Comparisons of the above formulas will show that the difference between the formulas for d- and l-glucose lies in the arrangement of the H atoms and the OH groups around the two asymmetric carbon atoms next the aldehyde end of the chain; while the d- and l-galactoses differ in that this arrangement is in the reverse order around all four of the asymmetric carbons. By similar variations in the grouping around the four asymmetric atoms, it is possible to produce the sixteen different space arrangements shown on [page 37] for the groups of an aldohexose. Sugars corresponding to fourteen of these different forms have been discovered, three of which are of common occurrence in plants, either as single mono-saccharides or as constituent groups in the more complex carbohydrates; the remaining two forms have only theoretical interest.
Similarly, for a ketohexose, which contains three asymmetric carbon atoms, there are eight possible arrangements. Three sugars of this type are known, only one (fructose) being common in plants; the others are of only theoretical interest.
FOOTNOTES:
[1] It is assumed that the reader, or student, is familiar with the theoretical and experimental evidence in support of the existence of the so-called "asymmetric" carbon atom and its relation to the effect of the compound which contains it, when in solution, in rotating the plane of polarized light. For purposes of review, or of study of this most interesting and important phenomenon, the reader is referred to any standard text-book on Organic Chemistry.
[2] Attention should be called, at this point, to the fact that such formulas as these cannot possibly accurately represent the actual arrangement of the constituent groups of a carbohydrate molecule around an asymmetric carbon atom. The limitations of a plane-surface formula prevent any illustration of the three-dimension relationships in space. Furthermore, there are certain facts in connection with the birotation phenomenon and the relation of the molecular configuration to biochemical properties (which see) that cannot be explained on the basis of the open-chain arrangement represented by the Fischer formulas used here. A closed-ring arrangement, showing the aldehyde oxygen as linked by its two bonds to the first and the fourth carbon atoms of the chain, thus forming a closed-ring of four carbon and one oxygen atoms, instead of being attached by both bonds to a single carbon atom, as in the above formulas, is undoubtedly a more nearly accurate representation of the actual linkage in the molecule than are the open-chain formulas used above.
The differences in conception embodied by these two types of formulas may be shown by the following formulas for glucose:
It will be observed that in the closed-ring formula there are five asymmetric carbon atoms, and the asymmetry of the terminal one forms the basis for the explanation of the existence of the so-called α and β modification of d-glucose (see [page 46]). However, the ordinary aldehyde reactions of the sugars are more clearly indicated by the open-chain formula. Some investigators are inclined to be that sugars actually exist in the open-chain arrangement when in aqueous solution, and in the closed-ring arrangement when in alcoholic solution. The closed-ring formulas will be used in this text in the discussions of the birotation phenomena and of biochemical properties, but for the explanations of the stereo-isomeric forms and similar phenomena, the open-chain formulas are just as useful in conveying an idea of the possibilities of different space relationships, and are so much simpler in appearance and in mechanical preparation, that it seems desirable to use these rather than the more accurate closed-ring formulas.
CHEMICAL CONSTITUTION OF MONOSACCHARIDES
The term "monosaccharides," as commonly used, refers to hexoses. It applies equally well, however, to any other sugar-like substance which either occurs naturally or results from the decomposition of more complex carbohydrates, and which cannot be further broken down without destroying its characteristic aldehyde-alcohol groups and sugar-like properties.