RATE PH PROFILE FOR THE FORMATION OF 1-HYDROXY-8-ACETYLAMINONAPHTHALENE FROM 2-METHYLNAPHTH[1,8-DE]-1.3-OXAZINE IN AQUEOUS-SOLUTION - ACID CATALYSIS AND INHIBITION AND COMPARISON WITH THE REACTION OF 1-AMINO-8-TRIFLUOROACETYLAMINONAPHTHALENE

Formation of 1-hydroxy-8-acetylaminonaphthalene from 2-methylnaphth[1, 8-de]-1,3-oxazine in aqueous solution shows a rate maximum at pH ca. 2 . The results are compatible with a mechanism involving addition of so lvent to the protonated oxazine to give a tetrahedral intermediate whi ch collapses to product. In the range pH 6 to 2 the rate increases wit h acidity because an increasing fraction of the oxazine is converted i nto the more reactive protonated species (pK(a) ca. 2). The unprotonat ed species is unreactive towards nucleophilic addition of water. Below pH 2 the reaction is inhibited by acid because collapse of the tetrah edral intermediate to the protonated oxazine is catalysed by acid. At pH > 12 the reaction is first-order in hydroxide ion showing that addi tion of hydroxide ion to the unprotonated oxazine occurs. The favourab le reaction of 2-methylnaphth[1,8-de]-1,3-oxazine to 1-hydroxy-8-acety laminonaphthalene contrasts with the behaviour of 1-amino-8-trifluoroa cetylaminonaphthalene for which the preferred direction of reaction is the elimination of water in favour of the cyclic product 2-trifluorom ethylperimidine.