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. It was originally prepared using peroxyacids as the oxidising agent. 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 peracids including peracetic acid and perbenzoic 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., [C5H5NOH]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.

Treatment of the pyridine-N-oxide with phosphorus oxychloride gives 4- and 2-chloropyridines.

Related pyridine-N-oxides
The N-oxides of various pyridines are precursors to useful drugs:
 * Nicotinic acid N-oxide, derived from nicotinic acid is a precursor to niflumic acid and pranoprofen.
 * 2,3,5-trimethylpyridine N-oxide is a precursor to the drug omeprazole
 * 2-chloropyridine N-oxide is a precursor to the fungicide zinc pyrithione

Safety
The compound is a skin irritant.