Editing Louis Pasteur (section)

=== Molecular asymmetry ===
[[File:Pcrystals.svg|thumb|Pasteur separated the left and right [[crystal]] shapes from each other to form two piles of crystals: in solution one form rotated light to the left, the other to the right, while an [[Racemic mixture|equal mixture]] of the two forms canceled each other's effect, and does not rotate the [[polarized light]].]]

In Pasteur's early work as a [[chemist]], beginning at the ''École Normale Supérieure'', and continuing at Strasbourg and Lille, he examined the chemical, optical and crystallographic properties of a group of compounds known as [[tartrates]].<ref name="Flack" />

He resolved a problem concerning the nature of [[tartaric acid]] in 1848.<ref>L. Pasteur (1848) [http://gallica.bnf.fr/ark:/12148/bpt6k2983p/f539.item.r=.zoom "Mémoire sur la relation qui peut exister entre la forme cristalline et la composition chimique, et sur la cause de la polarisation rotatoire"] {{Webarchive|url=https://web.archive.org/web/20161021035454/http://gallica.bnf.fr/ark:/12148/bpt6k2983p/f539.item.r=.zoom |date=21 October 2016 }} (Memoir on the relationship that can exist between crystalline form and chemical composition, and on the cause of rotary polarization)," ''Comptes rendus de l'Académie des sciences'' (Paris), '''26''': 535–538.</ref><ref>L. Pasteur (1848) [https://books.google.com/books?id=gJ45AAAAcAAJ&pg=PA442 "Sur les relations qui peuvent exister entre la forme cristalline, la composition chimique et le sens de la polarisation rotatoire"] (On the relations that can exist between crystalline form, and chemical composition, and the sense of rotary polarization), ''Annales de Chimie et de Physique'', 3rd series, vol. 24, no. 6, pp.&nbsp;442–459.</ref><ref>George B. Kauffman and Robin D. Myers (1998)[https://web.archive.org/web/20060117144722/http://192.129.24.144/licensed_materials/00897/papers/0003006/36kau897.pdf "Pasteur's resolution of racemic acid: A sesquicentennial retrospect and a new translation,"] ''The Chemical Educator'', vol. 3, no. 6, pp. 1–4, {{doi|10.1007/s00897980257a}}</ref><ref name=Gal>Joseph Gal: ''Louis Pasteur, Language, and Molecular Chirality. I. Background and Dissymmetry'', Chirality ''23'' ('''2011''') 1–16.</ref> A solution of this compound derived from living things [[Optical rotation|rotated]] the [[plane of polarization]] of light passing through it.<ref name="Flack">H.D. Flack (2009) [http://crystal.flack.ch/dox/sh5092.pdf "Louis Pasteur's discovery of molecular chirality and spontaneous resolution in 1848, together with a complete review of his crystallographic and chemical work,"] {{Webarchive|url=https://web.archive.org/web/20190908211420/http://crystal.flack.ch/dox/sh5092.pdf |date=8 September 2019 }} ''Acta Crystallographica'', Section A, vol. 65, pp.&nbsp;371–389.</ref> The problem was that tartaric acid derived by [[chemical synthesis]] had no such effect, even though its chemical reactions were identical and its elemental composition was the same.<ref name="cohn" />

Pasteur noticed that crystals of tartrates had small faces. Then he observed that, in [[racemic mixture]]s of tartrates, half of the crystals were right-handed and half were left-handed. In solution, the right-handed compound was [[dextrorotatory]], and the left-handed one was levorotatory.<ref name="Flack" /> Pasteur determined that optical activity related to the shape of the crystals, and that an asymmetric internal arrangement of the molecules of the compound was responsible for twisting the light.<ref name=Chirality>{{cite web|title=Louis Pasteur|url=https://www.sciencehistory.org/historical-profile/louis-pasteur|website=[[Science History Institute]]|access-date=20 March 2018|date=June 2016|archive-date=21 March 2018|archive-url=https://web.archive.org/web/20180321132243/https://www.sciencehistory.org/historical-profile/louis-pasteur|url-status=live}}</ref> The (2''R'',3''R'')- and (2''S'',3''S'')- tartrates were isometric, non-superposable mirror images of each other. This was the first time anyone had demonstrated [[Chirality (chemistry)|molecular chirality]], and also the first explanation of [[isomer]]ism.<ref name="Flack" />

Some historians consider Pasteur's work in this area to be his "most profound and most original contributions to science", and his "greatest scientific discovery."<ref name="Flack" />