174 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



It mixes in all proportions with water and alcohol but is insoluble 

 in ether and chloroform. 



Glycerol dissolves alkalies and many inorganic salts ; its presence 

 in a solution prevents the precipitation of cupric hydrate by alkalies. 

 This behaviour is common to other compounds which contain 

 several OH groups in their molecule, such as tartaric acid and the 

 sugars. 



Tests. 



(1) On account of its oily appearance it may be mistaken for fat. 

 On moistening a piece of paper with a drop of glycerol, the paper be- 

 comes marked as with a grease spot, but on washing the paper with 

 water and drying the spot is removed. 



(2) On heating a few drops of glycerol in a dry test tube with 

 potassium bisulphate or anhydrous phosphoric acid the pungent odour 

 of acrolein (p. 104) is noticed : 



CH 2 OH . CHOH . CH a OH = CH 2 : CH . CHO + 2H 2 O. 



(3) On adding an aqueous solution of glycerol (about 20 per cent.) 

 to a 5 per cent, solution of borax, to which sufficient phenolphthalein 

 solution has been added to produce a distinct red colour, the red colour 

 is discharged, but on boiling it returns if excess of glycerol has not 

 been used. This is known as Dunstan's test. 



Any polyhydric alcohol may give this reaction. Ammonium salts 

 also decolorise the solution, but the colour does not return on 

 heating. 



Several polyhydric alcohols, containing 4 carbon atoms, erythritols, exist. 

 They differ from one another in their stereochemistry (see tartaric acid). 

 The inactive, internally compensated, compound occurs naturally. 



Adonitol is the natural pentahydric alcohol : the other stereoisomeric 

 forms are obtained by the reduction of the pentoses. 



Mannitol. 



The hexahydric alcohols can exist in ten stereoisomeric forms, eight 

 of which are known. They are closely related to the monosaccharides 

 from which they are obtained on reduction and to which they give rise 

 on oxidation. Mannitol and sorbitol are found in nature, the others 

 have been prepared from the hexoses. 



Mannitol is obtained from manna, the juice of the manna ash, by 

 extracting it with hot water or hot dilute alcohol and crystallising the 

 solution. It crystallises from water in prisms, from alcohol in silky 

 needles and melts at 165-166. It requires 6 parts of water for 

 solution ; it is very slightly soluble in cold alcohol and almost in- 

 soluble in ether, 



