148 THE CARBOHYDRATES 



CH3CHO + CH^CHO = CH.,CHOH.CH.^.CHO 

 Aldol 



The analogous reaction with formaldehyde is, however, brought 

 about by dilute alkalis ; in this way two molecules of formal- 

 dehyde give rise to glycollic aldehyde, 



HCHO + HCHO = CH2OH.CHO 



Glycollic aldehyde 



or three molecules may combine together to produce glyceric 

 aldehyde, 



HCHO + HCHO + HCHO^CH^OH.CHOH.CHO 

 Glyceric aldehyde 



By repeatedly shaking a 4 per cent solution of formal- 

 dehyde for half an hour with an excess of lime water, and then 

 filtering the solution and setting it aside for some days until 

 the odour of formaldehyde had disappeared, Loew * was able 

 to obtain a crude mixture of sugars called formose, from which 

 true reducing hexose sugars have been isolated. This change 

 may be represented by the equation : — 



6HCH0 = CoHijOe 



Similarly H. and A. Eulerf have shown that when a 2 

 per cent solution of formaldehyde is heated for some hours 

 with calcium carbonate, a pentose sugar — arabinoketose — is 

 produced ; in addition to this substance, glycollic aldehyde and 

 dihydroxyacetone are produced, but in smaller quantity. 



FORMALDEHYDE. 



From the point of view of photosynthesis formaldehyde 

 is of outstanding interest ; as is well known, it is at ordinary 

 temperatures a colourless gas with a pungent odour ; when 

 cooled to — 21° it condenses to a liquid. It is usually met with 

 in the form of an aqueous solution, commercial formalin, 

 which contains about 40 per cent of the gas dissolved in water, 

 and is used as a disinfectant or as a hardening medium for 

 pathological and other specimens and occasionally as a pre- 

 servative for milk. It undergoes most of the general reactions 

 for aldehydes which have been mentioned above. 



* Loew : " Ber. deut. chem. Gesells,," 1887, 20, 142, 3039 ; 1881, 21, 

 270 ; 1889, 22, 470, 478. 



fEuler, H. and A. : id., 1906, 39, 36, 39. 



