52 PHYSIOLOGY 



On account of the ease with which this oxidation takes place, aldehydes act as 

 strong reducing agents. Warmed with an alkaline solution of cupric hydrate 

 they take up oxygen, reducing the cupric to a red precipitate of cuprous 

 f warmed with an ammoniacal solution of silver (i.e. silver nitrate 

 solution to which ammonia has been added until the precipitate first formed 

 s just redissolved), they reduce the silver nitrate with the formation of a mirror 

 of metallic silver on the surface of the glass vessel in which they are heated 



(2) On warming with phenyl hydrazine, they give the typical compounds, 

 hydrazones and osazones, which are also given by the sugars and will be 

 mentioned in connection with these bodies. 



(3) They also form addition products. With ammonia, they yield the group 

 of compounds known as aldehyde ammonia. Thus : 



CH 3 CH 3 



+ NH 3 = | 

 CHO C 



With sodium sulphite the following reaction takes place : 



CH 3 CH 3 



+ NaHS0 3 = OH 



CHO CH( x 



X S0 3 Na 



These compounds of aldehydes with sodium sulphite can be readily obtained 

 in a crystalline form and furnish a convenient means of separating the alde- 

 hydes from their solutions. 



(4) All the aldehydes possess a strong tendency towards polymerisation 

 Ethyl or acetic aldehyde treated with strong sulphuric acid gives the com- 

 pound paraldehyde. Thus : 



3C 2 H 4 = C 6 H 12 3 . 



(acetic aldehyde) (paraldehyde) 



If warmed with strong potash the polymerisation occurs to a still further extent 

 with the formation of resinous substances of unknown composition, but at 

 any rate of a very high molecular weight, the so-called 'aldehyde resin.' Formic 

 or methyl aldehyde, CH 2 O, may in the same way undergo polymerisation with 

 the formation of a mixture of substances belonging to the group of sugars 

 namely, the hexoses, as follows : 



6CH 2 = C 6 H 12 6 . 



This formation of sugar from formic aldehyde probably plays an important 

 part in the assimilation of the carbon from the carbonic acid of the atmosphere 

 by the green parts of plants. 



ACIDS. By the oxidation 'of the group CHO of the aldehydes we obtain 

 the group ( )OH, which is characteristic of an organic acid. Thus, formic 

 aldehyde on oxidation gives the compound HCOOH, formic acid Ethyl 



n r w a n e rl^ aldel ^ de ' CH ' CH0 ' Avith an ato <* oxygen, gives the compound 

 (JHgCOOH, acetic acid. 



CH 3 CH 3 



+ 0= | 

 CHO COOH. 



Since these acids are derived from the paraffins a whole series of them exista 



