The SN mechanism in aromatic compounds. XXXVI. Reactivity of monochlorodiazabenzenes

Abstract

An investigation has been made of the reactions of p-nitrophenoxide ion in methanol with equimolar quantities (0.01-0.02M) of the six monochlorodiaza-benzenes, viz. 3- and 4-chloropyridazine (1,2- diazabenzene); 2-, 4-, and 5-chloro-pyrimidines (1,3-diazabenzene); and 2-chloropyrazine (1,4-diazabenzene) in the presence of a tenfold excess (0.1-0.2M) of p-nitrophenol. Results have been compared with earlier results from the pyridine series, and reactivity shown to be in reasonable agreement with that predicted from kinetic data for the pyridine series assuming independent activation by each nitrogen. There is evidence for mild acid catalysis by the p-nitrophenol added to prevent concurrent methanolysis. It is estimated that this leads to an increase in Hammett substituent constant (σ-) values of only 0.2 units or less. The reactivity order is 2- > 4-chloro-N-methylpyridinium > 2- > 4- chloro-pyrimidine > 4-chloropyridazine≈3-chloropyridazine≈2- chloropyrazine>3-chloro-N-methylpyridinium≈5-chloropyrimidine, and this corresponds with the theoretical order as modified by mild acid catalysis, more effective in the order ortho-> meta- > para- to the point of substitution, and including the inverted order, 2- > 4- chloropyrimidine, whereas the usual reactivity order with anionic reagents is 4- > 2-chloropyrimidine.