B— CHEMISTRY. 39 



Among the more important developments of the theory of co-ordination 

 which must be expected in the near future, its systematic application to 

 organic chemistry must take a high place, for it is by the study of organic 

 compounds that we really can examine in minute detail the influence of 

 structure on properties. * The very existence of organic chemistry — the 

 fact that the compounds of carbon form a group at least as numerous 

 and important as all other chemical compounds together— can only be 

 fully explained by reference to the theory of co-ordination. Werner 

 pointed out long ago that the unique position of carbon was due to the 

 fact that its valency and its co-ordination number were identical. This 

 we should now express by saying that as it has four valency electrons it 

 can obtain a fully shared octet by normal covalency formation and without 

 the production of co-ordinate links. But this is not all. Since carbon 

 is in the first short period of the table, this octet is incapable of further 

 expansion. Hence the ordinary saturated quadrivalent carbon atom is 

 incapable of acting either as an acceptor or as a donor, and for this 

 reason it is peculiarly well protected from the attack of other atoms. 

 This is undoubtedly the chief cause of the remarkable sluggishness 

 (Tragheit, as Victor Meyer called it) so characteristic of carbon, a dis- 

 inclination to react which gives comparative stability to a large number 

 of thermodynamically unstable compounds. That this explanation is 

 sound may be seen by comparing the behaviour of the halides of carbon 

 with that of the halides of neighbouring elements. Most non-metallic 

 halides are readily hydrolysed by water, and we may assume that the 

 hydrolysis is preceded by a combination (through the formation of 

 co-ordinate links) of the water with the halide. In boron trichloride, for 

 example, the incomplete octet of the boron completes itself by sharing a 

 pair of electrons from the oxygen of the water, forming the compound 



>O^B-Cl , 



h/ \ci 



analogous to the ammonia compound discussed above. A hydrogen and 

 a chlorine atom then separate as hydrochloric acid, leaving a hydroxyl 

 group attached to the boron, and by the repetition of this process 

 the hydrolysis to boric acid is completed. The same reaction occurs 

 with silicon tetrachloride, because, although the silicon has already a 

 complete octet, it can expand this to a group of 12, since it is in the 

 second period. With nitrogen the position is not quite the same. In 

 the trichloride NC1 3 the octet of the nitrogen is complete, and it is incapable 

 of expansion ; but it is not fully shared, and contains a lone pair of elec- 

 trons. Hence, though it cannot be an acceptor, it can be a donor. It 

 forms a co-ordinate link not with the oxygen but with the hydrogen of 



the water, giving 



Civ XJ1 



CK ^H-O-H 



The chlorine then reacts with the hydroxyl, forming hypochlorous acid, 

 while the hydrogen remains attached to the nitrogen the ultimate product 



