Tartaric acid affords an interesting example in stereo-chemistry, not only on account of its structure, but also because it was upon this acid and its modifications that Pasteur per formed the classical experiments which led (1860) to the development. of the methods that are now used for the separation of optical iso mers. The constitutional formula for tartaric acid as ordinarily written is Each of the two central carbon atoms is here asymmetric, because each has its four bonds satisfied by four different substances. In the solid diagram for representing the structure of tartaric acid we shall, therefore, have to make use of two tetrahedra; and since the two nu clear carbon atoms are directly united by one bond, their corresponding tetrahedra will be symmetrically situated, and will have one vertex in common. In attaching the hydrogen, the hydroxyl (OH) and the carboxyl (COOH) to the vertices of the tetrahedra, we may adopt any one of three essentially different arrange ments, for representing the relations expressed by the foregoing structural formula. These are shown in Figs. 7, 8 and 9. It will be observed, in Fig. 7, that the radicals and atoms that are attached to the vertices are so situated that the two tetrahedra would be superposable in all respects, if they were separated from each other. According to stereo-chemical theory, this signi fies that the compound that Fig. 7 represents will rotate the plane of polarized light by an amount equal to the sum of the effects of the two constituent asymmetrical tetrahedra. Turn ing now to Fig. 8, it will be seen that the two tetrahedra in this diagram also admit of super position upon themselves, if they are separated; and hence the compound that this diagram rep resents will also rotate the plane of polarization by an amount that is equal to the sum of the effects that are due to its two constituent tetra hedra. It will be noted, however, that neither of the tetrahedra in Fig. 8 can be superposed upon either of those in Fig. 7; and this fact (when taken in connection with the identity of the two types of tetrahedra in all other re spects) signifies that the compounds that are represented by Figs. 7 and 8 will rotate the plane of polarization by equal amounts, but in opposite directions. Fig. 7 may, therefore, be
taken to represent dextro-rotary tartaric acid, while Fig. 8 represents the Imvo-rotatory va riety of the same substance. In the configura tion shown in Fig. 9, it will be observed that the two tetrahedra, if separated, cannot be superposed; for they are enantiomorphic, or re spectively right-handed and left-handed. Ac cording to the theories of stereo-chemistry, one of these tetrahedra tends to cause the plane of polarization of polarized light to rotate to the right, while the other tends to rotate it by an equal amount to the left; and hence the sub stance, as a whole, will be optically inactive. In addition to these three varieties of tartaric acid (all of which are known and can be ac tually prepared), there is a fourth variety which is optically inactive, but which is not really chemically distinct from the varieties that have been described. This is known as °racemic acid,* and it consists of a mere mixture of equal parts of the dextro-rotatory and lmvo rotatory acids.
exceedingly close, but certain slight differences are nevertheless observable, and these render the separation of the isomers possible. Special methods of separation may be adopted in spe cial cases; but the three general methods that we have, and which are due to Pasteur, are as follows: (1) The mixed isomers are caused to combine with some other substance, whose iso meric compounds exhibit differences in melting point, solubility or crystallization; and when the compounds have been separated by the usual methods applicable in such cases, they are separately decomposed so as to liberate the isomers of the original substance again.
The general facts of optical isomerism are thus stated by Nernst: (1) No compounds are optically active in the amorphorus, homogeneous state (whether solid, liquid or gaseous), save those which contain one or more asymmetrical carbon atoms in the molecule. (2) If several asymmetrical carbon atoms exist in the mole cule,— for example, two, — then the rotations that are due to these respective atoms may be denoted by A and B, and the following com binations are possible, in the isomeric forms which the substance may exhibit: