Benzidine rearrangements. 16. The use of heavy-atom kinetic isotope effects in solving the mechanism of the acid-catalyzed rearrangement of hydrazobenzene. The concerted pathway to benzidine and the nonconcerted pathway to diphenyline

Abstract

Kinetic isoptope effects (KIE) in the acid-catalyzed rearrangement of hydrazobenzene to benzidine and diphenyline have been measured. Nitrogen KIE were determined by whole-molecule mass spectrometry on each of the products obtained at low and 100% conversions from mixtures of hydrazobenzene and (/sup 13/N,/sup 15/N') hydrazobenzene. The results were k(/sup 14/N)/k(/sup 15/N) = 1.0222 for benzidine and 1.0633 for diphenyline. Carbon KIE were determined with both /sup 14/C and /sup 13/C labeling, using counting techniques for the former and whole-molecule mass spectrometry for the latter. Again measurements were made on both products isolated from low and 100% conversions. Use of mixtures of hydrazobenzene and (4-/sup 14/C) hydrazobenzene gave k(/sup 12/C)/k(/sup 14/C) = 1.0284 for benzidine and 1.0011 for diphenyline. Use of mixtures of hydrazobenzene and (4,4'-/sup 13/C/sub 2/) hydrazobenzene gave k(/sup 12/C)/k(/sup 13/C) = 1.0209 for benzidine and 1.000 for diphenyline. The results show that the formation of benzidine is a concerted process while the formation of diphenyline is a dissociative process involving the formation of an intermediate (possibly a ..pi.. complex or pair of caged radical ions) in the rate-determining step. Calculations of the KIE were made on simplified models of transition states for concerted and dissociative processes and weremore » found to be in reasonable agreement with the experimental results. In harmony with the concerted formation of benzidine (the major product) we found also by whole-molecule mass spectrometry and with the use of mixtures of hydrazobenzene and (4,4'-/sup 2/H/sub 2/) that the disappearance of hydrazobenzene has an inverse secondary deuterium KIE, k(/sup 1/H)/k(/sup 2/H) = 0.962.« less