Syn-Anti System (Syn-Anti Nomenclature) describes stereoisomers by the relative positions of substituents on adjacent atoms or double bonds.
Applicability:
- This system is less commonly used for simple alkenes and more common in:
- Cyclic compounds
- Organic reaction mechanisms (especially eliminations and additions)
- Carbohydrate and organometallic chemistry
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Rules and Definitions of Syn-Anti System
-
Syn:
- Two groups (atoms or substituents) are on the same face (or side) of a double bond or ring system.
-
Anti:
- Two groups are on opposite faces (or sides).
This system is based on spatial orientation, often discussed in 3D molecular structures and conformations.
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Usage in Reaction Mechanisms
The Syn–Anti terminology is especially valuable when describing stereochemical outcomes in:
-
Syn Addition:
- Two atoms/groups add to the same side of a double bond.
- Example: Catalytic hydrogenation of alkenes.
-
Anti Elimination:
- Two atoms/groups are eliminated from opposite sides of a molecule.
- Example: E2 elimination mechanism.
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Examples of Syn-Anti System:
- R1 and R2 on the same side → Syn
- R1 and R2 on opposite sides → Anti
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