Question

Arrange the following groups in order of decreasing \(-\mathrm{R}\) (or -M)power: (1) \(\mathrm{NO}_{2}\) (2) \(\mathrm{SO}_{3} \mathrm{H}\) (3) \(\mathrm{CF}_{3}\) (4) CHO (a) \(1>3>2>4\) (b) \(1>2>3>4\) (c) \(1>4>3>2\) (d) \(4>3>2>1\)

Short answer

(b) \(1>2>3>4\)

Step-by-step solution

Understand -R and -M Power

The -R (or -M) power refers to the ability of a substituent to withdraw electrons via resonance. Groups with high -R values are strong electron-withdrawing groups and stabilize negative charge by delocalizing electrons.

Recognize -R (or -M) Groups

Identify the groups in the question and their electron-withdrawing properties: 1. \(\mathrm{NO}_{2}\): Strong electron-withdrawing group due to resonance as it can accept electrons through its double-bonded oxygen atoms.2. \(\mathrm{SO}_{3} \mathrm{H}\): Similar to \(\mathrm{NO}_{2}\), can withdraw electrons via resonance.3. \(\mathrm{CF}_{3}\): Strong inductive effect due to electronegative fluorine atoms, but weaker resonance compared to \(\mathrm{NO}_{2}\) and \(\mathrm{SO}_{3} \mathrm{H}\).4. \(\mathrm{CHO}\): Aldehyde group, weaker -M effect than \(\mathrm{NO}_{2}\), relies on resonance from the oxygen.

Compare the Groups

Compare the resonance effects and inductive effects of these groups. \(\mathrm{NO}_{2}\) and \(\mathrm{SO}_{3} \mathrm{H}\) have strong resonance effects, \(\mathrm{CF}_{3}\) has a strong inductive effect but relatively weaker resonance effect, and \(\mathrm{CHO}\) has weaker resonance.

Order the Groups

Based on their electron-withdrawing resonance effects, arrange the groups in order of decreasing -R (or -M) power: \(\mathrm{NO}_{2}> \mathrm{SO}_{3} \mathrm{H}> \mathrm{CF}_{3}> \mathrm{CHO}\). This corresponds to option (b) in the question.

Key concepts

Electron-Withdrawing Groups

In chemistry, electron-withdrawing groups (EWGs) play a significant role in determining the reactivity and stability of molecules. These groups pull electron density away from the rest of the molecule, enhancing the positive character at certain points in the molecule.
Common electron-withdrawing groups you may encounter include nitro groups (\(\text{NO}_2\)), carboxyl groups (\(\text{COOH}\)), and sulfonyl groups (\(\text{SO}_3\text{H}\)).

• \(\text{NO}_2\) is a very strong electron-withdrawing group due to its ability to participate in resonance.
• \(\text{SO}_3\text{H}\) is also strong, because it can delocalize charge across its structure.
• \(\text{CF}_3\) primarily acts through its inductive effect, which is different from resonance.
• \(\text{CHO}\), or aldehydes, are generally weaker electron-withdrawing groups.

Understanding these groups helps predict how molecules will interact in chemical reactions, particularly where charge distribution is key.

Inductive Effect

The inductive effect is another essential concept in understanding chemical reactivity. It refers to the shifting of electrons in a molecule due to differences in electronegativity between atoms. This effect is transmitted through sigma bonds, leading to polarization in the molecule.
Think of it as a tug-of-war for electrons, where more electronegative atoms, such as fluorine, pull electrons towards themselves, creating a partial negative charge.

• Fluorine atoms in \(\text{CF}_3\) exert a strong inductive effect due to their high electronegativity.
• This effect decreases with distance; hence, the farther an atom is from the electronegative atom, the weaker the inductive effect.
• Inductive effect influences the stability of intermediates in chemical reactions, particularly those involving carbocations and carbanions.

Grasping the inductive effect helps you determine which atom or group holds a stronger electron-withdrawing power through sigma bonds.

Resonance Power

Resonance power is the capability of a substituent to delocalize electrons across different atoms in a molecule, stabilizing it through resonance structures. This effect is crucial for understanding why some groups are stronger electron-withdrawing agents than others.
Resonance involves the overlap of orbitals, which allows the movement of pi electrons, creating multiple forms of a molecule or ion, known as resonance structures.

• \(\text{NO}_2\) showcases strong resonance power by accepting electrons into its pi system on the double-bonded oxygen atoms.
• \(\text{SO}_3\text{H}\) also exhibits significant resonance by spreading out the charge across its oxygen atoms.
• \(\text{CHO}\) uses resonance through its oxygen, but its resonance power is weaker compared to other groups like \(\text{NO}_2\).

Resonance strongly affects the stability of molecules and ions, impacting reaction mechanisms and rates.

MCQ Chemistry Problem

Multiple-choice questions (MCQs) in chemistry are common way of testing your understanding of chemical principles, including electron effects like resonance and induction.
These questions often require you to compare and rank different groups or molecules based on their electronic effects.

• This particular problem asks for the ranking of groups by decreasing \(-R\) (or \(-\text{M}\)) power.
• The correct sequence, \(\text{NO}_2 > \text{SO}_3\text{H} > \text{CF}_3 > \text{CHO}\), reflects their differing capacities for resonance stabilization.
• Such questions test your understanding of how different substituents influence molecular behavior.

By practicing MCQs regularly, you improve your ability to quickly and accurately assess the relative strengths of various chemical effects.