Question

The electrophilic aromatic substitution of a compound \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Y}\) produces mainly a meta-disubstituted product. Among the following, which one could be the substituent \(\mathrm{Y} ?\) \((\) A \()-\mathrm{NH}_{2}\) (B) \(-\mathrm{CH}_{3}\) (C) \(-\mathrm{COOH}\) (D) \(-\mathrm{Cl}\)

Short answer

The correct substituent \(\mathrm{Y}\) that forms a meta-disubstituted product during electrophilic aromatic substitution is option C, \(-\mathrm{COOH}\) group (carboxylic acid), as it is an electron-withdrawing group that directs electrophile to the meta-position.

Step-by-step solution

A. \(\mathrm{-NH_2}\) group

The \(\mathrm{-NH_2}\) group is an electron-donating group due to the lone pair on the nitrogen atom that can resonate with the benzene ring. Hence, it will direct the electrophile to ortho- and para- positions.

B. \(-\mathrm{CH}_{3}\) group

The \(-\mathrm{CH}_{3}\) group (methyl group) is an electron-donating group due to the hyperconjugative effect. It will also direct the electrophile to ortho- and para- positions.

C. \(-\mathrm{COOH}\) group

The \(-\mathrm{COOH}\) group (carboxylic acid) is an electron-withdrawing group due to its electronegative oxygen atoms and the presence of a resonance structure where the benzene ring gets a positive charge. So, it will direct the electrophile to the meta-position.

D. \(-\mathrm{Cl}\) group

The \(-\mathrm{Cl}\) group (chlorine) is a deactivating group because of the strong electronegativity of the chlorine atom; however, it can exert some electron-donating resonance effect. Therefore, it directs the electrophile to the ortho- and para- positions due to partial donation through resonance. #Step 2: Finding the correct substituent # Since we are looking for a substituent that directs the electrophile mainly to the meta-position, we can now identify the correct option.

Answer

The correct substituent \(\mathrm{Y}\) that forms a meta-disubstituted product during electrophilic aromatic substitution is option C, \(-\mathrm{COOH}\) group (carboxylic acid), as it is an electron-withdrawing group that directs electrophile to the meta-position.

Key concepts

Electron-withdrawing Groups

Electron-withdrawing groups play a crucial role in determining how a substituent affects the location of incoming electrophiles during reactions like electrophilic aromatic substitution. These groups tend to pull electron density away from the aromatic ring.
They generally decrease the electron density around the benzene ring, making it less reactive.

Key Characteristics of Electron-withdrawing Groups:

• They often contain electronegative atoms such as oxygen or nitrogen.
• They may include additional features, like resonance, which stabilize the positive charge developed on the ring during the substitution process.
• The ability to pull away electrons often translates to the deactivation of the benzene ring for subsequent electrophilic reactions.

Understanding who these groups affect a ring can help predict reaction outcomes.
For example, the carboxylic acid group (-COOH) in the exercise is identified as an electron-withdrawing group due to its electronegative oxygen atoms.
These traits enable it to influence the reaction towards the meta-position when electrophiles approach.

Meta-directing Groups

Meta-directing groups are a specific subset of substituents that guide new additions on an aromatic ring to the meta position. This guidance is vital in reactions like electrophilic aromatic substitution, where location matters for the structure and properties of the resultant molecule.

Why Meta-direction Occurs:

• Electrophiles prefer the meta position in rings that are less electron-dense at ortho and para positions.
• The electron-withdrawing nature of these groups makes ortho and para positions less favorable for incoming positive charges.
• Meta-directing usually results in more stable intermediates when compared to ortho/para-directing substituents during these reactions.

In the supplied example exercise, the carboxylic acid group was identified as the meta-directing group.
This occurs because its electron-withdrawing oxygen atoms make the ortho and para positions on the benzene ring positively charged, thus discouraging electrophiles from attaching at those locations.

Resonance Effect

The resonance effect is a significant concept when understanding how substituents influence electrophilic aromatic substitution.
Resonance involves the delocalization of electrons within a molecule, which can impact its reactivity and stability.

Features of the Resonance Effect:

• In electron-donating groups, resonance can push electron density into the ring, activating it for further reactions.
• Conversely, electron-withdrawing groups can spread out positive charges across the ring, modifying its reactivity pattern.
• Resonance stabilization occurs when the delocalization leads to lower energy and more stability for reactive intermediates.

In the context of the exercise, the chloride group (-Cl) was a notable example because it shows how even an electron-withdrawing group can exert some resonance effect.
Despite being deactivating due to electronegativity, the resonance effect allows partial donation, thus initially directing the electrophile to ortho/para positions.
This highlights the complex interplay between resonance and position-directing tendencies in aromatic chemistry.