46 CARBOHYDRATES [ch. 



due to the fact that furfural is formed by the action of the acid on the pentose, and 

 the furfural then gives a red colour with aniline acetate solution : 



JOH Hi 



'1 1" CH = CH 



CH-CH-iOHi ouo - I \o 



CH— C<; '' CH=C 



.! l>^=o \c=o 



iOH >Hi I 1 



Arabinose Furfural 



This reaction, however, is also given by the hexoses but to a much less extent. 



(6) Warm a few c.c. of the sugar solution with an equal volume of concentrated 

 hydrochloric acid in a test-tube, and add a small quantity of phloroglucinol. A bright 

 red coloration is produced. 



(c) To a few c.c. of the sugar solution in a test-tube add an equal quantity of 

 concentrated hydrochloric acid, and then a little solid orcinol. Divide the solution 

 into two equal portions. Heat one portion. The solution becomes bluish changing 

 to reddish -violet and finally deposits a blue precipitate. To the other portion, after 

 heating for a time, add a few drops of 10% ferric chloride solution. A deep green 

 colour is at once produced. On the addition of a little amyl alcohol, the green 

 colour will be extracted by the alcohol. 



(d) a-Naphthol reaction. Add to a little of the sugar solution a few drops of a 

 1 % solution of Q-naphthol in alcohol. Mix the two solutions and then run in about 

 5 c.c. of concentrated sulphuric acid down the side of the test-tube. A violet colora- 

 tion is produced at the junction of the two liquids. The coloration is due to a 

 condensation product of a-naphthol with furfural, the latter being formed by the 

 action of the acid on the carbohydrate. This reaction is likewise given by laevulose 

 and cane-sugar (since it yields laevulose, see p. 54), and less strongly by glucose and 

 maltose ; also by some proteins which contain a carbohydrate group. 



(e) Boil a little of the arabinose solution with a few drops of Fehling's solution. 

 Keduction will take place. 



(/) Make the osazone of arabinose following the instructions given for glucosazone 

 (see p. 50). 



A solution of arabinose which will give the pentose reactions can also 

 be obtained by hydrolysis of Cherry Gum. The gum oozes from the bark 

 of various species of Frunus, such as the Cherry (Prunus Cerasus) and 

 the Bird Cherry (P. Padus), 



Expt. 40. Preparation of arabinose solution from Cherry Oum. The gum is heated, 

 on a water-bath in a round-bottomed flask fitted with an air condenser \ with 

 dilute sulphuric acid (1 pt. by wt. of gum: 7 pts. by wt. of 4% sulphuric acid) for 

 about 5 hours. The solution is then neutralized with calcium carbonate and filtered. 

 Perform the tests a, h and c of Expt. 39 on the solution. A positive result is obtained 

 in each case. Since the solution contains other sugars as inpurities, it cannot con- 



i.e. a wide piece of glass tubing about 3 ft. long passing through the cork. 



