Specify the reagents in which addition to alkene is syn or anti.

The addition of reagents to an alkene can occur in a syn (meaning addition occurs on the same side of the double bond) or anti (meaning addition occurs on opposite sides of the double bond) fashion, depending on the specific reagents and reaction conditions. Here are some common examples of each:

Syn Addition:

Hydrogenation: When an alkene is reacted with molecular hydrogen (H2) in the presence of a metal catalyst like palladium (Pd), platinum (Pt), or nickel (Ni), the addition of hydrogen occurs syn to the double bond, resulting in the formation of a saturated alkane.

Dihydroxylation: The use of reagents like potassium permanganate (KMnO4) or osmium tetroxide (OsO4) can result in syn dihydroxylation, where two hydroxyl groups (-OH) are added to the same side of the double bond.

Anti Addition:

Halogenation: When an alkene reacts with halogens (e.g., chlorine or bromine) in the presence of a solvent like chloroform (CHCl3) or carbon tetrachloride (CCl4), anti halogenation occurs. The halogen atoms add to opposite sides of the double bond, forming a vicinal dihalide.

Hydroboration-Oxidation: In the hydroboration-oxidation reaction, boron adds to one side of the double bond (anti-Markovnikov addition), and then the boron is replaced with a hydroxyl group (-OH) via an oxidation step. This results in anti addition of water across the double bond.

Ozonolysis: In ozonolysis, the alkene is treated with ozone (O3), followed by a reducing agent (e.g., zinc and water). This cleaves the double bond, and the resulting products are aldehydes or ketones, depending on the substituents. Ozonolysis typically results in anti addition of oxygen atoms across the double bond.

It's important to note that the outcome of an addition reaction can depend on various factors, including the reagents used, reaction conditions, and the presence of other functional groups or substituents on the alkene. The examples provided above represent some common cases of syn and anti addition reactions.