Two molecules can have every atom joined in exactly the same order and still be different, because their groups point in different directions. A rigid double bond or a carbon with four different groups is all it takes.
Stereoisomers have the same structural formula but a different arrangement of atoms in space. Cis-trans isomerism arises because rotation about a C=C bond is restricted; it needs two different groups on each carbon of the double bond. Optical isomerism arises when a carbon atom carries four different groups (a chiral centre); the two forms, enantiomers, are non-superimposable mirror images that rotate plane-polarised light equally in opposite directions. A 50 : 50 mixture is a racemic mixture and has no overall effect on the light.
Rotate the molecules in the 3D stereoisomerism simulator. Try to twist cis-but-2-ene into the trans form, then try to superimpose a molecule on its mirror image.
| Compound | Cis-trans isomers? | Reason |
|---|---|---|
| but-2-ene, CH3CH=CHCH3 | yes | each C=C carbon has H and CH3 |
| but-1-ene, CH2=CHCH2CH3 | no | C1 has two H atoms |
| 1,2-dichloroethene, CHCl=CHCl | yes | each carbon has H and Cl |
| 1,1-dichloroethene, CH2=CCl2 | no | each carbon has two identical groups |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which compound shows cis-trans isomerism?
A chiral centre is a carbon atom bonded to:
Enantiomers differ in:
A racemic mixture:
Look for carbon atoms with four different groups attached. Count the whole group, not just the first atom.
| Compound | Chiral centre? | Groups on the carbon |
|---|---|---|
| butan-2-ol | yes | H, OH, CH3, C2H5 |
| 2-hydroxypropanoic acid (lactic acid) | yes | H, OH, CH3, COOH |
| 2-bromobutane | yes | H, Br, CH3, C2H5 |
| propan-2-ol | no | two CH3 groups |
| 1-bromobutane | no | no carbon has four different groups |
Cis-trans isomerism needs two different groups on each carbon of the C=C, so but-1-ene does not show it. A chiral centre needs four different groups; a carbon with two identical groups, such as the central carbon in propan-2-ol, is not chiral.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which compound has a chiral centre?
Which compound shows both cis-trans and optical isomerism?
Why does but-1-ene not show cis-trans isomerism?
How many chiral centres are in CH3CHClCHClCH2CH3?
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