Tartarate Isomerism

From WikidChem

Pasteur, in order to learn crystallographic techniques, tried repeating the work of Eilhard Mitscherlich using a "goniometer" to measure crystal angles on crystals of tartrate and racemate.

He noticed that within a particular set of eight edges in the crystal which were sometimes truncated to generate small additional faces only four of the eight were so truncated in the sodium ammonium salt of tartaric acid (Mitscherlich had already made this obseration). Pasteur was surprised to discover that the crystals of sodium ammonium racemate looked just like those of sodium ammonium tartrate except that only half of the crystals had the same four edges truncated. The other half of the crystals had the other set of four edges truncated! One set of crystal was the non-superimposable mirror image of the others - they were "right and left handed".

When he dissolved the two different types crystals of crystals from Racemate separately and measured their influence on polarized light using Biot's polarization apparatus, he discovered that one group of the crystals rotated polarized light to the right , while the other group rotated polarized light to the left. The two groups were about equal in quantity.

In doing so, he discovered there were really two forms of tartaric acid- left (l) and right (d); the combination of the two in equal proportion formed racemic acid. He was unable to tell these tartrates apart on the basis of their melting points, but they could be distinguished by their mirror image shapes and/or their opposite effect on polarized light.

He concluded that l and d tartaric acids are just mirror images of one another and that racemic acid is their 50:50 mixture, but he still did not know what meso-Tartaric acid is. NOTE that Pasteur's work in 1840 predated tetravalence and self-linking of carbon by nearly a decade, so there was no possibility at that time for interpreting the isomerism in terms of molecular structure.

JYW 11/16/06 7:54-8:03 PM