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Baudisch Reaction
The Baudisch Reaction was discovered by Oskar Baudisch in 1939 and further developed by his colleague Cronheim in 1947. Baudisch proposed that o-nitrosophenols are formed when a nitrosyl radical, NOH, is inserted to one of the ethylene bonds of a benzene or benzene substituted ring followed by the addition of an oxygen atom at the position ortho to the nitroso group. The Baudisch reaction is of great importance in organic chemistry because it has posed a way of obtaining an o-substituted phenol that cannot be formed by mere electrophilic aromatic substitution which forms the more thermodynamically stable p-substituted phenol. In Baudisch reaction, which is done in the presence of a transition metal ion, the order of substitution of the ring is that the nitroso group (from the nitrosyl radical) adds first followed by the oxygen. The nitrosyl radical, NOH, used in the reaction can be formed in three different ways, all in the presence of Hydrogen peroxide: The metal ion is the bedrock of the Baudisch reaction for two reasons. First, it helps to stabilize the nitrosyl radical, enabling it to exist long enough for it to react and sencondly it forms a metal-ion complex with the o-nitrosophenol product that stabilizes it, preventing it from rearranging into the p-nitrosophenol. If these two phases of stabilization are absent, the reaction either doesn't occur or the p-substituted product is formed.
 * 1) oxidation of hydroxylamine hydrochloride with cupric ions.
 * 2) reduction of nitrous acid with cuprous ions.
 * 3) the action of cuprous ions and hydrogen peroxide on benzenesulfo-hydroxamic acid.

Properties of o-nitrosophenols

 * they are extremely volatile and readily oxidized in air. This makes them very difficult to isolate without stabilizing agents such as transition metals that form stable complexes with them.
 * they form deeply colored ions when in their metal-ion complex form.
 * they undergo a keto-enol tautomerization in the presence of water. The keto form is called o-quinonemonoxime.




 * o-nitrosophenols are soluble in water and/or ether depending on the kind of complex ion formed. Typically there are two kinds of coplex ions that can be formed as shown in the figure:

Me stands for the metal ion and A represent an anion like chloride, sulfate or nitrate. Type I was formed when the concentration of o-nitrosophenol to metal ions was small while Type II was formed when the concentration of o-nitrosophenol was high such that there were some uncoordinated o-nitrosophenol. It was found that Type I was soluble in water while Type II was not. This can be attributed to the presence of the anion in Type I which could seems to be solvated by the molecules of water.
 * the formation of o-nitrosophenol metal-ion complexes is highly dependent on the pH of the reaction. Higher pH's caused by the addition of pyridine favor the formation of o-nitrosopenol complexes. This is also called the pyridine effect because pyridine is the only base know to cause this increase in yield.

Substituted o-nitrosophenols
The Baudisch reaction has been done on a large number of benzene and benzene substituted compounds and certain trends have been observed:
 * In benzene substituted compounds with activating substituents, the nitrosyl group adds at the position ortho to the activating group then the OH group is added at the position ortho to the nitroso group.




 * In cases where an OH group is already present in the compound, as in the case of phenol, the NO group appears at the position ortho to the OH group.



It is to be noted that two groups of benzene derivatives don't undergo the Baudisch reaction: aromatic aldehydes and primary amines. Under the Baudisch reaction conditions, the aromatic aldhyde-substituted benzene forms either hydroxamic acid (with the nitrosyl radical) or an oxide (with hydroxylamine) while the promary amine is converted to a diazo compound.

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 of 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.