Online Encyclopedia

CH2 (OH)

Online Encyclopedia
Originally appearing in Volume V26, Page 35 of the 1911 Encyclopedia Britannica.
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CH2 (OH) + - - - COH = l-gulose. When xylose is combined with hydrocyanic acid and the cyanide is hydrolysed, together with l-gulonic acid, a second isomeric acid, 1-idonic acid, is produced, which on reduction yields the hexaldose 1-idose. When l-gulonic acid is heated with pyridine, it is converted into l-idonic acid, and vice versa; and d-gulonic acid may in a similar manner be converted into d-idonic acid, from which it is possible to prepare d-idose. It follows from the manner in which 1-idose is produced that its configuration is CH2(OH)+--+COH. The remaining aldohexoses discovered by Fischer are derived from d-galactose from milk-sugar. When oxidized this aldohexose is first converted into the monobasic galactonic acid, and then into dibasic mucic acid; the 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 l-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 I.earing to galactose the same relation that mannose bears to glucose. I t can be shown that d-galactose is CH2(OH) + — + — COH, and hence d-talose is CH2(OH)+—++ COH. The configurations of the penta- and tetra-aldoses have been determined by similar arguments; and those of the ketoses can be deduced from the aldoses. Disaccharoses. The disaccharoses have the formula C12H22011 and are characterized by yielding under suitable conditions two molecules of a hexose : C12HnOn+H2O=C6H,208+C,H,206. The hexoses so obtained are not necessarily identical: thus cane sugar yields d-glucose and d-fructose (invert sugar); milk sugar and melibiose give d-glucose and d-galactose, whilst maltose yields only glucose. Chemically they appear to be ether anhydrides of the hexoses, the union being effected by the aldehyde or alcohol groups, and in consequence they are related to the ethers of glucose and other hexoses, i.e. to the alkyl glucosides. Cane sugar has no reducing power and does not form an hydrazone or osazone; the other varieties, however, reduce Fehling's solution and form hydrazones and osazones, behaving as aldoses, i.e. as containing the group •CH(OH)•CHO. The relation of the disaccharoses to the a- and (3-glucosides was established by E. F. Armstrong (Journ. Chem. Soc., 1903, 85, 1305), who showed that cane sugar and maltose were a-glucosides, and raffinose an a-glucoside of melibiose. These and other considerations have led to the proposal of an alkylen oxide formula for glucose, first proposed by Tollens; this view, which has been mainly developed by Armstrong and Fischer, has attained general acceptance (see GLUCOSE and GLUCOseDE). Fischer has proposed formulae for the important disaccharoses, and in conjunction with Armstrong devised a method for determining how the molecule was built up, by forming the osone of the sugar and hydrolysing, whereupon the hexosone obtained indicates the aldose part of the molecule. Lactose is thus found to be glucosido-galactose and melibiose a galactosido-glucose. Several disaccharoses have been synthesized. By acting with hydrochloric acid on glucose Fischer obtained isomaltose, a disaccharose very similar, to maltose but differing in being amorphous and unfermentable by yeast. Also Marchlewski (in 1899) synthesized cane sugar from potassium fructosate and acetochloroglucose; and after Fischer discovered that acetochlorohexoses readily resulted from the interaction of the hexose penta-acetates and liquid hydrogen chloride, several others have been obtained. Cane sugar, saccharose or saccharobiose, is the most important sugar; its manufacture is treated below. When slowly crystallized it forms large monoclinic prisms which are readily soluble in water but difficultly soluble in alcohol. It melts at 160°, and on cooling solidifies to a glassy mass, which on standing gradually becomesopaque and crystalline. When heated to about 200° it yields a brown amorphous substance, named caramel, used in colouring liquors, &c. Concentrated sulphuric acid gives a black carbonaceous mass; boiling nitric acid oxidizes it to d-saccharic, tartaric and oxalic acids; and when heated to 16o° with acetic anhydride an octa-acetyl ester is produced. Like glucose it gives saccharates with lime, baryta and strontia. Milk sugar, lactose, lactobiose, C12H22011, found in the milk of mammals, in the amniotic liquid of cows, and as a pathological secretion, is prepared by evaporating whey and purifying the sugar which separates by crystallization. It forms hard white rhombic prisms (with 1H20), which become anhydrous at 14o° and melt with decomposition at 205°. It reduces ammoniacal silver solutions in the cold, and alkaline copper solutions on boiling. Its aqueous solution has a faiht sweet taste, and is dextro-rotatory, the rotation of a fresh solution being about twice that of an old one. It is difficultly fermented by yeast, but readily by the lactic acid bacillus. It is oxidized by nitric acid to d-saccharic and mucic acids; and acetic anhydride gives an octa-acetate. Maltose, malt-sugar, maltobiose, C12H22Ou, is formed, together with dextrine, by the action of malt diastase on starch, and as an intermediate product in the decomposition of starch by sulphuric acid, and of glycogen by ferments. It forms hard crystalline crusts (with 1H20) made up of hard white needles. Less important disaccharoses are: Trehalose or mycose, C12H22011.2H20, found in various fungi, e.g. Boletus edulis, in the Oriental Trehala and in ergot of rye; melibiose, C12H220n, formed, with fructose, on hydrolysing the trisaccharose melitose (or raffinose), C,8H82018.5H2O, which occurs in Australian manna and in the molasses of sugar manufacture; touranose, C12H220n, formed with d-glucose and galactose on hydrolysing another trisaccharose, melizitose, C,8HnO1e•2H2O, which occurs in Pinus larix and in Persian manna; and agavose, C,2H22011, found in the stalks of Agave americana. (X:)
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