Syn and anti addition
In organic chemistry, 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. This article will use alkenes as examples.
Syn addition is in direct contrast to anti addition, which is addition of substituents to opposite sides of the double 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, 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 is the reverse process of syn addition creating a new double bond.