Pyridine-N-oxide
Chemical compound (C5H5NO)
From Wikipedia, the free encyclopedia
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]
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| Names | |||
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| Preferred IUPAC name
1λ5-Pyridin-1-one | |||
| Other names
Pyridine-1-oxide | |||
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| ECHA InfoCard | 100.010.705 | ||
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| Properties | |||
| 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) | ||
| 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).
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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], sodium perborate in acetic acid[6], catalytic methylrhenium trioxide (CH
3ReO
3) with sodium percarbonate[7] or dimethyldioxirane.[8]
Reactions
Pyridine N-oxide is five orders of magnitude less basic than pyridine: the pKa of protonated pyridine-N-oxide is 0.8.[9] 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.[10] For example, 4-nitropyridine can be prepared from nitrating pyridine-N-oxide and subsequent deoxygenation with PCl3.[11] Deoxygenation can also be carried out with POCl3 to give 2-chloropyridines.[12]
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.[13]
The N-oxides of various pyridines are precursors to useful drugs:[14]
- 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]
See also
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.