48 SECTIONAL ADDRESSES. 



In the formula for butadiene shown above, the dotted line indicates 

 the interchange of an electron between the two end carbon atoms of the 

 system ; it is to be assumed that this condition provides a point of attack 

 and leads to the 1 : 4 addition which is characteristic of this substance. 

 Evidence has been sought in order to substantiate this view, and with, 

 this object an extended investigation on the properties of hexatriene has 

 been undertaken. 



Hexatriene contains a three-conjugated system, and must, in accordance 

 with stereo-chemical theory, exist in two forms, which may be represented 

 diagrammatically thus : 



=CH, 



In accordance with the Thiele hypothesis these would be conjugated 

 thus : 



CH-, : CH— CH CHa : CH— CH 



II II 



CH2 : CH-CH CH-CH=CHj 



• m 



m 



and there should thus be no difference in the behaviour of the two forms 

 towards additive reagents such as bromine. 



On the other hand, if these two forms are expressed in terms of the 

 electronic hypothesis thus : 



H H H H H H 



II II II II II II 



H=C=C=C H=C=C=C 



H=C=C=C C=C=C=H 



II 11 II II II II 



H H H H H H 



the cis form, in which the two terminal atoms are near together, might 

 be expected to share the electron, as shown by the dotted line, whilst the 

 trans form, having the terminal carbon atoms too remote from one another 

 to enable this interchange to take place, would react in the form 



H H H 



II II II 

 H=C=C=C H 



II II II 

 H H H 



There can be no doubt that the hexatriene prepared by van Romburgh 

 is the trans form. This, as its discoverer showed, adds on bromine in the 



positions 3 : 4 to give 



CH2=CH-CHBr 



CHBr-CH=CH, 



