OPTICALLY ACTIVE SUGARS. The property pos sessed by 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, lxvo-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 meter, is in common use for this purpose. For the 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 11. Kiliani and Emil Fischer. In the early eighties Kiliani demonstrated that d-glucose (dextrose or grape sugar) and d-fructose 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 ALCODOLS; ALDEIIYDES; KETONES.) He also showed that arabinose is an aldehyde of the pcntahydric al cohol, arabite, and has the formula The molecules of all carbohydrates 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, tetroses, pentoses, hexoses, heptoses, octoses, and ?onoses. According as the individual members of each of these classes contain aldehyde o• ke tone groups (see ALDEHYDES; KETONES), they are designated as aldoses or ketoses; for example, aldohexoses and ketohexoses. These classes to gether constitute time 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 o• 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 are called disaccha rides when each of their molecules yields two monosaccharide molecules; trisaccharides when one molecule yields three monosaccharide molecules; and polysaccharides when one mole cule yields several monosaccharide molecules 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. lie showed that. the monosaeebao•ides furnish excellent examples of the necessity for the chemist to consider the space relations of the atoms in the molecule.
(See STEREo-ellEIOISTRY.) According to the the ory of Le Bel and Van 't Hoff sixteen isomeric aldohexoses are possible, since their molecules contain each four asymmetric carbon atoms. Three of these occur in nature or arc obtainable from natural carbohydrates by hydrolysis. Eight others have been produced in the laboratory. The aldopeutose and ketobexose molecules each contain three asymmetric carbon atoms, there fore but eight stereo-isomers are possible in each case. No ketopentoses are known with certainty. Two aldopentoscs are readily obtainable from natural sources; three o• more others have been prepared in the laboratory. Only two natural ketohexoses are known; one or more others have been prepared in the laboratory. The character of the differences between the itemises is illus trated by the following stereo-chemical formuhe: Formic aldehyde and glycerin, both of which have been produced synthetically, arc the substances from which certain sugars have been synthesized by Fischer and others. Formic alde hyde, may be regarded as the simplest of the monosaccharides and he desig nated as morose. Bayer (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 satisfactorily demon strated, but it is known readily to undergo poly merization or condensation to form pa raform and trioxymethylene (CH,0),. In 1S61 Butleroff obtained by condensation of tri oxymethylene a sweet, sugar-like body which he called `inethylenitan.' Loew ( 1SS5) obtained in a similar manner 'formose' from oxymethylene, and later 'maltose.' 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 nucotg others d-acrose, which he also obtained from glyeerose, which is a mixture of gbyecraldohyde and dioxyaeetone, by the careful oxida tion of glycerol (glycerin). From d-aerose [(d+ 1) fructose] Tischer prepared d- f ru Mose, d-ghtcose, d-mannose, and the 1- modifications, or optical antimeres, of each of these bodies.
The synthesis of each one of these bodies was accomplished by a series of complicated reactions Nvhich are fully described in text-books on chem istry. Fischer's work was greatly facilitated by the use of phenylhydrazine, which forms readily crystallizable compounds with the hexoses, called osazones. These compounds are insoluble in water and eminently suited for the separation and identification of the different sugars. By means of the cyanhydrine reaction he changed hexose sugars into heptoses, and these in turn into octoses and nonoses. The combination of monosaccharides to form disaccharides has only recently been accomplished by Fischer. Among a number of his synthetic disaecharides is one which he designates galactosidoglucose (because it is made by the union of one molecule of d-galactose with one molecule of d-glucose), and which closely resembles the natural sugar meli biose. In an article published in September, 1902. he says that if further investigations con firm his conclusion that the two sugars are iden tical, "melibiose is the first natural disaccharide to he produced synthetically." The more important sugars are mentioned in the following table with brief descriptions of their success: