Nitration of phenol

1) Electron density

The lone pairs of the oxygen are delocalized towards the ring and increase the electron density at the ortho and para positions.

Electrostatic potential map of phenol: red marks the regions with the highest electron density and blue the poorest ones

The zone with the highest electron density is in fact the oxygen, because it is the most electronegative atom in the molecule. Within the ring, the electron density concentrates at the ortho and para positions.

Charge-separated resonance forms of phenol, showing how the electron density of the ring changes

The resonance forms with charge separation won't contribute much to the resonance hybrid. But they allow us to explain why the negative charge concentrates in ortho and para positions.

2) Relative reaction rate

Phenol reacts 1000 times faster than benzene. The OH group activates the aromatic ring towards SEAr and lowers the activation energy. The intermediate benzenonium ion is more stable when the OH is attached to it.

3) Direction of substitution

Nitration takes place almost exclusively at the ortho and para positions. This means that the ortho and para benzenonium ions must be much less unstable than the meta one. The resonance forms of each ion give us the answer.

Ortho substitution

Resonance forms of the benzenonium ion in the ortho nitration of phenol

The positive charge delocalizes along the ortho and para positions. In the third resonance form the positive charge lies on the carbon supporting the OH group. The oxygen is thus able to further delocalize the positive charge by sharing its electron lone pairs. Therefore, the resonance hybrid is described with four resonance forms.

Para substitution

Resonance forms of the benzenonium ion in the para nitration of phenol

The reasoning is similar to that used for the ortho position.

Meta substitution

Resonance forms of the benzenonium ion in the meta nitration of phenol

The positive charge in no case lies on the carbon attached to the OH group that is unable to further delocalize it. Therefore, the resonance hybrid is described with only three resonance forms and is more unstable than the preceding cases.

The energy diagram of phenol nitration is qualitatively similar to benzene's.

Energy diagram of the nitration of phenol compared with that of benzene: ortho, meta and para pathways

Phenol reacts faster than benzene. That experimental fact suggests that benzene reaction pathway has to be the highest. Of the three possible pathways of phenol, ortho, para and meta, the latter should be the second highest because it is the least stabilized by resonance (see above). The major isomer is para and hence, its pathway has to be the lowest in energy. Experimentally, the ortho:para ratio is 40:58, but there are two equivalent ortho positions. We have to remove this statistical factor in order to compare true rates. Therefore, the para position reacts 58/20 times as fast as the ortho, i.e. 2.9:1.

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