Markovnikov's rule
In organic chemistry, Markovnikov's rule or Markownikoff's rule describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870.[1][2][3]
Explanation
The rule states that with the addition of a
The same is true when an alkene reacts with water in an addition reaction to form an alcohol which involve formation of carbocations. The
The chemical basis for Markovnikov's Rule is the formation of the most stable
Anti-Markovnikov reactions
Also called Kharasch effect (named after
The anti-Markovnikov rule can be illustrated using the addition of hydrogen bromide to isobutylene in the presence of benzoyl peroxide or hydrogen peroxide. The reaction of HBr with substituted alkenes was prototypical in the study of free-radical additions. Early chemists discovered that the reason for the variability in the ratio of Markovnikov to anti-Markovnikov reaction products was due to the unexpected presence of free radical ionizing substances such as peroxides. The explanation is that the O-O bond in peroxides is relatively weak. With the aid of light, heat, or sometimes even just acting on its own, the O-O bond can split to form 2 radicals. The radical groups can then interact with HBr to produce a Br radical, which then reacts with the double bond. Since the bromine atom is relatively large, it is more likely to encounter and react with the least substituted carbon since this interaction produces less static interactions between the carbon and the bromine radical. Furthermore, similar to a positive charged species, the radical species is most stable when the unpaired electron is in the more substituted position. The radical intermediate is stabilized by hyperconjugation. In the more substituted position, more carbon-hydrogen bonds are aligned with the radical's electron deficient molecular orbital. This means that there are greater hyperconjugation effects, so that position is more favorable.[5] In this case, the terminal carbon is a reactant that produces a primary addition product instead of a secondary addition product.
A new method of anti-Markovnikov addition has been described by Hamilton and Nicewicz, who utilize aromatic molecules and light energy from a low-energy diode to turn the alkene into a cation radical.[6][7]
Anti-Markovnikov behaviour extends to more chemical reactions than additions to alkenes. Anti-Markovnikov behaviour is observed in the
Anti-Markovnikov behavior can also manifest itself in certain
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 stable secondary carbocation B. Chlorine can approach this center from two faces leading to the observed mixture of isomers.
Another notable example of anti-Markovnikov addition is hydroboration.
See also
- Zaitsev's rule
- Hofmann's rule
References
- .
- .
- S2CID 230570680.
- ISBN 9780840054548.
- ^ Clayden, Jonathan (2012). Organic Chemistry. Oxford University Press. pp. 977, 985.
- ^ Drahl, Carmen. "Light-Driven Reaction Modifies Double Bonds With Unconventional Selectivity – April 15, 2013 Issue – Vol. 91 Issue 15 – Chemical & Engineering News". cen.acs.org.
- PMID 23113557.
- ligand
- PMID 17134289.
- PMID 17134274..
External links
- Markovnikov's rule and carbocations – Khan Academy. 25 August 2010.