Electrophilic Substitution Reactions of Phenol

Phenol's $-\text{OH}$ group is a strongly activating, ortho/para-directing substituent on

the benzene ring, and this single fact governs essentially every electrophilic aromatic

substitution phenol undergoes.

Why $-\text{OH}$ activates and directs ortho/para. One of the lone pairs on phenol's oxygen is

conjugated into the ring by resonance -- the same delocalisation, in the neutral phenol molecule

itself rather than in its anion, that raises electron density specifically at the ring's ortho and

para carbons (extra resonance structures can be drawn placing a formal negative charge, balanced by

oxygen's formal positive charge, at exactly those positions). This makes the ring, and the ortho/

para positions in particular, considerably more electron-rich than benzene's ring is, so phenol

reacts with electrophiles both faster than benzene does and preferentially at the ortho and para positions.

Bromination. Reacting phenol with bromine water (aqueous $\text{Br}_2$) at room temperature,

with no Lewis-acid catalyst at all, gives an immediate white precipitate of

2,4,6-tribromophenol: because the ring is so strongly activated, all three of the ortho/para

positions (two ortho, one para) are substituted in a single fast reaction, without needing the

$\text{AlCl}_3$ or $\text{FeBr}_3$ catalyst that plain benzene would require to brominate at all.

This immediate, catalyst-free formation of a white precipitate is itself used as a simple

qualitative test for the presence of phenol. (Using only one equivalent of bromine in a non-aqueous,

non-polar solvent at low temperature can instead be controlled to give mainly the mono-brominated

product, predominantly p-bromophenol, but the readily observed qualitative test is the

uncontrolled, exhaustive tribromination in aqueous solution.)

Nitration. Under milder conditions -- dilute nitric acid at room temperature or below -- phenol

undergoes mono-nitration to give a mixture of the two possible ortho/para products,

o-nitrophenol and p-nitrophenol, exactly as the directing effect of $-\text{OH}$ predicts;

no meta-nitrophenol forms in any significant amount. The two isomers are, usefully, separable by

steam distillation: o-nitrophenol's $-\text{OH}$ and $-\text{NO}_2$ groups sit close enough to form

an intramolecular hydrogen bond, which (i) makes it more volatile, since that hydrogen bond …