NOxide

Noxide, in a chemical context, refers to organic compounds containing an N–O bond where either a tertiary amine has been oxidized to an amine oxide or a heteroaromatic nitrogen (such as in pyridine) has been oxidized to an aromatic N-oxide. The two main subclasses are amine oxides (R3N→O) and aromatic N-oxides (for example, pyridine N-oxide). In both cases resonance places a formal positive charge on nitrogen and a negative character on oxygen, with the overall molecule being neutral.

Preparation of N-oxides typically involves oxidation of the parent compound. Tertiary amines can be oxidized with peracids like mCPBA or with hydrogen peroxide under acidic conditions to give amine oxides (R3N+–O−). Aromatic N-oxides are formed by oxidation of heteroaromatic nitrogens using similar oxidants or related catalytic methods.

Reactivity and applications of N-oxides are diverse. In aromatic systems, the N-oxide group activates the ring toward electrophilic substitution and often directs new substituents to the para or ortho position relative to the nitrogen. After functionalization, the oxide can be removed by reduction to restore the original amine. Amine oxides can also undergo syn-elimination upon heating (the Cope elimination), yielding alkenes and hydroxylamines, a transformation used in certain synthetic sequences. The Polonovski reaction employs N-oxide intermediates to convert tertiary amines into carbonyl compounds. N-oxides serve as useful directing groups and reagents in medicinal chemistry, particularly for pyridine-containing frameworks.

Examples of common N-oxides include pyridine N-oxide and quinoline N-oxide.