Syn vs Anti Addition: Key Differences in Organic Chemistry
Syn addition is when two substituents add to the same face of a double bond; Anti addition places them on opposite faces.
Students confuse them because textbook drawings look alike and “syn” feels like “together” in everyday speech, so they assume opposite faces are “syn” too.
Key Differences
Syn addition yields eclipsed products and occurs in hydroboration. Anti addition gives staggered products and dominates halogenation and dihydroxylation with cold KMnO₄.
Which One Should You Choose?
Pick syn for hydroboration-oxidation to get anti-Markovnikov alcohols. Choose anti when forming vicinal diols or dihalides to secure predictable stereochemistry.
Examples and Daily Life
Think of syn as two stickers slapped on the same side of a card; anti is like one sticker on front, one on back. This decides whether your “card” twists clockwise or not.
Is hydroboration always syn?
Yes, the concerted four-center mechanism locks substituents to the same face.
Can a reaction switch from syn to anti?
No, mechanism dictates geometry; changing conditions only favors one pathway.
Why care in drug synthesis?
Stereoerrors can flip a molecule from therapeutic to toxic.