4.3.2Core Organic Chemistry
Stereoisomerism in Alkenes
5 min read 6 parts Lesson 12 of 29
Section 1 of 6Why Alkenes Have Stereoisomers
But-2-ene shows stereoisomerism but but-1-ene does not, because one carbon in but-1-ene has two hydrogens.
Stereoisomers
- Stereoisomers have the same structural formula but a different arrangement of their atoms in space.
Restricted rotation
- The π bond stops the two carbon atoms of a C=C bond from rotating freely relative to each other.
Fixed positions
- Groups attached to the double bond therefore stay in fixed positions above or below the double-bond axis.
Two conditions
- Each carbon of the double bond must carry two different groups for E/Z isomers to exist.
Section 2 of 6Assigning E and Z
1
For each carbon of the double bond, compare the two attached atoms and find which has the higher atomic number.
2
The atom with the higher atomic number takes higher priority; if atoms are the same, move outwards along the chain.
3
If the two higher priority groups are on the same side of the double bond, the isomer is Z (from the German 'zusammen').
4
If the two higher priority groups are on opposite sides, the isomer is E (from 'entgegen').
5
Write the letter, in brackets, at the start of the name, for example (Z)-but-2-ene.
Section 3 of 6Cis-trans Isomers
Cis-trans Isomers
Cis
- Cis means that two similar groups, such as two hydrogens, are on the same side of the double bond.
Trans
- Trans means that the two similar groups are on opposite sides of the double bond.
Link to E/Z
- Cis-trans is a special case where each carbon holds one identical group, often a hydrogen;
- E/Z can be used for any alkene.
Example
- The (Z)-but-2-ene isomer is cis-but-2-ene, and (E)-but-2-ene is trans-but-2-ene.
| Isomer | Arrangement | Approx. boiling point |
|---|---|---|
| (Z)-1,2-dichloroethene | Cl atoms on same side | 60 °C |
| (E)-1,2-dichloroethene | Cl atoms on opposite sides | 48 °C |
Section 4 of 6Properties of the Isomers
Physical properties
- E and Z isomers differ in melting point, boiling point and solubility because the molecules have different shapes.
Dipoles
- In (Z)-1,2-dichloroethene the C–Cl dipoles add up, so the molecule is polar with stronger forces between molecules.
Symmetry
- In the E isomer the dipoles cancel, so it is less polar and boils at a lower temperature.
Chemical properties
- Both isomers contain the same functional group, so they usually show very similar chemical reactions.
Section 5 of 6Worked Example
Question
- The molecule CH₃CH=CHCH₂CH₃ is drawn with the CH₃ group and the CH₂CH₃ group on opposite sides of the double bond.
- Give its full name with E/Z.
- The longest chain containing C=C is five carbons, and the double bond starts at carbon 2: pent-2-ene.
- On carbon 2 the groups are CH₃ and H; CH₃ has higher priority since C has higher atomic number than H.
- On carbon 3 the groups are CH₂CH₃ and H; CH₂CH₃ has the higher priority.
- The two higher priority groups are on opposite sides, so the isomer is E.
Answer (E)-pent-2-ene
Section 6 of 6Exam Checklist & Tips
4.3.2 – Key Points for the Exam
- Stereoisomers Compounds with the same structural formula but a different arrangement of atoms in space.
- E isomer The isomer with the higher priority groups on opposite sides of the double bond.
- Z isomer The isomer with the higher priority groups on the same side of the double bond.
- Restricted rotation The inability of atoms to rotate about a double bond because of the π bond.
Common Mistakes
- Using E/Z for an alkene that has two identical groups on one carbon of the double bond.
- Deciding E or Z from the position of the longest chain instead of CIP priority.
- Assuming a single compound is cis or trans without comparing the substituents.
- Forgetting that priority depends on atomic number, so Br outranks Cl and Cl outranks C.