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.^[1] The compound is used infrequently as an oxidizing reagent in organic synthesis.^[2]

Pyridine-N-oxide

Names
Preferred IUPAC name 1λ5-Pyridin-1-one
Other names Pyridine-1-oxide
Identifiers
CAS Number	694-59-7 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29136 Y
ChEMBL	ChEMBL3278446
ChemSpider	12229 Y
ECHA InfoCard	100.010.705
EC Number	211-774-6
PubChem CID	12753
UNII	91F12JJJ4H Y
CompTox Dashboard (EPA)	DTXSID3061007
InChI InChI=1S/C5H5NO/c7-6-4-2-1-3-5-6/h1-5H Y Key: ILVXOBCQQYKLDS-UHFFFAOYSA-N Y InChI=1/C5H5NO/c7-6-4-2-1-3-5-6/h1-5H Key: ILVXOBCQQYKLDS-UHFFFAOYAZ
SMILES c1cc[n+](cc1)[O-]
Properties
Chemical formula	C5H5NO
Molar mass	95.101 g·mol−1
Appearance	Colourless solid
Melting point	65 to 66 °C (149 to 151 °F; 338 to 339 K)
Boiling point	270 °C (518 °F; 543 K)
Solubility in water	high
Acidity (pKa)	0.8 (of conjugate acid)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

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 1.34 Å. The C–N–C angle is 124°, 7° wider than in pyridine.^[3]

Synthesis

The oxidation of pyridine can be achieved with a number of peroxy acids, including peracetic acid and peroxybenzoic acid.^[4] Oxidation can also be effected by a modified Dakin reaction using a urea–hydrogen peroxide complex,^[5] and sodium perborate^[6] or, using methylrhenium trioxide (CH
₃ReO
₃) as catalyst, with sodium percarbonate.^[7]

Reactions

Pyridine N-oxide is five orders of magnitude less basic than pyridine: the pK_a of protonated pyridine-N-oxide is 0.8.^[8] Protonated derivatives are isolable, e.g., [C₅H₅NOH]Cl.^[4] 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.^[9] For example, treatment of the pyridine-N-oxide with phosphorus oxychloride gives 4- and 2-chloropyridines.^[10] The oxygen atom can then be removed, e.g., using zinc dust.^[9]

Related pyridine-N-oxides

Pyridine-N-oxides are uncommon in nature. 2-(Methyldithio)pyridine-N-oxide and related compounds have been isolated from species of Allium.^[11]

The N-oxides of various pyridines are precursors to useful drugs:^[12]

• 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.^[2]

Further reading

• Synthesis of N-oxides from substituted pyridines: Youssif, Shaker (2001). "Recent trends in the chemistry of pyridine N-oxides". Arkivoc. 2001: 242–268. doi:10.3998/ark.5550190.0002.116. hdl:2027/spo.5550190.0002.116.