274 COMPOUNDS DERIVED FROM NITRIC ACID. 



2. It was found, after making all the necessary calculations, 

 that the theoretical reaction 



C 6 H 5 N0 2 (pure) + HN0 3 (pure) = C 6 H 4 (N0 2 ) 2 (crystal.) + H 2 0, 

 disengages -J- 36*45 and + 36*35 ; average, 4- 36*4. 



This result is to all intents the same as in the formation of 

 mononitrobenzene ; + 36'6 ; or, in other words, the heat dis- 

 engaged is proportioned to the number of equivalents of acid linked 

 on to the hydrocarbon. 



The complete formation of dinitrobenzene, starting from 

 benzene 



C 6 H 6 + 2HN0 3 = C 6 H 4 (N0 2 ) 2 + 2H 2 0, 



would give off + 73. 



3. These numerical values show that the formation of nitro- 

 compounds involves a considerable loss of energy ; it is much 

 greater than that entailed by the formation of nitric ethers, as 

 we shall presently show. 



We can therefore understand why the explosive energy of the 

 latter compounds is greater, and their stability less. We can 

 also understand why nitro-compounds do not act like ethers, 

 the latter being capable of decomposition by potash with re- 

 formation of an alcohol and acid. Potash, which, when 

 combined with dilute nitric acid, gives off only 137 Cal., 

 cannot furnish, by a simple reaction, the energy required for the 

 reproduction of the acid and benzene, the union of which, in 

 order to form nitrobenzene, has. disengaged 36*5 Cal. This 

 energy, on the contrary, is available in the case of nitric ether 

 and nitroglycerin, which require only 4 to 6 Cal. for the re- 

 generation of each equivalent of acid. 



4 Moreover, the figures -f 36 '5, relating to nitrobenzene, are 

 worthy of notice from another point of view. In fact, this 

 quantity is approximately three-quarters of the heat disengaged 

 in the action of hydrogen on dilute nitric acid, with the 

 formation of nitrous acid, which remains in solution. 



H 2 + HN0 3 (dissolved) = H 2 + HN0 2 (dissolved) gives off 



+ 50-5. 



In this reaction, the action of the hydrogen is, in certain respects, 

 similar to that of benzene in the formation of nitrobenzene. 



5. This shows that the formation of nitrobenzene and similar 

 substances may be compared to oxidation. 



On the other hand, the formation of nitric ether and nitro- 

 glycerin, which causes the liberation of much less heat, 

 represents a simple substitution of the elements of the acid for 

 the elements of water. 



6. The decomposition of nitrobenzene may be effected by 

 sudden heating ; but the products have not been studied. 



