STEREOCHEMISTRY is a branch of chemistry which is concerned with the arrangement in space of the atoms in the molecules of which substances are composed. (Gr. arepein, solid). Knowledge regarding the spatial configuration of mole cules has been acquired chiefly through the investigation of ex amples of stereoisornerism, a type of isomerism in which the isomers exhibit the same chemical reactions (and therefore possess the same chemical structure) and yet differ in certain physical properties; for differences between compounds of the same structure can only be due to differences in the arrangement of the atoms in their molecules in space.
The fundamental principles of stereochemistry were estab lished by Louis Pasteur in a series of investigations on tartaric acid, carried out chiefly during the years 1849-53. Tartaric acid is obtained as a by-product in the manufacture of wine, being deposited from the fermented must in the form of potassium hydrogen tartrate. The crude salt contains, however, a small pro portion of the potassium hydrogen salt of another acid of the same composition named racemic acid. Now tartaric and racemic acid not only have identical chemical compositions but they also possess the same chemical structure. Yet they show certain marked differ ences in properties. For example, racemic acid is much less soluble in water than tartaric acid and crystallizes with water of crystal lization, whereas tartaric acid separates in anhydrous crystals; but the most remarkable difference between the two substances is in their effect on plane-polarized light. Many naturally occurring substances of organic origin, such as sugar, camphor and oil of turpentine, possess, even in the liquid or dissolved state, the curious property of rotating the plane of polarization of a beam of plane-polarized light through an angle proportional to the thickness traversed. Tartaric acid and the tartrates have this property, being dextrorotatory (i.e., they rotate the plane of polar ization to the right, or in the sense which is clockwise when re garded in the direction contrary to that in which the light is travelling), while racemic acid and the racemates are optically inactive.
Pasteur's discoveries arose out of a crystallographic investiga tion of tartaric acid. He observed that there was a certain lack of symmetry in the crystalline form of this substance of such a kind that the crystal was non-superposable on its mirror-image. The
crystal and its image thus differ in the same kind of way as the right hand from the left hand. He prepared and examined 19 differ ent salts of tartaric acid and found that each of these showed a similar lack of symmetry in its crystalline form. Thus fig. Ia represents the crystalline form of one of these salts, sodium ammonium tartrate. If the crystal possessed the full symmetry of the system to which it belongs, two faces, corresponding with the two shaded, should be found in the positions indicated by dotted lines. These faces, instead of occurring 8 times in the crys tal, occur only 4 times (there are of course two corresponding faces on the back of the crystal not shown in the figures) and such faces, occurring only half as frequently as the full sym metry of the crystal demands, are termed hemihedral faces. The sodium ammonium tartrate crystal, on account of the presence of these hemihedral faces, is evidently non-superposable on its mirror-image represented in fig. 'b. In contrast to tartaric acid and the tartrates, racemic acid and such of its salts as Pasteur examined (with one exception, which will be considered below) crystallize in symmetrical forms superposable on their mirror images. The optically active tartrates thus crystallize in dissym metric forms, whilst the optically inactive racemates yield sym metrical crystals.
Pasteur's chief discovery was made through investigating the exceptional racemate referred to above, which was sodium am monium racemate. Crystallized under certain conditions this salt yielded crystals which unlike those of the other racemates ex hibited hemihedral faces, but the crystals were of two kinds. Some were identical in form with dextrorotatory sodium ammo nium tartrate (fig. ia) and were in fact crystals of that. salt. The other crystals differed from these only in that their hemihedrism was in the opposite sense; they were the realization of the mirror image of sodium ammonium tartrate: they had thus the form represented in fig. 'b. When picked out from the mixture and dissolved in water they gave a laevorotatory solution ; and the acid extracted from them had a crystalline form which was the mirror-image of that of tartaric acid, and its solutions were laevorotatory.