276 COMPOUNDS DERIVED FROM NITRIC ACID. 



5. Nitro-derivatives of the Aromatic Series in general. 



1. It has just been shown that the formation of a nitro- 

 compound, belonging to the aromatic series, is generally 

 accompanied by a liberation of heat approximately = +36 Cal. ; 

 this number was also obtained by Troost and Hautefeuille for 

 the derivatives of toluene and naphthalene. It will be shown 

 presently that it also holds for the formation of trinitrophenol, 

 otherwise called picric acid. 



2. This being admitted, it is easy to give general formulae for 

 calculating d, priori the heat of formation of a nitro-compound 

 from its elements, and also its heat of combustion, provided 

 that we possess these data for the original hydrocarbon. 



Let A be the heat of formation of the generating substance ; 

 4- 41'6 Cal. being that of nitric acid ; -f 364 the heat of 

 nitration ; and lastly + 69 the heat of formation of water ; 

 the equation representing nitration is as follows : 



K + HN0 3 = X + H 2 0, 



and from it we arrive at the expression for the heat of formation 

 of the nitro-compound, X, or 



A + 41-6 + 36-4 - 69 = A + 9 Cal. 

 For a binitrated, trinitrated, etc., compound, we shall get 

 A + 18 Cal. ; A + 27 Cal ; and generally, A + 9rc. 



3. In the same way, the heat of complete combustion of a 

 nitro-compound is deduced from that of the original hydro- 

 carbon. The latter being supposed = Q ; that of the mononitro- 

 compound, which contains one equivalent less of hydrogen, will 

 be Q - 34-5 - 9 = Q - 43'5 ; for a dinitro-compound, Q - 87 ; 

 for a trinitro-compound, Q 130 '5. These formulae must only 

 be regarded as approximate, as the effect of the nitration is 

 often complicated by the change of physical condition, which 

 should be taken into consideration separately. 



4. The large quantity of heat liberated in the formation of 

 nitro-compounds, when using pure nitric acid, enables us to 

 understand the formation of the same compounds when using a 

 mixture of nitric and sulphuric acids. We know for a fact that 

 this mixture is employed, in preference to pure nitric acid, for 

 the preparation of nitro-derivatives ; but this is an empirical fact. 



The theoretical explanation of it may be given; it results 

 from the difference between the heat of formation of sulphuric 

 derivatives and that of nitro-derivatives, joined to the tendency 

 of sulphuric acid to form a secondary hydrate with the water 

 resulting from the formation of the nitro-compound. For 

 instance, the formation of benzene-sulphonic acid 



C 6 H 6 + H 2 S0 4 = C 6 H 6 S0 3 + H 2 0, gives off + 14'3 - a; 1 



1 a represents the heat of solution of benzene-sulphonic acid in water ; a 

 positive quantity amounting to a few Calories. 



