Stereochemistry

This new acid has exactly the same solubility, melting point, specific gravity, etc., as tartaric acid; in all such properties the two acids are identical. They differ in that, (I) solutions of the new laevorotatory acid rotate the plane of polarization exactly as much to the left as equivalent solutions of tartaric acid ro tate it to the right, and (2) their crystals cannot be superposed; their hemihedrism is in opposite senses. In the two compounds, therefore, all properties are identical except those which have direction, and these are equal and opposite. The only relationship between their molecules that can give this relationship in proper ties is evident; the molecules must be related as object to mirror image. The reason why tartaric acid exists in two forms is thus that its molecule has a lack of symmetry that renders it non-super posable on its mirror-image, so that right- and left-handed forms of the molecule are possible. The dextrorotatory tartaric acid obtained directly from grape-juice is termed dextro-tartaric (or d-tartaric) acid, and the laevorotatory modification extracted from sodium ammonium racemate is known as laevo-tartaric (or l-tartaric) acid.

When concentrated solutions of d-tartaric and l-tartaric acids are mixed, the less soluble racemic acid crystallizes at once from the mixture. Racemic acid is evidently composed of molecules of d- and l-tartaric acids in equal proportions, and its optical inactiv ity is the result of the summation of the equal and opposite ac tivities of its components. In the crystal of racemic acid the B and /-molecules are packed together in pairs; such a crystal may therefore be compared with a large package of pairs of gloves. The salts of racemic acid similarly crystallize almost invariably in uniform crystals composed of pairs of the d- and /-molecules. Under exceedingly special conditions, however—in the case of a particular salt, the sodium ammonium double salt, and pro vided that crystallization takes place below 27° C—the constitu ent d- and l-tartrates crystallize in separate crystals, just as under exceptional circumstances gloves might be packed with right-hand and left-hand gloves in separate packages.

The discovery of l-tartaric acid thus revealed the significance of optical activity in liquid and dissolved substances. All natural objects can be divided into two classes: (i.) those which are super posable, and (ii.) those which are non-superposable on their mirror-images. It is evident that a substance of which the mole cules belong to the first class can only exist in one form and can not be optically active in the liquid or dissolved state, for in view of the symmetry of its molecules there can be nothing in them to cause the rotation of the plane of polarization in one direction rather than in the other. The molecules of substances that are op

tically active in the liquid or dissolved state must therefore belong to the second class, and such molecules are necessarily capable of existence in enantiomorphous (opposite-shaped) forms, and the optical activities of these forms must be equal and opposite; these complementary forms are termed enantiomorphs antimers or optical antipodes. Optically active substances must there fore occur in pairs. To every naturally occurring optically active compound there should be an optical antipode. Many of these are known as, for example, the laevorotatory antipodes of glucose and camphor, whilst others still await discovery.

To express the fact that a substance is composed of molecules which are non-superposable on their mirror-images, Pasteur intro duced a new term. He described such substances as possessing molecular dissymmetry. The fundamental principle established by his investigations may thus be stated as follows : If any sub stance exhibits optical activity in the liquid or dissolved state it possesses molecular dissymmetry. The converse principle is also to be regarded as well established (for it now rests on a very broad experimental basis), namely, Every molecularly dissym metric substance is capable of showing optical activity in the liquid or dissolved state. It would appear also that the d- and /-modifications of a molecularly dissymmetric substance crystal lize in enantiomorphous forms, though the enantiomorphism may be difficult to detect.

Separation of the Antimeric Forms.

When a molecularly dissymmetric substance is prepared in the laboratory from in active materials, an optically inactive mixture composed of equal parts of its dextro- and laevo-modifications must be formed, since there will be no asymmetric influence to favour the formation of one antimer rather than the other. The experimental demonstra tion of the molecular dissymmetry of a synthetic compound there fore always involves the resolution of the initially formed in active mixture into its active antimeric components. The separa tion of antimeric substances requires, however, the use of special methods, since the processes of fractional crystallization and dis tillation usually employed for the isolation of individual sub stances are inapplicable, for antimers have equal solubilities in all solvents and equal vapour pressures at all temperatures. The special processes by which the resolution of molecularly dis symmetric substances can be effected are of three types which may be entitled: (i.) The method of manual separation of the enantiomorphous crystals of the antimers, (ii.) the method of conversion of the mirror-image isomers into diastereoisomers, (iii.) the biochemical method.