Problem 73 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following groups in order of decreasing inductive effects: \(\mathrm{NO}_{2}, \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}, \mathrm{CH}_{3}, \mathrm{OCH}_{3}, \mathrm{Br}\) (a) \(\mathrm{NO}_{2}>\mathrm{Br}>\mathrm{OCH}_{3}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}>\mathrm{CH}_{3}\) (b) \(\mathrm{NO}_{2}>\mathrm{Br}>\mathrm{OCH}_{3}>\mathrm{CH}_{3}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\) (c) \(\mathrm{NO}_{2}>\mathrm{OCH}_{3}>\mathrm{Br}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}>\mathrm{CH}_{3}\) (d) \(\mathrm{NO}_{2}>\mathrm{OCH}_{3}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}>\mathrm{Br}>\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Option (a) is correct: \(\mathrm{NO}_{2} > \mathrm{Br} > \mathrm{OCH}_{3} > \mathrm{C}(\mathrm{CH}_{3})_{3} > \mathrm{CH}_{3}\).

Step 1: Understand Inductive Effect

Inductive effect refers to the electron-withdrawing or electron-releasing ability of a substituent in a molecule. Electron-withdrawing groups show a negative inductive effect \((-I)\) causing a pull of electron density towards themselves, while electron-donating groups show a positive inductive effect \((+I)\).

Step 2: Analyze Each Group

- **\(\mathrm{NO}_2\)**: Known for being a strong electron-withdrawing group due to its resonance and electronegativity. It has a strong \(-I\) effect.- **\(\mathrm{Br}\)**: A halogen that is moderately electron-withdrawing due to high electronegativity, but it has some electron-donating character through resonance.- **\(\mathrm{OCH}_3\)**: This group is generally considered to be electron-donating through resonance but is slightly electron-withdrawing inductively due to the electronegative oxygen atom.- **\(\mathrm{C}(\mathrm{CH}_3)_3\)** and **\(\mathrm{CH}_3\)**: Alkyl groups that typically show a \(+I\) effect by donating electron density.

Step 3: Arrange in Decreasing Order of Inductive Effect

Based on the strength of each group's \(-I\) effect:1. **\(\mathrm{NO}_2\)**: Strongest \(-I\) effect.2. **\(\mathrm{Br}\)**: Moderate \(-I\) effect.3. **\(\mathrm{OCH}_3\)**: Lesser \(-I\) effect than \(\mathrm{Br}\).4. **\(\mathrm{C}(\mathrm{CH}_3)_3\)** and **\(\mathrm{CH}_3\)**: Exhibit \( + I \) effect, not primarily electron-withdrawing.

Step 4: Compare with Options

Match the order to the provided options. - Option (a): \(\mathrm{NO}_2 > \mathrm{Br} > \mathrm{OCH}_3 > \mathrm{C}(\mathrm{CH}_3)_3 > \mathrm{CH}_3\)- Option (b): \(\mathrm{NO}_2 > \mathrm{Br} > \mathrm{OCH}_3 > \mathrm{CH}_3 > \mathrm{C}(\mathrm{CH}_3)_3\)- Option (c): \(\mathrm{NO}_2 > \mathrm{OCH}_3 > \mathrm{Br} > \mathrm{C}(\mathrm{CH}_3)_3 > \mathrm{CH}_3\)- Option (d): \(\mathrm{NO}_2 > \mathrm{OCH}_3 > \mathrm{C}(\mathrm{CH}_3)_3 > \mathrm{Br} > \mathrm{CH}_3\)The correct order is option (a).

Key Concepts

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in organic molecules. They attract electron density towards themselves, thereby generating a polar effect within the molecule. This is typically expressed as a negative inductive effect \((-I)\). Examples of such groups include \(\mathrm{NO}_2\) and \(\mathrm{Br}\). \(\mathrm{NO}_2\) is a classic EWG because of its ability to pull electrons toward itself through both resonance and inductive mechanisms, making it very strong in this role. The inductive effect is a static phenomenon where electronegativity differences between atoms in a molecule dictate the pull. This influences how the molecule interacts with other chemical entities, affecting reactivity and stability.
\(\mathrm{Br}\), on the other hand, though not as strong as \(\mathrm{NO}_2\), still serves as an EWG due to its high electronegativity.

Electron-Donating Groups

Electron-donating groups (EDGs) donate electron density to the rest of the molecule. This results in a positive inductive effect \(\left( +I \right)\). Alkyl groups like \(\mathrm{CH}_3\) and \(\mathrm{C}(\mathrm{CH}_3)_3\), for instance, are known for their \(\left( +I \right)\) effect because they push electron density through their sigma bonds.
\(\mathrm{OCH}_3\) is an interesting group, as it can act both as an electron donor and acceptor. Inductively, the electronegative oxygen slightly withdraws electrons, but through resonance, it can donate electrons. This dual behavior gives it a moderate electron-releasing characteristic in some contexts. EDGs enhance the electron density of molecular sections they are attached to, impacting the reactivity patterns in synthesis and reaction mechanisms.

Resonance Effect

The resonance effect involves the delocalization of electrons within a molecule through \(\pi\) orbitals. This occurs when lone pair electrons or double bonds can move through a conjugated system, allowing electron density to be spread throughout the molecule. It provides stability by allowing multiple resonance structures.
This effect is observed in groups like \(\mathrm{NO}_2\), where electron delocalization occurs, making the group strongly electron-withdrawing. Similarly, \(\mathrm{OCH}_3\) can interact through resonance, primarily when involved in aromatic systems, providing some degree of electron donation, despite being typically electron-withdrawing inductively.

Organic Chemistry

Organic Chemistry is the study of carbon-containing compounds' structure, properties, composition, and reactions. It's the branch of chemistry concerned with compounds commonly found in living organisms.
Critical concepts like electron-withdrawing and donating groups, resonance, and inductive effects are foundational in understanding organic reactions and mechanisms. These concepts help in predicting the behavior of organic molecules during chemical transformations. Organic chemistry plays a pivotal role in the development of pharmaceuticals, plastics, fuels, and many other materials we use daily.

JEE Chemistry

JEE Chemistry refers to the subject's preparation for the Joint Entrance Examination (JEE) in India, a significant challenge for students aiming to enter elite educational institutions like the IITs. This exam tests various chemistry topics, including organic chemistry principles like inductive effects, electron-withdrawing and donating groups, and resonance effects.
Aspiring students must grasp these concepts deeply, rationally understanding reactions, mechanisms, and synthesis processes outlined in the syllabus. Being familiar with these fundamental principles is essential for solving complex problems efficiently and effectively, like the one involving the arrangement of groups by inductive effect.