V] 



CAKBOHYDRATES 



45 



(see p. 10, Cole, 5, for stereoisomerism). It will be found on examination 

 that there are eight possible isomers of the formulae given above : 



CHO 



I 

 HO— C— H 



I 

 HO— C— H 



I 

 HO— 0— H 



I 

 CH2OH 



^-Ribose 



CHO 



I 

 H—C— OH 



HO— C— H 



i 

 -0— H 



HO- 



CH2OH 

 ^-Arabinose 



CHO 



i 

 H—C— OH 



I 

 H—C— OH 



i 

 H—C— OH 



I 

 CH2OH 



c?-Ribose 



CHO 



I 

 HO— C— H 



I 

 H_C— OH 



H—C— OH 



I 

 CH2OH 



c?-Arabinose 



CHO 



I 

 HO— C 



H 



H—C— OH 



i 

 HO— C— H 



I 

 CH2OH 



^Xylose 



CHO 



I 

 H_C— OH 



I 

 H—C— OH 



HO 



C— H 



I 

 CH2OH 



^-Lyxose 

 unknown 



CHO 



I 

 H—C— OH 



I 

 HO— C— H 



I 

 H—C— OH 



I 

 CH2OH 



o?-Xylose 



CHO 



I 

 HO— C— H 



I 

 HO— C— H 



H—C- OH 



I 

 CH2OH 



c?-Lyxose 



Of these only seven have been isolated. The pentoses which occur 

 in plants are ^arabinose, c?-xylose^ and (Z-ribose. The two former, how- 

 ever, are known almost solely as condensation products, pentosans, in 

 gums, woody tissue, etc.; the latter only as a component of nucleic acid. 

 The pentoses form osazones (see p. 51 for reactions and composition). 



Arabinose. This sugar occurs as the pentosan, araban, in various 

 gums, such as Cherry Gum, Gum Arabic, etc. (see p. 46). 



Some of the properties and reactions of the pentoses are demonstrated 

 in the following experiments. 



Expt. 39. Tests for arabinose. For reactions a-e use a 1 ^/o solution of arabinose : 

 for reaction / a 0*2 % solution. 



If pure arabinose is not available, a solution for tests a, h and c can be prepared 

 from gum arable. Boil 5 gms. of the gum in 100 c.c. of water with 10 c.c. of con- 

 centrated hydrochloric acid for 5 minutes and then neutralize to litmus with alkali. 

 Such a solution is only suitable for the specific tests for arabinose, since it also 

 contains galactose (see p. 63). For tests a, 6 and c small pieces of solid gum arable 

 may even be used. 



{a) Heat a few c.c. of the sugar solution in a test-tube with about half its volume 

 of concentrated hydrochloric acid. In the mouth of the test-tube place a piece of 

 filter-paper soaked with aniline acetate (made by mixing equal quantities of aniline, 

 water and glacial acetic acid). A pink colour will be produced in the paper. This is 



1 Known formerly as Z-xylose. 



