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Electrophilic Aromatic Substitution Reactions are those organic reactions in which an electrophile replaces one or more hydrogen atoms linked to an aromatic ring. These reactions include the substitution of a hydrogen atom from a benzene ring with an electrophile. Aromatic nitrations, aromatic sulphonation, and Friedel-Crafts reactions are examples. The aromaticity of the aromatic system is conserved in an electrophilic aromatic substitution process. When bromobenzene is made from the reaction of benzene and bromine, for example, the aromatic ring remains stable. Aryl halides or haloarenes can be produced through electrophilic aromatic halogenation reactions of aromatic rings with iodine, chlorine, or bromine.
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Key Terms: Electrophilic Substitution, Substitution Reaction, Chiral Carbon, Aromatic Ring, Phenols, Benzene, Electron, Nitration, Halogenation
Electrophilic Substitution Reaction of Phenols
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Because of their high electron density, phenols are prone to electrophilic substitution reactions. In phenol, the hydroxyl group linked to the aromatic ring promotes efficient charge delocalization in the aromatic ring. As a result, it uses resonance to stabilize the arenium ion. The hydroxyl group also produces ortho para directors.
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Nitration of Phenols
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After being treated with weak nitric acid, phenols are nitrated at a low temperature (298 K), yielding a mixture of ortho and para nitrophenols. Based on their volatility, the resulting mixture is further separated into ortho and para nitrophenols by steam distillation. Ortho nitrophenols, which include both intramolecular and intermolecular hydrogen bonding, are less volatile than para nitrophenols, which solely involve intermolecular hydrogen bonding.
Nitration of Phenols
When phenol is treated with strong nitric acid, 2, 4, 6-trinitrophenol is formed as a result of the nitration (commonly called picric acid).
Formation of 2, 4, 6-trinitrophenol
Halogenation of Phenols
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Even in the absence of Lewis acids, phenols undergo halogenation due to the highly activating impact of the hydroxyl group. Monobromophenols are generated when phenols are treated with bromine in the presence of a low-polarity solvent such as CHCl3 at low temperatures. A white precipitate of 2, 4, and 6-tribromophenol is generated when phenol is treated with bromine water.
Formation of 2, 4, 6-tribromophenol
Kolbe’s Reaction
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The phenoxide ion is generated when phenol is treated with sodium hydroxide. The generated phenoxide ion is extremely reactive in electrophilic substitution processes. It conducts an electrophilic substitution reaction with a weak electrophile (carbon dioxide) to create Ortho-hydroxybenzoic acid. Kolbe's reaction is the commonly used term or name for this reaction.
Kolbe’s Reaction
Reimer-Tiemann Reaction
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An aldehyde group is generated at the ortho position of the benzene ring when phenol is treated with chloroform in the presence of sodium hydroxide. The Reimer-Tiemann reaction is a well-known name for this reaction.
Reimer-Tiemann Reaction
Things to Remember
- The organic reactions in which an electrophile replaces one or more hydrogen atoms linked to an aromatic ring are known as electrophilic aromatic substitution reactions.
- Phenols are effectively and highly prone to electrophilic substitution reactions due to their rich electron density.
- In phenols, the hydroxyl group attached to the aromatic ring, facilitates the effective delocalization of the charge in the aromatic ring. Therefore, it helps in stabilising the arenium ion through resonance.
- The hydroxyl group acts as ortho para directors, thus, most of these substitutions are at ortho and para positions only.
- Some of the electrophilic substitution reactions of phenols are Nitration of Phenols, Halogenation of Phenols, Kolbe’s Reaction, and Reimer-Tiemann Reaction.
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Sample Questions
Ques. Why phenol does not undergo electrophilic substitution? (3 Marks)
Ans. The oxygen attaches to the C on the benzene ring due to the Resonance (+R) effect, forming a partial double bond that is difficult to break. It is tough to break the cycle. The bond length increases shorter when oxygen is linked to an sp3 hybridized carbon, making it more difficult to break. Due to repulsion, the approach of an electron-rich nucleophile to an electron-rich benzene ring is less likely.
Ques. Why does phenol undergo an electrophilic substitution reaction at the ortho and para positions? (3 Marks)
Ans. Because the lone pair on the oxygen atom stabilizes the intermediate carbocation, phenols undergo electrophilic substitution at ortho and para positions, the stability is greatest. As a result of the presence of the hydroxyl group, the ring is activated and reactive in both ortho and para positions.
Ques. What is the electrophilic substitution reaction mechanism? (3 Marks)
Ans. Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a molecule. Aromatic substitution reactions are frequent means of adding functional groups into benzene rings and are typical of aromatic compounds.
Ques. Why Phenols are very reactive towards electrophilic aromatic substitution? (3 Marks)
Ans. Because the nonbonding electrons on oxygen stabilize the intermediate cation, phenols are extremely reactive for electrophilic aromatic substitution. The hydroxyl group of phenol is believed to be activating (i.e., its presence causes the aromatic ring to be more reactive than benzene) and ortho- or para-directing since this stabilization is more effective for the attack at the ortho or para position of the ring.
Ques. Why is Phenol more reactive than Benzene in electrophilic substitution reactions? (2 Marks)
Ans. The -OH group on phenol and the lone pair of electrons on oxygen are given into the Benzene ring and activate it, making it more vulnerable to electrophile assault.
Ques. What is the directing effect of phenol towards electrophiles? (3 Marks)
Ans. Phenol is an ortho/para director, however, the reaction is faster in the presence of a base. This is owing to the phenolate anion's greater reactivity. The negative oxygen was 'pressed' to supply electron density to the carbons (it has an additional +I effect due to its negative charge).
Ques. What is the bromination of phenol? (3 Marks)
Ans. Bromination is a substitution process for phenol. Bromine is used to substitute hydrogen in the benzene ring of phenol. When phenol is treated with Br2 in a water solvent, it produces a polybromo derivative with all hydrogen atoms in ortho, meta, and para positions with respect to the OH.
Ques. What happens when phenol reacts with H2SO4? (3 Marks)
Ans. Phenol combines with sulfuric acid to form substituted phenols when it reacts with concentrated sulphuric acid. At normal temperature, a combination of phenol sulphuric acid and the resulting benzene ortho hydroxy sulfuric acid reaches equilibrium.
Ques. What is the oxidation of phenol? (3 Marks)
Ans: The oxidation process begins with the decomposition of phenol into aromatic compounds with two hydroxyl groups replaced in the benzene rings (hydroquinone, resorcinol, and catechol). The oxidation of these molecules produces the quinone chemicals p-benzoquinone and o-benzoquinone.
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