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In
organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and th ...
, Markovnikov's rule or Markownikoff's rule describes the outcome of some
addition reactionImage:Chlorine addition to ethene.svg, Addition of chlorine to ethylene An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one (the adduct).. Addition reacti ...

addition reaction
s. The rule was formulated by Russian chemist
Vladimir Markovnikov Vladimir Vasilyevich Markovnikov (russian: Влади́мир Васи́льевич Марко́вников), also spelled as Markownikoff, (December 22, 1838 – February 11, 1904), was a Russian chemist A chemist (from Greek ''chēm(ía)'' a ...
in 1870.


Explanation

The rule states that with the addition of a
protic acid An acid is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms ...
HX or other polar reagent to an asymmetric
alkene In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo dur ...

alkene
, the acid hydrogen (H) or electropositive part gets attached to the carbon with more hydrogen substituents, and the
halide A halide is a binary phase In materials chemistry, a binary phase or binary compound is a chemical compound containing two different elements. Some binary phase compounds are molecular, e.g. carbon tetrachloride (CCl4). More typically binary ph ...
(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 A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group ( ...

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 A carbocation () is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and Vinyl cation, vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are ...
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 Induction may refer to: Philosophy * Inductive reasoning, in logic, inferences from particular cases to the general case Biology and chemistry * Labor induction (birth/pregnancy) * Induction chemotherapy, in medicine * Induction period, the t ...
and
hyperconjugationIn organic chemistry, hyperconjugation (or σ-conjugation) refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electron The electron is a ...

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

Mechanisms 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 cation, vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are ...
intermediate may react through other mechanisms that have other regioselectivities not dictated by Markovnikov's rule, such as free radical addition. 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. 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
hyperconjugationIn organic chemistry, hyperconjugation (or σ-conjugation) refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electron The electron is a ...

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 of
phenylacetylene Phenylacetylene is an alkyne hydrocarbon containing a phenyl group. It exists as a colorless, viscous liquid. In research, it is sometimes used as an analog for acetylene; being a liquid, it is easier to handle than acetylene gas. Preparation In t ...

phenylacetylene
by auric catalysis, which gives
acetophenone Acetophenone is the organic compound with the chemical formula, formula C6H5C(O)CH3 (also represented by the Skeletal formula#Pseudoelement symbols, pseudoelement symbols PhAc or BzMe). It is the simplest aromatic ketone. This colorless, viscous l ...

acetophenone
; although with a special
ruthenium Ruthenium is a chemical element with the Symbol (chemistry), symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to ...

ruthenium
catalyst it provides the other
regioisomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a chemical compound, compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct chemical bond, bon ...
2-phenylacetaldehyde
2-phenylacetaldehyde
: Anti-Markovnikov behavior can also manifest itself in certain
rearrangement reactionA rearrangement reaction is a broad class of organic reactionImage:Claisen Rearrangement Scheme.png, 400px, The Claisen rearrangementOrganic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are ...

rearrangement reaction
s. In a
titanium(IV) chloride Titanium tetrachloride is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is not clearly ...
-catalyzed formal
nucleophilic substitution A nucleophilic substitution is a class of chemical reactions A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of ma ...
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
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 cation, vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are ...
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 In chemistry, hydroboration refers to the addition of a hydrogen-boron bond to C-C, C-N, and C-O double bonds, as well as C-C triple bonds. This chemical reaction is useful in the organic synthesis of organic compounds. The development of this tech ...

hydroboration
.


See also

*
Zaitsev's ruleIn organic chemistry, Zaitsev's rule (or Saytzeff's rule, Saytzev's rule) is an empirical rule for predicting the favored alkene , the simplest alkene. In chemistry Chemistry is the scientific discipline involved with Chemical element, e ...

Zaitsev's rule


References

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External links


Khan academy lecture
Physical organic chemistry Rules of thumb