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pairs corresponding to the first group are not known products at present but would be represented by a-d-gulose and a-d-idose, or by a-d-allose and a-d-altrose. In the hexose series the second group is represented by a-d-glucose and a-d-mannose, and by a-d-galactose and a-d-talose. The epimeric differences obtained from these pairs, +14,930 and +14,900, are in excellent agreement. The third group is represented by B-d-allose and B-d-altrose, and by B-d-a-glucoheptose and B-d-B-glucoheptose. The epimeric differences for these pairs are -5,940 and -6,010. Since these sugars have not been extensively studied, it is quite possible that B-d-altrose or B-d-B-glucoheptose may be improperly classified. The fourth group is represented in the hexose series by B-d-glucose and B-d-mannose, and by B-d-galactose and B-d-talose. The epimeric differences obtained from these pairs are +6,430 and +7,130. The epimeric differences obtained for various configurations bring out the need for considering the configuration of adjacent groups in making comparisons, and emphasize the complex character of the problem.

The rotational differences for carbon 3, obtained from sugars which represent three possible combinations for the configurations of the adjacent groups, give values of approximately 16,000, -3,000, and +8,000. The differences in these values show that the configurations of the adjacent carbons influence rotation. Data are not available for calculating the rotational differences for the fourth group. For two of the configurations, the comparisons give results in approximate agreement with one another.

The data available for calculating the rotational differences for carbon 4 in the hexose series are limited to only one combination for the configurations of the adjacent carbon atoms. Four calculations from the optical rotations of eight sugars give values in approximate agreement with each other, namely, -6,940, -6,140, -6,970, and -5,440. These values are in accord with those obtained in the heptose series for substances of like configuration, but they are not strictly comparable with those obtained from the pentoses, because the pentoses differ from the hexoses and heptoses in the substituent group on adjacent carbon 5.

The determination of the optical rotation of carbon 5 is complicated, because any change in its configuration affects the adjacent ring oxygen, which in turn determines the alpha and beta positions of the first carbon. Consequently, the rotational differences for carbon 5 (2R) include any changes which may be induced by the dissymmetry of the molecule as a whole. The data at hand are not sufficient to evaluate this factor. The comparisons involving the optical rotations of allose and altrose do not appear to be in accord. The discrepancy may be caused by improper classification, erroneous optical rotations, or unknown structural differences, such as differences in the conformation of the rings.

The rotational differences clearly show that optical rotation is not uniformly an additive property and that dissimilarity in the configurations of the contiguous atoms results in deviations from the Van't Hoff theory of optical superposition. The work of Tschugaeff, Kuhn, Lowry, and others [24, p. 429] shows that each asymmetric carbon in an optically active substance gives rise to one or more partial rotations, which may be correlated with absorption bands of characteristic frequency having their origin in particular electronic transitions

taking place in the molecule. These transitions are not influenced greatly by atoms or groups at some distance from the asymmetric carbon but are influenced by the neighboring groups. The optical rotation in the visible spectrum is chiefly governed by the absorption bands nearest the wave length used for the rotation measurements. Since the bands are not located at the same wave lengths for all sugars, the partial rotation varies in irregular fashion with the wave length. For this reason the difference in the rotations of two sugars depends in part on the light used for making comparison, and it is obvious that the optical rotations cannot be rigorously represented by the simple algebraic equations suggested by Van't Hoff.

Nevertheless, the active part that the principle of optical superposition has played in the development of carbohydrate chemistry is sufficient justification for continuing its use. It has been amply demonstrated that substances of similar structure and configuration give approximately like rotational differences.

For correlating optical rotation and structure, Hudson [32] has noted a number of approximations which are expressed in several empirical rules. The so-called first rule of isorotation relates to the optical rotation of the glycosidic carbon. If the formulas for alpha and beta glucose are written as ring structures differing solely in the configuration of carbon 1, and if the rotation due to the end asymmetric carbon is A, and the rotation due to the rest of the molecule is B, the molecular rotation of one isomer will be +A+B, and the rotation of the other isomer will be -A+B. The sum of the rotations is +2B and their difference +24. When the molecular rotations of the alpha and beta modifications of glucose, galactose, and lactose are compared on the one hand, and the molecular rotations of lyxose, rhamnose, mannose, and 4-glucosidomannose are compared on the other hand, it will be observed that the differences in the molecular rotations for the alpha-beta pairs in each group are nearly constant. The members of the first group have the configuration H-C-OH for the carbon adjacent to the glycosidic group, while the members of the second group have the configuration HO-C-H. Many similar comparisons reveal that the rotational difference, 24, is nearly constant for substances which have like glycosidic groups, like ring structures, and like configurations on the adjacent carbon atoms. This approximate equality is the basis of the first rule of isorotation which states that the rotation of the glycosidic group is affected in only a minor degree by changes in the structure of the remainder of the molecule provided the changes are not on the contiguous atoms.10

Hudson's second rule of isorotation relates to the optical rotation of the rest of the molecule. The sum of the molecular rotations of the alpha and beta sugars, +2B, varies from sugar to sugar, but if the sums of the molecular rotations of the sugars are compared with the sums of the molecular rotations of the methyl glycosides, it will be observed that the values of 2B obtained for the sugars are in close agreement with the values of 2B' obtained for the glycosides. This is the basis for the second rule of isorotation which states that changes in the structure of the glycosidic carbon affect in only a minor degree the rotation of the remainder of the molecule. As may be observed from data given in table 54, the values for 2B are in approximate agreement

40 Hudson's original rule does not exclude changes on the contiguous atoms.

for substances which do not have large differences in the glycosidic group.

