Final answer:
In alkenes, the addition that occurs specifically in a syn fashion is hydroboration, where hydrogen and boron add to the same side of the double bond.
Step-by-step explanation:
In alkenes, the addition that occurs specifically in a syn fashion is hydroboration. This is a type of addition reaction where borane (BH3) or a borane complex adds to the double bond of an alkene. The defining feature of hydroboration is that the hydrogen and boron add to the same side of the double bond, which is what we refer to as syn addition. This reaction is particularly useful because it sets the stage for subsequent reactions to form alcohols in an anti-Markovnikov fashion after oxidation of the borane complex.
Conversely, halogenation and hydration reactions involve the addition of halogen atoms or water across the double bond and may proceed with either an anti- or syn-addition mechanism depending on the specific conditions. Ozonolysis, on the other hand, is a reaction where ozone cleaves the double bond and typically does not lead to a syn addition pattern; it results in the formation of two carbonyl compounds from the alkene.