Electrophilic Substitution in Phenols Explained Simply

Phenol (C6H5OH) undergoes electrophilic aromatic substitution far more readily than benzene. The –OH group is a powerful activating group: its lone pair donates into the ring (+M effect), raising electron density especially at the ortho and para positions, so electrophiles attack there preferentially.

This matters because phenol is so reactive that its reactions look nothing like benzene's: bromine water gives an instant white precipitate of 2,4,6-tribromophenol at room temperature, while benzene needs a Lewis acid catalyst and still gives only one bromine. The same activation drives the industrial preparations of salicylic acid (aspirin precursor) and salicylaldehyde.

Key Concepts

• +M (mesomeric) effect: donation of the oxygen lone pair into the ring's π system, which outweighs oxygen's electron-withdrawing inductive effect.
• Ortho/para directing: resonance forms place extra negative charge at the 2, 4 and 6 positions, so electrophiles attack there.
• 2,4,6-tribromophenol: the white precipitate formed instantly when phenol meets bromine water — a classic test for phenol.
• Picric acid (2,4,6-trinitrophenol): the product of phenol with concentrated nitric acid.
• Reimer–Tiemann reaction: phenol + chloroform + NaOH gives salicylaldehyde (ortho product mainly).
• Kolbe's reaction: sodium phenoxide + CO2 (under pressure) gives salicylic acid.

How It Works

Bromination. In aqueous solution, phenol is ionized slightly to phenoxide, which is even more activated, so all three available ortho/para positions brominate immediately, giving white 2,4,6-tribromophenol. In a non-polar solvent (CS2) at low temperature (273 K), the reaction can be stopped at monobromination, giving a mixture of ortho- and para-bromophenol.

Nitration. With dilute HNO3 at low temperature, a mixture of ortho- and para-nitrophenol forms (separated by steam distillation — the ortho isomer is steam-volatile due to intramolecular hydrogen bonding). With concentrated HNO3, all three positions nitrate to give picric acid.

Reimer–Tiemann. Chloroform and NaOH generate dichlorocarbene (:CCl2), which attacks the ortho position of phenoxide; hydrolysis of the resulting intermediate gives salicylaldehyde (o-hydroxybenzaldehyde).

Worked Example

What mass of 2,4,6-tribromophenol precipitates when 94 g of phenol reacts completely with excess bromine water?

1. Molar mass of phenol (C6H5OH) = 72 + 6 + 16 = 94 g/mol, so 94 g = 1.00 mol.
2. Equation: C6H5OH + 3 Br2 → C6H2Br3OH + 3 HBr (1:1 product ratio).
3. Molar mass of 2,4,6-tribromophenol (C6H3Br3O) = 72 + 3 + 240 + 16 = 331 g/mol.
4. 1.00 mol gives 331 g of white precipitate.
5. Mass check: 94 + 3(160) = 574 = 331 + 3(81) — the books balance.

Common Misconceptions

• "Phenol brominates once, like benzene." Wrong. In water, phenol gives the tribromo product instantly at room temperature with no catalyst — its ring is enormously more activated than benzene's.
• "The –OH group deactivates the ring because oxygen is electronegative." Wrong. The +M lone-pair donation dominates over the −I inductive pull, making –OH one of the strongest activating groups.
• "Nitration of phenol always gives picric acid." Wrong. Dilute nitric acid at low temperature gives mono-nitrated ortho and para products; only concentrated acid pushes all the way to 2,4,6-trinitrophenol.

Real-World Applications

• Aspirin route: Kolbe's reaction converts phenol to salicylic acid, the direct precursor of aspirin.
• Antiseptics: chlorinated phenols (like chloroxylenol in Dettol) exploit phenol's germicidal action.
• Picric acid: 2,4,6-trinitrophenol was historically used as a yellow dye and an explosive.

To recap: the –OH group's lone pair floods the ring with electron density at ortho and para positions, so phenol reacts with electrophiles far faster than benzene — tribromination in bromine water, nitration to picric acid, the Reimer–Tiemann route to salicylaldehyde, and Kolbe's route to salicylic acid.