Isocyanates interchange with

A mechanism may be written for the reaction similar to the ones just described if we suppose that the driving force for the rearrangement is indeed a nitrogen atom with only six electrons in its valence shell. Separation of a nitrogen molecule from the azide (LXXII) leaves the electronically deficient nitrogen atom shown in (LXXIII). Interchange of the alkyl group and the electron deficiency leaves the completely polarized form of an isocyanate (LXXIV). 

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