Constitutionally unsymmetrical molecules

An acyclic, constitutionally unsymmetrical molecule can exist as 2 stereoisomers which are enantiomeric in pairs. In other words, such a molecule can exist as 2( -1) diastereoisomeric pairs of enantiomers. Any stereoisomer will thus have one enantiomer (that stereoisomer of opposed configuration on every chiral center) and 2" — 2 diastereoisomers. The latter may have as little as 1 and as much as n — 1 centers of opposed configuration. Those diastereoisomers which differ in the configuration of a single chiral center (i.e., which have identical configuration on n — 1 centers) are called epimers. Any stereoisomer in such a series has n epimers. 

In a molecule containing n centers of chirality, the number of possible stereoisomers varies depending whether the molecule is constitutionally unsymmetrical (nonidentical centers of chirality) or constitutionally symmetrical. 

No final or absolute decision is therefore yet possible. By analogy with other cases of a similar type amongst organic compounds, the possibility of a dynamic isomerism must not be left out of consideration. It is quite possible that in sulphurous acid solution, molecules of the symmetrical and the unsymmetrical constitution may be present, side by side, in equilibrium with one another, and that even in solutions of the salts a similar condition of equilibrium may exist. Indeed, the distinctly contradictory nature of some of the chemical evidence favours this view. 

Constitution.—The formulae must, on account of the properties of pyrophosphoric acid and its relation to orthophosphoric acid, show the phosphorus atoms as saturated. This can be done either by the electronic formulae or by those containing quinquevalent phosphorus, since as already shown the two kinds of formulae are interchangeable. The molecule may be (1) symmetrical or (2) unsymmetrical —... 

In each of the preceding Diels-Alder reactions, only one product is formed (disregarding stereoisomers) because at least one of the reacting molecules is synunetrically substituted. If both the diene and the dienophile are unsymmetrically substituted, however, two products are possible. The products are constitutional isomers. 

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