Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/780

This page needs to be proofread.



latter is optically inactive, so that its configuration must be one of those given in the sixth and seventh columns of the table. On reduction it yields an inactive mixture of galactonic acids, some molecules being attacked at one end, as it were, and an equal number of others at the other. On reducing the lactone prepared from the inactive acid, an inactive galactose is obtained from which 1-galactose may be separated by fermentation. Lastly, when d-galactonic acid is heated with pyridine, it is converted into talonic acid, which is reducible to talose, an isomeride bearing to galactose the same relation that mannose bears to glucose. As it can be shown that d-galactose is CH2(OH) + - + - COH, d-talose is CH2(OH)+ - + + COH. Apart from the value they possess as extending our knowledge of a particular group of compounds, and because they furnish a complete justification of the doctrine of isomerism due to asymmetry, ^”c'u/ar Fischer’s researches supply evidence which cannot be gainsaid that the systems with which the chemist has to deal are intrinsically of a high order of stability, and that the changes they undergo are rarely, if ever, simply the outcome of a state of “intramolecular wobble ”; in fact, they afford clear proof that such changes are dependent on and conditioned by extramolecular or catalytic influences. It will suffice to call attention to a single example in illustration—that afforded by the conversion of gluconic into mannonic acid, and vice versd. If in the case of a series of compounds such as the penthydroxypentanecarboxylic acids any tendency to “wobble” existed, it is to be supposed that of the numerous possible configurations one would prove to be the most stable, and that eventually this would be produced whichever of the isomerides were subjected to the influence of a degree of heat sufficient to cause a change. But nothing of the kind occurs, the change being strictly limited and confined to a particular region within the molecule—to a region which, on various grounds, is to be regarded as particularly susceptible to external influences. The change, in fact, affects only the carbon atom to which the carboxyl group is attached, and it is easy to understand and explain its character. There is every reason to suppose that in the first instance the carboxyl group combines with water—for it will suffice to regard the effect as produced by water alone—and becomes converted into the group •CO(OH) + OH2 = -C^OHjg. By the removal of one of the OH groups in conjunction with an atom of hydrogen from the contiguous CH.OH group, an ethenoid compound is formed containing the group and that consequently only one of the acids will be •C(OH):C(OH)2. In the final stage this compound optically active. As a matter of fact, only arabinose gives combines witbT water, but in two ways, the ethenoid an active product on oxidation • it is, therefore to be linkage being split partly on the one side and partly on supposed that arabinose is the — — — compound, and con- the other, so that two stereoisomeric acids are formed, but sequently— in unequal proportions, owing to the unequal influence CH2(OH) + COH = 1-glucose exercised by the C4H5(OH)4 and OH groups attached to CH9(0H) + COH = 1-gulose. the one member of the pair of ethenoid carbon atoms. The glucoses are much more sensitive to change than When xylose is combined with hydrocyanic acid and the cyanide is hydrolysed, together with 1-gulonic acid, a are the acids to which they give rise. Lobry de Bruyn second isomeric acid, 1-idonic acid, is produced, which on has, in fact, showm that when merely left in contact at reduction yields the hexaldose 1-idose. When 1-gulonic ordinary temperatures with weak alkali they undergo acid is heated with pyridine, it is converted into 1-idonic change in a manner precisely similar to that in which the acid, and vice versd; and d-gulonic acid may in a similar more stable acids are changed on heating with water. manner be converted into d-idonic acid, from which it is Thus glucose, mannose and fructose are reciprocally possible to prepare d-idose. It follows from the manner converted the one into the other, and although, owing to in which 1-idose is produced that its configuration is the formation of acid products, actual equilibrium L never established, the state of reciprocal transformation is CH9(OH)+ - -+COH. The remaining aldhexoses discovered by Fischer are practically that represented by the symbols— derived from d-galactose from milk-sugar. When oxidized glucose fructose ^ mannose. this aklhexose is first converted into the monobasic It is easy to conceive that changes may take place with galactonic acid, and then into dibasic mucic acid 5 the

acid on oxidation, the configuration in this and the Configura- corresponding 1-acid must be sought from t°oa of among those numbered 5-10 in the above table. isomeric Nos. 7 and 8 can be at once ruled out, howgiucoses. everj as acids so constituted would be optically inactive and the saccharic acids are active. If the configuration of d-saccharic acid were given by either 6 or 10, bearing in mind the relation of mannose to glucose, it would then be necessary to represent d-mannosaccharic acid by either 7 or 8—as the forms 6 and 10 pass into 7 and 8 on changing the sign of a terminal group ; but this cannot be done as mannosaccharic acid is optically active. Nos. 6 and 10 must, in consequence, also be ruled out. No. 5, therefore, represents the configuration of one of the saccharic acids, and No. 9 that of the isomeride of equal opposite rotatory power. As there is no means of distinguishing between the configuration of a dextro- and Isevo - modification, an arbitrary assumption must be made. No. 5 may therefore be assigned to the d- and No. 9 to the 1- acid. It then follows that d-mannose is represented by No. 1, and 1-mannose by No. 4, as mannose is produced by reversing the sign of the asymmetric system adjoining the terminal COH group. It remains to distinguish between 5 and 11, 9 and 15 as representing glucose and gulose. To settle this point it is necessary to consider the configuration of the isomeric pentoses—arabinose and xylose—from which they may be prepared. Arabinose being convertible into 1-glucose and xylose into 1-gulose, the alternative formulae to be considered are— CH9(0H) + COH CH“(OH) + + + - COH. If the asymmetric system adjoining the COH group, which is that introduced in synthesizing the hexose from the pentose, be eliminated, the formulae at disposal for the two pentoses are— CH9(0H) COH CH“(OH) + - - COH. When such compounds are converted into corresponding dibasic acids, C02H.[CH(0H)]3.C02H, the number of asymmetric carbon atoms becomes reduced from three to two, as the central carbon atom is then no longer associated with four, but with only three different radicles. Hence it follows that the “optical” formulae of the acids derived from two pentoses having the configuration given above will be— COOH - 0 - COOH COOH + O-COOH,