150 THE CARBOHYDRATES 



the same carbon atom is, as a rule, unstable, and at once loses 

 water. Exceptions to this rule are, however, occasionally met 

 with; for example, chloral CC1 3 CHO forms a stable com- 

 pound, chloral hydrate, of the formula 



OH 

 CC1 S C-OH 



H 

 [cf. Glucosides, p. ljo\. 



10. Aldehydes exhibit a tendency to polymerize, that is, 

 for two or more molecules to combine together to form new 

 compounds of higher molecular weight 



Thus two molecules of formaldehyde will combine together, 

 forming a compound known as paraformaldehyde (CH 2 O) 2 ; 

 this substance, which is a white solid, is obtained by evaporat- 

 ing an aqueous solution of formaldehyde. 



A second polymer formed from three molecules of formal- 

 dehyde is known as metaformaldehyde or trioxymethylene 

 (CH 2 O) 3 . This substance is produced by the spontaneous 

 polymerization of anhydrous formaldehyde. 



In the case of both the above polymers the molecules of 

 formaldehyde are probably connected together through oxygen 

 atoms as under : 



CH a CH 3 



/ \ / \ 



O O and O O 



\ / / \ 



CH S CH 2 O CH 2 



Paraformaldehyde Trioxymethylene or 



M etaformaldehyde 



which accounts for the fact that they are readily broken up 

 into the simple molecules of formaldehyde by heating. 



11. A different type of polymerization, involving the link- 

 ing together of molecules of formaldehyde through carbon, is 

 also known ; this type of polymerization, which is sometimes 

 known as aldol condensation, results in the formation of a 

 more stable complex which cannot be reconverted into the 

 simple substance. 



The reaction takes its name from the substance produced 

 by the action of dilute hydrochloric acid or zinc chloride on 

 acetic aldehyde. 



