6 Robinson, The Conjugation of Partial Valencies 



A. F. Hollemann and B. F. H. J. Matthes (Proc. K. Akad. 

 Wetensch., Amsterdam, 1918, 21, 90) have shown that in 

 bright light allyl bromide absorbs hydrobromic acid with 

 production of trimethylene dibromide, but that in the dark, 

 although the latter remains the main product, considerable 

 amounts of propylene dibromide are also formed. The 

 present writer, in view of the simplification in expression, 

 proposes to adopt the alternate labelling with + and - signs 

 to denote the existence of secondary conjugation, and in 

 accordance with Professor Lapworth's suggestion (this vol., 

 Memoir No. 3, p. 5) the " key " atom may be indicated by an 

 additional dot. Thus to follow out the examples already given, 

 the secondary conjugations involved in the production of 

 trimethylene bromide and propylenedibromide from allyl 

 bromide are : — 



+ - + + +. 



CH 2 = CHCH 2 -Br and CH 2 = CHCHBr-H 



respectively. Secondary conjugation is no doubt a wide- 

 spread phenomenon. It is concerned in meta substitution in 

 aromatic compounds and very frequently also reinforces the 

 effect of a primarv conjugation. 



N (X 



r/K 



::.o' + 



e.jr. H | +| H and 



H L J,- CH 3 —C^CH CO,£t 



H 



fj Secondary. 



Examples of primary and secondary conjugations could be 

 indefinitely extended, but it is not the present purpose to 

 survey the field of organic chemistry from this point of view, 

 but rather to indicate the general principles applicable to the 

 symbolisation of the mechanism of reactions. There is, 

 however, one group of reactions which occupies a somewhat 

 special position, namely, those which involve molecular 

 rearrangement. A theory of partial valency obviously offers 

 scope for the explanation of such changes, which, it may 

 plausibly be assumed, are in all cases due to an initial ring 

 formation by the aid of fractional valencies. A generalisa- 

 tion of the transformation, of which the change of hydrazo- 

 benzene into benzidine is an example, has been discussed 

 elsewhere (G. M. Robinson and R. Robinson, Trans, Chem. 

 Soc, 191 8, 113, 639), and it is of interest to note that the 

 benzidine-type change in the glyoxaline series discovered by 