TABLE 54.-Sum of the molecular rotations (2B) for some alpha and beta derivatives of d-glucose

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The value of 2B obtained for the alpha and beta phenyl glucosides (+28,140) differs considerably from the value (+24,220) obtained for the alpha and beta methyl glucosides and related substances. The introduction of an unsaturated chromophoric group appears to induce a change in the optical rotation of the rest of the molecule. The chromophoric group is particularly influential when adjacent to the asymmetric center. Its influence is small when it is at some distance from the asymmetric center, as for example, in the benzylglucosides. As may be observed from the data given in table 55, the sums for the molecular rotations of the alpha and beta sugars do not differ widely from the sums for the molecular rotations of the corresponding methyl glycosides.

TABLE 55.-Sums of the rotations of the alpha and beta sugars in comparison with those of the a- and 3-methyl glycosides (second rule of isorotation)

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1 The value for the rotation of 3-d-xylose was taken from Hudson's indirect measurements by means of solubility experiments.

The accumulation of a large amount of experimental data has brought out other approximations which are useful for correlating optical rotation and structure. One of the most obvious is the striking similarity in the optical rotations of substances which have like configurations for the carbon atoms comprising the pyranose ring. This generalization can be expressed in a rule which states that changes in the side chains attached to the pyranose ring affect in only a minor degree the rotation of the remainder of the molecule. The optical rotations of a few sugar derivatives, which differ merely in the groups attached to the pyranose ring, are given in table 56.

As might be anticipated from the close similarity in structure, the molecular rotations of the configurationally related hexoses and heptoses resemble one another more closely than they resemble the rotations of the configurationally related pentoses and methyl pen

toses. A number of unexplained variations may be noted in table 56; these call for further investigation to ascertain whether they arise from errors or from unknown differences in structure.

TABLE 56.-Molecular rotation of substances of like configuration of the pyranose ring

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Several relationships connecting the optical rotations and configurations of the sugar acids and their derivatives have been noted. first of these is the Hudson lactone rule [44, 45], which states that lactones, in which the oxygen ring lies to the right when the projectional formulas are written in the conventional manner, are more dextrorotatory than the parent acids, and if the oxygen ring lies on the left, the lactone is the more levorotatory. The rule was originally derived from the optical rotations of gamma lactones, but it appears to apply equally well to the delta lactones. With the exception of allonic, manno-nononic, and digitoxonic lactones, the sign of rotation in water solution corresponds with the configuration of the ringforming carbon. That is, lactones in which the oxygen ring lies to the right are usually dextrorotatory, while lactones in which the oxygen ring lies to the left are usually levorotatory. Levene [46, 47] and also Hudson [48] have pointed out that the phenylhydrazides and amides of the aldonic acids are dextrorotatory when the hydroxyl on the alpha carbon lies to the right. These relationships are very useful for determining the structures of new acids formed by extending the carbon chain, because frequently the phenylhydrazides and amides

are used for separating the products. Levene [46, 47] has shown that the alkali salts of the aldonic acids are more dextrorotatory than the free acids when the hydroxyl on the alpha carbon lies to the right, and Isbell [49] has shown that the lead salts are more levorotatory than the alkaline earth salts when the hydroxyl on the alpha carbon lies to the right. These empirical rules make the classification of newly prepared sugar acids a very simple matter.

TABLE 57.-Molecular rotation of the aldonic acids and related products

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1 Value derived from the enantiomorph by reversing the sign of the rotation.

The marked parallelism between the optical rotations and configurations which has been considered briefly is but one of many correlations which exist between the properties and the configurations of the sugars and their derivatives. In the next section, some of the chemical properties will be considered in relation to structure and configuration.

6. CORRELATIONS BETWEEN THE CONFIGURATIONS AND THE CHEMICAL PROPERTIES OF THE SUGARS

The alpha and beta sugars have diverse configurations for their reducing carbons and marked differences have been found in their properties. An extensive investigation of the alpha and beta sugars was initiated by Isbell, who sought to determine the effect of configuration on the relative reactivity of the alpha and beta sugars. Some of the data obtained in this investigation are given in table 58, from which it may be observed that the beta sugars are oxidized by bromine water more rapidly than the corresponding alpha sugars.

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