Pyridine- N -oxide

Structural formula
General
Surname	Pyridine- N -oxide
other names	Pyridine-1-oxide
Molecular formula	C 5 H 5 NO
Brief description	white, technically also brown solid
External identifiers / databases
CAS number 694-59-7 EC number 211-774-6 ECHA InfoCard 100.010.705 PubChem 12753 Wikidata Q287787
properties
Molar mass	95.10 g · mol -1
Physical state	firmly
Melting point	61-66 ° C
boiling point	90-92 ° C (0.13 Pa )
Dipole moment	4.25 D
safety instructions
GHS labeling of hazardous substances no GHS pictograms H and P phrases H: no H-phrases P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Pyridine- N -oxide is an organic compound which consists of a pyridine ring that is oxidized at the nitrogen atom .

In general, all derivatives of pyridine with an oxygen atom on the ring nitrogen are referred to as pyridine N-oxides , with pyridine-1-oxide being the parent compound.

presentation

The compound can be prepared by reacting pyridine with peroxycarboxylic acids . The peroxycarboxylic acids can be prepared in situ from the underlying acid and hydrogen peroxide . The reaction is mechanistically similar to the Prileshayev reaction . Another oxidizing agent for the production of pyridine- N -oxide is, for example, Caro's acid .

properties

The heat of decomposition determined by DSC is −102 kJ · mol ⁻¹ or −1076 kJ · kg ⁻¹ .

use

Electrophilic aromatic substitutions on pyridine proceed with good selectivity in the 3-position, which is due to the electronic structure of the pyridine ring. The carbon atoms in the 3-position have the highest electron density of all carbon atoms in the ring, which is why it is preferred to substitute at this position . However, the oxidized nitrogen atom in the N -oxide causes the electron density in the ring to increase and the electronic structure of the ring to be reversed. The carbon atoms in the 2- and 4-position thus have the highest electron density in contrast to pyridine. This leads to a reversal of the selectivity and in electrophilic substitutions the 2- and 4-substituted products are preferably obtained. For this reason, pyridine- N -oxide is an important starting compound for the synthesis of 2- or 4-substituted pyridines (e.g. 4-nitropyridine- N -oxide ), which are not accessible by nucleophilic aromatic substitution .

Reactions

After the substitution has taken place, the nitrogen atom can usually be reduced smoothly , the substituted pyridine derivative being obtained. Trivalent phosphorus compounds or divalent sulfur compounds, which are easily oxidizable, are generally suitable as reducing agents . Triphenylphosphine , which is oxidized to triphenylphosphine oxide, is often used as a cheap reagent . Furthermore, samarium (II) iodide , tin (II) chloride or chromium (II) chloride can be used at room temperature for the deoxygenation of pyridine N -oxide. The reduction using ammonium formate in the presence of palladium on activated carbon leads to the deoxygenation of the nitrogen and, at the same time, to the hydrogenation of the pyridine ring, so that a derivative of piperidine is obtained.

Individual evidence

1. ↑ Data sheet pyridine-N-oxide from Acros, accessed on May 1, 2010.
2. ↑ Data sheet pyridine-N-oxide from AlfaAesar, accessed on May 1, 2010 ( PDF )(JavaScript required) .
3. ↑ C. Rüchardt, O. Krätz, S. Eichler: Reactions of pyridine N-oxide with acid anhydrides. In: Chem. Ber. 102, 1969, pp. 3922-3946, doi: 10.1002 / cber.19691021137 .
4. ↑ ^{a b c d e} J. A. Joules, K. Mills: Heterocyclic Chemistry. 5th edition. Blackwell Publishing, Chichester, 2010, ISBN 978-1-4051-9365-8 , pp. 125-141.
5. ↑ ^{a b} Datasheet Pyridine N-oxide from Sigma-Aldrich , accessed on May 9, 2017 ( PDF ).
6. ↑ AR Gallopo, JO Edwards: Kinetics and mechanism of the oxidation of pyridine by Caro's acid catalyzed by ketones. In: J. Org. Chem. 46, 1981, pp. 1684-1688; doi: 10.1021 / jo00321a032 .
7. ↑ T. Grewer, O. Klais: Exothermic decomposition - investigations of the characteristic material properties. (= Humanization of working life. Volume 84). VDI-Verlag, Düsseldorf 1988, ISBN 3-18-400855-X , p. 9.
8. ↑ DT Davies: Basic Texts Chemistry: Aromatic Heterocyclen. 1st edition. Wiley-VCH, Weinheim, 1995, ISBN 3-527-29289-6 .