: 230
Four named reactions of phenols are frequently tested in JEE.
Kolbe-Schmitt reaction: Sodium phenoxide + CO2 (125°C, 4-7 atm) -> salicylic acid (ortho-hydroxybenzoic acid). At higher temperature, para-product predominates. Salicylic acid is the precursor to aspirin (acetylation with acetic anhydride).
Reimer-Tiemann reaction: Phenol + CHCl3 + NaOH -> salicylaldehyde (o-hydroxybenzaldehyde). The key intermediate is dichlorocarbene (:CCl2), an electrophile generated from CHCl3/NaOH. Critical JEE trap: using CCl4 instead of CHCl3 gives salicylic acid (not aldehyde).
Fries rearrangement: Phenyl ester + AlCl3 -> acylphenol. Temperature controls regiochemistry: low T favours para (kinetic), high T favours ortho (thermodynamic — stabilised by intramolecular H-bonding).
Claisen rearrangement: Allyl phenyl ether (heated, 200°C) -> o-allylphenol via [3,3]-sigmatropic shift. Thermal, concerted, no catalyst needed. If both ortho positions are blocked, allyl migrates to para with inversion.
These reactions are interconnected in JEE questions. The common thread is electrophilic attack on the activated phenol/phenoxide ring. Kolbe uses CO2, Reimer-Tiemann uses :CCl2, and Fries uses an intramolecular acyl migration.