8 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 variously 

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



CI - c - c = o 



+ - + 



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^dichloro-camphor alcoholic sodium hydroxide 

 actually causes replacement of one chlorine atom by hydrogen, 

 the product being mtmochlorocamphor. 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 



H - 



+ 



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

 in carbonyl compounds offers a striking contrast with this. 

 The a/3-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 



+ 



Br 



-C- 



-C = 



= o 



• - 





+ 



- 



+ 



- 



1 



+ 



Br 



i 







1 

 -C- 



-C- 



I 

 -C = 



=o 



1 + 



1 " 



+ 



— 



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 



