Amine oxide

  An amine oxide, also known as amine-N-oxide and N-oxide, is a chemical compound that contains the functional group R₃N⁺-O⁻ (sometimes written as R₃N=O or R₃N→O). In the strict sense the term amine oxide applies only to oxides of tertiary amines including nitrogen-containing aromatic compounds like pyridine, but is sometimes also used for the analogous derivatives of primary and secondary amines.

Amine oxides are used as protecting group for amines and as chemical intermediates. Long-chain alkyl amine oxides are used as nonionic surfactants and foam stabilizers.

Amine oxide are highly polar molecules have a high polarity close to that of quaternary ammonium salts. Small amine oxides are very hydrophilic and have an excellent water solubility and a very poor solubility in most organic solvents.

Amine oxides are weak bases with a pK_a of around 4.5 that form R₃N⁺-OH, cationic hydroxylamines, upon protonation at a pH below their pK_a.

Pyridine N-oxide is a crystalline solid with melting point 62-67°C and soluble in water N-Methylmorpholine N-oxide is an oxidant.

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Synthesis

Amine oxides are prepared by oxidation of tertiary amines or pyridine analogs with hydrogen peroxide (H₂O₂), Caro's acid or peracids like mCPBA in N-oxidation ^[1].

Reactions

• Pyrolytic elimination. Amine oxides, when heated to 150 to 200 °C eliminate a hydroxylamine, resulting in an alkene. This pyrolytic syn-elimination reaction is known under the name Cope reaction. The mechanism is similar to that of the Hofmann elimination.
• Reduction to amines. Amine oxides are readily converted to the parent amine by common reduction reagents including lithium aluminum hydride, sodium borohydride, catalytic reduction, zinc / acetic acid, and iron / acetic acid. Pyridine N-oxides can be deoxygenated by phosphorus oxychloride
• O-alkylation. Pyridine N-oxides react with alkyl halides to the O-alkylated product
• In the Meisenheimer rearrangement certain N-oxides R₁R₂R₃N⁺O^- rearrange to hydroxylamines R₂R₃N-O-R₁ ^{[2] [3]}

Metabolites

Amine oxides are common metabolites of medication and psychoactive drugs. Examples include nicotine, Zolmitriptan, and morphine.

Amine oxides of anti-cancer drugs have been developed as prodrugs that are metabolized in the oxygen deficient cancer tissue to the active drug.

See also

• Functional group
• Amine, NR₃
• Hydroxylamine, NR₂OH
• Sulfoxide, R₂S=O
• Azoxy, RN=N+(O–)R RN=N⁺RO⁻
• TEMPO (2,2,6,6-Tetramethylpiperidine-1-oxyl), a stable amine oxide radical

References