3D Stereoisomerism – Cis–Trans (E/Z) and Optical Isomers

What it shows

Stereoisomers have the same structural formula but a different arrangement of atoms in space. The first part shows why a C=C double bond cannot rotate: the π bond needs two parallel p orbitals, so twisting one end breaks it, which costs about 266 kJ/mol in a simple model, while a C–C single bond turns freely over a barrier near 12 kJ/mol. Cis and trans but-2-ene are therefore different substances. The second part places molecules with a chiral carbon next to their mirror images, labelled (R) and (S), and shows that no rotation can superimpose them.

How to use

In the Cis–trans isomers tab, choose a molecule and drag the θ slider, or click Turn one end 180°, to twist the right-hand end; the p orbitals button shows the p lobes, and the graph follows the energy. In the Optical isomers tab, choose a molecule, rotate the mirror image with the 120° buttons, click Superimpose to slide it onto the original, or click Find the best rotation to test every rotation.

Parameters you can change

• Part Cis–trans isomers, Optical isomers
• Molecule (cis–trans) But-2-ene, 1,2-Dichloroethene, But-1-ene (no cis–trans), 1,2-Dichloroethane (single bond)
• Starting form cis (Z), trans (E)
• Molecule (optical) Bromochlorofluoromethane, Butan-2-ol, Lactic acid, Alanine, Propan-2-ol (achiral)
• Show the p orbitals of the π bond
• Auto-rotate

Questions to explore

1. Why does but-2-ene have cis and trans isomers while but-1-ene does not?
2. Why can the cis and trans forms of 1,2-dichloroethene be separated, but not those of 1,2-dichloroethane?
3. Is the central carbon of propan-2-ol a chiral center, and what does its mirror image show?