SECTION II. 

 ALDEHYDES. 



IN view of the important part played by aldehydes in ques- 

 tions relating to the carbohydrates and other compounds, it 

 appears desirable here to draw attention to the chief properties 

 of these substances. 



It is well known that the aldehydes are the first products 

 of the oxidation of primary alcohols : 



CH 3 OH + O = HCHO + H 2 O 

 Methyl alcohol Formaldehyde 



CH 3 CH 2 OH + O = CH 3 CHO + H a O 

 Ethyl alcohol Acetic aldehyde 



The reconversion of formaldehyde into the alcohol can be 

 effected by means of nascent hydrogen obtained by sodium 

 amalgam and water. 



Chemically, the aldehydes are very active, undergoing a 

 number of reactions, some of which are of biological significance, 

 whilst others serve as valuable means of isolation or identi- 

 fication. 



I. Aldehydes are readily oxidized to the corresponding 

 acids by even such mild oxidizing agents as ammoniacal silver 

 hydroxide or Fehling's solution, or even atmospheric oxygen, 

 as is shown by the following experiments : 



(a) A few drops of caustic potash are added to some silver 

 nitrate solution in a test tube, ammonia is then care- 

 fully added, drop by drop, until the brown precipitate 

 has just redissolved. A little dilute acetaldehyde 

 Solution is poured in and the mixture is warmed 

 gently ; if the solution be sufficiently dilute, a silver 

 mirror will be deposited on the side of the test tube ; 

 otherwise a black precipitate will be formed : 



CH 3 CHO + Ag 2 = CH 3 COOH + aAg 

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