CYCLIC IMINE FORMATION BY INTRAMOLECULAR NUCLEOPHILIC-ADDITION AND ELIMINATION BETWEEN AN AMINO GROUP AND AMIDE CARBONYL - RATE PH PROFILE FOR THE REACTION OF 1-AMINO-8-TRIFLUOROACETYLAMINONAPHTHALENE TO 2-TRIFLUOROMETHYLPERIMIDINE

The kinetics of the cyclisation of 1-amino-8-trifluoroacetylaminonaphthalene to 2-trifluoromethylperimidine have been studied in the range pH 0.6-13 in 70% (v/v) Me2SO-H2O in the presence of buffers and in solutions of hydrochloric acid. The first-order rate coefficient (k) obtained in the presence of hydrochloric acid or determined by extrapolation of the observed rate coefficient in buffers to zero buffer concentration gives a pH dependence with two plateau regions. The results are explained by assuming that reaction of 1-amino-8-trifluoroacetylaminonaphthalene occurs by spontaneous and hydronium-ion catalysed pathways and that forms of the amide in which the amide group is ionised or the amino group is protonated are unreactive. The rate-pH profile is fitted by the expression k = K1(k0 + k(H)[H3O+]) [H3O+]/(K1K2 + K1[H3O+] + [H3O+]2) in which K1 is the acid dissociation constant of the protonated amino group in protonated 1-amino-8-trifluoroacetylaminonaphthalene and K2 is the acid dissociation constant of the amide group in 1-amino-8-trifluoroacetylaminonaphthalene. The rate coefficients k0 and k(H) refer, respectively, to spontaneous and hydronium-ion catalysed cyclisation pathways. Catalysis by buffer acids is also observed. The mechanism of the reaction involves intramolecular addition of the amino group in 1-amino-8-trifluoroacetylaminonaphthalene to the amide carbonyl followed by elimination of water.