HEXOSES. 205 



All hexoses, as also their anhydrides, yield levulinic acid, C 6 H 8 O3, 

 besides formic acid and humus substances on boiling with dilute min- 

 eral acids. Some of the hexoses, as above stated, are fermentable with 

 yeast. 



Some hexoses are aldoses, while others are ketoses. Belonging to the 

 first group we have MANNOSE, DEXTROSE, and GALACTOSE, and to the other 

 LEVULOSE, and possibly also SORBINOSE. 



The most important syntheses of the carbohydrates have been made 

 by E. FISCHER and his pupils, chiefly within the members of the hexose 

 group. A short summary of the syntheses of hexoses is given below. 



The first artificial preparation of a sugar was made by BUTLEROW. On 

 treating trioxymethylene, a polymer of formaldehyde, with lime-water he obtained 

 a faintly sweetish syrup called methylenitan. LOEW * later obtained a mixture 

 of several sugars, from which he isolated a fermentable sugar, called methose,, 

 by condensation of formaldehyde in the presence of bases. The most important 

 and comprehensive syntheses of sugar have been performed by E. FISCHER. 2 



The starting-point of these syntheses is a-acrose, which occurs as a condensa- 

 tion product of formaldehyde. The name a-acrose has been given to this body 

 because it is obtained from acrolein bromide by the action of bases (FISCHER). 

 It is also obtained admixed with {3-acrose on the oxidation of glycerin with 

 bromine in the presence of sodium carbonate and treating the resulting mixture 

 with alkali. On the oxidation with bromine a mixture of glyceric aldehyde, 

 CH 2 OH.CH(OH).CHO, and dioxyacetone, CH 2 (OH).CO.CH 2 OH, is obtained. 

 These two bodies may be considered as true sugars, namely, glyceroses or trioses. 

 It seems as if a condensation to hexoses takes place on treatment with alkalies. 



a-acrose may be isolated from the above mixture and obtained pure by first 

 converting it into osazone and then re transforming this into the sugar, a-acrose 

 seems to be identical with r-levulose. With yeast one half, the levogyrate 

 d-levulose ferments, while the dextrogyrate /-levulose remains. The r- and 

 Z-levulose may be prepared in this way. 



On the reduction of a-acrose we obtain a-acrite, which is identical with r- 

 mannite. On oxidation of r-mannite we obtain r-mannose, from which only 

 -mannose remains on fermentation. On further oxidation of r-mannose it 

 yields r-mannonic acid. The two active mannonic acids may be separated from 

 each other by the fractional crystallization of their strychnine or morphine salts. 

 The two corresponding mannoses may be obtained from these two acids, d- and 

 ^-mannonic acids, by reduction. 



d-Levulose is obtained from d-mannose by the method given on page 196, 

 using the osazone as an intermediate step. The* d- and Z-mannonic acids are partly 

 converted into d- and Z-gluconic acids on heating with quinoline, and d- or l- 

 glucose is obtained on the reduction of these acids; /-glucose is best prepared 

 from Z-arabinose by means of the cyanhydrin reaction, using /-gluconic acid as 

 the intermediate step. The combination of I- and d-gluconic acids, forming 

 r-gluconic acid, yields r-glucose on reduction. 



The artificial preparation of sugars by means of the condensation of formalde- 

 hyde has received special interest because, according to BAEYER'S assimilation 

 hypothesis, in plants formaldehyde is first formed by the reduction of carbon 

 dioxide, and the sugars are produced by the condensation of this formaldehyde. 



1 Butlerow, Ann. d. Chem. u. Pharm., 102; Compt. rend., 53; O. Loew, Journ. f. 

 prakt. Chem. (N. F.), 33, and Ber. d. deutsch. chem. Gesellsch., 20, 21, 22. 



2 Ber. d. d. chem. Gesellsch., 21, and 1. c., p. 193. 



