V] CARBOHYDRATES 51 



Warm gently until the mixture is dissolved and filter into another test-tube. Then 

 place the tube in a beaker of boiling water for at least ^ hour, keeping the water 

 boiling all the time. Let the test-tube cool slowly, and a yellow crystalline deposit 

 of phenylglucosazone will separate out. Examine this under the microscope and it 

 will be found to consist of fine yellow needles variously aggregated into sheaves and 

 rosettes. Glucosazone melts at 204-205°C. 



The osazone reaction takes place as follow^s : 



CH20H(CHOH)4CHO-hH2NNHC6H6=CH20H(CHOH)4CH : N • NHCeHg-l-HaO. 



Glucose phenylhydrazone 



The phenylhydrazone is very soluble, but if an excess of phenyl- 

 hydrazine is used, a second hydrazine complex is introduced and an 

 insoluble osazone is formed : 



CH2OH (CH0H)3— C— CH : N • NHCeHg 



II 

 N-NHCgHs 



Glucose reacts in this way by virtue of its aldehyde group. Phenyl- 

 hydrazine hydrochloride does not give an osazone when boiled with 

 glucose unless excess of sodium acetate be added. This acts on the 

 hydrochloride to form phenylhydrazine acetate and sodium chloride. 



Galactose. Galactose rarely, if ever, occurs free in plants, though it 

 is fairly widely distributed in the form of condensation products, the 

 galactans, in combination with other hexoses and with pentoses (see 

 p. 62). These galactans form constituents of various gums, mucilages, 

 etc. Agar-agar, which is a mucilage obtained from certain genera of the 

 Red Seaweeds (Rhodophyceae), yields a high percentage of galactose on 

 hydrolysis with acids. Galactose also occurs as a constituent of some 

 glucosides from which it may be derived on hydrolysis. 



One of the most important reactions of galactose is the formation of 

 mucic acid on oxidation with nitric acid. Mucic acid is practically in- 

 soluble in water and separates out as a crystalline precipitate on pouring 

 the products of oxidation into excess of water. 



Expt. 42. Preparation of galactose from agar-agar. Weigh out 50 gms. of agar- 

 agar. Put it into a round-bottomed flask fitted with an air condenser (see p. 46). 

 Add 500 c.c. of 2 ^/q sulphuric acid and heat on a water-bath for 4 hrs. Neutralize 

 the solution with calcium carbonate and filter. Concentrate on a water-bath to a 

 syrup. On standing, crystals of galactose will separate out. Then add a little 50-75 o/q 

 alcohol and warm gently on a water-bath. By this means much of the dark-coloured 

 product will go into solution and can be poured off leaving the crystalline residue. 

 Take up this residue in a little hot water, boil well with animal charcoal to decolorize 

 the solution and filter. Concentrate again on a water-bath. On cooling, colourless 

 prisms of galactose will separate out. 



4—2 



