It is seen that aldoses and ketoses which differ stereochemically in only the two final carbon atoms must yield the same osazone; and since d-mannose, d-glucose, and d-fructose do form the same osazone (d-glucosazone) differences either structural or stereochemical must be placed in the two final carbon atoms.3 It may here be noticed that in the sugars there are asymmetric carbon atoms, and consequently optical isomers are to be expected.
The one group included those isomers where the identity in composition was accompanied by identity in molecular weight, i.e.
Of the two systems of three carbon atoms into a chain of six carbon atoms. But it is not only the formation of different isomers which is included in their constitution, but also the different ways in which they will decompose or give other products.
Typical formulae are (R denoting 0, S or NH): Isomers are possible, for the condensation may be effected on the two carbon atoms symmetrically placed to the hetero-atom; these isomers, however, are more of the nature of internal anhydrides.
In general, isomers boil at about the same temperature, as is shown by the isomeric esters CH1802: Methyl octoate..
Thus glucose, containing four such atoms, can exist in 16 forms; and the realization of many of these isomers by E.
In reality such tautomeric compounds are apparently a mixture of two isomers in equilibrium, and indeed in some cases both forms have been isolated; then one speaks of desmotropy (Gr.
These isomers may frequently be distinguished by the facts that the cis-acids yield anhydrides more readily than the trans-acids, and are generally converted into the trans-acids on heating with hydrochloric acid.
Consequently, of each pair of isomers we may establish beforehand which is the more stable; either in particular circumstances, a direct change taking place, as, for instance, with maleic acid, which when exposed to sunlight in presence of a trace of bromine, yields the isomeric fumaric acid almost at once, or, indirectly, one may conclude that the isomer which forms under greater heat-development is the more stable, at least at lower temperatures.
It is found that isomers have nearly the same critical volume, and that equal differences in molecular content occasion equal differences in critical volume.