Manchester Memoirs, Vol. Ixh. (1920), No. 4 3 



The representation of the phenomena of conjugation and 

 addition to conjugated systems is much simplified by the use 

 of the theory of divisible and polar valency and the definition 

 of a conjugated system may be widely extended when the 

 matter is considered from this point of view. Conjugation is 

 the transfer of a free partial valency to an adjacent atom or 

 to the end of a chain of atoms — it is the explanation of action 

 at a distance in a molecule. The most elementary case may 

 be exemplified by the scheme B above where the disturbance 

 of the valency of a hydrogen atom of methane, due to the 

 proximity of" the reactive chlorine, involves the carrying- 

 through "of this effect to the carbon atom and the appearance 

 thereon of a negative free partial valency. There is clearly 

 no definite limit to this process, but it is certain that the larger 

 the number of saturated atoms in a chain the weaker will be 

 the effect which can be carried through. The facts in relation 

 to the azo-dyes which are substantive to cotton suggest the 

 existence of very long conjugated chains in which all the 

 atoms are unsaturated. A conjugated polarised complex 

 capable of taking part in reactions must clearly have free 

 partial valencies of opposite sign and the chain must accord- 

 ingly comprise an even number of atoms, the exception being 

 those systems which contain a nitrogen, oxygen or sulphur 

 atom in which part use is made of the latent valencies. The 

 even and the odd membered conjugations are shown below, 

 the examples chosen being butadiene and ethyl 0-amino- 

 crotonate. 



CHo = CH-CH = CH, CH 3 -C = CHCOOEt 



I 



o 



CH 2 "-CH— CH--CH 2 c^-C^CH-COfi 



It is convenient to classify as "primary " those conjugated 

 systems or partially dissociated complexes which take part in 

 reactions characterised by addition at the ends of the chain. 

 Thus, in reacting with bromine, butadiene undergoes a 



