E/Z and Alkene
Stereochemistry
Master the Cahn-Ingold-Prelog priority rules and learn to assign E/Z configurations to alkenes with confidence.
What Are Alkene Stereoisomers?
Alkenes contain a carbon-carbon double bond that prevents free rotation. When each carbon of the double bond carries two different substituents, two distinct spatial arrangements are possible: the groups can be on the same side or on opposite sides of the double bond.
Why Can't Alkenes Rotate?
A double bond consists of one sigma bond (head-on overlap) and one pi bond (side-on overlap of p orbitals). Rotation around the bond would break the pi overlap, requiring about 264 kJ/mol of energy: far too much at room temperature.
When Does E/Z Apply?
E/Z stereoisomerism only arises when each carbon of the double bond bears two different groups. If either carbon has two identical substituents, no stereoisomerism exists.
The CIP Priority Rules
To assign E or Z, you must first rank substituents using the Cahn-Ingold-Prelog (CIP) system. Compare the atoms directly bonded to each double-bond carbon:
Atomic Number Decides Priority
Assigning E vs Z
After ranking the two substituents on each double-bond carbon, look at where the higher-priority group on each carbon ends up:
E/Z vs Cis/Trans
Both naming systems describe alkene geometry, but they are not interchangeable. The older cis/trans system works only for simple cases; E/Z is universal.
| Cis / Trans | E / Z (CIP) | |
|---|---|---|
| Basis | Identical groups on same/opposite sides | CIP priority groups on same/opposite sides |
| Applicability | Only disubstituted alkenes | All alkenes with stereoisomerism |
| Ambiguity? | Yes: fails for tri- and tetrasubstituted | No: always unambiguous |
| Relationship | cis often \(=\) Z, trans often \(=\) E | Not always: depends on priority |
How to Assign E/Z
Example 1: (Z)-2-Butene
Example 2: A Trisubstituted Alkene
Assign E/Z to (Z)-1-bromo-1-chloropropene. This molecule has three different substituents and one H: cis/trans labels are ambiguous here.
Example 3: Breaking a Tie
Assign E/Z to (E)-3-methyl-2-pentene. On \( \text{C}3 \), both substituents start with C: we must look further along the chain.
Multiple Bonds as Phantom Atoms
When a substituent contains a double or triple bond, CIP rules treat it as if the bond were duplicated or triplicated into phantom atoms:
Why E/Z Matters
E/Z stereochemistry has profound consequences in biology, medicine, and materials science:
Common Mistakes
Summary at a Glance
| Concept | Key Point |
|---|---|
| Double bond | Restricted rotation creates stereoisomers |
| Requirement | Each C must have 2 different groups |
| CIP Rule 1 | Higher atomic number = higher priority |
| CIP Rule 2 | If tied, move outward until a difference |
| CIP Rule 3 | Multiple bonds = phantom atoms |
| Z (zusammen) | High-priority groups on same side |
| E (entgegen) | High-priority groups on opposite sides |
Question 1 of 5
Which condition is required for a carbon-carbon double bond to show E/Z stereoisomerism?
Question 2 of 5
Rank the following atoms from lowest to highest CIP priority: \( \text{H} \), \( \text{C} \), \( \text{Br} \), and \( \text{O} \).
Question 3 of 5
In the E/Z system, what does the designation "Z" mean?
Question 4 of 5
Using CIP rules, which substituent has higher priority: \( \text{-CHO} \) (aldehyde) or \( \text{-CH}_2\text{OH} \)?
Question 5 of 5
A student claims that cis-2-butene and (Z)-2-butene refer to different molecules. Is this correct?