How it began
Paul Walden was born in 1863 to poor Latvian farmers and was orphaned young. He studied chemistry at Riga Polytechnic Institute from 1882 and returned there to teach in 1892, becoming professor in 1894. A colleague, Carl Bischoff, suggested that he try organic chemistry. In 1896 Walden reported in the journal of the German Chemical Society that he had converted one form of chlorosuccinic acid into its mirror-image form in two steps, and back again in two further steps.
Before it existed
Shortly before Walden's work, Jacobus van't Hoff had proposed that the single bonds of a carbon atom could be modeled as a tetrahedron. According to chemist Erick Carreira, van't Hoff's work was purely theoretical. There was no key experiment showing how the spatial arrangement of a molecule behaves during a reaction.
How it works
A chiral molecule exists in two forms that are mirror images of each other, like left and right hands. The Walden inversion is a transformation that switches the configuration of a chiral molecule with a single stereocenter into its mirror image. Walden showed this by taking (+)-chlorosuccinic acid through two reactions to obtain (-)-chlorosuccinic acid, and then through two more to return to the starting form.
What lasted
Walden's experiment provided experimental verification for van't Hoff's stereochemical theory. In Carreira's words, it helped set a stage for how to use stereochemistry to gain insight into molecular structure and reactions. Walden himself spent only about five years on organic chemistry and did most of his life's work in physical chemistry, remaining at Riga for 25 years before he emigrated to Germany in 1919.
Where it stands today
The reaction still carries Walden's name. In Latvia a Walden medal is awarded for outstanding work in chemistry, and a Paul Walden Symposium on Organic Chemistry has been held since the late 1980s. For the 150th anniversary of his birth in 2013, two streets were named after him, one next to Riga Technical University, and the Latvian postal service issued a stamp.
