44 SECTIONAL ADDRESSES 



It may be noted as a matter of observation that six-co-ordination 

 compounds, which are, as far as is known, universally of octahedral 

 configuration, are more stable configurationally than tetrahedral four- 

 co-ordination compounds. If the movements involved in optical in- 

 version in the two types are considered, it is clear that this is what we 

 should expect. 



Finally, it would be anticipated that a compound would have a greater 

 configurational stability if the central atom had a resultant positive charge 

 than if its resultant charge was negative, since the field controlling the 

 octet would be stronger in the former case. Perhaps the difference 

 between the tertiary amines and the sulphonium compounds is partly to 

 be accounted for in this way. 



In the tertiary amines we may infer from the di-pole moment — and the 

 length— of the single carbon-to-nitrogen link that the external electro- 

 static effect of the nitrogen atom and its associated electrons is equivalent 

 approximately to that of a negative charge of o-^e at its centre, while the 

 three directly attached carbon atoms have resultant positive charges of 

 about o'le. 



In the corresponding sulphonium ions we have similarly to infer the 

 existence of positive charges of about 0-65^ on the sulphur atom and 

 o-i2e on each of the attached carbon atoms. Thus, whilst there is a 

 positive charge on the sulphur in the sulphonium compounds, there is 

 a negative charge on the nitrogen in the amines, and the former should 

 increase and the latter diminish the stability of the associated octet. 



On the other hand, if the constitution attributed by Kuhn to the salts 

 of the nitroparaffins is correct — and, as we have seen, there is good reason 

 to think that it is — then in these compounds we have very considerable 

 configurational stability of a tri-co-valent atom associated with the 

 presence of a negative charge. 



In view of this it is improbable that the difference in optical stability 

 between the amines and the sulphonium ions can be wholly attributed to 

 a difference in sign of the resultant charge on the central atom, though 

 doubtless this has a contributory effect. 



It is interesting to look back on some of the principal difficulties which 

 were encountered in interpreting stereochemical phenomena on the basis 

 of the older conceptions of valency, and to review them from our present 

 standpoint. 



Probably the most conspicuous example of these is afforded by the 

 problem of the Walden inversion , which Emil Fischer described in 1907 

 as being, ' since the fundamental investigations of Pasteur, the most 

 surprising observation which had been made in the field of optically 

 active compounds.' 



The difficulty of bringing the experimental observations on this in- 

 version within the scope of any fixed set of rules is well recognised. 

 I think, however, that if we review the evidence at present available, it 

 might be interpreted as indicating generally that there is at any rate one 

 type of reaction which, taking place at an asymmetric carbon atom, is 

 normally accompanied by an inversion of configuration. This type of 



