Question

Choose the correctly matched option(s): (Group attached to benzene) (Effect of group) (1) \(-\mathrm{CH}_{3}\) \((\mathrm{P})+\mathrm{R} /+\mathrm{M}\) (2) \(-\mathrm{NO}\) (Q) \(-\mathrm{R} /-\mathrm{M}\) (3) \(-\mathrm{CH}=\mathrm{NH}\) (R) \(-\mathrm{I}\) (4) \(-\mathrm{OH}\) \((\mathrm{S})+\mathrm{H}\) \((\mathrm{T})+\mathrm{I}\) (A) \(1-S, T\) (B) \(3-\mathrm{Q}, \mathrm{R}\) (C) \(2-\mathrm{P}, \mathrm{Q}, \mathrm{R}\) (D) 4-P,R

Short answer

The correct match is (B) \(3-Q, R\).

Step-by-step solution

Identify the effect of each group attached to benzene

To begin, let's identify the effect of each group on the benzene ring. 1. \(-CH_3\): This is a methyl group, which is electron-donating. So, its effects are +R /+M. 2. \(-NO\): Nitro group (\( -NO_2 \)), which is an electron-withdrawing group. Its effects are -R /-M. 3. \(-CH=NH\): This is an imine group, which is electron-withdrawing. Its effects are -I. 4. \(-OH\): This is a hydroxyl group, which is electron-donating. Its effects are +R /+M.

Match the groups with their effects

Now, let's match the groups with their respective effects in the given options. (A) 1-S, T: Methyl group with \( +H, +I\). This is incorrect because the effects are +R / +M instead. (B) 3-Q, R: Imine group with \( -R / -M, -I\). This is correct since the imine group has an -I effect. (C) 2-P, Q, R: Nitro group with \( +R / +M, -R / -M, -I\). This is incorrect as the nitro group has effects -R / -M only. (D) 4-P, R: Hydroxyl group with \( +R / +M, -I\). This is incorrect because the effects should be +R / +M only.

Choose the correct option(s)

Based on the above analysis, the correct match is: (B) \(3-Q, R\)

Key concepts

Group effects on benzene

Benzene, a fundamental aromatic compound, is greatly influenced by the groups attached to it. Depending on whether these groups donate or withdraw electrons, they can enhance or reduce the electron density within the benzene ring. This occurs through mechanisms known as resonance or inductive effects.
Understanding these group effects is essential, as they dictate the chemical behavior of benzene in various reactions. For example, if a group increases the electron density in certain parts of the benzene ring, it can make the ring more nucleophilic. Conversely, if a group withdraws electrons, the ring can become more electrophilic.
These effects are crucial for predicting how benzene will react with other substances, making the study of group effects an essential part of organic chemistry.

Electron-donating groups

Electron-donating groups (EDGs) are pivotal in increasing the electron density of benzene rings. These groups can donate electrons through resonance (+R effect) or inductive (+I effect) mechanisms.
Common electron-donating groups include alkyl groups like methyl ( -CH_3 ) and functional groups like hydroxyl ( -OH ). They stabilize positive charges through resonance by sharing their lone pair of electrons.
As a result, EDGs activate the benzene ring towards electrophilic aromatic substitution reactions, generally making them react faster and more easily with electrophiles. This activation is particularly pronounced at the ortho and para positions relative to the electron-donating group.

Electron-withdrawing groups

Electron-withdrawing groups (EWGs) have the opposite effect of EDGs, as they decrease the electron density in the benzene ring. They achieve this through resonance (-R effect) or inductive (-I effect).
Examples of EWGs include nitro ( -NO_2 ) and imine ( -CH=NH ) groups. These groups often withdraw electrons by pulling them towards themselves, leaving the ring more electropositive.
This withdrawal deactivates the benzene ring, making it less reactive towards electrophilic aromatic substitution. Instead, the benzene ring becomes more positive, and reactions can occur at positions that further stabilize the charge deficit.

Molecular identification

In organic chemistry, identifying functional groups attached to molecules like benzene is crucial for understanding their chemical behavior. Molecular identification involves recognizing specific functional groups that influence the chemical and physical properties of the compound.
This identification can be achieved through various experimental techniques such as NMR, IR spectroscopy, or even chemical tests that highlight the presence of particular groups.
Knowing how to identify these groups allows chemists to predict the compound's reactivity, stability, and compatibility with other substances, providing valuable insight for designing chemical reactions and understanding mechanisms.

Chemical reactions analysis

Analyzing chemical reactions involves understanding how different groups on benzene influence the reactivity and pathways of the reaction. It requires a careful examination of how electron-donating or withdrawing groups affect the nucleophilicity or electrophilicity of the benzene.
Reaction analysis might include drawing reaction mechanisms to visualize electron movement and identifying reaction intermediates or transition states.
Understanding these components is crucial for predicting reaction outcomes, optimizing conditions, and engineering new reactions. This type of analysis underscores the importance of stereochemistry and electronic effects in determining the direction and products of benzene-related chemical reactions.