Pyridine-N-oxide


Pyridine-N-oxide is the heterocyclic compound with the formula C5H5NO. This colourless, hygroscopic solid is the product of the oxidation of pyridine. Its synthesis was first reported by Jakob Meisenheimer, who used peroxybenzoic acid as the oxidant. The compound is used infrequently as an oxidizing reagent in organic synthesis.

Structure

The structure of pyridine-N-oxide is very similar to that of pyridine with respect to the parameters for the ring. The molecule is planar. The N–O distance is 1.34Å. The C–N–C angle is 124°, 7° wider than in pyridine.

Synthesis

The oxidation of pyridine can be achieved with a number of peroxy acids, including peracetic acid and peroxybenzoic acid. Oxidation can also be effected by a modified Dakin reaction using a urea–hydrogen peroxide complex, and sodium perborate or, using methylrhenium trioxide as catalyst, with sodium percarbonate.

Reactions

Pyridine N-oxide is five orders of magnitude less basic than pyridine: the pKa of protonated pyridine-N-oxide is 0.8. Protonated derivatives are isolable, e.g., Cl. Further demonstrating its basicity, pyridine-N-oxide also serves as a ligand in coordination chemistry. A host of transition metal complexes of pyridine-N-oxides are known.
Some electrophilic substitutions on pyridine rings are usefully effected using pyridine N-oxide followed by deoxygenation. Addition of oxygen suppresses further reactions at nitrogen atom and promotes substitution at the 2- and 4-carbons. For example, treatment of the pyridine-N-oxide with phosphorus oxychloride gives 4- and 2-chloropyridines. The oxygen atom can then be removed, e.g., using zinc dust.

Related pyridine-''N''-oxides

Pyridine-N-oxides are uncommon in nature. 2-pyridine-N-oxide and related compounds have been isolated from species of Allium.
The N-oxides of various pyridines are precursors to useful drugs:

Safety

The compound is a skin irritant.