Question

a. Draw resonance contributors to show why pyridine-N-oxide is more reactive than pyridine toward electrophilic aromatic substitution.

b. At what position does pyridine-N-oxide undergo electrophilic aromatic substitution?

Short answer

a. As electron density is more in pyridine-N-oxide.

b. At oxygen and 2 and 4-position.

Step-by-step solution

Resonance

The structures which shows the delocalization of pi- electrons or lone pairs of electron are called resonating structures.

More are the resonating structures more will be the stability of molecule.

Activator or deactivator

The substituent groups which are electron donating are called activators and the substituent groups which are electron withdrawing are called deactivators.

Activators increase reactivity for electrophilic aromatic substitution and deactivators decrease reactivity.

Effect of substituents on orientation

All activating substituents and the weakly deactivating halogens present on ring are ortho–para directors as the electron density is more at these positions, and all substituents that are more deactivating than the halogens are meta directors for electrophile as electron density is decreased at ortho and para position by deactivators.

Resonating structure for pyridine 

Pyridine has 5 resonating structures which are as follows:

Resonating structure of pyridine

It can be seen that nitrogen in ring is electron withdrawing and hence is a deactivator which decrease electron density in ring making electrophilic substitution difficult.

Resonating structure for pyridine-N-oxide

Pyridine-N- oxide has following resonating structures:

Resonating structure of pyridine-N-oxide

Here in the structures it can be seen that oxygen in ring is electron donating by resonance and hence is a activator which will increase electron density in ring making electrophilic substitution more easy.

Orientation for electrophilic aromatic substitution of pyridine-N-oxide

Here from the structures it can be seen that electron density in increased at position 2 and 4. Electron density is also more at oxygen so electrophilic attack can occur on any of the three positions.