Page:EB1911 - Volume 26.djvu/49

 beet, carrot and other roots were identical with cane sugar. The sugars obtained from honey were investigated by Lowitz and Proust, and the latter decided on three species: (1) cane sugar, (2) grape sugar, and (3) fruit sugar; the first has the formula C$12$H$22$O$11$, the others C$6$H$12$O$6$. This list has been considerably developed by the discovery of natural as well as of synthetic sugars.

It is convenient to divide the sugars into two main groups: monosaccharoses (formerly glucoses) and disaccharoses (formerly saccharoses). The first term includes simple sugars containing two to nine atoms of carbon, which are known severally as bioses, trioses, tetroses, pentoses, hexoses, &c.; whilst those of the second group have the formula C$12$H$22$O$11$ and are characterized by yielding two monosaccharose molecules on hydrolysis. In addition trisaccharoses are known of the formula C$18$H$32$O$16$; these on hydrolysis yield one molecule of a monosaccharose and one of a disaccharose, or three of a monosaccharose. It is found also that some monosaccharoses behave as aldehydes whilst others contain a keto group; those having the first character are called aldoses, and the others ketoses. All sugars are colourless solids or syrups, which char on strong heating; they are soluble in water, forming sweet solutions but difficultly soluble in alcohol. Their solutions are optically active, i.e. they rotate the plane of polarized light; the amount of the rotation being dependent upon the concentration, temperature, and, in some cases, on the age of the solution (cf. ). The rotation serves for the estimation of sugar solutions (saccharimetry). They are neutral to litmus and do not combine with dilute acids or bases; strong bases, such as lime and baryta, yield saccharates, whilst, under certain conditions, acids and acid anhydrides may yield esters. Sugars are also liable to fermentation. Our knowledge of the chemical structure of the monosaccharosés may be regarded as dating from 1880, when Zincke suspected some to be ketone alcohols, for it was known that glucose and fructose, for example, yielded penta-acetates, and on reduction gave hexahydric alcohols, which, when reduced by hydriodic acid, gave normal and secondary hexyliodide. The facts suggested that the six carbon atoms formed a chain, and that a hydroxy group was attached to five of them, for it is very rare for two hydroxy groups to be attached to the same carbon atom. The remaining oxygen atom is aldehyde or ketonic, for the sugars combine with hydrocyanic acid, hydroxylamine and phenylhydrazine. The correctness of this view was settled by Kiliani in 1885. He prepared the cyanhydrins of glucose and fruotose, hydrolysed them to the corresponding oxy-acids, from which the hydroxy groups were split out by reduction; it was found that glucose yielded) normal heptylic acid and fructose methylbutylacetic acid; hence glucose is an aldehyde alcohol, CH$2$OH·(CH·OH)$4$·CHO, whilst fructose is a ketone alcohol CH$2$OH·(CH·OH)$3$·CO·CH$2$OH. Kiliani also showed that arabinose, C$5$H$12$O$6$, a sugar found in cherry gum, was an aldopentose, and thus indicated an extension of the idea of a “sugar.”

Before proceeding to the actual synthesis of the sugars, it is advisable to discuss their decompositions and transformations.

1. Cyanhydrins.—The cyanhydrins on hydrolysis give monocarboxylic acids, which yield lactones; these compounds when reduced by sodium amalgam in sulphuric acid solution yield a sugar containing one more carbon atom. This permits the formation of a higher from a lower sugar (E. Fischer) 2. Oximes.—The oximes permit the reverse change, Le. the passage from a higher to a lower sugar. Wohl forms the oxime and converts it into an acetylated nitrile by means of acetic anhydride and sodium acetate; ammoniacal silver nitrate solution removes hydrocyanic acid and the resulting acetate is hydrolysed by acting with ammonia to form an amide, which is finally decomposed with sulphuric acid. Ruff effects the same change by oxidizing the sugar to the oxy-acid, and then further oxidizing this with Fenton's reagent, i.e. hydrogen peroxide and a trace of a ferrous salt: 3. Phenylhydrazine Derivatives.—Fischer found that if one molecule of phenylhydrazine acted upon one molecule of an aldose or ketose a hydrazone resulted which in most cases was very soluble in water, but if three molecules of the hydrazine reacted, (one of which is reduced to ammonia and aniline) insoluble crystalline substances resulted, termed osazones, which readily characterized the sugar from which it was obtained. On warming the osazone with hydrochloric acid the phenylhydrazine residues are removed and an osone results, which on reduction with zinc and acetic acid gives a ketose. A ketose may also be obtained by reducing the osazone with zinc and acetic to an osamine, which with nitrous acid gives the ketose: These reactions permit the transformation of an aldose into a ketose; the reverse change can only be brought about by reducing the ketose to an alcohol, and oxidizing this compound to an aldehyde. 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.

It may here be noticed that in the sugars there are asymmetric carbon atoms, and consequently optical isomers are to be expected. Thus glucose, containing four such atoms, can exist in 16 forms; and the realization of many of these isomers by E. Fischer may be regarded as one of the most brilliant achievements in modern chemistry. The general principles of stereochemistry being discussed in (q.v.), we proceed to the synthesis of glucose and fructose and then to the derivation of their configurations.

In 1861 Butlerow obtained a sugar-like substance, methylenitan, by digesting trioxymethylene, the solid polymer of formaldehyde, with lime. The work was repeated by O. Loew, who prepared in 1885 a sweet, unfermentable syrup, which he named formose, C$2$H$2$O$2$ and, later, by using magnesia instead of lime, he obtained the fermentable methose. Fischer showed that methose was identical with the -acrose obtained by himself and Tafel in 1887 by decomposing acrolein dibromide with baryta, and subsequently prepared by oxidizing glycerin with bromine in alkaline solution, and treating the product with dilute alkali at 0°. Glycerin appears to yield, on mild oxidation, an aldehyde, CH$2$OH·CH(OH)·CHO, and a ketone, CH$2$OH·CO·CH$2$OH, and these condense as shown in the equation:

CH$2$OH·CH(OH) + CHO + CH$2$OH·CO·CH$2$OH＝CH$2$OH·CH(OH)·CH(OH)·CH(OH)·CO.CH$2$OH + H$2$O.

The osazone prepared from -acrose resembled most closely the glucosazone yielded by glucose, mannose, and fructose, but it was optically inactive; also the ketose which it gave after treatment with hydrochloric acid and reduction of the osone was like ordinary fructose except that it was inactive. It was surmised that -acrose was a mixture of dextro and laevo fructose, a supposition which was proved correct by an indirect method. The starting point was ordinary (d)mannite (mannitol), C$3$H$3$O$3$, a naturally occurring hexahydric alcohol, which only differed from a.-acritol, the alcohol obtained by reducing -acrose, with regard to optical activity. Mannite on oxidation yields an aldose, mannose, C$4$,H$9$O$4$ which