90 



THE CARBOHYDRATES 



Only four pentoses have so far been recorded as occurring 

 in the combined state and entering into the composition of 

 plant materials ; these are arabinose, xylose, ribose, and 

 apiose. The structural formulae of these substances is given 

 below, in order that their relationship to the hexoses and to 

 each other may be appreciated : — 



OH 



CHO 

 Hi. 

 HOC.H 

 HOC.H 

 H . C— OH 



i 



Arabinose 



CHO 

 HCOH 

 HOC . H 



HCOH 



I 

 H . C . OH 



I 

 H 



Xylose 



CHO 

 HCOH 



■i 



HCOH 



HCOH 



I 

 HC— OH 



H 

 Ribose 



CHO 

 H . C . OH 

 C.OH 



CH,OH CHoOH 



Apiose 



GENERAL PROPERTIES OF PENTOSES. 



A number of colour reactions are available for the charac- 

 terization of pentoses. 



1. Thomas's Reaction.-\ — A freshly prepared 0*3 per cent 

 solution of ^-naphthol in concentrated sulphuric acid is care- 

 fully poured down the side of a test-tube containing a few 

 cubic centimetres of the solution to be tested ; if a pentose is 

 present a deep blue ultramarine ring is formed at the junction 

 of the two liquids on gently shaking ; the colour gradually 

 changes to green-brown. 



2. BiaVs Reaction. — To a few cubic centimetres of the 

 reagent raised to boiling-point in a test-tube, add a few drops 

 of the pentose or pentosan solution and raise again to boiling- 

 point. A green colour indicates a pentose, a methyl pentose, 

 or glucuronic acid. The colour is soluble in amyl alcohol, 

 and shows an absorption band between the C and D lines. 



3. Add a small quantity of powdered gum-arabic to a few 

 cubic centimetres of 18 per cent of hydrochloric acid together 

 with a few crystals of phloroglucinol ; place in a water bath 

 and gradually raise to boiling-point ; remove from time to 



* It will be seen that apiose represents an abnormal type of sugar 

 possessing a branched chain. 



t Thomas : " Bull. Soc. Chim. Biol.," 1925, 7, 102. 



