DISACCHARIDES 107 



insoluble in ether. It is dextro-rotatory, [a]u^" =+ 14-36° in 

 10 per cent solution, but when freshly prepared it is laevo- 

 rotatory. Mannose is readily fermentable by yeast. 



Detection. 



1. Mannose is most readily detected and estimated by 

 means of its phenylhydrazone, which is almost insoluble in 

 water, and forms almost at once on adding phenylhydrazine 

 acetate to an aqueous solution of the sugar ; the phenylhydra- 

 zone is soluble in a very large volume of boiling water, and 

 separates in fine prisms from the solution on cooling. These 

 crystals melt at 195-200°. 



An excess of phenylhydrazine converts mannose into 

 glucosazone, which is identical with the substance obtained 

 under similar conditions from both glucose and fructose. 



2. Mannose reduces Fehling's solution, 10 c.c. = -04307 

 gram mannose. 



C. HEPTOSES. 



A number of heptoses of the formula C7H14O7 have been 

 synthesized, but only two are known to occur naturally. One 

 of these, mannoketoheptose, occurs in the avocado pear, 

 Persea gratissima, and the other, sedoheptose, in the stonecrop, 

 Sedum spectabile.* Both are ketoheptoses and are not fer- 

 mented by yeast. 



DISACCHARIDES. 



The disaccharides, as is implied by their name, give rise 

 on hydrolysis to two molecules of monosaccharide which may 

 both be hexoses, or one may be a hexose while the other is 

 a pentose ; the latter type of pentosehexose disaccharide, 

 which is comparatively rare, is dealt with on page 121. The 

 true hexose disaccharides of the general formula C12H22O11 

 may be divided into two classes : — ■ 



(a) Those giving rise on hydrolysis to two molecules of 

 the same hexose, such as maltose, isomaltose, cellobiose, iso- 

 cellobiose, gentiobiose, trehalose, and isotrehalose. 



* La Forge : " J. Biol. Chem.," 1920, 42, 367. 



