Problem 170 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is/are correct? (a) Phenol gives paraquinol with \(\mathrm{S}_{2} \mathrm{O}_{8}^{-2} / \mathrm{OH}^{-}\)as a major product. (b) Phenol can give electrophilic aromatic substitution reactions in absence of lewis acid. (c) Salicylic acid can give white/yellow solution with \(\mathrm{Br}_{2} / \mathrm{CS}_{2}\) (d) Ipso substitution is possible in phenol derivative if it contains \(-\mathrm{COOH}\) or \(-\mathrm{SO}_{3} \mathrm{H}\) group at ortho or para position with respect to OH group.

Step-by-Step Solution

Answer

All statements (a), (b), (c), and (d) are correct.

Step 1: Verify Statement (a)

Phenol when treated with \(\mathrm{S}_2\mathrm{O}^{2-}_8/\mathrm{OH}^-\) gives para-benzoquinone as the major product. Para-benzoquinone (also simply called paraquinol) is obtained when phenol undergoes oxidation. The statement (a) is correct.

Step 2: Verify Statement (b)

Phenol can indeed undergo electrophilic aromatic substitution reactions even in the absence of a Lewis acid. The hydroxyl group in phenol is highly activating and can direct substitution to the ortho and para positions without the need for a catalyst. Thus, statement (b) is correct.

Step 3: Verify Statement (c)

Salicylic acid, when reacted with \(\mathrm{Br}_2/\mathrm{CS}_2\), forms a white or off-white precipitate of 2,4,6-tribromophenol. The solution initially becomes yellow due to the formation of the bromination products, which then precipitates as a white to off-white solid. Therefore, statement (c) is also correct.

Step 4: Verify Statement (d)

Ipso substitution refers to substitution at the same carbon atom where the substituent is located, usually at a position of high electron density. In phenol derivatives, an \(\text{-COOH}\) or \(\text{-SO}_3\text{H}\) group at the ortho or para position can enhance ipso substitution by providing sites for electrophilic or nucleophilic attack. Thus, statement (d) is correct.

Key Concepts

Electrophilic Aromatic Substitution

Phenol, a simple aromatic compound, is capable of undergoing electrophilic aromatic substitution (EAS) reactions. This process exhibits interesting chemistry due to the presence of an -OH group attached to the benzene ring. This hydroxyl group is an electron-donating group, rendering the aromatic ring more reactive towards electrophiles. Unlike many other aromatic compounds, phenol doesn't always require a Lewis acid catalyst to facilitate EAS reactions. The -OH group's ability to donate electrons makes the ring highly activated, allowing for the substitution to occur more readily. Reactions tend to direct incoming electrophiles to the ortho or para positions relative to the hydroxyl group. In summary, phenol's enhanced reactivity and lack of requirement for catalysis make EAS an interesting and frequently observed reaction in its chemistry.

Ipso Substitution

Ipso substitution is a fascinating concept, particularly in the context of phenol derivatives. This type of substitution occurs when an incoming group displaces an existing substituent from the same position, or 'ipso' position on the aromatic ring. In phenol derivatives, the presence of groups such as -COOH or -SO₃H at the ortho or para positions relative to the -OH group can make them excellent candidates for ipso substitution. These groups introduce sites of high electron density, facilitating the substitution process. The process is facilitated further by the electron-donating nature of the hydroxyl group, which enhances the electrophilic nature of the pathway, thus making ipso substitution a notable feature of phenolic chemistry.

Oxidation of Phenol

Phenol is known for its ability to undergo oxidation reactions, with one significant reaction being the oxidation to para-benzoquinone, also referred to as paraquinol. This reaction often involves the use of oxidizing agents like \( ext{S}_2 ext{O}_8^{2-}/ ext{OH}^- \).When phenol is oxidized, it forms para-benzoquinone as the major product. The reaction is interesting because it transforms the hydroxyl group into a carbonyl functionality, thus showcasing phenol's versatility. This reaction is essential not only in synthetic chemistry but also in biological processes where similar oxidations occur.

Bromination of Salicylic Acid

The bromination of salicylic acid is a classic reaction in organic chemistry. When salicylic acid is treated with bromine in carbon disulfide (\( ext{Br}_2/ ext{CS}_2 \)), it undergoes a transformation leading to the formation of a white to off-white precipitate of 2,4,6-tribromophenol. Initially, the solution may exhibit a yellow color due to the reaction intermediates. However, as the reaction completes, the tribrominated product precipitates out. This process demonstrates an important electrophilic substitution, with the bromine atoms attaching to the aromatic ring's ortho and para positions relative to the carboxyl and hydroxyl groups. This reaction not only highlights the reactivity of salicylic acid but is also a prime example of the bromination process in aromatic compounds.