REACTIONS OF THE MONOSACCHARIDES. 7 



Its crystals, which are difficult to obtain, are partly water-free 

 (C 6 H 12 ) and partly contain water of crystallisation (2C 6 H 12 6 .H 2 0). 

 Levulose is different in chemical constitution from the other sugars we 

 have studied in this group. It, however, gives the same general tests ; 

 but its specific rotatory power has not been satisfactorily determined. 



Galactose is obtained by the hydrolytic decomposition of lactose or 

 milk-sugar, and from many other carbohydrates, especially gums and 

 mucilages. It is obtained by the decomposition of a glucoside occurring 

 in the brain called cerebrin. 1 It crystallises in needles or plates, which 

 melt at 168 C. It is somewhat more difficult of solution in water than 

 dextrose, and more strongly dextrorotatory. 



Mannose or seminose is another monosaccharide which is of scientific 

 interest, as it is the aldehyde of the alcohol (mannite) of which levulose is the 

 corresponding ketone. 



It does not occur free in nature. It is obtained from mannite by 

 oxidation, 2 and also by the action of dilute sulphuric acid on the so-called 

 reserve cellulose. 3 



Reactions of the monosaccharid.es. (a) Fermentation. They are 

 directly fermentable by yeast into alcohol and carbonic acid (C G H 12 6 

 = 2C 2 H 5 .OIH-2C0 2 ) ; and by the Bacterium lactis into lactic acid (C 6 H 12 6 

 = 2CH 3 CH.OH. GOOH). But this property of fermentation is only 

 possessed by those which occur in nature. 



(&) Reducing power. Being aldehydes or ketones, they are easily 

 oxidisable, and reduce metallic oxides in alkaline media. 



They cause a deposition of metallic silver in an ammoniacal silver solution 

 containing some caustic soda ; and of metallic bismuth from basic bismuth 

 nitrate suspended in soda (Bottcher's test) and of the red cuprous oxide (Cu 2 0), 

 or of the yellow cuprous hydrate Cu(OH 2 ), from an alkaline solution of cupric 

 oxide, as in Trommer's and Fehling's tests. 4 



(c) When heated in the dry state, before they char, they yield a 

 brownish product called caramel. A similar substance is formed by 

 boiling with alkalies (Moore's test). 5 In the brown substance formed, 

 among other bodies is levulinic acid, CH 3 CO CH 2 CH 2 COOH. 



its rotatory power weak (D - 26) ; it forms an osazone. Neubauer and Vogel 

 (" Anleitung zur qualitativen mid quantitativen Analyse desHarns," 1890) suggest the name 

 "laiose " for it. 



1 Thierfclder, Ztschr. f. physiol. Glum., Strassburg, Bd. xiv. S. 209; Brown and 

 Morris, Proc. Glum. Soc., London, 1889, p. 167. 



2 Fischer and Hirschberger, Ber. d. deutsch. chem. Gesellsch., Berlin, Bde. xxi. S. 1805 ; 

 xxii. S. 1155 and 3218. 3 Reiss, ibid., Bd. xxii. S. 909 and 3218. 



4 In Allihn's method (Journ. f. prakt. Chem., Leipzig, Bd. xxii. S. 55) of estimating 

 the reducing power of a sugar, the cuprous oxide obtained by Fehling's method is collected 

 and weighed as metallic copper. Pfliiger (Arch. f. d. ges. Physiol., Bonn, 1877, Bd. Ixvi. ) 

 recommends that the cuprous oxide should be dried at 120 and weighed. O'Sullivan and 

 Stern (Journ. Chem. Soc., London, 1896, p. 1691), who have recently prepared dextrose 

 from several sources, have found that 1 gr. of CuO is reduced by 0'4535 gr. of dextrose 

 (1 gr. Cu 2 = 0*5045 gr. dextrose). On the relation between reducing power and specific 

 rotation see a series of papers by H. T. Brown, G. H. Harris, and J. H. Millar (Proc. 

 Chem. Soc., London, 1896, pp. 241-244). If the reducing power of dextrose is taken as 

 100, that of levulose is 92 to 94 (ibid., 1897, p. 4). 



5 F. Framm (Arch. f. d. gcs. Physiol., Bonn, 1896, Bd. Ixiv. S. 575) has found that 

 Moore's test is accompanied by the formation of products of oxidation, namely aldehyde 

 and formic acid. 



