Pyridine-N-oxide is the heterocyclic compound with the formula C<sub>5</sub>H<sub>5</sub>NO. 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.
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 NâÂÂO distance is 1.34à. The CâÂÂNâÂÂC angle is 124ð, 7ð wider than in pyridine.
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 ureaâÂÂhydrogen peroxide complex, sodium perborate in acetic acid, catalytic methylrhenium trioxide () with sodium percarbonate or dimethyldioxirane.
Pyridine N-oxide is five orders of magnitude less basic than pyridine: the pK<sub>a</sub> of protonated pyridine-N-oxide is 0.8. Protonated derivatives are isolable, e.g., [C<sub>5</sub>H<sub>5</sub>NOH]Cl. Further demonstrating its (feeble) 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, 4-nitropyridine can be prepared from nitrating pyridine-N-oxide and subsequent deoxygenation with PCl<sub>3</sub>. Deoxygenation can also be carried out with POCl<sub>3</sub> to give 2-chloropyridines.
Pyridine-N-oxides are uncommon in nature. 2-(Methyldithio)pyridine-N-oxide and related compounds have been isolated from species of Allium.
The N-oxides of various pyridines are precursors to useful drugs:
The compound is a skin irritant.