Bolton and McClelland reported the decomposition of 2-(hydroxyamino)imidazoles,
ID: 922721 • Letter: B
Question
Bolton and McClelland reported the decomposition of 2-(hydroxyamino)imidazoles, which have been of interest as possible drugs as radiation sensitizers of oxygen-deficient tumor cells. The reported mechanism in aqueous solution involves rate-limiting cleavage of an N-O bond (J. Am. Chem. Soc. 1989, 111, 8172 ): Grunwald-Winstein mY plots for this reaction in various aqueous mixed solvents are shown below. A common line was obtained in methanol-water with an m value of 0.60. In acetonitrile-water mixtures m = 0.46. Interpret the role of the solvent in each of these two cases. Also why is a lower m value reasonable in acetonitrile-water mixtures?Explanation / Answer
First understand the basic difference in Methanol-water and Acetonitrile-water system.
As you know acetonitrile is more polar than methanol. Acetonitrile does not associate strongly with water and thus, as opposed to methanol, acetonitrile-water mixtures remain binary in character.
So, nature of system determine the reactivity of water with compound, 2-(hydroxyamino)imidazole in your case.
A mechanism is proposed with the neutral form of the imidazole as the kinetically active species, undergoing rate-limiting cleavage of the N-O bond with no catalysis (OH- as leaving group) and with catalysis by the hydronium ion and by buffer acids. These reactions produce a resonance-stabilized imidazolenitrenium ion 7, which reacts with water and added nucleophiles leading to products.