
The same is true when an alkene reacts with water in an addition reaction to form alcohol. The hydroxyl group (OH) bonds to the carbon that has the greater number of carbon-carbon bonds, while the hydrogen bonds to the carbon on the other end of the double bond, that has more carbon-hydrogen bonds.
The chemical basis for Markovnikov's Rule is the formation of the most stable carbocation during the addition process. The addition of the hydrogen to one carbon atom in the alkene creates a positive charge on the other carbon, forming a carbocation intermediate. The more substituted the carbocation (the more bonds it has to carbon or to electron-donating substituents) the more stable it is, due to induction and hyperconjugation. The major product of the addition reaction will be the one formed from the more stable intermediate. Therefore, the major product of the addition of HX (where X is some atom more electronegative than H) to an alkene has the hydrogen atom in the less substituted position and X in the more substituted position. It is important to note, however, that the other less substituted, less stable carbocation will still be formed to some degree, and will proceed to form the minor product with the opposite attachment of X.
The rule may be summarized as "the rich get richer and the poor get poorer": a carbon rich in substituents will gain more substituents and the carbon with more hydrogens attached will get the hydrogen in many organic addition reactions.
Historic context
One of the organic reactions Markovnikov based his rule on (first performed in 1865) was that of hydrogen iodide with vinyl bromide. In another manifestation of his rule he observed that the halogen atom added to the carbon atom already carrying a halogen atom Geminal halide hydrolysis of the initial reaction product with moist or hydrogenated (rarely used) silver oxide to ethanal proved the 1,1 substitution pattern.
It has been observed [2] that the original 1869 Markovnikov publication was sloppy and that he did not do much experimental work himself. The rule itself appeared only as a four-page footnote in a 26-page article, which may also explain why his rule took 60 years to be accepted.
Anti Markovnikov rule
Mechanisms which avoid the carbocation intermediate may react through other mechanisms that are regioselective, against what Markovnikov's rule predicts, such as free radical addition. Such reactions are said to be anti-Markovnikov, since the halogen adds to the less substituted carbon.This reaction is exactly opposite of Markovnikov reaction,and hence the name. Again, like the positive charge, the radical is most stable when in the more substituted position.
Anti-Markovnikov behaviour extends to other chemical reactions than just additions to alkenes. One Anti-Markovnikov manifestation is observed in hydration of phenylacetylene that, gold-catalyzed, gives regular acetophenone but with a special ruthenium catalyst [4] the other regioisomer 2-phenylacetaldehyde [5]:
Anti-Markovnikov behaviour can also manifest itself in certain rearrangement reactions. In a titanium(IV) chloride catalyzed formal nucleophilic substitution at enantiopure 1 in the scheme below, two racemic products are formed 2a and 2b [6] :
This product distribution can be rationalized by assuming that loss of the hydroxy group in 1 gives the tertiary carbocation A which rearranges to the seemingly less attractive secondary carbocation B. Chlorine can approach this center from two faces leading to the observed mixture of isomers.
However, perhaps the most famous example of anti-Markovnikov addition is hydroboration.
References
- ^ W. Markownikoff (1870), "Ueber die Abhängigkeit der verschiedenen Vertretbarkeit des Radicalwasserstoffs in den isomeren Buttersäuren", Annalen der Pharmacie 153(1): 228–259, doi:
- ^ a b Was Markovnikov’s Rule an Inspired Guess? Peter Hughes 1152 Journal of Chemical Education • Vol. 83 No. 8 August 2006
- ^ Additions to Alkenes: Regiochemistry
- ^ catalyst system based on in-situ reaction of ruthenocene with Cp and naphthalene ligands and a second bulky pyridine ligand
- ^ Highly Active in Situ Catalysts for Anti-Markovnikov Hydration of Terminal AlkynesAurélie Labonne, Thomas Kribber, and Lukas Hintermann Org. Lett.; 2006; 8(25) pp 5853 - 5856; (Letter) doi:10.1021/ol062455k
- ^ TiCl4 Induced Anti-Markovnikov Rearrangement Mugio Nishizawa, Yumiko Asai, and Hiroshi Imagawa Org. Lett.; 2006; 8(25) pp 5793 - 5796; (Letter) doi:10.1021/ol062337x.
Wikipedia content modification information:
- This page was last modified on 22 September 2008, at 06:58.
Wikipedia Authorship and Review
Wikipedia content provided here is not reviewed directly by MedLibrary.org. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with MedLibrary.org.
Wikipedia Usage Guidelines
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Markovnikov's rule".
The URL for this specific entry is:
All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.