Same atoms. Different possibilities.
Structural isomers have the same molecular formula but different connectivity. Butane and 2-methylpropane are both C₄H₁₀. Stereoisomers share connectivity but differ in spatial arrangement.
A C=C bond prevents ordinary free rotation. Cis/trans isomerism requires each double-bond carbon to carry two different groups. But-2-ene has cis and trans forms; propene does not, because one alkene carbon has two hydrogens.
A tetrahedral carbon bonded to four different groups is a chiral centre. Its mirror-image arrangements can be non-superimposable. In a wedge–dash drawing, a solid wedge points towards the viewer and a dashed wedge away.
Same connections. Different geometry.
But-2-ene has two different groups on each double-bond carbon. Switching views compares isomers; it does not imply free rotation around C=C.
Explore: Rearrange the same atoms and distinguish structural from stereochemical changes.
Compare but-1-ene and but-2-ene: classify their structural relationship, identify which has cis/trans forms and explain the restriction.
- They share C4H8 but differ in double-bond position. [1 mark]
- Restricted rotation about C=C can support stereoisomerism. [1 mark]
- Each carbon of but-2-ene has two different substituents, H and CH3. [1 mark]
- The terminal carbon of but-1-ene has two H substituents, so distinct cis/trans forms cannot occur. [1 mark]
Answer: Position isomers; only but-2-ene has cis/trans forms.
Assumed knowledge
A single bond usually permits rotation; a double bond does not.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Distinguish structural, geometrical and optical isomers.
- Identify a chiral carbon.
- Apply cis/trans reasoning to simple alkenes.