Oxidation of Phenols
|Organic Chemistry 4e Carey|
|Online Learning Center|
Chapter 24: PhenolsSummary | Phenols | Preparation of Phenols | | Reactions of Aryl Ethers | Spectroscopic Analysis | Self Assessment | Quiz |
Reactions of Phenol
Chapter 24: Phenols
Electrophilic Aromatic SubstitutionReaction type: Electrophilic Aromatic Substitution
- Phenols are potentially very reactive towards electrophilic aromatic substitution.
- This is because the hydroxy group, -OH, is a strongly activating, ortho- / para- directing substituent (review)
- Substitution typically occurs para to the hydroxyl group unless the para position is blocked, then ortho substitution occurs.
- The strong activation often means that milder reaction conditions than those used for benzene itself can be used (see table below for a comparison)
|Nitration||dil. HNO3 in H2O or CH3CO2H||HNO3 / H2SO4|
|Sulfonation||conc. H2SO4||H2SO4 or SO3 / H2SO4|
|Halogenation||X2||X2 / Fe or FeX3|
|Alkylation||ROH / H+ or RCl / AlCl3||RCl / AlCl3|
|Acylation||RCOCl / AlCl3|
|Nitrosation||aq. NaNO2 / H+|
Consider the phenolate to be an enolate, hence reactions at the a-C are typically favored.
|MECHANISM FOR CARBOXYLATION OF PHENOLS|
The nucleophilic phenolate (reacting like an enolate) reacts with the electrophilic carbon of carbon dioxide in the ortho position (compare this with an Aldol reaction)
The non-aromatic cyclohexadienonecarboxylate intermediate tautomerises to the more stable aromatic enol which is further stabilized by an intramolecular hydrogen bond. An acidic work-up will generate the carboxylic acid.