Problem 154 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Phenol with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) at room temperature gives [MHCET 2000] (a) \(p\)-phenol sulphonic acid (b) \(m=p h e n o l\) sulphonic acid (c) 2-hydroxybenzene sulphonic acid (d) phenol trisulphonic acid

Step-by-Step Solution

Answer

Phenol with concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\) gives \(p\)-phenol sulphonic acid (option a).

Step 1: Understand the Reaction

Phenol reacts with concentrated sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)) to undergo sulfonation. The site of reaction on the phenol determines the product formed. Sulfonation introduces a sulfonic acid group \(-\mathrm{SO}_{3}\mathrm{H}\) to the aromatic ring of phenol.

Step 2: Identify the Sulfonation Position

When phenol is treated with concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\) at room temperature, sulfonation typically occurs at the para position relative to the hydroxyl \(-\mathrm{OH}\) group, due to the activating and directing effect of the hydroxyl group.

Step 3: Match Product to Options

The product is \(p\)-phenol sulphonic acid, where the sulfonic acid group is introduced at the para position relative to the \(-\mathrm{OH}\) group. This product matches option (a).

Step 4: Confirm the Correct Option

Since the para position is favored in this reaction situation and matches with option (a), we can confirm that the primary product of this reaction is \(p\)-phenol sulphonic acid.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a fundamental type of reaction in organic chemistry, widely observed in aromatic compounds like benzene and its derivatives.
In this reaction, an electrophile replaces a hydrogen atom in an aromatic ring. This is significant because aromatic rings are stable due to their delocalized electrons. Hence, only strong electrophiles can disrupt their structure to participate in such a reaction.
Some key features of EAS reactions include:

• Electrophile Generation: A necessary step where a strong electrophile is formed or introduced.
• Formation of an Arenium Ion: The electrophile attacks the aromatic ring, forming a positively charged intermediate called the arenium ion.
• Reformation of Aromaticity: The arenium ion loses a proton, restoring the aromatic system, with the electrophile now substituting for the hydrogen atom.

EAS reactions are categorized based on the type of electrophile. In the sulfonation of phenol, the electrophile is sulfur trioxide (\(\text{SO}_3\)). Understanding EAS is essential for predicting the behavior of aromatic compounds under various conditions.

Sulfonation Reaction

The sulfonation reaction is a specific type of electrophilic aromatic substitution that involves introducing a sulfonic acid group (\(-\mathrm{SO}_3\mathrm{H}\)) into an aromatic compound. Phenol sulfonation, in detail, showcases how the hydroxyl group \(-\mathrm{OH}\) acts as an activating group. This directs the sulfonation preferentially to certain positions on the benzene ring.
Key aspects of phenol sulfonation include:

• Use of Concentrated Sulfuric Acid: This acts as both a sulfonating agent and a solvent, providing the necessary \(\text{SO}_3\) for the reaction.
• Para Position Preference: Due to the \(-\mathrm{OH}\) group's electron-donating nature, the sulfonic acid group typically substitutes at the para position.
• Temperature Influence: Reaction conditions like temperature can affect the position and number of sulfonic acid groups introduced.

This reaction is important in industrial applications, notably in the production of detergents and dyes. It's also a gateway to understanding further complex aromatic transformations.

Organic Chemistry Reaction Mechanisms

Understanding reaction mechanisms in organic chemistry involves studying the step-by-step processes by which chemical reactions occur at the molecular level. A reaction mechanism provides a detailed pathway from reactants to products, highlighting the interactions and changes that happen throughout the process.
Main concepts of reaction mechanisms include:

• Reaction Intermediates: Such as carbocations, carbanions, radicals, and arenas ions in the case of EAS.
• Transition States: High-energy states during the conversion of reactants to products, providing insight into reaction kinetics and pathways.
• Activation Energy: The energy barrier that must be overcome for a reaction to proceed.

Studying mechanisms allows chemists to predict the outcome of reactions, optimize reaction conditions, and develop new reactions. In the context of sulfonation, grasping the mechanism helps one understand why phenol directs \(\text{SO}_3\mathrm{H}\) to the para position, aiding in developing efficient industrial processes and synthesizing desired compounds. This understanding forms the core of organic synthesis and applied chemistry.