Markovnikov Product
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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.


Explanation

The rule states that with the addition of a
protic acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
HX or other polar reagent to an asymmetric alkene, the acid hydrogen (H) or electropositive part gets attached to the carbon with more hydrogen substituents, and the
halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...
(X) group or electronegative part gets attached to the carbon with more alkyl substituents. This is in contrast to Markovnikov's original definition, in which the rule is stated that the X component is added to the carbon with the fewest hydrogen atoms while the hydrogen atom is added to the carbon with the greatest number of hydrogen atoms. The same is true when an alkene reacts with water in an addition reaction to form an alcohol which involve formation of carbocations. The
hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy g ...
(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 A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
during the addition process. The addition of the hydrogen ion 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 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. But the other less substituted, less stable carbocation will still be formed at some concentration, and will proceed to be the minor product with the opposite, conjugate attachment of X.


Anti-Markovnikov reactions

Also called Kharasch effect (named after
Morris S. Kharasch Morris Selig Kharasch (August 24, 1895 – October 9, 1957) was a pioneering organic chemist best known for his work with free radical additions and polymerizations. He defined the peroxide effect, explaining how an anti-Markovnikov orientation c ...
), these reactions that do not involve a
carbocation A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
intermediate may react through other mechanisms that have regioselectivities not dictated by Markovnikov's rule, such as
free radical addition In organic chemistry, free-radical addition is an addition reaction which involves free radicals. The addition may occur between a radical and a non-radical, or between two radicals. The basic steps with examples of the free-radical addition (al ...
. Such reactions are said to be anti-Markovnikov, since the halogen adds to the less substituted carbon, the opposite of a Markovnikov reaction. 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 Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
. 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. 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. Anti-Markovnikov behaviour extends to more chemical reactions than additions to alkenes. Anti-Markovnikov behaviour is observed in the
hydration Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * ...
of phenylacetylene by auric catalysis, which gives acetophenone; although with a special ruthenium catalyst it provides the other
regioisomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term metam ...
2-phenylacetaldehyde: Anti-Markovnikov behavior can also manifest itself in certain
rearrangement reaction In organic chemistry, a rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another ...
s. In a
titanium(IV) chloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds ...
-catalyzed formal nucleophilic substitution at enantiopure 1 in the scheme below, two products are formed – 2a and 2b Due to the two chiral centers in the target molecule, the carbon carrying chlorine and the carbon carrying the methyl and acetoxyethyl group, four different compounds are to be formed: 1R,2R- (drawn as 2b) 1R,2S- 1S,2R- (drawn as 2a) and 1S,2S- . Therefore, both of the depicted structures will exist in a D- and an L-form. :. This product distribution can be rationalized by assuming that loss of the hydroxy group in 1 gives the tertiary
carbocation A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
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 Hofmann elimination is an elimination reaction of an amine to form alkenes. The least stable alkene (the one with the least number of substituents on the carbons of the double bond), called the Hofmann product, is formed. This tendency, known as ...


References


External links

*{{cite video, url=https://www.khanacademy.org/video/markovnikov-s-rule-and-carbocations , title=Markovnikov's rule and carbocations - Khan Academy, date=25 August 2010 Physical organic chemistry Rules of thumb