Concept 06 / 06Alcohols, Phenols and Ethers
Chemical Reactions of Phenols: Electrophilic Substitution, Kolbe and Reimer-Tiemann Reactions
Reactions of phenols: esterification and O-acylation, electrophilic aromatic substitution activated by OH (nitration to nitrophenols and picric acid, bromination in CS2 and in water), Kolbe's reaction to salicylic acid, Reimer-Tiemann reaction to salicylaldehyde, reaction with zinc dust, oxidation to benzoquinone, and the ferric chloride test.
The OH group of phenol is a strong activating, ortho/para-directing group (), so phenol undergoes electrophilic substitution far more easily than benzene (see figure). Phenol also shows reactions of the O-H bond: it forms phenoxides with NaOH and esters with acid chlorides or anhydrides (not easily with carboxylic acids).
Nitration. With dilute nitric acid at , phenol gives a mixture of o-nitrophenol and p-nitrophenol. They are separated by steam distillation: the ortho isomer is steam-volatile (intramolecular hydrogen bonding), while the para isomer is not (intermolecular hydrogen bonding). With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol (picric acid), though the yield is poor because of oxidation; better yields come from sulfonating phenol first and then nitrating.
Halogenation. With bromine in a solvent of low polarity ( or ) at low temperature, monobromophenols form (mainly para). With bromine water, the phenoxide-like activation is so strong that 2,4,6-tribromophenol precipitates as a white solid, even without a Lewis acid catalyst (a test for phenol).
Kolbe's reaction. Sodium phenoxide, a more strongly activated ring than phenol, reacts with carbon dioxide (a weak electrophile) at about and to ; acidification gives ortho-hydroxybenzoic acid (salicylic acid), the precursor of aspirin.
Reimer-Tiemann reaction. Phenol heated with chloroform in aqueous NaOH introduces a CHO group at the ortho position: the intermediate substituted benzal chloride is hydrolysed in the alkali, and acidification gives salicylaldehyde (2-hydroxybenzaldehyde). The reactive electrophile is dichlorocarbene, , formed from chloroform and base.
Reaction with zinc dust. Heating phenol with zinc dust reduces it to benzene: .
Oxidation. Phenol is oxidised by chromic acid () to benzoquinone (1,4-benzoquinone), a conjugated diketone. On standing in air, phenol slowly turns dark pink or brown for the same reason.
Tests. Neutral ferric chloride gives a violet colour (complex formation). Bromine water gives a white precipitate. Phenol dissolves in NaOH but does not liberate from sodium hydrogencarbonate.
Key Takeaways:
- OH activates the ring; substitution at ortho and para positions.
- Dilute : o- and p-nitrophenol (separated by steam distillation); concentrated: picric acid.
- Bromine water: white 2,4,6-tribromophenol; in : monobromophenols.
- Kolbe: phenoxide + → salicylic acid; Reimer-Tiemann: /NaOH → salicylaldehyde (via ).
- Zn dust → benzene; chromic acid → benzoquinone; → violet.
- Pitfall: in Kolbe's reaction, the ring is attacked by , not by ; the phenoxide ion is essential.
Brain-Teaser 1: What is the mass of white precipitate formed when of phenol is treated with excess bromine water?
Solution: . phenol gives 2,4,6-tribromophenol (): .
Brain-Teaser 2: Why is the electrophile in the Reimer-Tiemann reaction dichlorocarbene, and why does it attack mainly at the ortho position?
Solution: Hydroxide removes the acidic proton of chloroform, and the resulting loses to give neutral, electron-deficient . The phenoxide ion, strongly activated at ortho and para positions, attacks it; ortho substitution is favoured by interaction of the sodium phenoxide oxygen with the incoming group. Hydrolysis of the group then gives CHO.
Brain-Teaser 3: How would you convert phenol into aspirin?
Solution: Kolbe's reaction: sodium phenoxide with at , to , then acid, gives salicylic acid. Acetylation of the phenolic OH with acetic anhydride (acid catalyst) gives acetylsalicylic acid (aspirin).
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