Phenol SMILES: SMILES:Oc1ccccc1

Resonance Stabilization of Phenoxide Ion: The negative charge on oxygen delocalizes into the ring at ortho and para positions:

$O^{-}$         O           O           O           O
|          ‖           |           |           ‖
$C_{6}H_{5}$  ↔  $C_{6}H_{4}^{-}$  ↔   $C_{6}H_{4}^{-}$  ↔  $C_{6}H_{4}^{-}$  ↔  $C_{6}H_{4}^{-}$
         (ortho)      (para)     (ortho)     (ipso)

Electrophilic Substitution Positions:

• -OH is an ortho/para director and strong activator
• Bromination: positions 2, 4, 6 all substituted (2,4,6-tribromophenol)
• Kolbe and Reimer-Tiemann: both give ortho product preferentially

Comparison of ring activation:

Benzene < Toluene < Phenol (fastest electrophilic substitution)

Substituent effects on acidity (pKa direction):

• -$NO_{2}$ (para): DECREASES pKa → MORE acidic (p-nitrophenol)
• -$CH_{3}$ (para): INCREASES pKa → LESS acidic (cresol)
• -Cl (para): slightly decreases pKa (inductive -I effect)