PHYSIOLOGICAL CHEMISTRY 273 



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

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

 CH 3 - CHO + = 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 produce 

 these reactions, it is necessary that the liquid be alkaline in reaction. 



EXPERIMENT I. Demonstrate the reducing power of a simple alde- 

 hyde, such as formaldehyde, on cupric salts in alkaline solution. Place 

 one drop of a weak solution of cupric sulphate in a test tube. Add 

 about ten drops of formalin (40% formaldehyde), and then, drop by 

 drop, a strong solution of caustic soda. The first drop or so of the 

 latter produces a precipitate of cupric hydroxide, but this afterwards 

 becomes redissolved, provided there be only a trace of copper present. 

 Now boil and note that a red or yellow precipitate of cuprous oxide is 

 produced. This is called Trommer's test. The chemistry of the 

 reaction is illustrated by the following equations : 



1st Stage. CuSO 4 + 2NaOH = Cu(OH) 2 + Na 2 SO 4 . 



Cupric sulphate + caustic soda. Cupric hydroxide + sodium sulphate. 



The cupric hydroxide is kept in solution by the aldehyde to form a 

 clear blue solution. By heating it is believed that a hypothetical 

 cuprous hydroxide is formed, thus : 



2nd Stage. 2Cu(OH) 2 + R* . CHO = Cu 2 (OH) 2 + R . COOH + H 2 0. 

 Cupric hydroxide + aldehyde. Cuprous hydroxide + acid. 



The cuprous hydroxide then loses a molecule of water and changes into 

 the oxide : Cu 2 (OH) 2 - H 2 = Cu 2 O. 



EXPERIMENT II. Demonstrate the reduction of silver nitrate by a 

 simple aldehyde. Place about 5 c.c. of an ammoniacal solution of silver 

 nitrate (prepared by adding ammonia to a solution of silver nitrate till 

 the precipitate formed just redissolves) in a test tube, and add to it 

 about ten drops of formaldehyde. Boil. Reduction takes place, and 

 the metallic silver is deposited as a mirror on the wall of the test tube : 

 Ag 2 O + R CHO = 2 Ag + R . COOH. 



* R stands for the more or less complicated group or radicle to which the - CHO 

 group is attached. In the case of formaldehyde this is H. 



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