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The oxymercuration reaction is an
electrophilic addition In organic chemistry, an electrophilic addition reaction is an addition reaction where a chemical compound containing a double or triple bond has a π bond broken, with the formation of two new σ bonds.March, Jerry; (1985). Advanced Organic Chemi ...
organic reaction Organic reactions are chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, mat ...
that transforms an
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
into a neutral
alcohol In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...
. In oxymercuration, the alkene reacts with
mercuric acetate
mercuric acetate
(AcO–Hg–OAc) in
aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute an ...
to yield the addition of an acetoxymercury (HgOAc) group and a hydroxy (OH) group across the double bond.
Carbocation A carbocation () is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
s are not formed in this process and thus rearrangements are not observed. The reaction follows
Markovnikov's rule 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, prop ...

Markovnikov's rule
(the hydroxy group will always be added to the more substituted carbon) and it is an
anti addition In organic chemistry, syn and anti addition are different ways in which two substituents can be added to a double bond In chemistry, a double bond is a covalent bond between two atom An atom is the smallest unit of ordinary matter In c ...
(the two groups will be trans to each other). Oxymercuration followed by reductive demercuration is called an oxymercuration–reduction reaction or oxymercuration–demercuration reaction. This reaction, which is almost always done in practice instead of oxymercuration, is treated at the conclusion of the article.


Mechanism

Oxymercuration can be fully described in three steps (the whole process is sometimes called ''deoxymercuration''), which is illustrated in stepwise fashion to the right. In the first step, the nucleophilic double bond attacks the mercury ion, ejecting an acetoxy group. The electron pair on the mercury ion in turn attacks a carbon on the double bond, forming a ''mercuronium ion'' in which the mercury atom bears a positive charge. The electrons in the highest occupied molecular orbital of the double bond are donated to mercury's empty 6s orbital and the electrons in mercury's dxz (or dyz) orbital are donated in the lowest unoccupied molecular orbital of the double bond. In the second step, the nucleophilic water molecule attacks the more substituted carbon, liberating the electrons participating in its bond with mercury. The electrons collapse to the mercury ion and neutralizes it. The oxygen in the water molecule now bears a positive charge. In the third step, the negatively charged acetoxy ion that was expelled in the first step attacks a hydrogen of the water group, forming the waste product HOAc. The two electrons participating in the bond between oxygen and the attacked hydrogen collapse into the oxygen, neutralizing its charge and creating the final alcohol product.


Regioselectivity and stereochemistry

Oxymercuration is very regioselective and is a textbook Markovnikov reaction; ruling out extreme cases, the water nucleophile will always preferentially attack the more substituted carbon, depositing the resultant hydroxy group there. This phenomenon is explained by examining the three
resonance structure In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they und ...
s of the mercuronium ion formed at the end of the step one. By inspection of these structures, it is seen that the positive charge of the mercury atom will sometimes reside on the more substituted carbon (approximately 4% of the time). This forms a temporary
tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology ...
carbocation A carbocation () is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
, which is a very reactive
electrophile 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 during a ...
. The
nucleophile 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 during a ...

nucleophile
will attack the mercuronium ion at this time. Therefore, the nucleophile attacks the more substituted carbon because it retains a more ''positive character'' than the lesser substituted carbon. Stereochemically, oxymercuration is an ''anti'' addition. As illustrated by the second step, the nucleophile cannot attack the carbon from the same face as the mercury ion because of steric hindrance. There is simply insufficient room on that face of the molecule to accommodate both a mercury ion and the attacking nucleophile. Therefore, when free rotation is impossible, the hydroxy and acetoxymercuri groups will always be
trans Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of". Used alone, trans may refer to: Arts, entertainment, and media * Trans (festival) trans was an annual, non-commercial arts festival held in Belfast, Northern Irel ...
to each other. Shown below is an example of regioselectivity and stereospecificity of the oxymercuration reaction with substituted cyclohexenes. A bulky group like ''t''-butyl locks the ring in a
chair conformation
chair conformation
and prevents ring flips. With 4-''t''-butylcyclohexene, oxymercuration yields two products – where addition across the double bond is always ''anti'' – with slight preference towards acetoxymercury group ''trans'' to the ''t''-butyl group, resulting in slightly more ''cis'' product. With 1-methyl-4-''t''-butylcyclohexene, oxymercuration yields only one product – still ''anti'' addition across the double bond – where water only attacks the more substituted carbon. The reason for ''anti'' addition across the double bond is to maximize orbital overlap of the lone pair of water and the empty orbital of the mercuronium ion on the opposite side of the acetoxymercury group. Regioselectivity is observed to favor water attacking the more substituted carbon, but water does not add ''syn'' across the double bond which implies that the transition state favors water attacking from the opposite side of the acetomercury group.


Oxymercuration–reduction

In practice, the mercury adduct product created by the oxymercuration reaction is almost always treated with
sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to ...

sodium borohydride
(NaBH4) in aqueous base in a reaction called ''demercuration''. In demercuration, the acetoxymercury group is replaced with a hydrogen in a stereochemically insensitive reaction known as reductive elimination. The combination of oxymercuration followed immediately by demercuration is called an oxymercuration–reduction reaction. Therefore, the oxymercuration-reduction reaction is the net addition of water across the double bond. Any stereochemistry set up by the oxymercuration step is scrambled by the demercuration step, so that the hydrogen and hydroxy group may be cis or trans from each other. Oxymercuration reduction is a popular laboratory technique to achieve alkene hydration with
Markovnikov Markovnikov (russian: Марковников) is a Russian masculine surname, which originates from ''морковь'' (''carrot''); its feminine counterpart is Markovnikova. It may refer to * Nikolai Markovnikov (1869–1942), Russian architect and ...

Markovnikov
selectivity while avoiding carbocation intermediates and thus the rearrangement which can lead to complex product mixtures.


Other applications

Oxymercuration is not limited to an alkene reacting with water to add hydroxyl and mercury groups. The carbon–mercury structure can undergo spontanous replacement of the mercury by hydrogeon, rather than persisting until a separate reduction step. In this manner, the effect is for mercury to be a
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty Non-bonding orbital, orbital which is capable of accepting an electron pair from a Lewis Base (chemistry), base to form a Lewis ad ...

Lewis acid
catalyst. For example, using an
alkyne In organic chemistry Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . ...

alkyne
instead of an alkene yields an enol, which tautomerizes into a ketone. Using an
alcohol In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...
instead of water yields an ether. In both cases, Markovnikov's rule is observed. Using a
vinyl ether
vinyl ether
in the presence of an alcohol allows the replacement of one alkoxy group (RO–) for another by way of an
acetal An acetal is a functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the sa ...

acetal
intermediate. An allyl alcohol and a vinyl ether under the conditions of oxymercuration–reaction can give R–CH=CH–CH2–O–CH=CH2, which is suitable for a
Claisen rearrangement The Claisen rearrangement is a powerful carbon–carbon chemical bond, bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of an allyl Vinyl group, vinyl ether will initiate a Sigmatropic reaction, ,3sigmatropic r ...
.


See also

* Hydroboration–oxidation reaction *
Mukaiyama hydration
Mukaiyama hydration


References

{{DEFAULTSORT:Oxymercuration Reaction Addition reactions Carbon-heteroatom bond forming reactions