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 solvent to the protonated oxazine to give a tetrahedral intermediate which collapses to product. In the range pH 6 to 2 the rate increases with acidity because an increasing fraction of the oxazine is converted into the more reactive protonated species (pKaca. 2). The unprotonated species is unreactive towards nucleophilic addition of water. Below pH 2 the reaction is inhibited by acid because collapse of the tetrahedral intermediate to the protonated oxazine is catalysed by acid. At pH > 12 the reaction is first-order in hydroxide ion showing that addition of hydroxide ion to the unprotonated oxazine occurs. The favourable reaction of 2-methylnaphth[1,8-de]-1,3-oxazine to 1-hydroxy-8-acetylaminonaphthalene contrasts with the behaviour of 1-amino-8-trifluoroacetylaminonaphthalene for which the preferred direction of reaction is the elimination of water in favour of the cyclic product 2-trifluoromethylperimidine.