E/Z Stereoisomerism in Alkenes

Because rotation around the C=C double bond is restricted (breaking the π bond costs too much energy), the four groups attached to the two doubly-bonded carbons are locked in place. When certain conditions are met, this allows a molecule to exist as two distinct stereoisomers — the E-isomer and the Z-isomer. This lesson covers OCR A-Level Chemistry A (H432) specification 4.1.3 (b)–(c).

1. Stereoisomerism — A Reminder

Key Definition — Stereoisomers: Compounds with the same molecular formula and the same structural formula (same atom connectivity) but a different arrangement of atoms in space.

Unlike structural isomers (Lesson 3), stereoisomers have their atoms joined in the same order. It is only the three-dimensional arrangement that differs.

At A-Level you meet two types of stereoisomerism:

E/Z isomerism (geometrical / cis-trans) — this lesson.
Optical isomerism — covered in A2 (not this topic).

2. The Origin of E/Z Isomerism

Because the C=C bond cannot rotate, the groups attached to the double-bond carbons stay on the same side or opposite sides of the bond permanently. If those groups are different, this gives rise to two non-superimposable arrangements.

2.1 Conditions for E/Z Isomerism

For a molecule to exhibit E/Z isomerism, both of the following must be true:

It must contain a C=C double bond (restricted rotation).
Each carbon of the C=C must be attached to two different groups.

If either carbon of the C=C is attached to two identical groups (e.g., two hydrogens), E/Z isomerism is not possible.

2.2 Which Molecules Show E/Z?

Molecule	Formula	E/Z?
Ethene	CH₂=CH₂	No — each C has two identical H
Propene	CH₂=CHCH₃	No — left C has two identical H
But-1-ene	CH₂=CHCH₂CH₃	No — left C has two identical H
But-2-ene	CH₃CH=CHCH₃	Yes — each C has one H and one CH₃
1,1-dichloroethene	CCl₂=CH₂	No — left C has two identical Cl
1,2-dichloroethene	CHCl=CHCl	Yes — each C has one H and one Cl
2-methylbut-2-ene	(CH₃)₂C=CHCH₃	No — left C has two identical CH₃

graph TD
  A[Alkene with C=C] --> B{Each C=C carbon has<br/>two different groups?}
  B -->|Yes| C[E/Z isomerism possible]
  B -->|No| D[No E/Z isomerism]
  C --> E[Use CIP rules<br/>to assign priorities]
  E --> F[Same side of C=C<br/>high-priority groups]
  F --> G[Z-isomer]
  E --> H[Opposite sides<br/>high-priority groups]
  H --> I[E-isomer]

3. The Old Way — cis and trans

Before the IUPAC E/Z system, geometrical isomers were named cis or trans:

cis — "same side": the two higher-priority groups (often the two alkyl groups or the two "main chain" groups) are on the same side of the C=C.
trans — "opposite sides": they are on opposite sides.

For simple molecules like but-2-ene, cis/trans still works:

cis-but-2-ene: both methyls on the same side — now called Z-but-2-ene.
trans-but-2-ene: methyls on opposite sides — now called E-but-2-ene.

But for alkenes with four different groups, cis/trans becomes ambiguous. The modern E/Z system uses rigorous priority rules and always gives an unambiguous name.

4. The Cahn–Ingold–Prelog (CIP) Priority Rules

The IUPAC Cahn–Ingold–Prelog rules are used to assign priority to the groups attached to each C=C carbon.

4.1 Rule 1 — Atomic Number

Compare the atoms directly attached to the double-bond carbon. The one with the higher atomic number has higher priority.

Priority (highest → lowest for common atoms):

I > Br > Cl > S > P > F > O > N > C > H

Examples:

Br beats Cl, which beats C, which beats H.
O beats C (in CH₃OH vs CH₃, the O-containing group wins).

4.2 Rule 2 — Next Atoms (if Rule 1 is a Tie)

If the two atoms directly attached are the same, move to the next atoms and compare them in the same way. Continue until a difference is found.

Example: compare –CH₂CH₃ (ethyl) and –CH₃ (methyl). Both carbons are directly attached. The ethyl carbon is attached to C, H, H; the methyl carbon is attached to H, H, H. (C, H, H) beats (H, H, H), so ethyl has higher priority than methyl.

4.3 Rule 3 — Double/Triple Bonds

A double bond to an atom counts as two single bonds to that atom (once for each carbon involved). Example: –CH=O counts as if the C is attached to (O, O, H).

4.4 Assignment of E or Z

Once priorities are assigned on both carbons of the C=C:

Z isomer (from German zusammen = "together"): the two higher-priority groups are on the same side of the C=C.
E isomer (from German entgegen = "opposite"): the two higher-priority groups are on opposite sides of the C=C.

Memory aid: "Z = Zame Zide".

5. Worked Examples

5.1 But-2-ene

CH₃CH=CHCH₃

Lesson 8: E/Z Stereoisomerism in Alkenes