Anti addition

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In organic chemistry, anti addition is the addition of two substituents to opposite sides (or faces) 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. This article will uses alkenes as examples.

Anti addition is in direct contrast to syn addition, which is addition of substituents to the same side of the double or triple bond.

Depending on the substrate double bond, addition can have different effects on the molecule. In a straight-chain alkene, after addition, 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 bond 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, cycloalkenes anti addition allows stable addition of substituents to opposite sides of the ring, where they will remain. The cyclic locked ring structure prevents free rotation.