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Syn and anti addition
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In organic chemistry, syn and anti addition are different ways in which two substituents can be added to a double bond or triple bond. This article will use cycloalkenes as examples.
Syn addition is the addition of two substituents to the same side (or face) of a double bond or triple bond, resulting in a decrease in bond order but an increase in number of substituents. Generally the substrate will be an alkene or alkyne. An example of syn addition would be the oxidation of an alkene to a diol via a suitable oxidizing agent such as Osmium tetroxide OsO4 or Potassium permanganate KMnO4.
Anti addition is in direct contrast to syn addition. In anti addition, two substituents are added to opposite sides (or faces) of a double bond or triple bond, once again resulting in a decrease in bond order and increase in number of substituents. The classical example of this is bromination (any halogenation) of alkenes.
Depending on the substrate double bond, addition can have different effects on the molecule. After addition to a straight-chain alkene such as C2H4, the resulting alkane will rapidly and freely rotate around its single sigma bond under normal conditions (i.e. room temperature). Thus whether substituents are added to the same side (syn) or opposite sides (anti) of a double can usually be ignored due to free rotation. However, if chirality or the specific absolute orientation of the substituents needs to be taken into account, knowing the type of addition is significant. Unlike straight-chain alkenes, cycloalkene syn addition allows stable addition of substituents to the same side of the ring, where they remain together. The cyclic locked ring structure prevents free rotation.
Syn elimination and anti elimination are the reverse processes of syn and anti addition. These result in a new double bond, such as in Ei elimination.
External links[edit]
IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "endo, exo, syn, anti".
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t
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Stereochemistry
Topics
Chirality
Racemic mixture
Configuration descriptors
cis–trans isomerism
cis-, trans-
E–Z notation
(E)-, (Z)-
Arene substitution pattern
o-, m-, p- (ortho, meta, para)
Endo-exo isomerism
endo, exo
Syn and anti addition
syn, anti
Three identical ligands
fac, mer (facies, meridonal) [obsolete]
In carbon skelets
n, iso, neo, cyclo
Secondary and tertiary
sec-, tert-
Spiro compound
spiro
Catenane
catena
Absolute configurations
CIP (Cahn–Ingold–Prelog) priority rules
(R), (S)
Pseudoasymmetric (pseudochiral) centers
(r), (s)
Optical rotation
(+)-, (−)- or d-, l-
Relative configuration
D-, L-
Category:Stereochemistry
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