E/Z Nomenclature
The E/Z nomenclature is a system used to describe the configuration of double bonds in alkenes (or other compounds with double bonds) when there are two different groups on each carbon of the double bond. It is based on the Cahn-Ingold-Prelog (CIP) priority rules, similar to the R/S system for chiral centers.
Key Concepts:
- Cis/Trans vs. E/Z:
- Cis/Trans: Used when the two groups on each carbon of the double bond are the same or similar.
- E/Z: Used when the two groups on each carbon of the double bond are different.
- E and Z Definitions:
- E (from German Entgegen, meaning “opposite”): The higher-priority groups are on opposite sides of the double bond.
- Z (from German Zusammen, meaning “together”): The higher-priority groups are on the same side of the double bond.
Steps to Assign E/Z Configuration:
- Identify the Double Bond:
- Locate the carbon-carbon double bond ((C=C)).
- Assign Priorities to the Groups:
- Use the CIP rules to assign priorities to the groups attached to each carbon of the double bond:
- Rule 1: Higher atomic number = higher priority.
- Rule 2: If two atoms are the same, compare the next atoms in the group.
- Rule 3: Double or triple bonds are treated as if each bond is connected to a separate atom.
- Compare the Priorities:
- For each carbon of the double bond, determine which group has higher priority.
- If the two higher-priority groups are on the same side, the configuration is Z.
- If the two higher-priority groups are on opposite sides, the configuration is E.
Example 1: Assigning E/Z Configuration to a Simple Molecule
Consider the molecule 2-butene:
[
CH_3-CH=CH-CH_3
]
- The double bond is between (C_2) and (C_3).
- Each carbon of the double bond has two groups:
- (C_2): (CH_3) and (H)
- (C_3): (CH_3) and (H)
Step-by-Step Assignment:
- Assign Priorities:
- For (C_2): (CH_3) (higher priority) > (H) (lower priority).
- For (C_3): (CH_3) (higher priority) > (H) (lower priority).
- Compare the Priorities:
- The two higher-priority groups ((CH_3) and (CH_3)) are on the same side of the double bond.
- Configuration:
- The molecule is Z-2-butene.
Example 2: Assigning E/Z Configuration to a Complex Molecule
Consider the molecule 1-bromo-2-chloro-1-iodoethene:
[
Br-CH=CH-Cl
]
- The double bond is between (C_1) and (C_2).
- Each carbon of the double bond has two groups:
- (C_1): (Br) and (I)
- (C_2): (H) and (Cl)
Step-by-Step Assignment:
- Assign Priorities:
- For (C_1): (I) (higher priority, atomic number 53) > (Br) (lower priority, atomic number 35).
- For (C_2): (Cl) (higher priority, atomic number 17) > (H) (lower priority, atomic number 1).
- Compare the Priorities:
- The two higher-priority groups ((I) and (Cl)) are on opposite sides of the double bond.
- Configuration:
- The molecule is E-1-bromo-2-chloro-1-iodoethene.
Special Cases:
- Double or Triple Bonds:
- Treat double or triple bonds as if each bond is connected to a separate atom.
- Example: (C=O) is treated as (C) bonded to two (O) atoms.
- Cyclic Compounds:
- Treat the ring as a single group and assign priorities based on the atoms directly attached to the double bond.
Summary of E/Z Nomenclature:
- E: Higher-priority groups are on opposite sides of the double bond.
- Z: Higher-priority groups are on the same side of the double bond.
- The E/Z system is essential for describing the stereochemistry of double bonds in organic chemistry.
Practice Problem:
Assign the E/Z configuration to the following molecule:
[
CH_3-CH=CH-CH_2Cl
]
- The double bond is between (C_2) and (C_3).
- Each carbon of the double bond has two groups:
- (C_2): (CH_3) and (H)
- (C_3): (CH_2Cl) and (H)
Solution:
- Priorities:
- For (C_2): (CH_3) (higher priority) > (H) (lower priority).
- For (C_3): (CH_2Cl) (higher priority) > (H) (lower priority).
- Compare the priorities:
- The two higher-priority groups ((CH_3) and (CH_2Cl)) are on the same side of the double bond.
- Configuration: Z-2-butene.