Question

\(+\mathrm{R}\) power for the following groups decreases in the \(\begin{aligned}&\text { order } \\\&\text { (1) }-\mathrm{NH}_{2} & \text { (2) }-\mathrm{O}\end{aligned}\) (3) \(-\mathrm{OH}\) (4) \(-\mathrm{NHCOCH}_{3}\) (a) \(2>1>3>4\) (b) \(2>3>1>4\) (c) \(1>2>3>4\) (d) \(3>2>1>4\)

Short answer

The correct order is (a) 2>1>3>4.

Step-by-step solution

Understand Resonance (+R) Effect

The +R or positive resonance effect refers to the ability of a substituent to donate electrons through resonance. Groups with lone pairs can donate more effectively via resonance, enhancing the electron density within a molecule.

Evaluate Electron Donating Groups

In the given groups, we need to assess which groups can donate electrons via resonance the most efficiently based on their structure: -NH₂ (amine), -O (oxide anion), -OH (hydroxyl), and -NHCOCH₃ (acetamide).

Compare Group Resonance Donation

The -O group, being an oxide ion, has a strong negative charge and hence possesses a high +R effect. -NH₂, while less charged, has a high +R effect because the nitrogen can delocalize its lone pair. -OH, with its single oxygen bound to hydrogen, has a lesser +R effect than -O or -NH₂. Finally, -NHCOCH₃, where the amide linkage reduces the availability of the nitrogen lone pair for resonance, has the weakest +R effect.

Order Groups by +R Power

Based on the evaluations from the previous step, order the groups from highest to lowest +R effect: -O > -NH₂ > -OH > -NHCOCH₃.

Match with Given Options

From the order determined, compare against the options provided: (2) -O > (1) -NH₂ > (3) -OH > (4) -NHCOCH₃. This matches with option (a) 2>1>3>4.

Key concepts

Electron Donating Groups

Electron donating groups (EDGs) play a pivotal role in understanding the resonance effect in organic chemistry. These groups are characterized by their ability to push electrons towards a conjugated system. By doing so, they enhance the electron density within a molecule. This is particularly significant in aromatic compounds, where EDGs influence the compound's reactivity and stability.

Electron donating groups often possess atoms with lone pairs or are bonded to hydrogen or alkyl groups via single bonds, such as -NH₂ or -OH, making them capable of donating electrons through resonance or inductive effects. The presence of EDGs in an organic molecule can lead to increased resonance stabilization, affecting the molecule's electronic structure and chemical behavior. Understanding how these groups donate electrons is key in predicting the behavior of organic compounds during chemical reactions.

Lone Pairs

Lone pairs are a common feature in many organic molecules, contributing to electron donation through resonance. A lone pair is a pair of valence electrons that are not shared with another atom and are located on an atom within a molecule. These pairs play a critical role in enabling atoms, such as nitrogen and oxygen, to participate in resonance, where they donate their electrons into a conjugated system, increasing its electron density.

This delocalization of electrons from lone pairs can help stabilize charges and radicals within a molecule, significantly impacting its reactivity and structure. For instance, in amines like -NH₂, nitrogen's lone pair can participate in resonance, affecting the substituent's ability to donate electrons. This resonance donation capability is a key factor in determining the molecule's stabilization and reactivity in chemical reactions.

Positive Resonance Effect

The positive resonance effect, or +R effect, is an essential concept in the study of organic chemistry. It describes the phenomenon where certain substituents increase the electron density within a molecule by donating electrons through resonance. The +R effect is attributed to groups that enhance resonance, often those with lone pairs or pi bonds.

Substituents with a strong +R effect can stabilize positive charges or other electron deficient areas in a molecule. This electron-donating capability is crucial in determining the reactivity and stability of the molecule, especially in reactions involving aromatic compounds. For example, -O groups, due to their lone pairs and negative charge, demonstrate a strong +R effect, indicating their ability to enhance the electron density significantly in adjacent groups.

+R Effect

The +R effect, or positive resonance effect, is observed when specific groups increase electron density by contributing electrons via resonance into a conjugated system. Groups like -O, -NH₂, and others with lone pairs are said to exhibit a strong +R effect because they can delocalize their lone pairs efficiently into the system they are attached to.

The strength of the +R effect is central to understanding how these groups impact the molecule's overall electron distribution, stability, and reactivity. For instance, in the given exercise, substituents like -O, which have an oxide ion, show a high +R effect due to their ability to strongly delocalize electrons. In contrast, groups like -NHCOCH₃ have a weaker +R effect because their structure limits the availability of lone pairs for resonance. Knowing the order and strength of the +R effect assists in predicting reaction outcomes and the stability of aromatic systems.