What is the Difference Between Syn and Anti Addition?

The difference between syn and anti addition lies in the orientation of the substituents added to a double or triple bond in a molecule. Here are the main differences between the two types of addition:

Syn Addition: In syn addition, two substituents are added to the same side (or face) of a double or triple bond, resulting in a decrease in bond order and an increase in the number of substituents. This type of addition is observed in reactions such as the oxidation of an alkene to a diol using OsO4 or KMnO4.
Anti Addition: In anti addition, two substituents are added to opposite sides (or faces) of a double or triple bond, once again resulting in a decrease in bond order and an increase in the number of substituents. A classical example of anti addition is the bromination (or any halogenation) of alkenes, which results in a trans-isomer of the products.

The type of addition that occurs depends on multiple factors of a reaction and is defined by the final orientation of the substituents on the parent molecule. Syn and anti addition reactions are related to Markovnikov's rule for the orientation of a reaction.