Manchester Memoirs, Vol. Ixiv. (1920), No. 3 5 



by the oxygen atom or atoms, for the rest of the molecule 

 consists of carbon and hydrogen atoms only. The influence 

 of — N0 2 , -S0 2 — , and — CN when in attachment to carbon 

 differs, however, only in degree from that of carbonyl ; hence 

 it would appear that with the first two the effect of the group 

 on the rest of the molecule is again determined by the oxygen 

 atoms and in the last by the nitrogen atom. The analogy 

 between the four different groups may therefore be indicated 

 as follows, where in each case the " key-atom" is suggested 

 by addition of a " dot " to the sign used for its latent polarity. 



C-C-C = C-C-N = 2 



+ - +, - +-+- 



C-C-S = 2 C-C-C = N 



+ -+- +-+- 



The writer has long held that certain atoms, and especially 

 divalent oxygen and tervalent (negative) nitrogen, tend to 

 produce such an alternation of latent polarities within the 

 molecules in which they occur. These two< atoms are as a 

 rule much more effective than the halogens and, where they 

 are in competition, the influence of the oxygen usually appears 

 greater than that of the nitrogen. A perceptible influence of 

 the opposite kind appears to be exercised by hydrogen. 

 Carbon itself (in part no doubt because it is the standard of 

 reference) appears nearly indifferent, as also does quinque- 

 valent nitrogen. 



The extension of the influence of the directing, or (( key- 

 atom," over a long range seems to require for its fullest 

 display the presence of double bonds, and usually in conju- 

 gated positions ; consequently the principle must find 

 ample scope in the aromatic series where conjugation is the 

 rule. The ready substitution of the y-hydrogen atoms in 

 ethyl crotonate was shown by the writer in 1901 to provide an 

 exact parallel with the behaviour of the hydrogen atoms in the 

 methyl group of 0- and ^-nitrotoluene, and both were shown 

 to be cases of " meta-substitution." The scheme 



O, 







CH=CH 









/ \ 





H- 



-CH 2 - 



-C=C-CH=C- 



-N 



+ 





+ I - + 

 H 

 + 



+ 



