the Constitution of Carbon Compounds, 85 



The heat of combustion of a hydrocarbon C a H 26 is expressed 

 by the equation 



» nu C x ,02a? 7 2H 2 ,0 2 p, rr 



/.C a H 2 6 = a. — - — +o . — s C a , ±L2&. 



Inverting the values for S^°H* and 2H ^° 2 ; viz. 96960 



and 68360 units, and simplifying as far as possible, we obtain 

 the equation 



/.C a H 26 = a. 135340 + 6. 37780-n. 14200 + 580, 



from which the heats of combustion of hydrocarbons generally, 

 in the gaseous state, may be calculated. 



By comparing experimental values with those calculated 

 from the above equations, it is possible to distinguish between 

 alternative formulae in the case of hydrocarbons in which there 

 may be a difference in the number of " single (or double) 

 bonds." For example, Thomsen argues that, if benzene has 

 the constitution represented by Kekule's formula, its heat 

 of formation would be 3 . 30000 + 6 . 14200 = 175200 units ; 

 whereas, if each carbon atom be regarded as associated with 

 three others (as in the prism formula), its heat of formation 

 should be 3.30000 + 9.14200 = 217800 units. There is 

 thus a difference of 42600 between the two values. Actually 

 Thomsen finds 216610*, which agrees well with the higher 

 value ; and he therefore concludes that the six atoms of 

 carbon in benzene are linked together by nine bonds. A 

 comparison of the observed and calculated heats of forma- 

 tion of chlorobenzene, aniline and methylphenyl ether serves 

 entirely to confirm this conclusion. In the case of dipro- 

 pargyl, the observed value, 133080 units, agrees well with 

 that calculated on the assumption that it has the formula 



CH i C . 0H 2 . CH 2 . C : CH, 

 viz. 



132600 = 3 .30000 + 3 . 14200 units. 



Thomsen cites 18 cases of hydrocarbons, in 15 of which the 

 heat of formation thus reckoned is within 3 per mille of the 

 observed heat of combustion, a degree of accuracy which 

 closely approaches that obtainable experimen tally f. 



* Compare footnote, § 2, p. 78. 



t In three cases the difference is somewhat greater — 



Calc. Observed. Diff. 



Diisopropyl 281000 287880 6880 



Mesitylene 350400 343010 -7390 



Pseudocumene 350400 343830 —6570 



But is very difficult to obtain diisopropyl pure, unless a large quantity be 



