48 SECTIONAL ADDRESSES 



valency. The third, which is the immediate consequence of the second, 

 is the liberation of the acet-ion. 



The determining factor which causes the unequal readiness of cis- and 

 frflHJ-elimination is the linear configuration of benzonitrile. The centres 

 of the carbon and nitrogen atoms of the cyanogen group and of the 

 carbon atom of the phenyl group to which it is attached lie in a straight 

 line. The formation of benzonitrile therefore entails the movement of the 

 nitrogen nucleus in the direction shown. 



In the derivative of the p-oxime, represented in the diagram, this 

 movement is directly away from the acetoxy-group and consequently 

 results in the liberation of this as the acet-ion. The successive events 

 in the chain are correlated, and they proceed readily. 



In the corresponding derivative of the a-oxime, the movement of the 

 nitrogen nucleus entails no similar withdrawal from the acetoxy-group. 

 There is no corresponding opportunity for the latter to escape as the 

 acet-ion, and the chain of events does not take place. 



In the P-oxime derivative the possibility of the complete withdrawal 

 of the proton is conditioned by the rapid breakdown of the resultant 

 negative ion into benzonitrile and acet-ion. The a-oxime acetate would 

 yield a negative ion which would possess no corresponding tendency to 

 split off an acet-ion. In accordance with its lack of acidity the a-acetate 

 could not, therefore, give up a proton to the alkaline solution. 



The greater facility of ^ra«^-elimination in the ethylene series is evidently 

 capable of an analogous explanation. 



Not only were the configurations formerly assigned to the aldoximes 

 based on a false assumption, but we now know from the work of 

 Meisenheimer that those attributed to the stereo-isomeric ketoximes have 

 also to be interchanged. In the Beckmann transformation the radicals 

 which migrate do not lie on the same side, but on opposite sides, of the 

 CN group. 



It is usual to think of the migration of the groups. But if we imagine 

 the change occurring in an isolated molecule and remember that moment 

 of momentum must be conserved, it is clear that most of the movement 

 by which the relative displacement is brought about would be executed 

 by the nitrogen atom. We therefore get a truer picture of the change by 

 regarding the groups as relatively stationary and directing our attention 

 on the movement of the nitrogen atom. 



Inspection of the diagrams which indicate the alternative movements 

 of the nitrogen atom corresponding with cis- and iraw- migration, shows 

 at once how much more probable the latter is. 



The Beckmann change is brought about by energetic reagents, but if 

 we view the phenomenon broadly, disregarding intermediate stages and 

 looking at the final result, it is clear that the principal source of the energy 

 difference between the oxime and the anilide is the replacement of the 

 weak oxygen-to-nitrogen link in the former by the strong oxygen-to-carbon 

 link in the latter compound. 



We may accordingly see, as the driving force which brings about 

 the change, the affinity of the oxygen for the central carbon atom. It 

 may therefore be concluded that the first step in the process of actual 



