Pyridine N-oxide: practice and mechanism
The peracid transfers an O atom to the lone pair of the ring nitrogen: pyridine N-oxide (N⁺–O⁻) forms. The ring stays aromatic and unchanged.
Example
The lone pair on the pyridine N lies in the ring plane and is not part of the aromatic system. It attacks the O of the peracid, giving the N-oxide. The ring itself is not attacked.
Mechanism step by step
- 1
The lone pair of the pyridine N attacks the terminal O of the peracid.
- 2
The O–O bond breaks; the carboxylic acid leaves (m-chlorobenzoic acid).
- 3
Pyridine N-oxide forms with N⁺–O⁻; the aromatic ring is unchanged.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- The ring is not attacked at carbon. The lone pair on nitrogen is the nucleophile, so the nitrogen reacts.
- The ring is not attacked at carbon. The lone pair on nitrogen is the nucleophile, so the nitrogen reacts.
Often confused with: N-alkylation of pyridine · Epoxidation
Frequently asked questions
What are N-oxides made for?
The N-oxide is more reactive than pyridine in SEAr and directs to C4 (e.g. nitration to 4-nitropyridine N-oxide). The oxygen is removed afterwards.
How does pyridine differ from benzene?
N is more electronegative than C and pulls electron density out of the ring: electrophiles react poorly (as with nitrobenzene), nucleophiles readily at C2 and C4. The lone pair on N lies in the ring plane and makes pyridine basic (pKa of pyridinium ≈ 5).