Oxygen substituents activate a benzene ring
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص480-482
2025-06-10
561
To brominate phenol, all we had to do was to mix bromine and phenol—if we do this with benzene itself, nothing happens. We therefore say that, relative to benzene, the OH group in phenol activates the ring towards electrophilic attack. The OH group is both activating and ortho, para-directing. Other groups that can donate electrons also activate and direct ortho, para. Anisole (methoxybenzene) is the ‘enol ether’ equivalent of phenol. It reacts faster than benzene with electrophiles.
2,4-D The multiple chlorination of another oxygen-substituted compound, phenoxyacetic acid, leads to a useful product. Chlorination with two equivalents of chlorine provides 2,4-dichlorophenoxy acetic acid, which is the herbicide 2,4-D. The oxygen substituent again activates the ring and directs the chlorination to the ortho and para positions.
Nitration of phenol is also very fast and can be problematic under the usual nitration conditions (conc. HNO3, conc. H2SO4) because concentrated nitric acid oxidizes phenols. The solution is to use dilute nitric acid. The concentration of NO2 + will be small but that does not matter with such a reactive benzene ring.
The product is a mixture of ortho- and para-nitrophenol from which the ortho compound can be separated by steam distillation. A strong intramolecular hydrogen bond reduces the availability of the OH group for intermolecular hydrogen bonding so the ortho compound has a lower boiling point.
Paracetamol from a phenol The remaining para-nitrophenol is used in the manufacture of the painkiller paracetamol (also known as acetaminophen).
The phenoxide ion is even more reactive towards electrophilic attack than phenol. It man ages to react with such weak electrophiles as carbon dioxide. This reaction, known as the Kolbe–Schmitt process, is used industrially to prepare salicylic acid (2-hydroxybenzoic acid), a precursor in making aspirin.
The O− substituent is ortho, para-directing but the electrophilic substitution step with CO2 gives mostly the ortho product. There must be some coordination between the sodium ion and the oxygen atoms of both the phenoxide and CO2 that delivers the electrophile to the ortho position.
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