82 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Paracetaldehyde or Paraldehyde. 



On adding a drop of concentrated sulphuric acid to about I c.c. of 

 acetaldehyde, a violent reaction occurs and the liquid becomes hot. Par- 

 aldehyde (CH 3 . CHO) 3 separates out as an oil on diluting with water. 

 Acetaldehyde is re-formed if the acid aqueous liquid be heated. 



Paraldehyde is a colourless liquid which boils at 124. 



Metaldehyde. 



The polymer, metaldehyde, is formed from acetaldehyde when it is 

 treated with hydrochloric acid gas or dilute sulphuric acid at a low 

 temperature. 



Aldol Condensation. 



Solutions of formaldehyde and acetaldehyde when kept with dilute 

 solutions of lime or potassium carbonate undergo aldol condensation. 



Formaldehyde gives a sweet syrup which contains monosaccharides,. 

 especially dl-fructose : 



HCHO + HCHO = HCHOH . CHO 



HCHO + HCHO + HCHO = HCHOH . CHOH . CHO, 



etc. 



Acetaldehyde gives aldol : 



CH 3 . CHO + CH 3 . CHO = CH 3 . CHOH . CH 2 . CHO. 



These reactions of aldehyde probably take place in nature. In 

 plants, under the "action of light and chlorophyll, carbon dioxide is 

 reduced to formaldehyde which undergoes aldol condensation into 

 sugars. The higher fatty acids are probably formed from acetaldehyde 

 jn this way in both animals and plants. 



Action of Ammonia. 



Hexamethylene Tetramine. 



Formaldehyde behaves differently to the other aldehydes. 



On adding ammonia gradually to formalin (i c.c. in 5 c.c. water) it 

 is absorbed. On now adding excess of ammonia and evaporating on 

 the water-bath hexamethylene tetramine, or urotropin, remains as a 

 white solid : 



6CH 2 O + 4 NH 3 = (CH 2 ) 6 N 4 + 6H 2 O. 



Hexamethylene tetramine consists of colourless crystals soluble in 

 about 1-5 parts of hot or cold water and 10 parts of alcohol. It is 

 volatilised on heating and it is converted into ammonium sulphate 

 and formaldehyde on treatment with strong sulphuric acid. 



Aldehyde Ammonia. 



On passing dry ammonia gas into a dry ethereal solution of 

 acetaldehyde, acetaldehyde ammonia is formed : 



/OH 

 CH 3 . CHO + NH 3 = CH 3 . CH/ 



^NH 2 . 



Acetaldehyde ammonia is a white crystalline compound easily 



