206 PRACTICAL PHYSIOLOGY 



attached to the last carbon atom of the molecule as in primary propyl 

 alcohol, 



CH 3 - CH 2 - CH 2 OH, 



and it contains the end group CH 2 OH. If, on the other hand, the hydroxyl 

 group be attached to a central carbon atom as in secondary propyl alcohol, 



CH 3 - CHOH - CH 3 , 

 the alcohol is called secondary, and contains the group CHOH. 



If ethyl alcohol be heated with potassium bichromate and sulphuric acid, 

 it is oxidised and acetic aldehyde is formed : 



CH 3 - CH 2 OH + O = CH 3 - CHO + H 2 O. 



Ethyl alcohol. Acetic aldehyde. 



This group, CHO, is, however, not a stable one, but very readily undergoes 

 further oxidation to produce the acid (carboxyl) radicle COOH, 

 CH 3 - CHO + O = CH 3 - COOH. 



Acetic aldehyde. Acetic acid. 



As a consequence of this tendency to absorb oxygen aldehydes are strong 

 reducing agents, and it is this property which constitutes one of their most 

 important group reactions, for the reaction is frequently accompanied by a 

 visible change in the colour of the solution. 



Their power of reducing cupric hydroxide, which is blue in colour, to cuprous 

 oxide, which is red, and of reducing silver nitrate to metallic silver, is of 

 especial value as a test. Similar reactions are obtained with certain bismuth 

 and mercury salts. In order to obtain these reactions, it is necessary that 

 the liquid be alkaline in reaction. 



Reactions of Monosaccharides depending on the fact that they are 

 aldehydes. /. Their Reducing Power. Dextrose is the aldehyde 

 corresponding to the hexatomic x alcohol, sorbitol. 



CH 2 OH - (CHOH) 4 - CH 2 OH, CH 2 OH - (CHOH) 4 - CHO. 



Sorbitol. Dextrose. 



It, therefore, manifests strong reducing powers on metallic oxides 

 in alkaline solution. 



EXPERIMENT I. Demonstrate the reducing power of a mono- 

 saccharide, such as dextrose on cupric salts in alkaline reaction. 



T rammer's Test. Place a few drops of a weak solution of copper 

 sulphate in a test tube ; add about 5 c.c. of a 1 per cent, solution of 

 dextrose, and then, drop by drop, a 20 per cent, solution of caustic 

 soda until the precipitate of cupric hydroxide, which at first forms, 

 becomes redissolved, and a clear blue solution is obtained. Boil. 

 Reduction is effected, a red precipitate of cuprous oxide resulting. 



Repeat experiment without the addition of dextrose. A black 

 precipitate of cupric oxide is obtained on boiling with excess of 

 caustic soda. 



EXPERIMENT II. Fehling's Test. This differs from Trommer's 

 test in that tartrate of sodium and potassium (Rochelle salt) is added 

 to the mixture of CuSO 4 and NaOH. 2 Rochelle salt has the 

 property of dissolving cupric hydroxide, forming a blue solution, 

 which is unaltered on boiling, and is therefore of especial value 



1 A hexatomic alcohol is one which contains six OH groups. Glycerine is 

 called tri-atomic, because it contains three such groups. Ethyl alcohol is 

 mon-atomic, because it contains one. 



2 For the exact formula for Fehling's solution see p. 275. 



