114 Prof. Louis Henry on the Polymerization 



Appendix. 



Dehydration and Condensation of the Hydroxides 

 by Heat. 



It has been already stated that the hydroxides, under the 

 action of a gradually increasing temperature, undergo a pro- 

 gressive dehydration, whilst at the same time their molecules 

 become more and more condensed by the accumulation of the 

 residual oxy hydroxides of the radical of the primitive compound. 

 What is the cause of this general fact ? This phenomenon is 

 analogous, both in appearance and result, to that of direct 

 etherification, L e. to the action of an acid on an alcohol. In 

 fact, etherification is an example of the production of an 

 anhydrous oxide from its hydroxide. The most simple case 

 is that in which we can follow step by step the work of 

 dehydration, as with ordinary lactic acid. 



Lactic acid, C 3 H 6 3 , is the dihydroxide of the radical 

 lactyl, C 3 H 4 0, which we may represent by La. On heating, 

 lactic acid gives successively two anhydrides, the so-called 

 lactic anhydride, 



2C 3 H 6 3 -H 2 = HO . La . . La . OH, 



and lactide, 



2C 3 H 6 3 -2H 2 = La . . La . 0. 



The dehydration thus takes place in two steps, and requires 

 at least two molecules of acid — a fact which is easily under- 

 stood from the usual formula of lactic acid, 



CH 3 .CH(OH).COOH. 



The two hydroxyl groups are not equivalent ; the one is acid 

 and the other alcoholic ; so that the action of heat on lactic 

 acid is really the action of an acid on an alcohol, which 

 mutually and successively undergo etherification, giving the 

 so-called lactic anhydride, which is at the same time an acid, 

 an alcohol, and an ether, 



CH 3 . CH(OH) . CO . . CH . COOH, 



CH 3 



and which furnishes lactide, 



CH 3 



CH 3 . CH . CO . . CH . CO . 0, 



which is only but doubly an ether. If the radical lactyl were 

 known only en bloc, the reason of this difference in the nature 

 of these hydroxyls would totally escape us. 



