Structure and Acidity: Phenol (SMILES:Oc1ccccc1) has -OH directly bonded to a benzene ring carbon. This makes it fundamentally different from alcohols. Its pKa is approximately 10, compared to ~16-18 for aliphatic alcohols. The dramatic acidity difference arises from resonance stabilization of the phenoxide ion ($C_{6}H_{5}O^{-}$): the negative charge delocalizes across five resonance structures into the ring (at ortho and para positions), dramatically reducing its energy and making the equilibrium favor ionization.

Substituent Effects on Phenol Acidity: Substituents on the benzene ring modulate the stability of the phenoxide ion and thus the acidity:

• p-$NO_{2}$: strong -M (mesomeric withdrawal) and -I (inductive withdrawal) effects → further stabilizes phenoxide → pKa decreases → MORE acidic than phenol
• p-Cl: -I effect dominates → slightly stabilizes phenoxide → slightly more acidic
• p-$CH_{3}$: +I effect → slightly destabilizes phenoxide → less acidic than phenol
• p-O$CH_{3}$: +M effect → pushes electrons toward $O^{-}$ → destabilizes phenoxide → less acidic

Acidity order: p-nitrophenol > p-chlorophenol > phenol > p-cresol (p-methylphenol)

Electrophilic Aromatic Substitution (EAS): -OH is a strong ortho/para director. The ring is highly activated — much more reactive than benzene toward EAS. Bromination with $Br_{2}$/$H_{2}O$ (no $FeBr_{3}$ needed) gives 2,4,6-tribromophenol quantitatively as a white precipitate. This is also a qualitative test for phenol.

Kolbe Reaction (Carboxylation): Sodium phenoxide + $CO_{2}$ under 125°C and 4-7 atm pressure → sodium salicylate → acidification → salicylic acid (SMILES:OC(=O)c1ccccc1O). The -COOH group enters at the ortho position. Salicylic acid is the precursor to aspirin.

Reimer-Tiemann Reaction (Formylation): Phenol + $CHCl_{3}$ + NaOH (reflux) → salicylaldehyde (SMILES:O=Cc1ccccc1O). The key mechanistic feature is generation of the electrophilic intermediate dichlorocarbene (:$CCl_{2}$) from $CHCl_{3}$ by the base. :$CCl_{2}$ attacks the ortho position of phenoxide to give, after hydrolysis, the ortho-aldehyde (salicylaldehyde).

Esterification: PhOH + $CH_{3}$COCl (acyl chloride) → phenyl acetate (SMILES:CC(=O)Oc1ccccc1) + HCl. Unlike aliphatic esters (Fischer esterification), phenyl esters are formed using acyl chlorides or anhydrides because phenol is a weaker nucleophile than aliphatic alcohols.

Key Comparison — Phenol vs Benzyl Alcohol: Benzyl alcohol (PhC$H_{2}O$H) is NOT phenol — its -OH is on a sp3 carbon (a side chain), so its pKa (~15) is close to other aliphatic alcohols. Phenol's -OH is directly on the ring, hence the low pKa (~10).