Upper secondary · 15 min

Isomers you can rotate: same formula, different molecules

Inspect three isomer pairs in one rotating scene — chain isomers of $\ce{C4H10}$ , cis/trans $\ce{C4H8}$ and the enantiomers of $\ce{CHBrClF}$ .

Goal

Distinguish constitutional isomerism (different connectivity) from stereoisomerism (same connectivity, different 3D arrangement) and spot a stereogenic centre.

Apparatus and reagents

The model viewer below; optionally a physical ball-and-stick kit to rebuild each pair by hand.

Procedure

Rotate the top pair (n-butane vs isobutane): count how many carbons each carbon touches — the connectivity differs.
Middle pair: try to rotate a bond of cis-but-2-ene so it becomes the trans form — notice the π bond blocks it.
Bottom pair: rotate the (S) molecule through every orientation; it never lands on the (R) one — they are mirror images.
Name each pair: constitutional, cis–trans (diastereomers) or enantiomers, and write both IUPAC names.

What to observe

Chain isomers differ in a measurable property — isobutane boils at −11.7 °C, n-butane at −0.5 °C — because branching changes the contact surface between molecules.
The enantiomers differ only in handedness; they share every scalar property but rotate plane-polarised light in opposite directions.

Explanation

Isomers share a molecular formula. Constitutional isomers reconnect the atoms — n-butane is a straight chain, isobutane a branched one. Stereoisomers keep the same connectivity: cis/trans pairs differ around a rigid $\ce{C=C}$ , while enantiomers are non-superposable mirror images around a carbon bearing four different groups (a stereogenic centre). IUPAC names encode all of this: locants for the connectivity, cis/trans or (R)/(S) for the 3D part.

Chemists behind it

Jacobus Henricus van ’t Hoff Vladimir Prelog