ALDEHYDES 81 



flask by a well-fitting cork, a mixture of 25 gm. (30 c.c. of absolute 

 alcohol and 35 gm. (20 c.c.) of concentrated sulphuric acid is slowly 

 added to the contents of the flask which have been gently warmed and 

 the flame removed. During the addition the contents of the flask, 

 which darken in colour, are occasionally shaken. A mixture of alde- 

 hyde, alcohol and water distils over. When the mixture has been 

 added the flask is heated until all the aldehyde (recognised by smell) 

 has distilled over. 



The tests for acetaldehyde can be carried out with this distillate 



( P . 8 4 ). 



Purification. 



The solution is redistilled through an inverted condenser filled with water 

 at 30-35. Water and alcohol are condensed, but the aldehyde passes on. 

 The aldehyde vapour is passed through a 100 c.c. pipette into about 30 c.c. 

 of pure dry ether contained in a bottle standing in ice. 



Pure ammonia, prepared by gently heating concentrated ammonia solution 

 with a small flame and dried by passage through a tower containing quick- 

 lime, is passed into the ether until it is saturated. Aldehyde ammonia 

 crystallises out. After standing for one hour the ether is decanted off, the 

 crystals are drained on a Buchner filter and washed with ether. 



The crystals of aldehyde ammonia are dissolved in an equal weight of 

 water, and the solution is distilled with a mixture of 1*5 parts of sulphuric 

 acid and 2 parts of water from a water-bath, which is gradually raised to 

 boiling. The receiver is cooled in ice. The distillate is dried with calcium 

 chloride from which the aldehyde is distilled in a bath at 20 and collected 

 in a receiver in ice. The aldehyde must be preserved in a well-stoppered 

 bottle. 



Properties of Aldehydes. 



Formaldehyde is a gas at the ordinary temperature easily soluble 

 in water and alcohol and with a peculiar pungent smell. 



Acetaldehyde is a colourless liquid having a fruity pungent smell 

 and boils at 21. It is easily soluble in water, alcohol and ether. 



The next members of the series of aldehydes are also liquids and 

 resemble acetaldehyde very closely in their properties. 



Polymerisation. 



Paraformaldehyde or Paraform. 



On evaporating formalin (about I c.c.) in a watch-glass on a 

 water-bath a solid mass of paraformaldehyde is left. 



If a portion of the solid be heated in a test tube dissociation occurs 

 at about 100, the mass melts between 153 and 172, a white sublimate 

 is formed and formaldehyde is evolved. 



