E–Z Nomenclature System (Cahn–Ingold–Prelog Priority Rules)

E–Z Nomenclature System uses Cahn–Ingold–Prelog rules to name isomers based on priority groups around a double bond (E = opposite, Z = same side).

Why Use E-Z Nomenclature System?

• When each carbon in the double bond has two different substituents, the cis-trans system is not sufficient. In such cases, the E-Z system provides a consistent method to assign configuration.

Step-by-Step Rules:

1. Identify the double bond and the two carbon atoms involved.
2. Assign priorities to the substituents on each carbon using Cahn–Ingold–Prelog rules:
   • Higher atomic number = higher priority.
   • If the first atoms are identical, move outward along the chain until a difference is found.
   • Double/triple bonds are treated as multiple single-bonded atoms for priority purposes.
3. Determine configuration:
   • If the two highest-priority groups are on the same side, the isomer is Z (from the German Zusammen, meaning together).
   • If they are on opposite sides, the isomer is E (from the German Entgegen, meaning opposite).

Advantages:

• Applicable to any alkene, including complex cases.
• Avoids ambiguity present in the cis-trans system.

Example of E-Z Nomenclature System:

1-bromo-1-chloro-2-fluoroethene

• Left carbon (C1):
  • Br (Z = 35) > Cl (Z = 17) → Br is higher priority.
• Right carbon (C2):
  • F (Z = 9) > H (Z = 1) → F is higher priority.
• Since Br and F are on opposite sides of the double bond → E-isomer.
• If Br and F were on the same side, it would be the Z-isomer.

Advantages:

• Applicable to all alkenes, even asymmetric or complex ones.
• Avoids ambiguity of the cis-trans naming system.
• Provides clear stereochemical information.

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