S Lapworth, Latent Polarities of Atoms 



The influence of one atom in a molecule in modifying the 

 polar character of another is well illustrated by the properties 

 conferred on halogen atoms by oxygen atoms yariously 

 situated with regard to them. For example, with the grouping 



CI - C - C = O 



+ •--+■ T 



there is abundant evidence that the chlorine atom has an 

 enhanced tendency to act as a positive, and a diminished 

 tendency to react as a negative, component ; it is, for example, 

 frequently difficult to replace it except by positive hydrogen. 

 Thus, the a-chloroderivatives of camphor are not converted by 

 alkalies or silver hydroxide into hydroxycamphor, and in the 

 case of aa^ichloro-camphor alcoholic sodium hydroxide 

 actually causes replacement of one chlorine atom by hydrogen, 

 the product being monochlorocamphor. Again, the a-halogen 

 derivatives of ketones, and especially of i :3-diketones and 

 i :3-ketonic esters, react excessively readily with hydrogen 

 iodide in dilute solution (Kurt Meyer's test for enol forms), 

 and even with hydrogen bromide, free halogen being liberated 



On the other hand the properties of halogen in the /3-position 

 in carbonyl compounds offers a striking contrast with this. 

 The a/?-dihalogen derivatives, when halogen hydride is with- 

 drawn, lose the halogen atom from the /3-position, which fact 

 indicates that the latent polarity of the /3-halogen in these 

 compounds is negative. Combining this result with that 

 above deduced for the a-halogen derivatives, it is seen that 

 the scheme 



Br 



I I l 



_c-c-c=o 



1+ I"" + -r 



Br H 



expresses all the data drawn attention to, and is one in which 

 the oxygen atom functionates as " key-atom." It is perhaps 

 doubtful whether the influence of the oxygen atom extends so 



