The cyclic locked ring structure prevents free rotation. Unlike straight-chain alkenes, cycloalkene syn addition allows stable addition of substituents to the same side of the ring, where they remain together. However, if chirality or the specific absolute orientation of the substituents needs to be taken into account, knowing the type of addition is significant. 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. After addition to a straight-chain alkene such as ethene ( C 2H 4), the resulting alkane will rapidly and freely rotate around its single sigma bond under normal conditions (i.e. An anti addition reaction results in a trans-isomer of the products, as the substituents are on opposite faces of the bond.ĭepending on the substrate double bond, addition can have different effects on the molecule. The classical example of this is bromination (any halogenation) of alkenes. 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. Īnti addition is in direct contrast to syn addition. An example of syn addition would be the oxidation of an alkene to a diol by way of a suitable oxidizing agent such as osmium tetroxide, OsO 4, or potassium permanganate, KMnO 4. Generally the substrate will be an alkene or alkyne. 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.
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