Cis-trans terminology requires two identical groups.
In cases with four different groups attached to a double bond, the E-Z system is used instead.
E-Z System Basics
Priority Assignment:
Based on atomic number: higher atomic number = higher priority.
E vs. Z Configuration:
Draw a line across the double bond.
E Configuration:
Higher priority groups on opposite sides (E for 'entgegen', German for 'opposite').
Z Configuration:
Higher priority groups on the same side (Z for 'zusammen', German for 'together').
Example Analysis
Example 1
Right Side:
Compare H (atomic number 1) and OH (atomic number 8).
Oxygen wins; OH = 1, H = 2.
Left Side:
Compare Br (atomic number 35) and CH3 (atomic number 6).
Bromine wins; Br = 1, CH3 = 2.
Result:
Higher priority groups (OH and Br) on opposite sides = E configuration.
Example 2
Swap CH3 and Br positions.
Result:
Higher priority groups (OH and Br) on the same side = Z configuration.
Detailed Example
Assigning Configuration
Left Carbon:
Compare Br (35) and Cl (17).
Bromine wins; Br = 1, Cl = 2.
Right Carbon:
Tie between two carbons, compare next bonded atoms.
Priority goes to the group with the double-bonded oxygen.
Result:
Same sides = Z configuration.
Naming with E-Z System
Find Longest Carbon Chain:
Includes double bond, prioritize substituents.
Example: 7-carbon chain = heptene.
Double Bond Location:
Start number from end closer to double bond or substituents.
Example: double bond starts at carbon 3 = 3-heptene.
Substituents:
Order in alphabetical sequence.
Example: 4-ethyl, 2-methyl.
Configuration:
Determine E or Z; place in parentheses before the name.
Example Final Name: (E)-4-ethyl-2-methyl-3-heptene.
Conclusion
The E-Z system provides a more comprehensive way to describe double bond configurations compared to cis-trans, especially with multiple different groups.