CYANOGEN COMPOUNDS 153 



Reactions. 



Cyanogen and the other compounds of this group resemble the 

 halogens very closely in their properties, thus on passing cyanogen 

 into alkali it is converted into alkali cyanide and cyanate : 



(CN) 2 + 2KOH = KCN + KOCN + H 2 O 

 C1 2 + 2KOH = KC1 + KOC1 + H 2 O. 



Cyanogen is converted by hydrolysis, on boiling with acids, into 

 oxalic acid and ammonia : 



CN COOH 



| + 4 H 2 = | +2NH 3 . 



CN COOH 



The compounds of this group which contain the CN radicle are 

 termed nitriles, because on hydrolysis with acids they are converted 

 into the corresponding acid : cyanogen is the nitrile of oxalic acid or 

 oxalonitrile. 



(2) Hydrogen Cyanide, or Prussia Acid. HCN. 



Hydrogen cyanide is present in numerous plants, e.g. in laurel 

 leaves, bitter almonds, cherry and peach kernels, usually in combina- 

 tion with glucose and benzaldehyde as the glucoside amygdalin (p. 2 1 5), 

 which is hydrolysed by acids, or by enzymes in the plant, into its con- 

 stituents. It is formed by the oxidation of glycine and other amino 

 acids ; it may thus be produced in animals and plants : the latter prob- 

 ably then combine it with glucose to form a glucoside, the former 

 apparently combine it with sulphur to form sulphocyanide or thio- 

 cyanate, which is present in saliva and other secretions. 



Preparation. 



A dilute solution of hydrogen cyanide is obtained by distilling 

 potassium ferrocyanide, or a cyanide, with dilute sulphuric acid : 



2K 4 Fe(CN) 6 + 3H 2 SO 4 = 6HCN + K 2 FeFe(CN) 6 + 3K 2 SO 4 



Potassium 

 ferrous ferrocyanide. 



A small flask is connected with a condenser by means of a bent 

 tube ; the open end of the condenser is dipped into water in a 

 test tube containing a drop of strong caustic soda. 10 c.c. of a 

 cold saturated solution of potassium ferrocyanide and 20 c.c. of 20 

 per cent, sulphuric acid are put in the flask. On heating, hydrogen 

 cyanide distils over and is converted into sodium cyanide. Its 

 presence is tested for by converting it into Prussian blue by heating 

 in alkaline solution with a ferrous salt, acidifying and adding a drop 

 of ferric chloride. The distillation should be carried out in the fume 

 cupboard. 



