xvii MOLECULAR ARCHITECTURE 433 



(4) In compounds of the ammonia type, three radicals 

 could be linked together, as in 



CH 3 | 

 CHj 



Trimethylamine. Trimethylphosphine. Dimethylaniline 



Hofmann had also represented the compounds of the 

 amines and phosphines with HI and with CH 3 I by formulae 

 which showed five radicals associated with an atom of 

 nitrogen or of phosphorus (Phil. Trims., 1850, 357-398). 



H 

 H 



Tetramethylammonium iodide Phosphonium iodide 



from N(CH 3 ) 3 +CH 3 I. from PH 3 +HI. 



Kekule (1857) on substitution-value or atomicity, 



These considerations, arising from the study of Gerhardt's 

 simple types, were expressed very clearly by Kekule, who 

 proposed in 1857 to describe the elements according to 

 their " substitution-value " or combining power as MON- 

 ATOMIC, DIATOMIC, and TRIATOMIC. In the first of his two 

 papers, "On Copulated Compounds and the Theories of 

 Polyatomic Radicals," he writes : 



" The molecules of chemical compounds consist of aggre- 

 gations of atoms." 



" The number of the atoms of other elements (or radicals) 

 combined with one atom (of an element. ... or of a radical) 

 is dependent on the basicity or substitution-value of the 

 constituents." 



"From this point of view the elements fall into three 

 principal groups : 



prepared them by the same method that Williamson had used for the 

 synthesis of the ethers, e.g. 



C 7 H 5 O'C1 + C 7 H 5 OOK = KCl + (C 7 H 5 O) 2 O 

 C 2 H 3 0-C1 + C 2 H 3 OOK = KCl + (C 2 H 3 O) 2 O. 



F F 



