Question

The order of electrophilic substitution reaction in the following compound is (I) Anisole (II) Acetophenone (III) Phenylethanoate (IV) Benzene (A) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (B) \(\mathrm{I}>\mathrm{III}>\mathrm{II}>\mathrm{IV}\) \((\mathrm{C}) \mathrm{I}>\mathrm{III}>\mathrm{IV}>\mathrm{II}\) (D) II > I > IV > III

Short answer

The correct order of electrophilic substitution reaction for the given compounds is \(\mathrm{I}>\mathrm{III}>\mathrm{IV}>\mathrm{II}\), which corresponds to option (C).

Step-by-step solution

Understand electrophilic substitution reactions

Electrophilic substitution reactions involve the replacement of an atom (usually hydrogen) in a compound with an electrophile. In the case of these aromatic compounds, the electrophile attacks the aromatic ring and forms a new bond.

Identify the presence of activators or deactivators

An activator is an electron-donating group, which increases the electron density in the aromatic ring, making it more reactive towards electrophiles. A deactivator is an electron-withdrawing group, which decreases the electron density in the aromatic ring, making it less reactive towards electrophiles. For each compound, we will identify the presence of activators or deactivators: (I) Anisole has a methoxy group (OCH3), which is an electron-donating group (activator). (II) Acetophenone has a carbonyl group (C=O) attached to the ring, which is an electron-withdrawing group (deactivator). (III) Phenylethanoate has an ester group (RCOOR'), which is a deactivator (electronegativity of R) but less deactivating than ketone. (IV) Benzene has no substituents, meaning it has no activators or deactivators present.

Arrange the compounds in the order of reactivity

Based on the presence of activators or deactivators, we can now determine the order of reactivity towards electrophilic substitution reactions: (I) Anisole: Activator present, making it the most reactive (III) Phenylethanoate: Deactivator present (RCOOR'), but less deactivating than ketone (IV) Benzene: No substituents, neutral reactivity (II) Acetophenone: Deactivator present (C=O), making it the least reactive So the order of reactivity is: \(\mathrm{I}>\mathrm{III}>\mathrm{IV}>\mathrm{II}\)

Match the order with the given options

Comparing our order of reactivity with the given options, we find that it matches with option (C): \((\mathrm{C}) \ \mathrm{I}>\mathrm{III}>\mathrm{IV}>\mathrm{II}\) Therefore, the correct answer is option (C).

Key concepts

Aromatic Compounds Reactivity

Understanding the reactivity of aromatic compounds is essential when studying their behavior in electrophilic substitution reactions. Aromatic compounds, characterized by their stable ring structure—known as the benzene ring—undergo reactions that preserve this stable ring system.

Reactants that seek out this stability are called electrophiles, which are electron-poor species. During an electrophilic substitution reaction, an electrophile replaces a hydrogen atom on the benzene ring. The rate at which these reactions occur can be dramatically affected by substituent groups attached to the benzene ring.

Role of Aromatic Ring Substituents

Substituents on the aromatic ring can either accelerate or decelerate an electrophilic substitution reaction. Substituents that donate electrons to the benzene ring enhance its reactivity by increasing its electron density, making it more attractive to electrophiles. Conversely, substituents that withdraw electrons from the ring make it less reactive, as the decreased electron density makes interactions with electrophiles less favorable.

This concept of reactivity is typically introduced in educational materials for students preparing for competitive examinations like JEE Main and Advanced, where a comprehensive understanding is crucial for success.

Electron-Donating and Withdrawing Groups

The concept of electron-donating and withdrawing groups is at the heart of predicting the reactivity of aromatic compounds towards electrophilic substitution reactions. These groups influence the electron density of the aromatic ring and ultimately determine the site and rate of electrophilic attack.

Electron-Donating Groups (EDGs)

Electron-donating groups (EDGs), such as the methoxy group in anisole (I), release electron density into the ring through resonance or inductive effects, making the compound more nucleophilic. This enhanced nucleophilicity makes the ring more susceptible to attack by electrophiles. Common EDGs include alkyl, hydroxyl, and amino groups.

Electron-Withdrawing Groups (EWGs)

On the other hand, electron-withdrawing groups (EWGs) pull electron density away from the aromatic ring. Groups like the carbonyl in acetophenone (II) are typical EWGs. They can decrease the electron density through resonance or inductive effects, thus making the aromatic compound less reactive. Examples of EWGs include nitro, carbonyl, and sulfonic acid groups.

Recognizing the presence and impact of these substituents is a key skill when solving problems related to electrophilic substitution reactions, especially for competitive exams like JEE.

JEE Main and Advanced Organic Chemistry

The Joint Entrance Examination (JEE) Main and Advanced are critical exams for students in India aiming to secure admission into premier engineering institutes. Organic chemistry, including electrophilic substitution reactions in aromatic compounds, forms a significant part of the chemistry syllabus for these competitive exams.

Students preparing for these exams must delve beyond memorizing reactions and develop a robust conceptual understanding. This includes recognizing the effects of different substituents on the reactivity of aromatic rings, predicting the products of reactions, and identifying mechanisms.

Strategy for JEE Organic Chemistry Preparation

A strategic approach to mastering organic chemistry for the JEE involves solving numerous practice problems, reviewing reaction mechanisms, and understanding the underlying principles that govern these reactions. Electrophilic substitution is one such principle, often intertwining with concepts of electron donation and withdrawal, as well as reaction kinetics and thermodynamics.

Through continuous practice and conceptual clarity, students can greatly improve their problem-solving abilities and performance in the organic chemistry section of the JEE Main and Advanced exams.