Page:The New International Encyclopædia 1st ed. v. 18.djvu/777

* SUGABS. 675 SUGARS. upon the amount of a given sugar present; the. process is tlierefore used for the quantitative de- termination of reducing sugars. Among the im- portant reducing sugars arc d-glucosc (dextrose or grape sugar), d-fructose (hevulose or fruit sugar), maltose, lactose {milU sugar), and 'in- vert sugar' (see below). Sucrose (cane sugar), rallinose, melicito.se, and stachyose are the prin- cipal non-reducing sugars known. Optically Active Sugars. The property pos- sessed b.v sugars of rotating the plane of polar- ized light to the right or left when passed through their solutions has long served as a basis for classification. Sugars possessing this prop- erty are said to be optically active, and those which do not are said to be inactive. Those that turn the plane of polarization to the right are called dextro-rotatory; those to the left, Ifevo-rotatory. The amount of rotation is proportional to the amount of the sugar in a given volume of the solution, and this is taken advantage of for the quantitative determination of sugars. A special form of the polariscope (q.v.), called a sacchari- Uictcr, is in common use for this purpose. For tlie polarimetric determination of sucrose (cane sugar), a weighed quantity of the sugar, syrup, or other material is dissolved in water; the solution is treated with lead acetate or other clarifying agent, diluted to 100 cubic centi- meters, filtered, and placed in the observation tube of the polariscope. The observer then ascer- tains the percentage of sugar contained in the material under investigation by simply looking into the instrument, adjusting it to compensate for the change in the field of vision caused by the presence of the sugar solution, and by read- ing the percentage of sucrose directly from the scale of the instrument. Since 18S0 the sugars have formed the subject of numerous brilliant researches, notably those of H. Kiliani and Emil Fischer. In the early eighties Kiliani demonstrated that d-glucose (dextrose or grape sugar) and d-fruetose (Inevu- lose or fruit .sugar) are aldehyde and ketone de- rivatives, respectively, of hexahydric alcohols, the former containing one aldehyde group and the latter one ketone group. (See Alcoiiols; Aldehydes; Ketones.) He also showed that arabinose is an aldehyde of the pentahydric al- cohol, arabite, and has the formula C,-,H,„0;,. The molecules of all earboh.ydrates had been believed to contain six carbon atoms or some multiple thereof. But sugars containing three, four, five, six, seven, eight, and nine atoms of carbon in their molecules are now known; these are desig- nated respectively by the class names trioses, tetroscs, pentoses, hexoses, hcptoses, octoses, and nonoses. According as the individual members of each of these classes contain aldehyde or ke- tone groups ( see Aldehydes ; Ketones ), they are designated as aldoses or ketoses; for example, aldohexoses and ketohcxoses. These classes to- gether constitute the group of carbohydrates known as monosaccharides. Their molecules con- tain as many atoms of oxygen as atoms of car- bon, except in the case of certain of their syn- thetic derivatives. There are other groups of sugars, in the mole- cules of which there are fewer atoms of oxygen than of carbon. When the latter are treated with acids or enzymes they are hydrolyzed. i.e. each one of their molecules combines with one or more molecules of water and simultaneously splits up into two or more monosaccharide mole- cules. These more complex sugars arc called disaccharides when each of their molecules yields two monosaccharide molecules; trisaccharides when one molecule yields three monosaecliaride molecules; and polysaccharides when one mole- cule yields several monosaccharide mcilecules on hydrolysis; for examples, see descriptions of in- dividual sugars below. In the course of his work, Emil Fischer pre- pared a number of hitherto unknown sugars by purely synthetic processes. He showed that the monosaccharides furnish excellent examples of the necessit.y for the chemist to consider the space relations of the atoms in the molecule. (See Stekeo-Chemistey.) According to the the- ory of Le Bel and Van 't Holf sixteen isomeric aldohexoses are possible, since their molecules contain each four asymmetric carbon atoms. Three of these occur in nature or are obtainable from natural carboliydrates by hydrolysis. Eight others have been produced in the laboratory. The aldopentose and ketohexose molecules each contain three asymmetric carbon atoms, there- fore but eight stereoisomers are possible in each case. No ketopentoses are known with certainty. Two aldopentoses are readily obtainable from natural sources ; three or more others have been pre|iared in the laboratory. Only two natural ketohcxoses are known; one or more others have been prepared in the laboratory. The character of the dilTerenccs between the hexoses is illus- trated by the following stereo-chemical formulte: d-glucoae l-glucope d-mannose d-galnotose CHO CHO CUO CHO I I ■■ I I H— C— OH HO— C— H HO— C— H H— C— OH HO— C— H H— C— OH HO— C— H HO— C— H I 1 I I H— C— OH HO— C— H H— f— OH HO— ('— H I I I I H— C— OH HO— C— H H— C— OH H— C— OH I I I I CHjOH CHjOH CH^OH CH.OH Formic aldehyde and glycerin, both of which have been produced synthetically, are the substances from which certain sugars have been sj'nthesized by Fischer and others. Formic alde- hyde, CHjO or H-COH, may be regarded as the simplest of the monosaccharides and be desig- nated as monose. Ba.ver (1870) suggested that this substance is the first product of the reduc- tion of carbon dioxide in the green parts of plants, and that starch and other carbohydrates are formed by its polymerization. Its formation in plants has never been satisfactorilv demon- strated, but it is known readily to undergo poly- merization or condensation to form paraform (CH.O)„ and triox.vmethylene (CH.O),. In 1S61 Butleroff obtained bv condensation of tri- oxymethylene a sweet, sugar-like body which he called 'methylenitan.' Loew (1885) obtained in a similar manner 'formose' from ox.vmethylene, and later 'methose.' The last named is more sugar-like than 'formose' in that it is fermented with yeast. Fischer regards these substances as mixtures of different hexoses. including among others d-aerose, which he also obtained from glyeerose, which is a mixture of glvceraldehyde and dioxyacetone. obtained by the careful oxida- tion of gl.vcerol (glycerin). From d-aerose [(d-f 1) fructose! Fischer ))repared d-fruclose, dglucosc. d-mannose, and the 1- modifications, or optical antimercs, of each of these bodies.