E/Z and Cis/Trans
Imagine a fence running across your yard. You and your neighbor both have dogs. If both dogs are on the same side of the fence, they can play together - that is the cis (or Z) arrangement. If the dogs are on opposite sides, they are separated - that is the trans (or E) arrangement. The fence is the double bond, and the dogs are the substituents. Because you cannot rotate around a fence post without tearing it down, the dogs stay where they are.
Double bonds do not rotate freely the way single bonds do. The pi bond locks the two carbons in place, preventing rotation. This means groups attached to a double bond are trapped on one side or the other. If two groups can be on the same side or on opposite sides, you have geometric isomers.
Why Double Bonds Restrict Rotation
A C=C double bond consists of one sigma bond (head-on overlap) and one pi bond (side-by-side overlap of p orbitals). The pi bond can only exist when the p orbitals are parallel. Rotating around the bond would break the pi bond, requiring roughly 250 kJ/mol of energy. At room temperature, molecules do not have enough energy to do this, so geometric isomers are stable and do not interconvert.
This is fundamentally different from single bonds, where rotation is essentially free (the energy barrier is only about 12-25 kJ/mol).
The Cis/Trans System
The cis/trans naming system is simple but limited:
- Cis - identical (or similar) substituents are on the same side of the double bond
- Trans - identical (or similar) substituents are on opposite sides of the double bond
Example: 2-butene (CH3CH=CHCH3)
- cis-2-butene: Both methyl groups are on the same side of the double bond
- trans-2-butene: Methyl groups are on opposite sides
Limitation: Cis/trans only works when each carbon of the double bond has one hydrogen and one non-hydrogen substituent (or two identical substituents). If a double bond carbon has two different non-hydrogen groups, cis/trans becomes ambiguous - you need the E/Z system instead.
The E/Z System
The E/Z system uses CIP priority rules (the same rules from R/S assignment) and works for ALL substituted alkenes, including those where cis/trans is ambiguous.
Step 1: For each carbon of the double bond, rank its two substituents by CIP priority (higher atomic number = higher priority).
Step 2: Determine whether the two higher-priority groups are on the same side or opposite sides of the double bond.
- Z (zusammen, German for “together”): Higher-priority groups are on the same side
- E (entgegen, German for “opposite”): Higher-priority groups are on opposite sides
Worked Example: Assigning E or Z
Consider 2-bromo-1-chloropropene: ClCH=CBrCH3
Carbon 1 of the double bond has two substituents: Cl (atomic number 17) and H (atomic number 1). Higher priority: Cl.
Carbon 2 of the double bond has two substituents: Br (atomic number 35) and CH3 (C, atomic number 6). Higher priority: Br.
Now compare the positions of Cl and Br (the two higher-priority groups). If they are on the same side of the double bond, the isomer is Z. If on opposite sides, it is E.
When Cis = Z and When It Doesn’t
For simple disubstituted alkenes (one substituent and one H on each carbon), cis and Z typically correspond, and trans and E correspond. But this is NOT always true for more complex alkenes.
Example where cis does not equal Z:
Consider (Z)-1-bromo-2-chloroethene: BrCH=CHCl
On carbon 1: Br (priority 1), H (priority 2). On carbon 2: Cl (priority 1), H (priority 2).
If Br and Cl are on the same side, this is the Z isomer. But Br and Cl are different groups, so calling this “cis” is ambiguous. The E/Z system resolves the ambiguity.
Stability of Geometric Isomers
Trans (E) alkenes are generally more stable than cis (Z) alkenes because of steric strain. In the cis arrangement, the two larger groups are on the same side, bumping into each other. In the trans arrangement, they are on opposite sides, minimizing steric interactions.
Evidence: Trans-2-butene has a lower heat of hydrogenation (-115.5 kJ/mol) than cis-2-butene (-119.7 kJ/mol). Since both give the same product (butane), the difference in energy released reflects the difference in starting energy. Cis-2-butene starts at higher energy (less stable), so it releases more energy when hydrogenated.
| Property | cis (Z) | trans (E) |
|---|---|---|
| Stability | Less stable | More stable |
| Heat of hydrogenation | Higher (releases more energy) | Lower (releases less energy) |
| Boiling point | Often higher (dipole moment) | Often lower (symmetrical, less dipole) |
| Dipole moment | Larger (substituents on same side) | Smaller (substituents cancel) |
Geometric Isomers in Rings
Cyclic compounds can also show cis/trans isomerism. In a ring, “cis” means two substituents are on the same face of the ring (both pointing up or both pointing down). “Trans” means they are on opposite faces (one up, one down).
This is particularly important for cyclohexane derivatives, which we will explore in detail in Section 2.11.
Geometric Isomers in Biological Molecules
Geometric isomerism has major biological consequences:
Fatty acids: Naturally occurring unsaturated fatty acids are almost always cis. The cis double bond creates a “kink” in the chain, preventing tight packing and keeping cell membranes fluid. Trans fats (artificial) pack more tightly, increasing rigidity and cardiovascular risk.
Vision: Retinal, the light-absorbing molecule in your eyes, switches from 11-cis-retinal to all-trans-retinal when it absorbs a photon. This cis-to-trans isomerization triggers a conformational change in the rhodopsin protein, initiating the signal cascade that lets you see.