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In organic chemistry, the Baudisch reaction is a process for the synthesis of nitrosophenols using metal ions. Although the products are of limited value, the reaction is of historical interest as an example of metal-promoted functionalization of aromatic substrates.
Contents
History
As described originally by Oskar Baudisch in 1939 and further developed by his colleague Cronheim in 1947, benzene, hydroxylamine, and hydrogen peroxide are combined. Baudisch proposed that o-nitrosophenols are formed when a "nitrosyl radical" NOH, inserts a C-H bond followed by the oxygenation at a position ortho to the nitroso group. The nitrosylating agent can be generated from three different nitrogen sources, all requiring presence of hydrogen peroxide:
- oxidation of hydroxylamine hydrochloride with cupric ions.
- reduction of nitrous acid with cuprous ions.
- the action of cuprous ions and hydrogen peroxide on benzenesulfo-hydroxamic acid.
Properties of o-nitrosophenols
Free nitrosophenols are readily oxidized in air. This makes them very difficult to handle in the absence of stabilization provided by the transition metal. Their metal complexes are deeply colored and water-soluble.
They undergo a keto-enol tautomerization in the presence of water. The keto form is called o-quinonemonoxime.
Substituted o-nitrosophenols
The Baudisch reaction is more profitably applied to substituted aromatic substrates, especially phenols. In these cases the NO group is introduced at the position ortho to the OH group. Nitrosylation using conventional electrophilic aromatic substitution occurs para to the OH group. In the Baudisch reaction, the transition metal ion directs the substitution. The metal ion binds the nitrosyl group and phenoxy oxygen, directing the reaction to occur ortho instead.
For non-phenolic substrates, nitrosylation generally first, and according to usual electrophilic aromatic substitution selectivity. Hydroxylation the occurs ortho to the nitroso group due to the directing effect of the metal.
Of the benzene derivatives that do not undergo the Baudisch reaction, aromatic aldehydes tend to form hydroxamic acid or oximes. Aniline derivatives are converted to a diazo compounds.
Revisions to The Baudisch Reaction
In years following the work Baudisch and Cronheim did, Konecny and Marayuma et al. suggested possible mechanisms for the Baudisch reaction. Konecny proposed that instead of the nitroso group adding first followed by the addition of a hydroxyl group as Baudisch predicted, the OH group added first through a similar radical mechanism with the hydroxyl radical coming from Hydrogen Peroxide after which the nitroso group is added by nitrous acid which he proposed was an intermediate under the reaction conditions. Marayuma et al. pointed some flaws in Konecny's proposal. They maintained that the nitrosation of a phenol (as Konecny's mechanism posed) through the intermediate nitrous acid was impossible under the reaction conditions because it would involve an ortho substitution of a phenol which is thermodynamically unfavorable which was the reason Konecny observed very low percent yields.