Three Werner's models illustrating racemization, by Science Museum Workshops, South Kensington, London, England, 1920-1924

These three models were constructed to illustrate a theory of racemisation proposed by A Werner (1866-1919) in 1904.

When optically active compounds are heated, they often become inactive or racemise. That is half the active form is converted to its enantiomorph (the active form of opposite rotation) cancelling the optical activity. The first model represents one optically active form with four different groups disposed tetrahedrally about a central carbon atom. These groups tend to vibrate, with increasing amplitude on heating. They may thus achieve the planar configuration of the second model. From the second model, they can return to the first model or pass to its mirror image, the third model.