B.— CHEMISTRY 45 



reaction is one that might be described as an ionic interchange, and 

 includes such changes as the replacement of halogens by hydroxyl by 

 means of aqueous alkalies, or of the arylsulphoxy-group by the acetoxy- 

 group by means of alkali metal acetates. 



Closely allied to reactions of this type — and also apparently normally 

 associated with an inversion of configuration — are the interactions of 

 halogen compounds with ammonia, leading to the formation of a sub- 

 stituted ammonium ion and a halogen ion. 



Reactions of these types can be represented with extreme simplicity 

 with the aid of the tetrahedral octet, and it is then seen that they would 

 naturally be accompanied by an inversion of configuration. 



Let us take as an example of such an interchange of radicals at an 

 asymmetric carbon atom the reaction commonly referred to as the 

 ' replacement of chlorine by the amino-group ' by means of ammonia. 



CI 



:ci: 



''^J:;:-. 





7*,R; 



H ; N : H H : N : H 



■ H H 



Before reaction. After reaction. 



Fig. 5. 



In representing this reaction the experimental facts to which we have 

 to give expression are : (i) the asymmetric carbon atom unites with a 

 molecule of ammonia, forming a substituted ammonium ion ; (ii) the 

 configuration is inverted ; (iii) a chlorine ion is set free. 



In order to represent these three facts with the tetrahedral octet we 

 have simply to move the nucleus of the carbon atom away from the 

 chlorine and towards the ammonia (suitably situated) through a distance 

 of half the height of the tetrahedron. It is a movement analogous to that 

 which we have to assume for the nitrogen nucleus in the amines to account 

 for the inversion of configuration which these compounds so readily 

 undergo. When the essential features of the change can be thus so simply 

 and naturally represented, it is difficult to resist the belief that the 

 representation corresponds closely with the reality. 



Thermodynamically considered, the reason for the change is to be found 

 in the loss of free energy by which it must be attended, and this is the 

 same whether the process results in an inversion of configuration or not. 



