Stereoisomerism in Alkenes

Full Notes

Stereoisomers have the same structural formula but atoms are arranged differently in space. There are different types of stereoisomer, however at this stage you need to be confident with stereoisomerism in alkenes.

Alkenes show E/Z isomerism, a type of stereoisomerism due to restricted rotation around the C=C bond, meaning groups bonded to each carbon atom are ‘locked’ in place relative to each other.

Example: 1-bromo-2-chloroethene

The Br and Cl groups bonded to the C=C double bond can have two different relative positions.

E/Z Isomerism

To name these kinds of stereoisomers we use E and Z notation.

The highest priority groups on each carbon determine the name using Cahn–Ingold–Prelog (CIP) rules.

• If highest priority groups are on opposite sides = E Isomer (opposite sides)
• If highest priority groups are on the same side = Z Isomer (same side)

Example: But-2-ene

• E-But-2-ene: CH₃ and H on opposite sides of C=C.
• Z-But-2-ene: CH₃ and H on the same side of C=C.

Cahn–Ingold–Prelog (CIP) Priority Rules

1. Look at the atoms directly attached to the C=C bond.
2. Assign higher priority to the atom with the higher atomic number. If the highest priority atoms bonded to each carbon are the same (for example C) you then assign the highest priority of the groups bonded to that atom (for example CH₂CH₃ group has higher priority than CH₃ group).
3. If the higher priority groups are on the same side, it is Z. If the higher priority groups are on opposite sides, it is E.

Cis and Trans Isomerism

If two groups bonded to each carbon atom are the same, cis and trans notation can be used instead of E and Z.

• Cis = the same groups on each carbon are on the same side of the double bond
• Trans = the same groups on each carbon are on opposite sides of the double bond

Example: But-2-ene

• Cis-but-2-ene: CH₃ groups on same side of C=C
• Trans-but-2-ene: CH₃ groups on opposite sides of C=C

Matt’s exam tip

Don’t get confused by E and Z, Cis and Trans isomerism. Just remember if two of the groups bonded to each carbon in the C=C are the same, you can describe the isomers using Cis and Trans. If they are all different, you need to use E and Z, based on the highest priority of each group bonded to each carbon.

Predicting Stereoisomers

Given a structural formula, you can predict the number of possible E/Z isomers by checking for two different groups on both sides of the C=C bond.

Summary

• Stereoisomerism occurs when molecules have the same molecular and structural formula but different spatial arrangements.
• Alkenes show E/Z isomerism due to restricted rotation around the C=C bond.
• CIP rules are used to assign priorities to groups and determine E or Z configuration.
• Cis–trans isomerism is a special case of E/Z where identical groups are present on each carbon of the C=C bond.