RELATIVE DISTANCE IN MOLECULE 



99 



CH 3 CHO, with separation of water. On the other hand, 

 compounds containing the group C(OH)C(OH), such as 

 ethylene glycol, CH 2 (OH)CH 2 (OH), are mostly very stable. 

 It must be remarked, however, and it adds to the difficulty 

 of making rational use of the principle, that not only 

 mutual distance, but other influences as well, affect the 

 separation of water from two hydroxyl groups, as is clear 

 from the case of chloral hydrate 1 . 



An interesting example from the aromatic compounds 

 may be added, in which the method discussed led to 

 a direct and reliable determination of constitution. The 

 three phthalic acids, C 6 H 4 (COOH) 2 , have the configurations 



H 



H 



c 



3 



C 



H 



COOH 



H 



HC 4 



HOOC 



H 



a CCOOH, 



2 



C 

 H 



H 



H 



C 



and 



HOOCC 4 jCCOOH. 



H 



H 



Now only one of them easily forms the anhydride, 



CO 



C c H 4r ,~O, with separation of water ; this one was therefore, 

 OU 



and correctly, assumed to be orthophthalic acid (1.2), whilst 

 the other formulae were assigned to isophthalic (1.3) and 

 terephthalic acid (1.4); in the latter the carboxyl groups, 

 COOH, containing the hydroxyl, are more widely separated. 

 It may be concluded in general that such condensation, 

 i. e. formation of an internal anhydride with separation of 

 water, occurs most easily in the orthoderivatives (1.2). 



1 See amongst others, measurements of velocity in the conversion of 

 chlorhydrins. Evans, Zeitschr. f. Phys. Chem. 7. 356. 



G % 



