Electrophilic aromatic substitution reactions are the reactions where an electrophile replaces one or more hydrogen atoms attached to the aromatic ring. Phenols are highly prone to electrophilic substitution reactions due to rich electron density.
The hydroxyl group attached to the aromatic ring in phenol facilitates the effective delocalization of the charge in the aromatic ring. Thus, it stabilizes the arenium ion through resonance. The hydroxyl group also acts as ortho para directors. Hence, we acknowledge most of these substitutions at ortho and para positions only.
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Some of the electrophilic substitution reactions of phenols are explained below:
1. Nitration of Phenols
Phenols upon treatment with dilute nitric acid undergo nitration at low temperature (298 K) to give a mixture of ortho and para nitrophenols. The mixture formed is further separated into ortho and para nitrophenols by steam distillation on the basis of their volatility.
o-nitrophenol is more volatile due to intramolecular hydrogen bonding. p-nitrophenol is less volatile and has a high boiling point due to intermolecular hydrogen bonding.
When phenol is treated with concentrated nitric acid, the nitration results in the formation of 2, 4, 6-trinitrophenol (commonly called picric acid).
2. Halogenation of Phenols
Due to a highly activating effect of the hydroxyl group in phenols, they undergo halogenation even in the absence of Lewis acids. When phenols are treated with bromine in the presence of a solvent of low polarity like CHCl3 at low temperatures, monobromophenols are formed.
When phenol is treated with bromine water, a white precipitate of 2, 4, 6-tribromophenol is formed.
3. Kolbe’s Reaction
When phenol is treated with sodium hydroxide, phenoxide ion is formed. This phenoxide ion formed is highly reactive towards electrophilic substitution reactions. Upon treatment with a weak electrophile (carbon dioxide), it undergoes electrophilic substitution reaction to form Ortho-hydroxybenzoic acid. This reaction is popularly known as Kolbe’s reaction.
4. Reimer-Tiemann Reaction
When phenol is treated with chloroform in the presence of sodium hydroxide, an aldehyde group is formed at the ortho position of the benzene ring. This reaction is popularly known as the Reimer-Tiemann reaction.
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Explain orienting influence of phenol towards electrophilic substitution reaction ?
Phenols are highly prone to electrophilic substitution reactions due to rich electron density. The hydroxyl group attached to the aromatic ring in phenol facilitates the effective delocalization of the charge in the aromatic ring.